Michael Atzmon, Univ of Michigan
Pascal Bellon, Univ of Illinois-Urban
Evan Ma, Louisiana State Univ
Rohit Trivedi, Iowa State Univ
Energy Conversion Devices, Inc.
- Philips Electron Optics
1997 Fall Exhibitor
Proceedings published as Volume 481
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
SESSION B1: SOLIDIFICATION I
Chairs: Brian Cantor and Rohit Trivedi
Monday Morning, December 1, 1997
America South (W)
8:00 AM *B1.1
PHASE FIELD MODELING OF SOLIDIFICATION MICROSTRUCTURES*. Alain Karma, Department of Physics, Northeastern University, Boston, MA. *Supported by DOE and NASA.
The phase-field method is rapidly emerging as a method of choice for simulating interfacial pattern formation phenomena in solidification and other systems. The appeal of this approach, which initially generated much enthusiasm in the community, is to avoid to track macroscopically sharp phase boundaries by introducing a non-conserved order parameter, or `phase-field', to distinguish between different phases. However, the initial enthusiasm for this method was seriously diminished after quantitative computations by independent groups showed that there are stringent computational constraints associated with a spatially diffuse interface description. These constraints essentially limit the method to large growth rates for the purpose of quantitative modeling. I will discuss the results of an analysis of the phase-field model that makes it possible to overcome some of these limitations, and thereby to recover a good part of this initial enthusiasm. I will then discuss how these results can be implemented to model quantitatively the three-dimensional growth of dendrites from a pure melt and the directional solidification of a dilute binary alloy. Finally, I will illustrate the extension of the approach to model more complex `multiphase' problems involving both growth and nucleation by focusing on peritectic solidification.
8:30 AM B1.2
PARAMETER FREE TEST OF ALLOY DENDRITE GROWTH THEORY. C.B. Arnold1, M. Schwarz2, D.M. Herlach2, M.J. Aziz1, 1 Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA; 2 Institut fúr Raumsimulation, Deutsche Forschungsanstalt fúr Luft- und Raumfahrt, GERMANY.
Predictions for the dendrite tip velocity (v) vs. tip temperature (T) function depend sensitively on the amount of deviation from local interfacial equilibrium during rapid solidification. These deviations (solute trapping; kinetic undercooling) have been characterized experimentally for plane-front growth but never in a system for which the dendrite tip v and T could be measured. Until now, in fitting v(T) data to dendritic growth models, important material parameters such as the diffusive speed for solute trapping (vd) and the liquid diffusivity (DL) have been used as fitting parameters. Here we report v(T) measurements and solute trapping measurements in the same alloy system in order to permit a parameter-free test of theory. The parameters vd and DL were measured for a dilute Ni(Zr) alloy by pulsed laser melting experiments. The dendrite v(T) function was measured independently using containerless electromagnetic levitation processing. The resultant theoretical desription of the growth velocity versus undercooling relation gives an excellent description of the experimental results.
8:45 AM B1.3
MEASUREMENT OF THE DYNAMICS OF SOLIDIFICATION PATTERN SELECTION THROUGH SPATIALLY PERIODIC PERTURBATIONS. Wolfgang Losert, Daniel A. Stillman, and Herman Z. Cummins, City College of CUNY, Dept of Physics, New York, NY.
The stability and dynamics of crystal growth patterns in directional solidification are studied with the model alloy Succinonitrile-Coumarin 152. Using the UV absorption of Coumarin 152 a brief spatially periodic UV perturbation is applied to the solid-liquid interface. The subsequent evolution of the periodically modulated interface pattern is measured with computer assisted videomicroscopy. This technique is applied to a new steady state doublet cellular pattern that was recently found in numerical calculations and also observed experimentally. During the planar-cellular transition a stable or unstable doublet structure is induced for a range of growth velocities and spatial periodicities of the perturbation. The range of parameters for which stable doublets are obtained is measured and compared to unperturbed experiments. The experimental results show remarkable qualitative agreement in the dynamic evolution and steady state structure to new phase-field model simulations by Kopzynski and Rappel.
9:00 AM B1.4
SUBCRITICAL BIFURCATION FROM PLANAR TO CELLULAR INTERFACE IN Al-0.5 wt% Cu DIRECTIONALLY SOLIDIFIED. Osvaldo Fornaro, Hugo A. Palacio, IFIMAT-CIC, Univ. Nacional del Centro, Tandil, ARGENTINA; Heraldo Biloni, LEMIT-CIC, La Plata, ARGENTINA.
To obtain the necessary experimental evidence needed to study the role that plays the instabilities in the process relatives to the microstructure selection for a given values of interface velocity, V and thermal gradient, GL, directional growths with the Al-0.5 wt. % Cu system were carried out under controlled conditions. By means of an interface quenching technique and metallographic analysis in longitudinal and transversal cuts of the samples, we determine the transition mechanisms between the different stages of growth, and associates them to the stability of the solidification front. We study the planar to cellular transition in different conditions, found that else solidification parameters values that are in good accord with the perturbations theory, when we study the amplitude of the perturbations during the planar to a cellular transition, the same theory is not able to predict certainly the critical wavelength in this case. Also, we found a subcritical behavior during the planar to cellular transition for the diluted Al-Cu system. We detect a hysteretic behavior for the amplitude of the perturbations when it is increasing and then decreasing the interface velocity, through the threshold. In adding to that, we study the microstructure behavior for different solidification conditions. passing from planar, cellular and dendritic microstructures. Then, we analyze the change of the microstructure for variations made in the initial solidification conditions. We conclude that the local solidification parameters play a fundamental role during the lateral growth of the cells, controlling the formation or disappearance of cellular walls, needed for the primary spacing regulation. With the obtained information would be possible to got an improvement in the existing models, both theoretical and numerical, through the inclusion of the mentioned mechanisms, so as it would permit a more accurate knowledge about the microsegregation map associated with the bulk crystalline growth of alloys.
9:15 AM B1.5
MELTING VS. SOLIDIFICATION OF A PURE METAL ANALYSED BY DSC. N. Clavaguera, Fac. Fisica, Univ. Barcelona; M.T. Clavaguera-Mora, J. Fontan, J.L. Touron, C. Comas, Univ. Autonoma Barcelona, SPAIN.
Kinetics of phase transformation studies include several experimental techniques, In particular, optical or electron microscopy, X-ray or neutron diffraction, electrical resistivity, magnetic susceptibility, M–ssbauer spectroscopy, thermal analysis and dilatometry. One of the thermal analysis methods, specifically differential scanning calorimetry (DSC) is of particular interest in first order phase transformations since it provides the rate of transformation as a function of time or temperature by measuring the heat released or absorbed during the phase change. Under equilibrium conditions the melting temperature, Tm, is a well defined quantity. The aim of the present paper is to analyse the change of Tm resulting from non-equilibrium conditions. Attention is focused to direct measurement of the temperature of both melting of an overheated solid and solidification of an undercooled melt. The non-equilibrium transformations are monitored by DSC under isothermal, continuous heating and continuous cooling regimes. The dependence of Tm on thermodynamic factors and , is explored. Here, is the Gibbs free energy difference and is the interfacial energy between the liquid and the crystal. In particular an investigation is performed on the melting-solidification behaviour of In and Pb.
9:30 AM B1.6
TRANSITORY METASTABLE PHASES IN REFRACTORY METALS-BASED SYSTEMS SOLIDIFIED BY DROP-TUBE PROCESSING A FIRTS-PRINCIPLES APPROACH. Alain Pasturel, LPMMC, Grenoble, FRANCE.
The solidification path of highly undercooled refractory metals and their alloys in a ultrahigh vacuum drop tube is shown to involve transitory metastable phases. For pure metals like Re and Ta, it may be thought that the high undercooling of a liquid is favorable to the formation of metastable phases. For alloys like Re-W system, the undercooled-induced metastable phased displays a wide range of composition. First-principles calculations of the structural stability of these systems are developed to appreciate the possibilities of obtaining such transitory metastable phases. Some prototype transition metal crystal structures based on the Frank-Kasper phases are examined and are shown to be possible candidates for the observed metastable phases. For those found in alloys, the broad range of composition is related to site occupation in the Frank-Kaspesr phases, assuming a mean-field treatment of order-disorder phenomena.
10:00 AM *B1.7
David Turnbull Award Lecture
SOLIDIFICATION SCIENCE AND ENGINEERING PRACTICE. Merton Flemings, MIT, Dept of Materials, Science & Engineering, Cambridge, MA.
Solidification science and the engineering practice of solidification processing have advanced arm-in-arm over the last 40 years, each sometimes leading the other. Today, mathematical modelling and computer controls are greatly aiding the marriage of the two. This talk will focus on dendritic solidification of metals and ceramics. We know a great deal today about dendrite structure and microsegregation; these features influence in important ways the properties of castings, ceramic abrasives, spray formed materials, and wrought materials. . Modern macrosegregation theory is critical to design and conduct of newer continuous casting processes. Directional solidification theory places limits on, and illustrates opportunities in, processing of metals and ceramics (e.g. high Tc materials). Elimination of dendrites by vigorous agitation is making possible more reliable, stronger die formed parts for the automotive industry. Elimination of dendrites by rapid solidification is making possible a variety of new products, including improved golf clubs. Concluding remarks will relate to the importance of mathematical modelling and computer controls in today's solidification research and practice.
10:45 AM *B1.8
SOLIDIFICATION AND SOLID STATE TRANSFORMATION IN PERITECTIC Fe-Ni ALLOYS. W. Kurz and M. Vandyoussefi, Swiss Federal Institute of Technology Lausanne, Lausanne, SWITZERLAND.
During solidification of peritectic alloys various phase transformations may be observed. Depending on composition i) two phases crystallize successively at different temperatures, leading, in a positive temperature gradient, to two distinct interfaces, ii) two phases, one stable, the other metastable, compete. In the latter case the metastable phase often wins at high solidification velocities due to its smaller solute rejection. Experimental evidence for these phenomena in Fe - Ni alloys will be presented. The results are discussed in the light of modern solidification theory.
11:15 AM B1.9
MECHANISMS OF LAYERED STRUCTURE FORMATION IN PERITECTIC SYSTEMS. P. Mazumder, J.S. Park, A. Karmw*, R. Trivedi, Ames Laboratory, US-DOE and Department of Materials Science and Engineering, IA; *Department of Physics and Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, MA.
The solidification of two phase microstructures in peritectic systems has recently received quantitative examination. Models based on unidirectional diffusion of solute are developed to predict the conditions under which alternate bands of primary and peritectic phases can form. Experimental studies, however, have shown that these models are not operative in the experiments carried out in the Pb-Bi and Sn-Cd systems. The frequently observed layered structure , is a complex structure that is interconnected in three dimensions and can occur for a wide range of initial composition as opposed to that predicted by the previous models. We investigate the effect of both convective and diffusive transport on peritectic microstructure through semi-analytical and numerical modeling. The model consists four different mechanisms under which banded or layered structures can form: the diffusive, the steady convective, the unsteady convective and the turbulent mixing. Discrete bands can form under diffusive or turbulent mixing conditions We shall show that the interconnected structures, which appear as bands on a vertical section, evolve when unsteady convection is present in the melt. In this case the oscillatory behavior of the solute profile in the liquid can give rise to the simultaneous growth of the primary and peritectic phases in which the primary phase forms as a macroscopic Christmas-tree. The importance of oscillatory convection will be demonstrated through experimental results in the Pb-Bi and Sn-Cd system in which the oscillatory structures disappear when the alloy is solidified in finer tubes in which convection effects are minimized. Only in the limiting conditions of no convection or turbulent mixing banded microstructures may develop according to the earlier `diffusive' or `boundary layer' models.
11:30 AM B1.10
PERITECTIC SOLIDIFICATION OF SUPERCONDUCTION RE-123 FACETED OXIDES. Yuh Shiohara, Div.IV, Superconductivity Research Lab.(SRL), ISTEC, Tokyo, JAPAN.
Since discovery of high temperature superconductive oxides in 1986, solidification processing has been intensively investigated for syntheses of bulk pellets and single crystals. This solidification route has been well recognized as a promising process for achieving high performance in superconductivity properties including higher critical temperature, T-c, and higher critical current, Jc. Further improvement of these properties could be attained through deep understandings of phase transformation. In this talk, phase equilibria and solidification processing of RE-123 (REBa2Cu3O6+8, RE;Y,Nd,Sm) are reviewed, focusing on peritectic reaction mechanisms of these oxides. The solidification of the 123 phase has been observed to be coupled to the disappearance of the high temperature stable 211 phase by diffusion through the liquid. Temperature gradient and solidification rate were shown to influence the morphologies of the 211 and the peritectic 123 phases. Various aspects of the peritectic reaction mode have been considered, including the coarsening of the 211 phase particles in the liquid, the likelihood of the 211 particles being pushed by the advancing 123 interface, the effects of capillarity, temperature gradient, and undercooling on the compositional difference which drives the diffusion. Single crystal pulling, unidirectional solidification, and bulk isothermal solidification with undercooking are the processes to be discussed.
11:45 AM B1.11
MICROSTRUCTURE OF Ni2B LASER INDUCED SURFACE ALLOYED -Fe. Gerhard Dehm, Menachem Bamberger, Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa, ISRAEL.
Laser alloying; of -Fe with borides is a well suited method to increase the chemical and abrasive resistance of the substrate surface. In this study Ni2B powder with a particle size of 100 m to 150 m was injected into the molten bath of the substrate surface created by irradiation with a CW-CO2 laser possessing a beam diameter of 2mm. The 10mm thick -Fe plate was scanned with a 50% overlap between successive passes. Single laser scans without any overlap were also conducted in order to investigate the influence of Ni2B content on the microstructure. Scanning and transmission electron microscopy investigations reveal an off eutectic microstructure in the melting zone which extends over a thickness of 500 m to 800 m. In the case of a single laser scan the matrix consists of an -Fe(Ni) matrix and eutectic -Fe(Ni) and Fe3B lamellas. Electron energy-loss spectroscopy measurements show that the -Fe(Ni) phase possesses 90.5 2at% Fe and 9.5 2at% Ni in solid and the Fe3B phase consists of 72 6at% Fe, 6.6 3at% Ni and 21.4 7at% B. In contrast, a 50% overlap between successive laser paths leads to the formation of FeNi dendrites and lamellas of -FeNi and (Fe,Ni)3B due to the larger amount of Ni2B injected to the sample. The -FeNi phase consists of 60 5at% Fe and 40 5at% Ni and the (Fe,Ni)3B lamellas possess 46 5at% Fe, 32 6at% Ni and 22 3% B. While neighbouring -FeNi dendrites and -FeNi lamellas show the same crystallographic orientation, no defined orientation relationship exists between -FeNi lamellas and (Fe,Ni)3B lamellas.
SESSION B2: SOLIDIFICATION II
Chairs: Alain S. Karma and John H. Perepezko
Monday Afternoon, December 1, 1997
America South (W)
1:30 PM *B2.1
THE NUCLEATION OF SECONDARY PHASES. K.A.Q. O'Reilly and B. Cantor, Department of Materials, University of Oxford, Oxford, UNITED KINGDOM.
This paper describes the nucleation of secondary phases in inter-dendritic liquid, simulated by a combination of DSC, TEM and XRD. These techniques are applied to multi-component Al alloys and demonstrate the significance of ppm levels of impurities in controlling the nucleation behaviour.
2:00 PM B2.2
GRAIN REFINEMENT IN UNDERCOOLED METALS. Jian-Zhong Xiao, Hua Yang, Hin-Wing Kui, The Chinese University of Hong Kong, Dept of Physics, Shatin, HONG KONG.
Recently, we demonstrated that grain refinement in undercooled Ni is brought about by dynamic nucleation while that of Cu30Ni70 is due to the remelting of a novel dendrite formed during the early stage of the solidification process. Compared Ni and Cu30Ni_70, it is clear that by continuously removing Cu from Cu_30Ni_70, a transition of the grain refinement mechanism from remelting to dynamic nucleation is anticipated. Now grain growth after solidification in Cu-Ni is serious. The transition is therefore not clearly displayed. However, such a transition is vividly exhibited in undercooled Ni-B. Interesting and unique micrographs will be shown here. finally, the origin of the grain refinement transition from remelting to dynamic nucleation is also discussed.
2:15 PM B2.3
MECHANISMS, MODES, AND RATES OF SOLID NUCLEATION IN RAPIDLY QUENCHED LIQUIDS. Vikas V. Gupta and James S. Im, Department of Chemical Eng., Materials Science and Mining Eng., Columbia University, New York, NY.
Rapid quenching of liquids is an effective and established means of obtaining amorphous and microcrystalline materials. Here, a proper manipulation of solid nucleation in supercooled liquids is essential-this is true regardless of whether the nucleation of solids is desired in copious amounts (e.g., formation of nanocrystals), is selectively controlled (e.g., formation of metastable phases), or is to be avoided as much as possible (e.g., formation of glasses). During the quench, the size of a critical nucleus of a solid phase with lower volumetric free energy than the liquid phase decreases as a function of time; this in turn-according to classical nucleation theory - can make the athermal nucleation mechanism an increasingly potent route through which nucleation can proceed. In this paper, we quantitatively illustrate the precise relationships among athermal and thermal mechanisms as well as the transient and steady state modes of nucleation that manifest themselves during rapid quenching of liquids. We accomplish this by computationally simulating the evolution of a small cluster population for conditions corresponding to - as a model system-those that are encountered in pulsed-laserinduced melting and rapid quenching (108 to 1012 K/s) of thin Si films on an SiO2 surface. The results, which are calculated using currently available physical parameters, clearly reveal that the nature of nucleation goes from an essentially steadystate mode/thermal-mechanism-dominated scenario at lower quenching rates to a transient mode/athermal-mechanism-dominated scenario at higher rates. The results also show that despite having a substantially smaller density of critical clusters, the total/athermal nucleation rates in deeply and rapidly supercooled Si can actually exceed that of the steady state values.
2:30 PM B2.4
IMPURITY EFFECTS ON NUCLEATION - COMPARISONS BETWEEN BULK SAMPLES AND DROPLET DISPERSIONS. C. W. Morton, W. H. Hofmeister, R. J. Bayuzick, Vanderbilt University, Dept. of Applied and Engineering Sciences, Nashville, TN; A. J. Rulison, J. L. Watkins, Space Systems/Loral, Palo Alto, CA.
Undercooling experiments have been conducted using 1.5 mm diameter samples of zirconium processed in the electrostatic levitator at Space Systems/Loral. The samples were prepared from three different grades of zirconium with stock purities of 99.8, 99.95, and 99.995% by weight. About 100 consecutive undercooling measurements were obtained for each sample. Histograms of the nucleation temperatures were constructed and revealed narrow distributions with mean temperatures that decreased with sample purity. The preexponential and exponential factors of the classical nucleation rate equation have been calculated using a statistical technique for fitting the distribution of undercoolings obtained on single bulk samples. The preexponential factors increased from 1032 to 1045 while the exponential term increased from 64 to 93 kT with increasing purity. The extremely high terms and the trend with purity suggest that modifications must be made to the theoretical framework. The undercooling results are discussed in light of similar observations concerning impurity effects in droplet dispersions. Kinetic analysis of differential thermal analysis (DTA) and differential scanning calorimetry (DSC) curves bears some similarity to the statistical analysis of bulk sample undercooling data. The similarities and differences between the results of droplet dispersion experiments and bulk experiments will be discussed in terms of impurity effects which have been present in one form or another in all undercooling experiments.
SESSION B3: NUCLEATION
Chairs: Alain S. Karma and John H. Perepezko
Monday Afternoon, December 1, 1997
America South (W)
3:00 PM B3.1
ANALYTICAL DESCRIPTION OF TIME-DEPENDENT NUCLEATION AND CRYSTALLIZATION. Vitaly A. Shneidman, University of Arizona, Dept of Materials Science, Tucson, AZ.
In the limit of a high nucleation barrier, , asymptotically accurate solutions to the time-dependent nucleation equation often can be derived using matched asymptotic technique. These solutions also incorporate post-nucleation growth of particles. The matched asymptotic approach is straightforward for the transient (isothermal) nucleation problem as well as for the description of a quench. Adding the heating stage which is required to describe devitrification of glasses complicates the problem, but it still can be described analytically. Results are compared with numerical solutions of the Turnbull-Fisher nucleation equations as obtained by Kelton, Greer and co-workers. Relevant experimental data for organic and inorganic glass formers are also discussed.
3:15 PM B3.2
DERIVATIONS OF THE KINETICS OF PHASE TRANSFORMATIONS WITH NUCLEATION AND GROWTH MECHANISM. Ge Yu, S.T. Lee and J.K.L. Lai, City University of Hong Kong, Dept of Physics and Materials Science, Kowloon, HONG KONG.
A new method based on the probability theory is developed for the theoretical treatment of the kinetics of phase transformations with nucleation and growth mechanism. From the calculation of the survival probability step by step, a number of problems can be analytically studied. In comparison to the derivation of Avrami equation, which has been used for the interpretation of the experimental results for an half of century, the new derivation is transparent and convincing. It is demonstrated, that in our treatment both nucleation and growth rates are allowed to vary in time and space. Moreover, by calculating the probability of the correlated occurrences, the problems of grain size distribution during the transformation can be analytically solved.
3:30 PM B3.3
TIME-DEPENDENT NUCLEATION IN PARTITIONING SYSTEMS. K.F. Kelton and K. Lakshmi Narayan, Department of Physics, Washington University, St. Louis, MO.
Nucleation in multi-component systems is poorly understood. This is particularly true when long-range diffusion becomes competitive with interfacial processes. Studies of the time-dependent nucleation rates in glasses that crystallize to a phase of the same composition have previously allowed fundamental investigations of the nucleation kinetics. To date, however, no similar studies have been made of the compositional dependence of the nucleation rate. The first measurements of the time-dependent nucleation rates in Na2O.2CaO.3SiO2 glasses made as a function of [SiO2] are presented. The rates obtained from multi-step annealing treatments agree reasonably well with predictions from the classical theory of nucleation, assuming a composition-dependent interfacial free energy. Quantitative deviations are evident, however. A new model that accounts for the simultaneous stochastic processes of atomic diffusion to the region of the cluster and attachment to the cluster interface is presented and applied to these data and to nucleation data in bulk metallic glasses.
3:45 PM B3.4
DISCRETE AND CONTINUUM ANALYSES OF ATHERMAL AND THERMAL MECHANISMS OF NUCLEATION IN CONDENSED SYSTEMS. James S. Im, Department of Chemical Eng., Materials Science and Mining Eng., Columbia University, New York, NY.
First-order phase transformations that proceed far from equilibrium tend to do so under highly transient conditions. In such an environment, nucleation of the product phase invariably figures more prominently in the transformation than would otherwise be the case under milder circumstances. It is known that when a first-order transformation proceeds in a system whose state variables change temporally so as to make the critical nucleus size change as a function of time, classical nucleation theory predicts that the athermal nucleation mechanism can become a path through which subcritical (supercritical) clusters are promoted (demoted) to the supercritical (subcritical) status. In this paper, by focusing on the details of the formation of supercritical clusters within the framework of classical nucleation theory, we examine the nature of nucleation in condensed systems. It is shown that the discrete formulation of the theory yields a complete and general description of the nucleation phenomenon in terms of the thermal and atherrnal mechanisms. An analysis of the total nucleation rates using the continuum approximation further reveals that the magnitudes of the thermal and athermal nucleation rates depend similarly on the density of critical clusters. One simple and important consequence of this is that the athermal nucleation mechanism can become relatively significant whenever the rate of change in critical cluster size, dn^+/dt, becomes comparable to the adatom attachment frequency evaluated at the critical size,k_n*^+
4:00 PM B3.5
COMPETITION OF THE NUCLEATION MODES IN THE CONCENTRATION GRADIENT. Fiqiri Hodaj, LTPCM-UMR CNRS/INPG/UJF, Saint Martin d'Heres, FRANCE; Andrei M. Gusak, Andriy O. Kovalchuk, Department of Theoretical Physics, Cherkassy State University, Cherkassy, UKRAINE; Pierre J. Desre, LTPCM-UMR CNRS/INPG/UJF, Saint Martin d'Heres, FRANCE.
Several works on reaction kinetics in bimetallic multilayers have demonstrated that sharp unidirectional concentration gradient, which develop as interdiffusion proceeds in the phase formed at the interface (supersaturated solid solution or amorphous phase) are able to delay or to suppress nucleation of intermetallics. It have been found that the existence of a critical gradient beyond which nucleation is inhibited is strongly dependent on the mechanism of formation of the embryo.
In this work a mechanism based on a combination between diffusion in the concentration gradient ( C) direction and diffusion in a direction perpendicular to C is proposed and treated on the basis of the Fokker-Planck equation for the distribution in the size space. The frequency of one atom's joining/departure the embryo is calculated as the superposition of the above mentioned two nucleation modes. The ``critical nucleus'' corresponds to the minimum of the distribution function found as the solution of the stationary Fokker-Planck equation and depends on the ratio of diffusivities in the parent and new intermediate phases.
The influence of the aspect ratio of the embryo on the critical concentration gradient is studied. Due to the fluctuations of the embryo shape, it is shown that the minimization of the thermodynamic potential leading to the equilibrium aspect ratio of the parallepiped-shaped embryo is only possible above a certain value of the concentration gradient. If the shape optimization is made, the nucleation becomes formally possible at any gradient when diffusion in the C direction is allowed. The main point remains the dependence of the nucleation barrier on atomic diffusivities in both phases. Application is presented to the nucleation of the compound Al3Ni and Ni10Zr7 in a supersaturated solid solution Al-Ni and in an amorphous layer Ni-Zr respectively.
4:15 PM B3.6
PARTICLE NUCLEATION IN DISSIPATIVE SYSTEMS. Kenneth C. Russell, Mat Sci Eng and Nucl Eng Dept, MIT, Cambridge, MA.
Conventional nucleation theory describes metastable systems, wherein the only energy loss is due to the trickle of critical nuclei over the activation barrier. This work addresses dissipative systems with a continuous influx of energy. Systems undergoing irradiation or intense plastic deformation are used as examples. The dissipative processes may either destroy or create embryos. Equations are derived for such cases and applied to bulk and surface irradiation and to plastic deformation.
4:30 PM B3.7
PHASE-SEPARATION: FROM THE INITIAL NUCLEATION STAGE TO THE FINAL OSTWALD RIPENING REGIME. Celeste Sagui, Dean Stinson O'Gorman, and Martin Grant, Centre for the Physics of Materials, Physics Department, McGill University, Montreal, CANADA.
In this work we have re-examined the classical problem of nucleation and growth. We introduce a new, self-consistent model that combines steady-state homogeneous nucleation theory with the classical Ostwald Ripening mechanism. The model, formulated in terms of a set of interface equations, naturally incorporates the correlations which originate in the overlapping of the diffusional fields corresponding to the different precipitates. The time evolution proceeds in three stages: a nucleation, a diffusive growth and a coarsening stage. Nucleation of droplets produces the initial depletion of the supersaturation. Diffusive growth occurs when the nuclei grow by seizing material from the background supersaturation. When the supersaturation is sufficiently reduced, growth proceeds through Ostwald ripening, and becomes a global, interactive phenomenon.
4:45 PM B3.8
DOMAIN-SPATIAL CORRELATION FUNCTIONS AND SCALING RELATIONS DURING NUCLEATION AND GROWTH. Tao Huang, Tomohiro Tsuji, M.R. Kamal,and A.D.Rey, McGill Univ, Dept of Chemical Engineering, Montreal, Quebec, CANADA.
Nucleation and growth in 2D semi-crystalline polymer films during free solidification experiments have been investigated via direct imaging in real-time in-situ observation and measurements. To characterize the experimental data and elucidate the governing scaling laws, we propose a novel domain-spatial correlation function G(r,t). The correlation function directly explores the time-dependent domain-size distribution function and the spatial correlation function of domain core centers simultaneously throughout the entire process, including the post-nucleation, domain growth and grain formation stages. The scaling relation: G(r,t)/G(r=0,t)=F(r/Rmax(t)), where Rmax(t) is the maximum domain size at time t, has been defined and evaluated from experimental data. It is exact for free growth during the post-nucleation stage, and it also provides a basis for the interpolation between the impingement stage and grain structures.
SESSION B4: MICROSTRUCTURAL EVOLUTION AND STRESS EFFECTS
Chairs: Michael Atzmon and Frans Spaepen
Tuesday Morning, December 2, 1997
America South (W)
8:15 AM *B4.1
STABILITY AND TRANSITIONS OF ANISOTROPIC TRIPLE JUNCTIONS: A GENERALIZATION OF THE TU-TURNBULL REPLACEMENT MECHANISM. John W. Cahn, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD; Jean E. Taylor, Mathematics Department, Rutgers University, Piscataway, NJ; Hubert I. Aaronson, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA; Chandra S. Pande, Physical Metallurgy Branch, Naval Research Laboratory, Washington DC.
In an evolving microstructure, interfacial free energies are changing either due to changes in temperature, composition or stress, or due to changes in orientations of anisotropic moving interfaces. We explore various factors in the existence, structure and stability of junctions of three surfaces when one or more is anisotropic, identify possible transitions in the stability of junctions, and investigate some consequences on phase transformation mechanisms. Experimental observations of the initiation of motion of a grain boundary-precipitate triple junction from one side of the precipitate to another was linked to the initiation of cellular precipitation by Tu and Turnbull, who called it the replacement mechanism, and linked it qualitatively to changes in interfacial free energy. We suggest that this is one possible example of a loss of stability of a triple junction.
8:45 AM B4.2
IN SITU PHASE MAPPPING AND MICROSTRUCTURE UNDER STEEP THERMAL GRADIENTS USING SPATIALLY RESOLVED DIFFRACTION WITH SYNCHROTRON RADIATION. Joe Wong, J.W. Elmer, and T. Ressler, Lawrence Livermore National Laboratory, Livermore, CA.
We have developed a SRXRD (spatially resolved x-ray diffraction) technique using intense synchrotron x-radiation and applied it to map the phases present and the microstructural changes in the heat affected zone (HAZ) of fusion welds in situ and in real-time. In this paper, the SRXRD instrumentation utilizing a 180 micron synchrotron beam will be described in some detail. Results for complete phase mapping of the - transformation in the HAZ of a commercially pure Ti arc weld will be presented. This material exhibits an allotropic transformation from the hcp -phase to a bcc -phase at 922 C. The microstructure at temperature may be deduced from texturing at different regions in the thermal gradient as reviewed by various (hkl) reflections (preferred orientation) and corresponding linewidth. This experimental phase data corroborated well with that calculated from a heat flow model . Concentration profiles derived from the SRXRD data revealed co-existence of both the - and -phases in the HAZ . Novel SRXRD experiments using imaging plates together with a Soller slit assembly to afford a more efficient row-by-row mapping will also be discussed.
9:00 AM B4.3
STRUCTURAL TRANSFORMATION OF GRAIN BOUNDARIES BY VACANCY SEGREGATION. A. Maiti, M. F. Chisholm, S. J. Pennycook, Oak Ridge National Laboratory, Oak Ridge, TN; and S. T. Pantelides, Vanderbilt University, Nashville, TN and Oak Ridge National Laboratory, Oak Ridge, TN.
We report atomic-scale simulations of a Si grain boundary and show that a structural transformation with very low activation energy can be induced by the segregation of vacancies in certain pairs of columns. The vacancies are completely absorbed so that all atoms retain fourfold coordination with a few long bonds along low-density pipes. The transformation can be induced spontaneously by the application of moderate uniaxial stress. In the early stages of vacancy segregation, prior to the structural transformation, preferential vacancy segregation occurs in certain atomic columns. These results provide an explanation of atomic-resolution electron microscopy images of a Si tilt grain boundary that find preferential modification of the same atomic columns during electron irradiation.
9:15 AM *B4.4
STRESS EFFECTS ON ATOMIC AND INTERFACIAL MOBILITIES. Michael J. Aziz, Division of Engineering and Applied Sciences, Harvard University, Cambridge MA.
Stress can affect phase transformations by altering the equilibrium state and altering the kinetic rate constants, or mobilities. To date much work has been done on the effects of stress on the energetics of phase transformations but relatively little is known about its effects on the atomic or interfacial mobilites, which can be expected to dominate the behavior in systems driven far from equilibrium. The first measurements of these effects have been made for atomic diffusion in silicon and for solid phase epitaxy of silicon. To understand the effects observed on an atomistic level, Transition State Theory has been extended to nonhdrostatic stress states; the result is that the Activation Volume, which characterizes the effect of hydrostatic pressure, generalizes to the Activation Strain Tensor. Experimental tests of the predictive capabilities of the theory will be reported.
10:15 AM *B4.5
OSTWALD RIPENING IN ELASTICALLY STRESSED SOLIDS. N. Akaiwa*, M.E. Thompson and P.W. Voorhees, Department of Materials Science and Engineering, Northwestern University, Evanston, IL; *National Research Institute for Metals, JAPAN.
Elastic stress is present in a wide variety of two-phase solids. A major challenge in predicting the evolution of microstructures in thes systems is that the difference in lattice parameters between the particle and matric engenders a long-ranged elastic stress field. Unlike stress-free systems, where the evolution of the microstructure is driven by a decrease in the total interfacial energy of the two-phase mixture, the ripening process in these materials is driven by a decrease in the sum of the elastic and interfacial energies. This change can give rise to qualitatively new phenomena, such as inverse Ostwald ripeining wherein small particles grow at the expense of large particles, large-scale particle migration and particle shape bifurcations. It is now possible to calculate numerically the morphological evolution of many thousands of elastically and diffusionally interacting particles. Using these calculations we can make predictions on the evolution of statistically averaged properties of these coarsening ensembles that are of relevance to the design of high temperature alloys, sucha as the growth rate of the average particle size. A discussion of these results will be given.
10:45 AM B4.6
KINETIC LATTICE MONTE CARLO SIMULATIONS OF DIFFUSION AND DECOMPOSITION KINETICS IN IRON-COPPER ALLOYS: EMBEDDED ATOM VERSUS NEAREST NEIGHBOUR PAIR POTENTIALS. Brian D. Wirth, G. Robert Odette, Dept of Mechanical and Environmental Engineering, University of California, Santa Barbara, Santa Barbara, CA.
In principle, Kinetic Lattice Monte Carlo (KLMC) methods can accurately simulate the precipitation of coherent phases by tracking the motion of a vacancy and the corresponding diffusion and clustering of solutes in a lattice simulation cell. However, the fidelity of the KLMC simulations depends on the validity of the assumed interatomic potentials. These potentials must not only reproduce the correct energetic interactions, but also the physical mechanism paths. For example, solute diffusion in bcc alloys requires jumps between first and second nearest neighbors, and is governed by at least five independent jump frequencies. Multiatom embedded-atom-method (EAM) potentials can, in principle, reproduce the physical sequence of events, while nearest neighbor (NN) potentials cannot. Further, at small sizes cluster energetic and morphologies, critical to nucleation processes, are somewhat different for NN versus EAM potentials. This work systematically explores EAM versus NN based KLMC models in terms of isolated solute diffusion coefficients, clustering paths and overall decomposition kinetics. Jump frequencies and configuration dependent cluster energetics for the EAM model are derived from molecular dynamics and statics calculations.
11:00 AM B4.7
A POSSIBLE MECHANISM FOR ATOMIC TRANSPORT IN AMORPHOUS METALS, Leon van Ee, Jult Sietsma and Barend J. Thijsse, Delft University of Technology, Laboratory of Materials Science, Delft, NETHERLANDS.
How does diffusion take place in a solid that contains no grain boundaries or atomic-size vacancies and does not allow interstitial atomic transport? Recent results from molecular dynamics simulations of amorphous Ni81B19 shed new light on the mechanism by which collective atomic motion can lead to small diffusive jumps of a group of atoms. The key idea is to look for locations where the atomic mobility is higher than average, not by inspecting the local atomic environments, but by computing the spectrum of vibrational modes and look for low-frequency modes strongly localized on only a few atoms. Following a new approach we excite such modes and find that if the excitation energy is high enough, the system can make a transition to a new state. Such a ``jump'' involves some tens of atoms and call be reversible or irreversible. The total square displacement is usually more than 1 Å2. After the jump the system contains other locations with similar dynamical properties, which may be the sites of new diffusion events. The process is a truly cooperative one: the potential-energy changes during a jump indicate that the new configuration is stable only because the displacements of all atoms create a new potential-energy minimum during the transition; this new potential-energy minimum is not yet present before the jump. Several atomic-level features of this possible diffusion mechanism are discussed.
11:15 AM B4.8
THEORETICAL MODELLING AND COMPUTER SIMULATIONS OF A CHESSBOARD-LIKE MICROSTRUCTURE DEVELOPMENT DRIVEN BY TRANSFORMATION INDUCED ELASTIC STRAIN. Yann Le Bouar***, Armen G. Khachaturyan*, Annick Loiseau** ;* Department of Ceramics and Material Science, Rutgers University, Piscataway, NJ; ** Office National d'Etudes et de Recherches Aerospatiales,Chatillon, FRANCE.
The understanding and control of the microstructure evolution of multiphase alloys is of critical importance to synthesize advanced materials with given properties. Recently, numerous experimental descriptions of the microstructure of several multiphase alloys have been performed using Transmission Electron Microscopy. We have focused on puzzling observations in the Co39.5-Pt60.5 magnetic alloy and the CuAu-based dental alloys where a fascinating microstructure consisting of a regular tiling has been found. Up to now, the precise understanding of the role of the transformation induced elastic strain in this microstructure formation has never been investigated in details. Firstly, we present a computational method based on the continuum stochastic field equation, able to describe a first-order phase transition with a cubic-> tetragonal symmetry reduction and a multicomponent long-range order parameter. No a priori constraints are made on the possible configurations and sequences of structural patterns. Then, 2D computer simulations are performed for an elastically isotropic and homogeneous crystal. The simulations predict the formation of the chessboard-like microstructure whose edges are aligned with the elastic soft directions, and show that the coarsening of such a microstructure is only possible with the disappearance of an entire band of the pattern. These simulation results are in very good agreement with experimental observations in both the CuAu-based dental alloys and the Co39.5-Pt60.5 binary alloy.
11:30 AM B4.9
EVOLUTION OF PRECIPITATION KINETICS AND MORPHOLOGY IN A MONOCRYSTALLINE Ni-Al ALLOY AGED UNDER UNIAXIAL COMPRESSION. Sergey V. Prikhodko, Alan J. Ardell, Dept. of Materials Science and Engineering, University of California, Los Angeles, CA.
The influence of externally applied uniaxal compression on coarsening of precipitates in a Ni-12.57 at.% Al alloy at 650 is currently under investigation. The stress is applied to cylindrical, doubly-tapered, monocrystalline specimens with axes of revolution parallel to . The magnitudes of the stresses for 35 to 140 MPa) are such that the strains are purely elastic except perhaps where the cross section has its smallest value of 3mm. Dark-field transmission electron microscopy, coupled with image-analysis software, are used to characterize the evolution of the precipitate morphology and the kinetics of coarsening. Our preliminary work reveals a totally unexpected non-monotonic dependence of coarsening behavior on the applied stress. The kinetics are accelerated at an intermediate value of the stress, in the neighborhood of 55 to 60 MPa. At higher and lower stresses the morphological evolution and coarsening kinetics are comparable to those under stress-free conditions. The obvious experimental variables (unexpected temperature gradients, etc.) can be eliminated as a cause of the unusual behavior observed.
11:45 AM B4.10
INDENTATION-INDUCED PHASE TRANSFORMATIONS IN DIAMOND. Yury G. Gogotsi, Univ. of Illinois at Chicago, Dept. of Mechanical Engineering, Chicago, IL; Klaus G. Nickel, Andreas Kailer, Univ. of Tubingen, Inst. of Mineralogy, Petrology and Geochemistry, Tubingen, GERMANY.
The stability of diamond under pressure and the structure of hypothetical high-pressure phases have been a controversial issue for a long time Theoretically predicted values for the Mott-transition (band-gap closure) and a number of other transformations under hydrostatic and uniaxial compression vary from hundreds to thousands of GPa. The conventional structure of diamond seems experimentally to be stable up to the highest static pressures that the modern high-pressure technology cart achieve. We addressed the problem by a different way, namely by decreasing the contact area of pressurization instead of increasing the total load. Experimentally this can be easily done in indentation tests using a sharp diamond indenter. Such a test creates in addition to hydrostatic stresses, shear stresses as well. Here deformations and transformations may be realized, which are either impossible or would require much higher pressures when utilizing only hydrostatic stresses. Coupling the indentation loading with micro-Raman spectroscopy we were able to drive and monitor phase transformations in diamond, and a number of new phases have been found. The new features observed on the Raman spectra can be ascribed to graphitic carbon, lonsdaleite, other hexagonal or rombohedral diamond polytypes, as well as to high-pressure phases of diamond R8 or BC8). We observed several other bands that have been assigned to twins and stacking faults in diamond. These may be taken as evidence for a transition of cubic diamond into several structurally different phases via the metallic state. A very similar phenomenon can be observed by scratching a diamond with another diamond. Thus, the metallization of diamond may in fact be a very common feature of wear.
SESSION B5: SOLID STATE TRANSFORMATIONS
Chairs: James S. Im and Peter W. Voorhees
Tuesday Afternoon, December 2, 1997
America South (W)
1:30 PM B5.1
LOCAL ENVIRONMENT EFFECTS IN THE VIBRATIONAL PROPERTIES OF DISORDERED ALLOYS: AN EMBEDDED-ATOM METHOD STUDY OF Ni3Al. Jeffrey Althoff, Dane Morgan, Didier de Fontaine, University of California, Berkeley, CA; Mark Asta, Stephen Foiles, Andrew Quong, Sandia National Laboratories, Livermore, CA; Duane Johnson, University of Illinois, Urbana, IL.
Recent work had suggested that vibrational effects can play a significant role in determining alloy phase equilibria. In order to better understand these effects, we investigate the vibrational properties of disordered Ni3Al using the Embedded-Atom Method. We examine different projections of the density of states in order to understand the influence of disorder, particularly the role played by local environment effects. We also examine various approximations to disorder (virtual crystal, special quasirandom structures) and assess their effectiveness.
1:45 PM B5.2
Recent experimental and theoretical studies have confirmed and predicted the formation of the -phase in transition-metal aluminides. We have performed first-principle calculations for investigating the formation of this phase in TiAl and VAl binary systems. The lattice constants and relative positions of the planes in -phase were calculated for each system. To better understand the formation of this phase, we have used a simple spring model. Our model confirms that the second nearest-neighbour interactions are the trigger mechanism for this transition.
2:00 PM B5.3
ENERGETICS OF STABLE AND METASTABLE LOW TEMPERATURE IRON OXIDES AND OXYHYDROXIDES. Christel Laberty and Alexandra Navrotsky, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis CA.
Iron oxides and oxyhydroxides are widespread in nature. Understanding the thermodynamic properties of the transformation in the iron oxide/hydroxide system permits the characterization of numerous geochemical processes including weathering, soil chemistry, diagenesis in continental and oceanic environments. The aim of this study is to measure the energetics of a series of metastable oxides and oxyhydroxides in order to determine the effects of polymorphism, of oxidation state and of hydration. High temperature solution calorimetry using sodium molybdate as a solvent and transposed temperature drop calorimetry at 979 K were used. Enthalpies of formation from the elements of the metastable iron oxides and oxyhydroxides were calculated from the measured values of the enthalpies of drop solution. The experiments allow an estimate of the enthalpy of formation from the elements of -Fe_2O_3 (-804.9 ± 1.2 kJ/mol) and -FeOOH (-561.4 ± 3.1 kJ/mol) which are currently unavailable in the literature. Thus the maghemite is 18.7 kJ/mol higher in energy than hematite.
2:15 PM B5.4
AB INITIO STUDY OF VACANCY PROPERTIES IN BCC TRANSITION METALS. Alessandra Satta, Francois Willaime, Section de Recherches de Metallurgie Physique, Centre d'Etudes de Saclay, FRANCE; Stefano De Gironcoli, Istituto Nazionale per la Fisica della Materia (INFM) and Scuola Internazionale Superiore di Studi Avanzati (SISSA), Trieste, ITALY.
The self-diffusion constants for the monovacancy mechanism in the 5d-transition metals with bcc structure ( -Hf, Ta, and W) are investigated by first-principles pseudopotential calculations within the framework of the Local Density Functional Theory (LDFT). The formation and migration energies, calculated for relaxed configurations using supercells containing up to 54 atomic sites, are in quite good agreement with experimental data, when available. The vibrational contribution to the formation entropies, as well as the migration entropies and the attempt frequencies, as defined in the Transition State Theory, are calculated in the harmonic approximation. The electronic contribution to the activation entropy is also taken into account: in the particular case of tungsten it results to be large, positive and comparable to the vibrational entropy at high temperature. Finally, the self-diffusion coefficients obtained from the above mentioned quantities are presented in comparison with experiments.
2:30 PM B5.5
FIRST-PRINCIPLES STUDY OF B2 ORDERING IN TERNARY INTERMETALLIC ALLOYS. Mark Asta, D. D. Johnson and J. J. Hoyt, Sandia National Laboratories, Livermore, CA; J. D. Althoff, University of California at Berkeley, Dept of Materials Science, Berkeley, CA.
The structural and thermodynamic properties associated with B2 ordering transitions in ternary intermetallic alloy systems have been studied from first-principles. The approach used in this work combines the results of ab-initio total-energy calculations with statistical thermodynamic computations based upon the formalism of the cluster variation method. In ternary alloy systems the state of order in the B2 phase is characterized by two long-range-order parameters associated with the values of the two independent concentration variables on each of the two sublattices. We present results illustrating the effects of concentration and temperature upon the relative values of these long-range-order parameters and compare with recent experimental results obtained for Cu-Al-Mn and Ti-Al-Nb B2 alloys.
2:45 PM B5.6
DISPLACEMENT EFFECTS IN CuAu ALLOYS. O. Malis, K. Ludwig, Boston University, Dept of Physics, Boston, MA; B. Chakraborty, Brandeis University, Dept of Physics, Waltham, MA.
Stoichiometric CuAu has a sequence of two first order phase transformations: from a high temperature disordered phase to a modulated ordered phase at and then to a simple ordered phase at . The transformation from the disordered to the ordered phases occurs with a lattice change from cubic to tetragonal. The 7% tetragonality in the ordering direction is caused by the significant difference in size between Cu and Au atoms. The influence of the size effects on the kinetics of phase transformation was investigated using in-situ time-resolved x-ray scattering experiments. We examined in detail the seamless crossover from an incoherent nucleation of the tetragonal distortion above the coherent phase boundary to a continuous process below it. Information about the shape and relative sizes of the ordering and coherent domains was extracted. We also investigated size-effects in the disordered phase by performing Monte Carlo simulations based on the Effective Medium Theory. The simulations allow for atomic relaxation, thermal vibrations and global lattice relaxation. The short range order parameters and the atomic static displacements were evaluated and compared with the data available from diffuse scattering experiments. There is a clear correlation between local order and bond length for the Cu-Cu, Au-Au and Cu-Au pairs. Finally the difference in vibrational entropy between the ordered and disordered phases was examined.
3:15 PM B5.7
ON REAL TIME MONITORING OF STRUCTURAL TRANSITION OF Fe/Cu METASTABLE SOLID SOLUTION UPON HEATING. Francesco Cardellini, Vittoria Contini, Gregorio D'Agostino, ENEA, Inn. Dept, Roma, ITALY; Adriano Filipponi, ESRF, Grenoble, FRANCE.
Present contribution is aimed at reporting on X-ray diffraction and Extended X-ray Absorption Fine Structure characterisation of non-equilibrium iron copper solid solution. Interest in Mechanical Alloying is mostly related to its capability of scanning non equilibrium conditions for a broad variety of systems. Immiscible binary systems do frequently experience increase in the solubility range upon ball milling or other forms of non-equilibrium preparation techniques. In this respect, Fe/Cu represents a sort of prototype binary systems that acquaints solubility upon mechanical alloying. To the purpose of further exploring the evolution and stability of the iron rich (bcc) phase under heat treatment, QEXAFS (Quick EXAFS) measurements have been performed. By this means one is able to, real time, monitoring the evolution of the structural short range environment of Cu atoms upon heating; thus providing insights on the atomistic arrangements of atoms and the dynamics of the process.
3:30 PM B5.8
HYDROGEN-INDUCED PHASE SEPARATION OF PALLADIUM-RHODIUM ALLOYS. David F. Teter, Robert D. Field and Dan J. Thoma, Los Alamos National Laboratory, Los Alamos, NM.
The palladium-rhodium system has been extensively studied for its hydrogen absorption characteristics. A solid solution miscibility gap exists in the Pd-Rh system with a peak temperature of 1188 K, but the lower temperature boundaries (below 800 K) are not conclusively defined due to sluggish kinetics. As a result, metastable phase retention for periods of at least ten years can occur in alloys lean in Rh. However, exposures to hydrogen have been proposed to cause the metastable solid solution to phase separate and therefore alter the hydrogen absorption behavior. In this work, three palladium-rhodium compositions were investigated (Pd-10%Rh, Pd-30%Rh, and Pd-40%Rh) to evaluate the phase stability under thermal treatments with hydrogen. Samples were exposed to various pressures of hydrogen to induce phase separation in bulk alloys. Both x-ray diffraction and transmission electron microscopy were used to characterize the phase separation. Also, an environmental cell transmission electron microscope was used to expose thin foils of the alloys to low pressures of hydrogen gas. The in situ observations revealed that both the Pd-10%Rh and Pd-30%Rh alloys phase separated after a few minutes of exposure to 3kPa of hydrogen gas. The mechanism by which hydrogen increases the kinetics of phase separation in the Pd-Rh system will be discussed.
3:45 PM B5.9
EVOLUTION OF PRECIPITATE IN IN-713C FOR LONG AGING TREATMENTS. Alejandro M. Ges, Osvaldo Fornaro, Hugo A. Palacio, IFIMAT-CIC Univ. Nacional del Centro, Tandil, ARGENTINA.
The strength of a Nickel base superalloy hardened through precipitation is related to the volume fraction, particle size and distribution of the precipitated phase . These parameters may vary as consequence of a heat treatment, or by high temperatures service. The information obtained, describing the influence of time and temperature on such precipitated phase, is of special importance owing to the technological application of superalloys at high temperatures. The information could be used not only for the design of superalloys heat treatments, but also to understand the microstructure as a consequence of work at high service temperatures. The results of the present study describe the analysis of particle coarsening, through the Oswald ripening process, in IN-713C during long ageing time at constant temperature (T=1223 K), given an initial size and volume fraction distribution of the precipitate phase, and the evaluation of two different heat treatments throughout the microstructure analysis and morphology. We found that for short ageing times, t< 2500 hours, the coarsening can be approximated by a linear volumetric growth as predicted by LSW theory. For a time greater than 2500 hours the growth rate of precipitate shows an asymptotic behaviour in both heat treatments. This fact suggest the LSW theory could not be secure to predict the coarsening behaviour for long ageing times. Concerning with the performed heat treatments we can say that the kinetics growth of has different speed of volumetric growth which depends on the respective heat treatments carried out.
4:00 PM B5.10
DISSOLUTION CHARACTERISTICS OF PRECIPITATES AT HIGH TEMPERATURES IN NI-BASE SUPERALLOY IN738LC. E. Balikci, A. Raman, and R.A. Mirshams, Materials Group, Mechanical Engineering Dept., LSU, Baton Rouge, LA.
IN738LC is a Ni-base superalloy used in gas turbines and aero-space applications. precipitates contribute to strengthening of this superalloy at high temperatures. In this study, the authors investigate the characteristics and mechanisms of precipitate dissolution into the matrix solid solution. The precipitates grow in cuboidal shape up to 1130 C, above which a duplex-size precipitate microstructure sets in. Duplex-size precipitate microstructure consists of two very distinct sizes (fine and coarse) of precipitates, and is present in the range 1120-1150 C. In this range, the fine precipitates of the duplex microstructure do not grow. Very coarse precipitates seem to undergo corner dissolution initially and lead to the formation of the fine precipitates. At later stages, through agglomeration of closely located precipitates on to the flat faces of the coarse ones, the latter continue to grow. Through these processes the morphology of the coarse precipitates changes to a spheroidal shape. Activation energy calculations show that in the range 1140-1150 C the precipitates are primarily in the dissolution mode. At and above 1160 C, the duplex microstructure transforms to a single-size fine precipitate morphology, which is found to be stable up to 1225 C. A complete dissolution of the precipitates into the matrix solid solution occurs only above this temperature, and at 1235 C and 1250 C single phase solid solution is obtained. The dissolution of coarse precipitates and formation of the fine ones or the single phase solid solution condition are found to be fast processes in the corresponding temperature ranges, and as little as five minutes has been found to be adequate.
4:15 PM B5.11
GRAIN BOUNDARY DIFFUSION-CONTROLLED PRECIPITATION AND DISSOLUTION AS NON-EQUILIBRIUM PHASE TRANSFORMATIONS. I. G. Solurzano, DCMM, PUC-Rio, Rio de Janeiro, BRAZIL.
Discontinuous precipitation (DP) lamellar products of Al-22at Zn, Cu-7at In and Ni-8at Sn have been studied by TEM/STEM techniques, confirming that it is a transformation controlled by grain boundary diffusion. Although the precipitate corresponds to the equilibrium phase, the transformation as a whole does not reach the thermodynamic equilibrium. High resolution microanalysis of aged and quenched microstructures reveal that a significant amount of supersaturation is retained in the depleted matrix. On the other hand, TEM observations has revealed that the DP product, besides generating a high density of interfaces, is able to incorporate strain energy in the wake of its growth. Long aging treatments lead to discontinuous coarsening, which is a process controlled, as well, by interface diffusion. The dissolution of lamellar microstructures has been observed to be both continuous and discontinuous. The former, dominated at high dissolution temperatures, is controlled by volume diffusion, and gives rise to new grains in the original DP colonies. The latter, at temperatures close to the alloy solvus, is controlled by diffusion along the grain boundaries migrating backwards to restore an inhomogeneous single-phase structure. The nature of the driving force for the observed phenomena is discussed in terms of its chemical, interfacial and strain energy components.
4:30 PM B5.12
COMPUTER SIMULATIONS ON PHASE DECOMPOSITION IN REAL ALLOY SYSTEMS BASED ON A DISCRETE TYPE PHASE FIELD METHOD. Toru Miyazaki and Toshiyuki Koyama, Dept of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya, JAPAN.
Since the recent remarkable development in computer has made the numerical analysis of non-linear diffusion equation possible, it appears that the kinetic simulations become very useful in understanding the dynamics of phase transformation. In the present, a new calculation method of the phase decomposition process through the phase equilibria is proposed on the basis of ``the phase field method'' of discrete type non-linear diffusion equation, recently proposed by us. The composition dependencies of atomic interchange energy and of elasticity are also taken into account so as to be applicable for the micro-structure formation of the real alloy system. The computer simulations are performed for the phase decompositions of the Fe-Mo and Fe-Co-Al alloy systems. The time-development of phase decomposition and the final phase diagrams theoretically given are quantitatively consistent with the experimental one in the real alloys. The new calculation method proposed here is considered to be very useful for the basic understanding of the whole thermodynamic process of phase decomposition through the phase equilibrium, since various types of ordering parameters such as the degree of order s and the crystallographic tetragonality can be used in the equation.
4:45 PM B5.13
THERMALLY ACTIVATED GROWTH OF NANOSIZED LIQUID Pb INCLUSIONS IN Al. E. Johnsona,b, A.V. Olesena,b, A. Johansena, U. Dahmenb and S.Q. Xiab. (a) Laboratory, Niels Bohr Institute, University of Copenhagen, Copenhagen, DENMARK. (b) National Center for Electron Microscopy, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA.
The growth of nanosized, liquid lead inclusions fully embedded in an aluminum matrix has been studied by in-situ TEM heating experiments. The possibility of following the development of the same ensemble of inclusions through the entire annealing time - typically 3 to 6 hours - has provided a unique opportunity for an unambiguous determination of the growth mechanism. In the temperature range from 615 K to 725 K the inclusions are nearly immobile, coalescence is minimal and the growth of the inclusions follows the Lifshitz-Slyozov-Wagner model. Due to an inevitable loss of lead to the surfaces of the thin TEM samples used in the experiments, it is not straightforward to analyze the data in conventional terms using average size versus time plots. However, application of a modification to the original analysis given by Ardell using inclusion density versus time plots has been very successful. In this way it has been possible by an appropriate choice of parameters to determine the diffusion coefficient and the interface energy for each experiment. The average value for the liquid lead/solid aluminum interface energy was found to 0.18 0.07 Jm-2, and from an Arrhenius plot of the diffusion coefficients the activation energy for diffusion of lead in aluminum has been found to be 1.9 eV 0.5 eV.
SESSION B6: POSTER SESSION: Chairs: Michael Atzmon, Pascal Bellon, Evan Ma and Rohit Trivedi
Tuesday Evening, December 2, 1997
America Ballroom (W)
MECHANICAL PROPERTIES OF KINETICALLY DISORDERED NI3AL THIN FILMS. Yucong Huang, Michael Aziz, Harvard Univ, Div of Engineering and Applied Sciences, Cambridge, MA.
Ordered intermetallic compounds are generally brittle, whereas chemically disordered alloys tend to be more ductile. Rapid solidification can produce kinetically disordered intermetallics for phases that in equilibrium are chemically ordered all the way to the melting point, permitting a comparison of mechanical properties of microstructurally and compositionally identical ordered and disordered intermetallic compounds. Kinetically disordered Ni3Al thin films produced by rapid solidification following pulsed laser melting are tested for fracture resistance by plastic deformation of the substrate. The results are compared to those for films re-ordered by furnace annealing and used to asses the effect of ordering on mechanical properties.
OF HYDRIDE FORMATION IN Zr-Nb ALLOYS. D. Srivastava, G.K. Dey and S. Banerjee, Materials Science Division, Bhabha Atomic Reserach Center, Mumbai, INDIA.
The crystallographic aspects associated with the formation of hydride phase (fct) from the phase and the phase of Zr-Nb alloys have been studied in two distinct situations, namely, in the (hcp) matrix in pure Zr and Zr-2.5 ) Nb and in the (bcc) matrix in stabilized Zr-20 Nb alloy. While the transformation can be treated primarily as a simple shear on the basal plane involving a change in the stacking sequence, the transformation has been conceptually broken into an transformation following the Burgers correspondence and the simple shear process. In this paper the transformation has been considered in terms of the phenomenological theory of martensite crystallography in order to predict the crystallographic features of the hydride in the transformation. The lattice invariant shear (LIS) (110) [ _(111_), [121_] has been considered and the crystallographic parameters associated with bcc-fct martensitic transformation, such as the habit plane and the magnitude of LIS and the shape strain have been computed. The prediction made in the present analysis has been found to match very closely to the experimentally observed habit planes. The possibility of thetransition through the formation of a transient Al)-Nb ALLOYS.
Various kinds of phase transformations, viz., spinodal decomposition, omega transformation, precipitation reactions and martensitic transformation can be induced in ternary (Zr3Al)-Nb alloys in conditions far from equilibrium. Transformation sequences in two alloys containing 3 and 10 Nb are described and rationalized in terms of some basic tendencies such as phase separation and chemical ordering in the (bcc) phase, the displacive omega and the (hcp) transformations. Microstructures of rapidly solidified alloys of both compositions showed a distribution of cuboidal Zr5Al3 (D88 structure) particles in the matrix. The periodic arrangement of these particles along directions is indicative of the spinodal transformation which preceded their formation. The D88 transformation can be accomplished by superimposition of three processes, namely, chemical ordering, lattice collapse akin to transformation and vacancy ordering. During isothermal aging, the Zr5Al3 particles transform into Zr2Al (B82 structure) particles. The observed lattice correspondence and transformation morphology suggest that the D88-B82 structural change involves replacement of structural vacancies in the former by zirconium atoms without reconstruction of the lattice. The evolution of the equilibrium Zr3Al (Ll2 structure) phase during prolonged aging has also been studied.
PRECIPITATION KINETICS IN AN AIR-COOLED ALUMINUM ALLOY: A COMPARISON OF SCANNING AND ISOTHERMAL CALORIMETRY MEASUREMENT METHODS. George W. Smith, General Motors Research and Development Center, Physics and Physical Chemistry Dept., Warren, MI.
Kinetics and energetics of precipitation in an air-cooled aluminum alloy have been determined for the first time using both differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC). The DSC data were analyzed by two methods: a modified Kissinger equation and a relation due to Thakur. For the analysis of DIC data several we employed methods developed in our laboratory. Activation energies and time constants from the DSC/Kissinger analysis agreed well with those from DIC provided DSC temperature scan rates were slow compared to the calorimeterís instrumental time constant. The Thakur method yielded an activation energy 8% higher than that derived from the Kissinger method. The latter analysis is preferred on theoretical grounds.
EFFECTS OF AGING AND THERMAL CYCLING TREATMENTS ON THE DAMPING CAPACITY OF CU-MN ALLOYS. Tae-Shin Chung, Agency for Defense Development, Daejon, KOREA; Young-Kook Lee, Chong-Sool Choi, Yonsei Univ, Dept of Metallugical Engineering, Seoul, KOREA.
The effects of the aging and thermal cycling treatments on the damping properties of the three Cu-Mn binary alloys containing manganese of 45, 55, 65 wt pct and Cu-65Mn-5Ni alloy have been investigated. The microstructures of aged and thermal-cycled specimens were observed using an optical and transmission elctron microscope. The phase constituents and lattice parameters were analyzed by X-ray diffraction test, and the damping capacity was measured by a cantilever type damping measuring appratatus. The maximum damping capacity was achieved after aging at 400oC for 18 hours in Cu-45Mn,8 hours in Cu-55Mn, 4 hours in Cu-65Mn, and 36 hours in Cu-65Mn-Ni alloys, respectively. But the storage at 100oC slightly reduced the damping capacity of those Cu-Mn alloys. This might be ascribed to Mn precipitation within the microtwin and the microstructual change from tweed to mottled structure. The thermal cycling treatments between room temperature and 250oC led to an increase in damping capacity, which is probably due to the refinement of tweed structure.
UNDERPOTENTIAL-OVERPOTENTIAL PHASE TRANSFORMATION PHENOMENA IN COPPER DEPOSITION PROCESSES ON GLASSY CARBON. Tatiana Arzhanova, Inst of Chemistry, Far-Eastern Div, Russian Academy of Sciences, Vladivostok, RUSSIA.
The structure, porosity, corrosion stability, and mechanical and electrical properties of thin metal films significantly depend on kinetics of nucleation and crystal growth. The electrochemical phase transformation phenomena and kinetics of nucleation and growth in copper deposition processes on glassy carbon were studied by potentiostatic pulse experiments from the underpotential to the overpotential range including the potentiostatic current transients in acidic copper sulphate solutions. The pure copper wire was used as a reference electrode. The experimental results were treated according to three- dimensional multiple nucleation model with diffusion controlled growth proposed by Scharifker and Hills. The mechanism of copper nucleation on glassy carbon was found to depend on the overpotential. Thus, the analysis of the current-time transients indicates the models of progressive and instantaneous nucleation with diffusion controlled growth in the respective overpotential ranges of -60 - -160 mV and -180 - -400 mV. Furthermore, with the increase in the overpotential, the nucleation rate and the number of nuclei increase. It was found that there is a strong influence of the initial state of glassy carbon surface in the underpotential range on the nucleation rate. Moreover, with the decrease in the underpotential, the nucleation rate decreases at constant overpotential chosen from the range of progressive nucleation, and it increases at constant overpotential chosen from the range of instantaneous nucleation
DETECTION OF PHASE TRANSFORMATIONS BY RESONANT PIEZOELECTRIC PHOTOACOUSTIC DETECTION. Yanina Cesa, Nelly Mingolo , Laboratorio de Haces Dirigidos, Departamento de Fisica, Facultad de Ingenieria, Universidad de Buenos Aires, Buenos Aires, ARGENTINA; Jose Maria Cordero Larriera and Oscar E. Martinez, Laboratorio de Electronica Cuantica, Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, ARGENTINA.
A new technique is presented that determines the temperature of phase transitions by monitoring the changes in the thermoelastic properties of the material using a resonant piezoelectric detection of the photoacoustic signal. The technique is particularly sensitive to changes in the thermal expansion and heat capacity, yielding an ideal method for the detection of glass-liquid transitions. The method consists in the detection of the acoustic signal arising from the thermal expansion of the surface layer of the material under test, that is shined by a modulated low power diode laser. The sample is placed inside an oven that is ramped at the desired speed. The modulation frequency is tuned at the resonant frequency of a piezoelectric detector, placed outside the oven. The technique differs from previously published photoacoustic schemes, in that no gas cell in contact with the sample is used. A 10mW near infrared diode laser modulated at 50kHz was used. The results obtained with Mg-Zn and Fe-B amorphous alloys are discussed. As set, the method allows the measurement of samples of less than 0.1mg, and in principle, detection with samples 1000 times smaller should be possible. The use of the technique for establishing the phase stability by determining the transition activation energy is also discussed.
CHEMICALLY DRIVEN SPINODAL-LIKE DECOMPOSITION. G. Beaucage, S. Sukumaran, Dept. of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH; B. Viers, J. E. Mark, Dept. of Chemistry, University of Cincinnati, Cincinnati, OH; M. Saraf, Dept. of Chemistry, University of Cairo, Cairo, EGYPT.
Thermally driven spinodal decomposition is often observed through the growth of a scattering peak under deep quench conditions. Depending on the critical size for a system this peak can be observed in the small-angle x-ray (SAXS, typical for atomic species) or light scattering regimes (SALS, typical for polymeric species). Phase growth is usually related to an increase in scattered intensity with time following Cahn-Hillard theory. Latter stage deviations from this thermodynamic prediction are described by Oswald ripening mechanisms. A parallel to thermally driven phase separation is seen in some systems where miscibility is governed not by a thermal quench but by a chemical reaction which leads to immiscibility of reaction products. In this study hydroxyl terminated, low molecular weight polydimethyl siloxane (PDMS) is reacted with multi-functional silicon based crosslinking agents to form a two phase gel with silica-like inclusions. A spinodal structure develops slowly (on the order of hours) in the PDMS gel as evidenced by optical microscopy and SALS. This structure differs from conventional thermal decomposition in that the process is irreversible and the spinodal-like structure is quite stable (i.e. Ostwald ripening is not observed). By varying the molecular weight of the PDMS precursor, the catalyst type and other reaction conditions the phase separation process can be drastically altered. A modified Cahn-Hillard approach will be described where the extent of reaction substitutes for the thermal driving force.
IRON PRECIPITATION IN FLOAT-ZONE SILICON. Deepak A. Ramappa, Worth B. Henley, Center for Microelectronics, University of South Florida, Tampa, FL.
Metallic impurities degrade device performance by either forming silicides or by acting as recombination centers. Iron is the chief heavy metal contaminant in silicon. This is because of its high solubility and diffusivity in silicon even at moderate temperatures and the prevalence of stainless steel equipment in the IC fabrication line. Iron precipitation in the device region has been known to degrade gate oxide integrity(GOI) and yield. A quantitative study of phase changes of iron in silicon from interstitial to precipitate phase and vice-versa, is presented. Temperature dependent iron precipitation and dissolution is float-zone grown silicon wafers have been experimentally investigated. Results of iron precipitation experiments over a wide thermal process temperature range and time are presented. A quantitative analysis of iron silicide precipitate stability and dissolution with respect to time and temperature is also presented. Iron precipitation and dissolution in silicon was analyzed by a quantitative assessment of change in interstitial iron using a surface photo voltage minority carrier lifetime analysis technique. It is concluded that maximum iron precipitation occurs in the temperature range of 500 C to 600 C. Iron precipitation is rapid in this region where more than 90% of the interstitial iron precipitates in a period of 30 minutes. This provides valuable information to model cooling procedures for thermal processes. Iron silicide precipitates, were found to dissolve, above a temperature or 760 C. Dissolution of FeSi2 precipitates, renews iron back to an interstitial phase in the silicon matrix. The amount of precipitate dissolved was found to be a function of dissolution process temperature and time. It is concluded that the precipitate phase of iron, FeSi2, is thermally unstable in the temperature interval of 760 C to 920 C. This characteristic of iron silicide could have direct implications on gettering mechanism.
THE FEATURES OF MODIFICATIONS IN PHASE COMPOSITION AND ELECTRICAL PROPERTIES IN SEMICONDUCTIVE BARIUM TITANATE DURING SOLID-STATE INTERACTIONS. V.N. Tsigankov, V.V. Safonov, V.V. Podolyaka, Moscow Academy of Fine Chemical Technology name by Lomonosov M.V., RUSSIA.
The effect of thermal treatment on the pahse composition and electrical properties was studied for positive temperature coefficient ceramic of general composition (Ba1-mYm) (Ti1-nFen)O kSiO2. The influence of iron impurity on the electrical properties was detected for Fe concentrations more than approx. 0.002 mol%. Iron impurity depresses anomalous grain growth in the samples with 0.2 mol% of yttrium. Characteristic properties of the samples fired in air at 1100 C (after sintering at1370 C) are: reduced content of non-base phases (Ba6Ti17O40,Ba2TiSi2O8) and presence of ``channels'' which extend into the volume of the sample from surface. The above results are discussed on the base of the model proposed in literature.
STRUCTURAL PROPERTIES AND CHARGE ORDERING TRANSITION IN LaSr2Mn2O7. J.Q. Li, Y. Matsui, National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki, JAPAN; T. Kimura and Y. Tokura, Joint Research Center for Atomic Technology, Tsukuba, JAPAN.
Transmission electron microscopy measurements characterizing the structures and charge ordering transition in LaSr2Mn2O7 single crystalline sample is presented. The crystal structure of this phase has been identified as well defined 327 layered perovskite structure by high resolution electron microscopy investigation and image simulation. At low temperature, electron diffraction observations reveal the presence of additional superstructural reflections along  direction. The low temperature structure of LaSr2Mn2O7 is found to be a quasicommensurate modulated and pseudotetragonal structure. The systemic extinction conditions associated with the main and satellite reflections allow this modulated structure to be described with the superspace symmetry group of PFmmms11. The possible ordering models based on the charge and dz2 (Mn3+) orbital ordering have been proposed.
PHASE TRANSITIONS IN DEFECT ALUMOSPINEL. A. Men and J. Zak, Department of Physics, Technion-Israel Institute of Technology, Haifa, ISRAEL.
The existence of a set of stable crystalline modifications ( ) for defect alumospinel is investigated. The structures of this modification and the models of phase transformations between them is of interest. A model suggested for a phase transition from a defect alumospinel Al64/3V8/3O32 (V-vacancy) with a cubic unit cell (V0) in to a defect alumospinel Al64V8O96 with an orthrhombic unit cell (V1). In this model the transition is connected with the modification the occupation probability of Al ions inequivalent positions (j) of space group m3m (PAl^j; j-a,b,c,d,f-the type of inequivalent positions ). Thus for ``cubic'' spinel the probability P_Alj can be less than 1 for the certain j-positions (PAlj<1- effect of smearing or model of effective ions). For orthorhombic spinel Al-ions occupy the determined positions (ni) in each of the type j /PAlj(ni) =1, disappears the smearing effect or model of fixed ions/. The positions (nk) of unoccupied by Al are vacant (PVj(nk) =1). The choice of positions ni in orthorhombic spinel is determined by energy stability. In this work is investigates the 3 cases of such transitions, which are characterized by choosing the defect structure of cubic spinel. Taking into account that in the space group m3m Al-ions can be occupy inequivalent positions 8a and 16d, these 3 cases differ from each other by the occupation these positions:
1. A116/3^aV_8/3aAll6^dO_32e -spinel
2. A18^aAl_40/3dV8/3^dO_32e -spinel
3. A1 ^aV_aAl ^dO_32e -spinel
For each of these case the analysis of chain spase groups is given that take part in this transition.
NON-EQUILIBRIUM DIFFUSION-KINETIC EFFECTS IN TRANSITIVE METAL-CHALCOGENIDE SYSTEMS. Victor P. Solntsev, Alexander A. Semenov-Kobzar, Myroslav O. Korbutyak, Valery V. Kartuzov, Institute for Problems in Materials Science, NASU, Kyiv, UKRAINE.
The arising of spatially organized dissipative structures was found out while studying the processes of high temperature interaction in high dense compositions at the basis of transitive metals with chalcogenides of IVA, VA, VIA groups of periodic table. Given autolocalized structures occur as result of competition between chemical reactions and diffusion processes. This leads to higher stability of easy sublimated metals and easy decomposing chalcogenides in high vacuum. For all this the system does not achieve the equilibrium state at arbitrary long exothermal expositions. At some reactionary compositions unstable oscillation modes are observed with following creation of stable stationary nonequilibrium states.
FABRICATION OF MECHANICALLY ENHANCED POROUS TiB2 BODIES BY PULSE ELECTRIC CURRENT SINTERING. Norimitsu Murayama, National Industrial Research Institute of Nagoya, Nagoya, JAPAN.
The essence of Pulse Electric Current Sintering (PECS) is the joule heating of conductive or semiconductive compact bodies. The contract area between particles is preferentially heated, because the electrical resistance at the contact area between particles is higher than that in the inner area of each particle. Therefore, PECS is an effective technique for the local melting-solidification reaction at the interparticle contact region resulting in the enhancement of neck growth. We have investigated the microstructure and mechanical properties of porous TiB2 bulk bodies prepared by PECS. The pulse electric current of an average value of 4 MA/m2 was made to flow through a sample with a relative density of 58% under the uniaxial pressure of 0.9 MPa in vacuum for 60 s. The temperature of the sample rose rapidly to about 1700 K. The relative density of this sample was 68%. The remarkable neck growth was observed. On the other hand, no appreciable neck growth was observed in a sample annealed at 1800 K in a conventional electric furnace. This result suggests that the temperature at the interparticle contact region is higher than that at the inner region of each particle. The flexural strength of the sample with the relative density of 72% was 300 MPa at room temperature. This value was 50% higher than that of the sample with the same relative density prepared by hot pressing. This result indicates that PECS is a useful method for preparing mechanically enhanced porous bulk bodies.
CHARACTERIZATION OF COMPOSITE CERAMIC HIGH LEVEL WASTE FORMS. Steven M. Frank, Stephen G. Johnson, Mitchell K. Meyer, Tonya L. Moschetti and Thomas
8:30 AM *B7.1
UNIFIED FORMALISM FOR ALLOY STABILITY UNDER IRRADIATION. G. Martin and C. Abromeit, CEA, DECM-SRMP, Gif-sur-Yvette, FRANCE; and HMI, TF/NM, Berlin, GERMANY.
Alloys under irradiation differ from alloys under classical thermal conditions by three major points: they experience a sustained detect supersaturation, steady point defect fluxes and ballistic mixing driven by nuclear collisions. Up to recently, the effect of point defect fluxes (irradiation induced precipitation) was handled by models for the inverse Kirkendall effect, while the effect of ballistic mixing was treated with a Lyapunov function formalism (effective temperature). Taking advantage of an adiabatic relaxation of the fast variables (point defects supersaturation), the Lyapunov function formalism is extended and incorporates now the effect of the coupling between point defects and solute fluxes. Beyond describing the various effects of irradiation on phase stability in a single frame work, the theory predicts new features of microstructural evolution under irradiation.
9:00 AM *B7.2
ORDER-DISORDER REACTIONS IN IRRADIATED ALLOYS. Robert S. Averback, L.C. Wei, Department of Materials Science and Engineering, University of Illinois; Y.S. Lee and C. P. Flynn, Department of Physics, University of Illinois.
Studies of ordering and disordering on Cu3Au have been used for nearly fifty years for elucidating the mechanisms of defect production and defect diffusion in irradiated alloys. We have renewed such studies, but now employing highly perfect single crystals, grown by MBE, which make possible precise experiments using ion irradiation and electrical resistivity measurements that provide a quantitative understanding of irradiation driven order-disorder reactions. A few of the topics discussed are: the disordering reaction in energetic displacement cascades as a function of temperature; the coupling between point defect fluxes and long-range order; the instantaneous concentrations of point defects, and linear and non-linear response to pulsed irradiation. These experimental studies are complemented by molecular dynamics simulations of displacement processes in ordered alloys and Monte Carlo simulations of disorder driven by defect fluxes.
9:30 AM B7.3
ATOMIC-SCALE COMPUTER SIMULATION OF DAMAGE EVOLUTION IN GOLD UNDER IRRADIATION. Eduardo Alonso, Tomas Diaz de la Rubia, Lawrence Livermore National Laboratory, Dept of Chemistry and Material Science, Livermore, CA.
The fraction of defects that escape the nascent cascade is considered to be one of the driving mechanisms of the microstructure evolution of materials under irradiation. We investigate this fraction of freely migrating defects (FMD) in gold by Kinetic Monte Carlo (KMC) computer simulations. The primary state of damage as well as the defect energetics and kinetics are calculated by Molecular Dynamics (MD) with the embedded atom method potential. We clearly observe a decrease of the FMD with increasing recoil energies. Preliminary calculations also show an important dependence of the number of escaping vacancies on temperature. This same correlation is not found for the number of freely migrating interstitials.
9:45 AM B7.4
PHASE FORMATION IN Zr/Fe MULTILAYERS DURING IRRADIATION AND THERMAL ANNEALING. A.T. Motta, Dept. of Nuclear Engineering, Pennsylvania State University; A. Paesano Jr., Instituto de Fisica, Universidade Estadual de Maringa, BRASIL; R.C. Birtcher and E.A. Ryan, Materials Science Division, Argonne National Laboratory; M.E. Bruckmann, S.R. Teixeira, and L. Amaral, Instituto de Fisica, Universidade Federal do Rio Grande do Sul, BRASIL.
The kinetics of phase selection and phase formation in vapor-deposited Zr-Fe multilayers are studied in-situ using the IVEM/Tandem Facility at Argonne National Laboratory. Previous studies have shown that phase selection and formation in metallic multilayers of elements depend on the irradiation conditions (ion type, irradiation temperature, dose, and dose rate), and on the multilayer characteristics, especially the overall composition and wavelength. In this study we irradiate Zr/Fe multilayers of different wavelengths (both with a 50-50 composition and an Fe-rich composition) with 300 keV Kr ions at temperatures from 15 to 573 K; we also subject the same samples to thermal annealing without irradiation. By performing these experiments in-situ we can follow the kinetics of phase formation by examining the diffraction patterns.
Amorphization was achieved for most of the conditions studied but thermal annealing of Fe-rich samples with large wavelength formed the crystalline intermetallic compounds. The results show that different equilibria are achieved at different compositions, and the rate of approach to the final steady-state depends on the modulation wavelength. For the 50-50 compositions, the dose to full reaction (amorphization) increases with the square of the Zr layer thickness and increases exponentially with temperature. This is well explained by a model that relates the dose-to-amorphization to achieving a critical Fe concentration in the Zr layer through a diffusion process including thermal, radiation-enhanced and ballistic contributions. The onset of the thermal regime is near room temperature. Under thermal annealing, the 50-50 samples form the amorphous phase, while the Fe-rich samples form either the amorphous phase or the crystalline intermetallic compounds depending on the wavelength. We discuss these results in terms of the Zr/Fe phase diagram and of the model proposed by Gosele and Tu. We also relate these results to previous studies of the Zr/Fe system that did not use in-situ methods.
10:30 AM *B7.5
STRAINED ION TRACKS IN AMORPHOUS SOLIDS: ORIGIN OF PLASTIC FLOW. Helmut Trinkaus, Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, GERMANY.
Bombardement with energetic heavy ions is an efficient way to drive solid systems into states far from equilibrium. Extreme conditions may be realized by this method: intense overall defect production as well as strongly localized thermal spikes followed by rapid quenching at rates of the order of 1015 K/s. These conditions may result in amorphization of crystalline solids and in modifications and plastic flow of amorphous solids.
In the present paper, it is argued that such effects may still be described in terms of classical continuum concepts including linear non-equilibrium thermodynamics (transport theory) and elasticity theory. This is illustrated by treating track formation and plastic deformation (creep and anisotropic growth) of amorphous solids under energetic ion bombardement in terms of visco-elastic shear stress relaxation in thermal spike regions followed by the freezing-in of the associated strain. In this model, the thermal spike regions which are strained in the frozen state represent the mesoscopic defects responsible for the macroscopic deformation. The lower symmetry of ion tracks explains the anisotropy in the deformation characteristics, i.e. the anisotropic growth of stress-free foil samples (expansion perpendicular to the ion beam direction) and the anisotropic creep resistance under stress similar as in nematic fluids. The elastic energy stored in tracks may be considered to be responsible for their etchibility even in the amorphous state. The quantitative predictions of the model concerning plastic flow (creep as well as growth) agree well with experimental results. This may be considered as a justification of the basic model assumptions.
11:00 AM B7.6
EFFECTS OF STRESS ON PREFERENTIAL AMORPHIZATION OF GRAIN BOUNDARIES IN POLYCRYSTALLINE Si DURING IRRADIATION WITH 1.5-MeV Xe IONS. S. Ohnuki and M. Takeda, Department of Materials Engineering, Hokkaido University, Sapporo, JAPAN; P.R. Okamoto and N.Q. Lam, Materials Science Division, Argonne National Laboratory, Argonne, IL.
Previous in situ HVEM-ion irradiation studies on unstressed polycrystalline Si samples have shown that preferential amorphization of grain boundaries occurs between 150 and 225 C during irradiation with 1.5-MeV Xe ions and that the growth rate of the amorphous phase is higher for high-energy grain boundaries than for low-energy boundaries . Similar in situ irradiation investigations have been performed on polycrystalline Si under Mode-I (tensile) loading using the ANL variable temperature straining stage. Irradiations carried out at 175 C on stressed and unstressed samples show that the applied stress causes a substantial increase in the growth rate of the radiation-induced grain-boundary amorphous phase. A stress-induced melting model is used to interpret the effect.
11:15 AM B7.7
ROLE OF SELF-INTERSTITIAL AND INTERSTITIAL-IMPURITY INTERACTION ON IRRADIATION-INDUCED SEGREGATION. Maylise Nastar, Georges Martin, C.E.A., DECM-SRMP, Gif-sur-Yvette, FRANCE; Pacal Bellon, Illinois Univ, Dept of Materials Science and Engineering, Urbana, IL.
Irradiation-induced segregation may result from both vacancy and interstitial fluxes. Interstitials form dumb-bells, i.e. two atoms occupy a single site and diffuse with a complex jump-rotation mechanism. Phenomenological models of segregation in austenitic steels generally do not take into account the segregation resulting from interstitial fluxes. We describe here a kinetic model where the jump frequencies of both vacancy and interstitial are functions of the local composition of alloys. Thermodynamics and kinetics are both treated in the mean field approximation. Change of segregation profiles with alloy composition in ternary systems on a fcc lattice is studied and the role of interstitials is discussed. It has been shown experimentally that oversized-impurities can annihilate segregation in austenitic steels. Our simulations assuming a strong interaction between interstitials and impurities dramatically reduce both interstitial and vacancy fluxes and then, segregation is almost vanished.
11:30 AM B7.8
-LIKE NANOSCALE STRUCTURES AND FULLERENE-TYPE CAGES FORMED BY ELECTRON IRRADIATION OF TURBOSTRATIC BxC1-x (x 0.2). Dmitri Golberg, Yoshio Bando, Keiji Kurashima, Takayashi Sasaki, and Minoru Akaishi, National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki, JAPAN.
Flakes of CVD grown BxC1-x (x 0.2) thin films were exposed to intense electron irradiation (dose 100 A/cm2) in a high resolution electron microscope equipped with a field emmision gun. The composition of the starting materials was determined by using Electron Energy Loss Spectroscopy (EELS). The flake compositions varied from approximately BC4 to BC9. A typical initial flake structure was a mixture of amorphous and turbostratic BxC1-x. Occasionally, BxC_1-x)%% single wall nanotube clumps were observed. Flash irradiation of the turbostratic BxC1-x regions led to formation of onion- and semi-onion-like structures (d 10 nm), and fullerene-type cages (d 0.7 nm) along with amorphization of the entire irradiated area. Such structures did not form during irradiation of initially amorphous regions. It is thought that the onions and fullerenes may arise from a continious bending of the hexagonal BxC1-x sheets during electron irradiation. Finally, some possible structural models of BxC1-x fullerenes are considered.
11:45 AM B7.9
FROM SEMICONDUCTOR TO METAL IN A FLASH: OBSERVING ULTRAFAST LASER-INDUCED TRANSFORMATIONS. Li Huang, J. Paul Callan, Eli N. Glezer, Albert Kim, and Eric Mazur, Physics Department and Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA.
We use a new broadband spectroscopic technique to measure ultrafast changes in the dielectric function of GaAs over the spectral range of 1.5 ñ 3.5 eV following intense 70-fs laser excitation. The results clearly reveal the nature of the phase transformations in the material resulting from the excitation, and provide the most detailed information thus far on the electron and lattice dynamics up to and above the fluence threshold for permanent damage, F = 1.0 kJ/m2.
Changes in the dielectric function are observed to occur within hundreds of femtoseconds. These rapid changes arise from the laser-induced plasma, primarily via changes in the bandstructure due to ionic screening and many-body effects. On a timescale of a few picoseconds, the behavior depends on the excitation strength. For excitations below 0.5 F, the excited carriers heat the lattice through phonon emission. The lattice temperature increases with a rise time of 7 ps. At 0.6 ñ 0.8 F, we observe disordering of the lattice after several picoseconds. This observation is consistent with our time-resolved measurements of the second-order susceptibility which show a loss of long-range order for these fluences. Above 0.8 F, the observed dielectric function indicates a semiconductor-to-metal transformation. The metallic state occurs earlier for higher fluence, arising within 1 ps for fluences above 1.6 F. The semiconductor-to-metal transformation is too fast to be thermal in nature; our results support theories that structural changes result from destabilization of the covalent bonds between the atoms by the laser excitation.
Note that lattice disordering and the semiconductor-to-metal transformation can occur below F, for which fluences the changes are completely reversible. Preliminary results for ultrafast laser-induced phase transformations in silicon will also be presented.
SESSION B8: AMORPHOUS SOLIDS
Chairs: P. R. Okamoto and Ricardo B. Schwarz
Wednesday Afternoon, December 3, 1997
America South (W)
1:30 PM *B8.1
STUDIES OF THE DEEPLY UNDERCOOLED LIQUID STATE IN MULTICOMPONENT BULK GLASS FORMING ALLOYS. William L. Johnson, California Institute of Technology, Pasadena, CA.
Recently discovered bulk metallic glass forming alloys exhibit an unusually high degree of resistance to nucleation and growth of crystalline phases throughout the entire temperature range of the undercooled liquid state. This has permitted extensive study of the crystallization kinetics as well as thermophysical properties of the undercooled liquid. Measurements of specific heat, thermodynamic functions, atomic diffusion, viscosity, and liquid phase separation have been the subject of several investigations. This talk review these experimental results and discuss their importance in understanding the glass forming ability of these multicomponent alloys.
2:00 PM B8.2
BULK METALLIC GLASS FORMATION IN MULTICOMPONENT ALLOYS WITH STRONG LIQUID BEHAVIOR. Ralf Busch, Andreas Masuhr and William L. Johnson, W.M. Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, CA.
The kinetics and thermodynamics of bulk metallic glass (BMG) formation are investigated in the Zr-Ti-Cu-Ni-Be system and compared with other glass formers.
The viscosity of Zr-Ti-Cu-Ni-Be BMG forming alloys was determined from the melting point down to the glass transition over fourteen orders of magnitude. The temperature dependence of the viscosity obeys a Vogel-Fulcher-Tammann (VFT) relation. The data reveal that these bulk glass forming alloys exhibit kinetics that is similar to sodium silicate glass forming liquids which are called ''strong'' liquids. The bulk metallic glass forming liquids are very viscous at the melting point with viscosities of about 100 poise. The high viscosity at the melting point (and upon undercooling) implies that crystallization kinetics is retarded.
The thermodynamic functions of the undercooled liquids were determined in differential scanning calorimetry experiments. We find a reduced Gibbs free energy difference between liquid and crystal compared to previous metallic glass forming systems. This mainly originates from a relatively small entropy of fusion in BMG. Both, kinetics (viscosity) and thermodynamics favor glass formation and can account for the smaller critical cooling rate compared to previous metallic glass forming alloys, since they lead to a drastic reduction of the nucleation and growth rate of crystals.
2:15 PM B8.3
SHEAR FLOW EFFECTS IN BULK METALLIC GLASS FORMING ALLOYS. Andreas Masuhr, Ralf Busch, William L. Johnson, California Institute of Technology, Pasadena, CA.
Multicomponent bulk metallic glass (BMG) forming alloys allow for measurements of thermodynamics and kinetics of the supercooled liquid over a wide temperature range. While many experimental investigations have been carried out in temperature and time regimes in the vicinity of the glass transition, little information is available about physical properties in the equilibrium melt and supercooled liquid at high tempreratures.
Heterogeneous nucleation of crystals at container interfaces as well as chemical reaction with the container material pose severe problems and have required levitation experiments or fluxing of the sample surface. For the Zr-Ti-Cu-Ni-Be sysytem we demonstrate the possibility of processing the melt and supercooled liquid in high purity graphite crucibles without increasing the critical cooling rate. This allows studies of the flow behavior and crystallization kinetics in the temperature range from 700 K to 1300 K. In a circular Couette type high temperature viscometer we measure extremely high melt viscosities of several 10 poise for BMG forming alloys in the Zr-Ti-Cu-Ni-Be system. The data can be described by Vogel-Fulcher-Tammann (VFT) relations and ''strong'' liquid behavior is observed in accordance with previous viscosity measurements around the glass transition. Non-Newtonian effects prior to solidification are are observed and the influence of continuous shearing with rates varying from 10-3 s-1 to 102 s-1 on solidification will be discussed.
2:30 PM B8.4
INVESTIGATION OF PHASE FORMATION DURING COLD-ROLLING OF ELEMENTAL ZR-AL-NI-CU-CO FOILS WITH BULK GLASS FORMING COMPOSITION. A.Sagel, Technical University of Berlin, Department for Metal Physics, Berlin, GERMANY; H.Sieber, J.Perepezko, University of Wisconsin - Madison, Department of Material Science and Engineering, Madison, WI; H.J.Fecht, University of Ulm, Department for Magnetic and Electronic Materials, Ulm, GERMANY.
Elemental Zr-Al-Ni-Cu-Co foils of bulk glass forming composition were cold-rolled at ambient temperatures. The phase transformations during the cold deformation process were monitored with electron and X-ray diffraction and thermal analysis. After 120 deformation cycles a fully amorphous sample was obtained, as indicated by a large exothermic crystallization reaction and the absent of any crystalline fractions in X-ray and high resolution electron microscopy. The characteristics of the amorphous phase are compared with a liquid quenched metallic glass and mechanical alloyed elemental powder mixtures of similar composition. The amorphization reaction during cold-rolling is similar to mechanical alloying and proceeds by the formation of a fine layered structure of the constituents, a rapid reduction of their layer thickness and of the average grain size to a nanometer scale and dissolution reactions in the hcp Zr lattice. While in the beginning of the cold-rolling process the elemental foils were ductile, after several rolling passes the multilayered sample became brittle and, finally, displayed large elastic flexure in the amorphous state. Several thermal treatments of the multilayered and amorphous samples were performed in order to study thermally activated solid-state amorphization reaction as well as phase formation during crystallization.
3:15 PM B8.5
EFFECT OF Cu SOLUTE ADDITION ON GRAIN BOUNDARY MELTING AND AMORPHIZATION IN ALUMINUM. J. K. Heuer(a,b), H. Tanigawa(a,c), P. R. Okamoto(a), N. Q. Lam(a), and J.F. Stubbins(b), (a) Materials Science Division, Argonne National Laboratory, Argonne, IL; (b) Department of Nuclear Engineering, University of Illinois, Urbana, IL; (c) Department of Nuclear Engineering, Kyoto University, Kyoto, JAPAN.
Recent studies show that crystal-to-glass transformations obey a generalized form of the Lindemann melting criterion which predicts that any crystal can undergo disorder-induced melting if sufficient atomic displacements are introduced. This is significant because it implies that melting is independent of the physical origin of atomic disorder. This study explores the hypothesis that displacements caused by thermal excitation and tensile stress interact with displacements caused by impurity solutes at grain boundaries (GB's) to lower a material's local melting temperature. Previous study on pure Al showed that its GB's melt slightly below the bulk melting temperature (660 C) . The present investigation examines the effect of Cu solute addition on the GB melting temperature in Al. Al-16at%Cu thin films prepared by co-evaporation were annealed in an Hitachi-9000 TEM to allow Al grain growth and Cu segregation to GB's. Subsequent in-situ temperature increases were applied until GB melting was evident. The addition of Cu to Al was found to lower the GB melting temperature to below 550 C due to the formation of an eutectic phase. The correlation between atomic disorder-induced thermal melting and tensile stress-induced amorphization is also examined. Crack tips and edges were examined for amorphization using diffuse dark field imaging. The distinct difference in the fracture of Al and Al-Cu films is discussed.
3:30 PM B8.6
RE-ENTRANT MELTING BEHAVIOR OF Zr-Al SOLID SOLUTIONS UNDER MECHANICAL ALLOYING H.W. Sheng and E. Ma, Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA; K. Lu, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, CHINA.
Metastable Zr1-xAlx (x=15, 20, etc.) hcp solid solutions were obtained by mechanical alloying of pure Zr and Al powders at elevated temperatures (e.g., at 573 K). During the subsequent ``cooling'' process (either ball-milling at 293 K or annealing at 423 K), the Zr(Al) solid solutions transformed into an amorphous phase. The exothermic nature of the crystal-to-amorphous transformation was verified using calorimetry measurements. The reverse transformation was also realized by milling experiments at 573 K of an amorphous phase directly formed by milling at 293 K or inverse-melted from the solid solution precursor. The critical Al concentration for amorphization (``melting'') of supersaturated Zr(Al) solid solutions has been determined at different milling temperatures. The melting temperature of the solutions was observed to increase with increasing Al concentration, an experimental evidence suggesting the existence of the re-entrant melting phenomenon. The thermodynamic (especially the entropic effects) and dynamic conditions leading to this behavior under ball milling are analyzed.
3:45 PM B8.7
NON-EQUILIBRIUM FORMATION OF AN AMORPHOUS SixNy PHASE DURING BOTH MECHANOCHEMISTRY AND ION BOMBARDMENT. Z.L. Li, M. Petravic, A. Calka*, M. Giersig+, D. Llewellyn and J.S. Williams, Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, AUSTRALIA. *Advanced Materials Procesing Research Centre, University of Wollongong, Wollongong, AUSTRALIA. +Hahn Meitner Institute, Berlin, GERMANY.
Our previous studies have shown  that -Si3N4 can be formed from ball milling of Si in NH3 gas after thermal annealing of the milled powder at temperatures around 800 C. After milling it is clear that the silicon powder has absorbed up to 40 at% nitrogen and also contains Fe contamination, but no distinct phases could be identified from X-ray data. In this study, TEM confirms that the as-milled powder consists of an amorphous phase which crystallizes to -Si3N4 on annealing. THe crystallization kinetics of -Si3N4 have been examined and found to depend quite sensitively on the level of Fe contamination. To study the crystallization of metastable amorphous SixNy, a ``pure'' sample has been prepared by nitrogen ion implantation into silicon. Comparing ball milled and ion implanted cases suggests that Fe can speed up crystallization whereas the presence of other impurities (e.g. carbon) appears to retard the formation of -Si3N4.
4:00 PM B8.8
CHANGES IN MASS DENSITY AND SOUND VELOCITY OF AMORPHOUS Pd80Si20 BY LOW TEMPERATURE IRRADIATION. G.Schumacher*, R.C. Birtcher, D.P. Renush, M. Grimsditch, L.E. Rehn, Argonne National Laboratory, Materials Science Division, Argonne, IL; *Technische Universitaet, Berlin, GERMANY.
Changes in mass density and sound velocity have been measured (during low temperature irradiation of amorphous Pd80Si20 with different light ions. Changes in mass density have been monitored in-situ by measurements of the specimen dimensions. The mass density decreased as a function of ion fluence and showed a tendency to saturate at -1.2 %. The initial swelling rate was determined to be 2.3 atomic volumes per displaced atom resulting in a relaxation volume for interstitial-like defects of about 2.5 atomic volumes. Complete recovery of the mass density was observed during annealing at room temperature. A comparison of the recovery of mass density with positron-lifetime measurements of electron-irradiated amorphous Pd80Si20 suggests correlated annealing of interstitial-like and vacancy-like defects. The sound velocity decreased as a function of the ion fluence and saturated a -4.7%. The value deduced for the changes in elastic modulus per interstitial-like defect are compared with predicted values. The results are also compared with changes in mass density and elastic modulus during thermal treatment to the glass temperature. The relevance of the results for the process of ion-induced plastic flow will be discussed.
4:15 PM B8.9
DEFECT MEDIATED DIFFUSION AND SEGREGATION AT THE AMORPHOUS-CRYSTAL Si INTERFACE. A. Polman, J.S. Custer, P.M. Zagwijn, A.M. Molenbroek, FOM-Institute for Atomic and Molecular Physics, Amsterdam, NETHERLANDS; P.F.A. Alkemade, DIMES, Delft, NETHERLANDS.
In the presence of impurities, solid phase epitaxial crystallization of amorphous Si on crystal Si often results in both segregation of the impurity into the amorphous layer and trapping in the growing crystal. We have studied this effect for the particular case of erbium, a slow diffusor in Si, with a surprizing result.
Si(100) was doped and amorphized by 250 keV Er ion implantation at fluences in the range 1016-5x1020 cm-2, and then recrystallized at 600 ƒC. High resolution (0.2 nm) RBS as well as SIMS were used to study the Er depth profile around the amorphous-crystal interface for partly regrown layers, and for fully regrown layers.
The Er segregation coefficient is strongly concentration dependent, ranging from 0.01 to 0.2. Moreover, the Er segregation spike shows a flat-topped Er profile near the interface that can not be explained using classical segregation and diffusion. The data are in agreement with a model in which defects, that are known to be intrinsically present in the amorphous Si network, act as traps for Er near the interface and thereby affect both the diffusion and segregation. The model will be described in detail and the results will compared with earlier data on diffusion and segregation of Cu, Ag, Au, Cu, and Pd in Si.
4:30 PM B8.10
DIFFUSION-GOVERNED-ASYMMETRIC GROWTH OF AMORPHOUS LAYER UNDER SOLID-STATE REACTION IN Ni-Zr MULTILAYERS. B.X. Liu, W.S. Lai, Department of Material Science and Engineering, Tsinghua University, Beijing, CHINA.
A molecular-dynamics simulation with an n-body potential was performed to study solid-state amorphization in the Ni-Zr system. The models for simulation were composed of fcc Ni/fcc Zr or fcc Ni/hcp Zr lattices. These models were subjected to isothermal annealing at medium temperatures ranging from 300 to 600 C to trace the detailed process of amorphization. Two quantitative methods were employed to monitor the disordering process, i.e. the planar structure factor and the pair correlation function. Our simulation demonstrated that the formation and subsequent growth of an amorphous layer were initiated at the interface through diffusion and alloying. An important observation, for the first time, was that mutual diffusion of Ni (Zr) atoms into Zr (Ni) lattice resulted in an alloying process between Ni and Zr prior to amorphization. Such diffusion-induced alloying caused the deformation of the original crystalline lattices, which changed gradually to disorder and eventually transformed into an amorphous phase. Kinetically, the growth of the amorphous layer followed exactly a t1/2 law and the growing speeds towards two opposite directions featured an asymmetric behavior. Accordingly, the diffusion coefficient of Ni atoms was found to be greater than that of Zr atoms m the amorphous layer.
Another important result was that the initiation of amorphization upon annealing was dependent on the interfacial structure, e.g. amorphization was readily to take place when the interfaces were composed of low density atomic planes, and was suppressed while the interfaces consisted of high density planes like fcc(l l l)Ni/hcp(00l)Zr.
4:45 PM B8.11
CRYSTAL NUCLEATION AND GROWTH IN A Ni-Ti-Cu SHAPE MEMORY RIBBON. Fan Yang,K. H. Wu, and Z. J. Pu, Florida International University, Department of Mechanical Engineering, Miami, FL.
Studies of crystallization of melt-spun NiTi-Cu shape memory metallic glass by isothermal and non-isothermal differential calorimeter ( DSC) and transmission electron microscopy will be presented. The overall crystallization kinetics have been studied using DSC and analyzed in terms of the John-Mehl-Avrami model. The results of our study indicate that the DSC data shows a good agreement with the John-Mehl-Avrami model. A series of TEM observations have been conducted to study the crystal nucleation and growth behavior in this Ni-Ti-Cu shape memory ribbon. The annealing was used to crystallize the Ni-Ti-Cu shape memory ribbon from amorphous to crystal phase. The annealing was carried out in an vacuum environment and the annealing temperature ranges from 225 0C to 550 oC. TEM observation shows that the crystal nucleates from the amorphous matrix uniformly. During the growth, the crystals maintain a perfect sphereic shape. This phenomenon was observed in all the testing annealing temperature range. To understand and explain why the crystal growths in a perfect sphere shapeic, a thermodynamically model was developed and will be presented.
SESSION B9: INTRFACES AND INTERFACIAL REACTIONS
Chairs: Evan Ma and G. Brian Stephenson
Thursday Morning, December 4, 1997
America South (W)
8:30 AM *B9.1
THERMODYNAMICS OF SOLID-SOLID INTERFACES. Frans Spaepen, Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA.
Experimental methods for determining the interfacial tension and stress of solid-solid interfaces are reviewed. Special attention is paid to the interface stresses determined from in situ curvature measurements during UHV deposition of metallic multilayers . The contribution of coherency dislocations to the interface stress is analyzed.
9:00 AM B9.2
A GRADIENT CHEMO-ELASTIC CONSTITUTIVE EQUATION FOR SOLID SURFACES. Joerg Weissmueller, Universitaet des Saarlandes, Technische Physik, Saarbruecken, GERMANY.
Chemical and mechanical equilibrium in solids with a fine-scaled microstructure, such as thin films, can be measurably influenced by capillarity. The equilibrium conditions of theory depend decisively on the state variables in the fundamental equation for the excess free energy, . In solids, is generally considered to depend on the local conditions at the surface through the excess variables, and on the state of the volume phase near the surface through the tangential strain. However, at least when there is continuous solid solubility, then the fundamental equation should reflect the difference in the surface excess free energies of the pure phases. This would seem to require that the composition of the volume is considered as an additional state variable for , that is generally neglected. A variational derivation of the fundamental equation for a two-dimensional curved solid surface will be presented, that illuminates the assumptions underlying the selection of state variables. When the free energy of the volume phase depends on gradients of composition, then must depend explicitly on the composition of the volume near the surface, and local equilibrium requires gradients along the surface normal. The approach then combines elements of both sharp interface theory, that is adequate for localized topological defects at solid surfaces, and of diffuse interface theory, that describes variations of composition on a larger length-scale.
9:15 AM B9.3
SURFACE ENERGIES AND PHASE STABILITY IN NANOCRYSTALLINE ALUMINAS. J. M. McHale, A. Navrotsky, Princeton Materials Institute and Department of Geosciences, Princeton University, Princeton, NJ; A. Auroux, Institut de Recherches sur la Catalyse, CNRS, Villeurbanne Cedex, France; A. J. Perrotta, Aluminum Company of America, ALCOA Technical Center, Alcoa Center, PA.
Corundum, alpha-Al2O3, is the thermodynamically stable phase of coarsely crystalline aluminum oxide at standard pressure and temperature conditions, but attempted syntheses of nanocrystalline Al2O3 usually result in gamma-Al2O3. New data from adsorption microcalorimetry, thermogravimetric analyses and BET adsorption experiments will be presented, which when coupled with our high temperature solution calorimetry data, clearly prove that gamma-Al2O3 becomes energetically stable with respect to alpha-Al2O3 at specific surface areas >125 m2/g. Furthermore, as gamma-Al2O3 has a greater entropy than alpha-Al2O3, it becomes thermodynamically stable with respect to alpha-Al2O3 at even smaller specific surface areas (e.g. 75 m2/g at 800 K). The results are in excellent agreement with the predictions of recent molecular dynamics simulations, and provide the first conclusive experimental evidence that surface energy differences can favor the formation of a particular polymorph.
9:30 AM B9.4
THE CHARACTERIZATION OF INTERFACES OF GROWING PRECIPITATES USING 3-D ATOM PROBE MICROSCOPY. Ian Rozdilsky, A. Cerezo, and G.D.W. Smith, Department of Materials, University of Oxford, Oxford, UNITED KINGDOM.
The three-dimensional atom probe can reconstruct the positions of the majority of atoms within a small volume of material to sub-nanometer resolution. This technique has been used to characterize nanometer scale precipitates during diffusional growth/coarsening. The model systems of Cu-Co and Ni-Al have been studied with particular emphasis on determining the atomic scale composition profile at the interface of precipitates. Computer models of the growth/coarsening process have been compared to the atomic scale composition profiles obtained from the three-dimensional atom probe.
9:45 AM B9.5
INTERFACE DIFFUSION IN POLYSYNTHETICALLY-TWINNED TiAl. David E. Luzzi, Dept of Materials Science and Engineering, Univ of Pennsylvania, Philadelphia, PA; Dai Imamura, Haruyuki Inui, Dept of Materials Science and Engineering, Kyoto Univ, Kyoto, JAPAN; Masahiro Kawasaki, JEOL, Ltd., Akishima, JAPAN; Hidehiro Yasuda, Hirotaro Mori, Res. Center for UHVEM, Osaka Univ, Osaka, JAPAN; Masaharu Yamaguchi, Dept. of Materials Science and Engineering, Kyoto Univ, Kyoto, JAPAN.
Polysynthetically-twinned (PST) TiAl, containing a high density of parallel, atomically-flat interfaces of identical crystallographic orientation, is an excellent model system for a detailed investigation of interface diffusion. Macroscopic PST crystals were grown in an optical float zone furnace. Thin films were cut from oriented crystals and polished with <110> and <112> directions normal to the film. After sputter cleaning, either Ag or W was deposited on one side of the TiAl thin films. The atomic transport of the Ag and W through the films along the interfaces was measured using in-situ heating experiments in an Auger microprobe, the well-known Hwang-Balluffi accumulation experiment. The Auger results were correlated with high-resolution structural and chemical analyses of the interfaces using transmission electron microscopy. The goals of these experiments are to make the first absolute determination of an interface diffusion coefficient by using the TEM to characterize the interface width, and to study the influence of interface crystallography on interface diffusion.
10:30 AM B9.6
INITIAL PHASE FORMATION DURING INTERDIFFUSION. J.H. Perepezko, J.S. Park, K. Landry, H. Sieber, M.H. da Silva Bassani, Univ. of Wisconsin, Dept. Mat. Sci. & Engr., Madison, WI; A.S. Edelstein, Naval Research Laboratory, Washington, DC.
In multiphase materials systems involved in coatings, composites or multilayered structures, diffusion treatments often result in the development of intermediate phases at the reaction interfaces. While diffusional growth of phases has received much attention, the initial phase evolution involves a nucleation stage as well. The development of metastable phases during solid state interdiffusion demonstrates that the nucleation reaction can be controlling in some cases. For alloy systems with extensive solubility, intermediate phase nucleation is preceeded by interdiffusional mixing in order to achieve the required supersaturation. This leads to the identification of a critical concentration gradient for the onset of phase nucleation. The concentration gradient and the relative magnitudes of the component diffusivities provide a basis for a phase selection strategy and the application of a kinetic bias to modify the phase selection. For multicomponent alloy systems, the identification of the operative diffusion pathway is central to the control of phase formation. Experimental access to the nucleation stage of reaction is facilitated in thin film multilayer samples where the results from systems with both extensive and limited solubility offer new insight into the phase formation kinetics.
11:00 AM B9.7
METASTABLE PHASE FORMATION AND MICROSTRUCTURAL EVOLUTION DURING SELF-PROPOGATING REACTIONS IN Al/Ni AND Al/MONEL MULTILAYERS. D. Van Heerden, A.J. Gavens, A.B. Mann, T.P. Weihs, Johns Hopkins University, Materials Science and Engineering Dept., Baltimore, MD.
Multilayer films can produce self-sustaining reactions (after an initial thermal pulse) provided that the heat of mixing of the constituent layers is sufficiently large and the thickness of the alternate layers are sufficiently small. The rates at which the new phases grow perpendicular to the layering during these reactions are typically 10-3 m/s and can give rise to local heating rates of the order of 109K/s. These rates are much faster than the growth rates (10-8 m/s) and heating rates (100K/s) typically found in DSC experiments. Self-propagating reactions, therefore, provide a novel opportunity to contrast the reaction paths and metastable phases found in very rapid diffusion-based transformations with those seen in DSC experiments for much slower transformations. To study the microstructures and phases produced by self-sustaining reactions we have quenched reactions in Al/Ni multilayers and in A1/Monel (68% Ni, 30% Cu, 2%Fe) multilayers, and we have compared the results to those producced in DSC scans. Cross-sectional TEM examinations of self-propagating reactions that have been quenched using liquid nitogen reveal that, as in DSC studies, a relatively small number of second phase grains nucleate at the interface, and then coarsen until a continuous layers of the second phase is formed. Further growth then occurs by 1-dimensional thickening of this layer. However, in the quenched reaction the nucleated grains grow with a relatively, equiaxed shape (indicating considerable bulk diffusion), while those in the DSC traces generally grow with an ellipsoidal shape along the layer interfaces (indicating the prodominance of boundary diffusion). The reaction paths in the quenched reactions, as determined from the crystal structure along the flame front, will also be contrasted with that seen for DSC experiments. The results give, for the first time, an insight into the processes occurring in self-propaging reactions, and their implications for our understanding of these transformations will be discussed.
11:15 AM B9.8
IN-SITU TEM PHASE FORMATION IN COLD ROLLED ALUMINUM-NICKEL MULTILAYERS. H.Sieber and J.Perepezko, University of Wisconsin - Madison, Department of Material Science and Engineering, Madison, WI.
Multilayer samples of Nickel and Aluminum with the composition of Al-20Ni were prepared by cold rolling of elemental foils. The microstructure and phases were characterized by XRD, SEM and TEM/SAED and the phase formation reactions were studied by DSC and DTA measurements in multilayer foils rolled for different passes. The rolling procedure results only in a decrease of the layer thickness of the elemental foils and in the decrease of the individual grains size of both elements. DSC measurements show the first phase formed at around 200 C for extensively rolled foils. The phase formation shows a double peak related to a 2-dimensional and a 3-dimensional phase growth, well known for NiAl3 formation in sputter deposited multilayer samples. While XRD measurements show only the NiAl3 phase and unreacted aluminum in the multilayer samples after DSC scans, TEM studies and microdiffraction show also areas of an amorphous Ni-Al phase with a composition near to Al-20Ni. Heating under in-situ conditions with the electron beam in the microscope yields the crystallization of the amorphous phase into a B2 structure phase and the growth of grains up to 100 nm in diameter. While the equilibrium NiAl B2 phase exists only in the range of no more than 10 around the stoichiometric composition, the in-situ formed B2 phase grains show a composition near to Al-25Ni. The amorphous to B2 transition is discussed by comparison with the phase formation sequence observed in sputtered Ni-Al multilayers and ball milled powder of Al-Ni.
11:30 AM B9.9
SEQUENCE OF PHASE FORMATION IN THE REACTION OF NI/AL MULTILAYER FILMS. C. Michaelsen, GKSS Research Center, Geesthacht, GERMANY; G. Lucadamo, K. Barmak, Lehigh Univ., Dept. Materials Science and Engineering, Bethlehem, PA.
In the early stages of solid-state reactions between two elements, a phase selection is commonly observed in which product phases appear sequentially and not simultaneously and, in addition, certain equilibrium phases are absent while metastable phases are readily formed. Although the prediction of phase formation at interfaces is extremely important in many technological applications and has prompted numerous investigations, there is still no universally accepted theory of phase selection. The phenomenon of phase selection is studied using Ni/Al multilayer films as a model system. It is found that an initial formation of B2 NiAl already takes place during the multilayer deposition. Upon annealing at low temperatures, this B2 phase continues to grow to a thickness of approximately 5 nanometers. The reduction of driving force by this B2-phase growth gives rise to significant nucleation barriers for the formation of other phases, and explains why the formation of the second product phase, NiAl3, is observed to take place by a nucleation-and-growth process. The results indicate that the phase-formation sequence in the Ni/Al system is controlled by nucleation barriers and hence by nucleation kinetics.
11:45 PM B9.10
STRUCTURAL STABILITY DIGRAMS FOR THIN-FILM MULTILAYERS. S. A. Dregia, R. Banerjee, and H. L. Fraser, Dept of Materials Science and Engineering, The Ohio State University, Columbus, OH.
Structural stability in thin-film multilayers is described in terms of classical thermodynamics, involving the competition between bulk and interfacial energies. A new type of phase diagram is introduced, the biphase diagram, in which concurrent phase stabilities are mapped as a function of two degrees of freedom, corresponding to two independent layer thicknesses in a periodic multilayer. The model is illustrated with experimental results from Al/Ti thin-film multilayers. As a function of increasing the periodic size scale of the multilayer, the two metals were found to coexist as hcp/hcp, fcc/fcc, and fcc/hcp biphases. Thus, the behavior of the Ti layers appeared to be unconventional, as they transformed from their bulk structure (hcp) to a metastable structure (fcc) on thickening. The model, including the effects of coherency strains, will be presented to explain the Al/Ti results, as well as to offer a tool for designing and developing metastable multilayered materials.
SESSION B10: NANOPHASES AND MECHANICAL ALLOYING
Chairs: Georges Martin and James S. Williams
Thursday Afternoon, December 4, 1997
America South (W)
1:30 PM *B10.1
KINETICS OF STRUCTURAL TRANSFORMATIONS IN SEMICONDUCTOR NANOCRYSTALS. Paul Alivisatos, Univ of California, Berkeley, Dept of Chemistry, Berkeley, CA.
Recent work on the kinetics of solid-solid phase transitions in nanocrystals will be described. The kinetics are much simpler in nanocrystals as compared to extended solids. In extended solids nucleation is defect dominated, and multiple kinetic processes proceed in parallel. In contrast, nanocrystals transform between structures via single nucleation events, accompanied by shape change. Recent experiments and simulations of details of the CdSe wurtzite to rocksalt transition in nanocrystals will be used as an example. The results suggest that there is a strong analogy between solid-solid phase transitions in nanocrystals, and the well known physics of magnetization reversal in nanometer size magnetic particles.
2:00 PM B10.2
PHASE TRANSFORMATION AS A FUNCTION OF PARTICLE SIZE IN NANOCRYSTALLINE ZIRCONIA. Tomas Chraska(1), Alexander H. King(1), Christopher C. Berndt(1) and J. Karthikeyan(2); (1) Dept. of Materials Science & Engineering, State University of New York, Stony Brook, NY; (2) Heany Industries, Inc., Scottsville, NY.
Bulk zirconia undergoes a pressure-induced transformation from a (low pressure) monoclinic phase to a high pressure tetragonal phase, at around 3GPa (above 900K). We have studied the structures of zirconia nanoparticles formed by plasma-spraying an organo-metallic precursor. Inspection of the particles in the TEM reveals that they adopt one of two distinct crystal structures, depending upon their size. The smallest particles have the tetragonal structure, while larger ones are monoclinic. Interpolation of the data reveals a critical size above which the monoclinic structure is stable. Upon annealing, the zirconia particles coarsen and the small tatragonal particles transform to the monoclinic structure at about the critical size. We demonstrate that the critical size corresponds to the diameter at which the surface-energy induced internal pressure in the particle is equal to the bulk phase transition pressure. Coarsening under these conditions produces irregular particle size distributions, resulting from a discontinuity in dG/dr at the size where the phase transformation occurs.
2:15 PM B10.3
GRAIN GROWTH IN SEGREGATION-STABILIZED NANOCRYSTALLINE MATERIALS. C. E. Krill, H. Ehrhardt, R. Birringer, Universität des Saarlandes, Saarbrücken, GERMANY.
The thermodynamic driving force for grain growth in polycrystalline materials is the reduction in excess Gibbs free energy that occurs with a decrease in the grain-boundary area per unit sample volume. Since the latter is inversely proportional to the average grain size, the driving force for grain growth is much larger in nanocrystalline materials than in their coarse-grained polycrystalline counterparts--a major limiting factor to the technological application of nanocrystalline materials, since their enhanced grain-size-dependent properties (i.e, mechanical, catalytic, magnetic) are lost once significant grain growth occurs. Strategies for preventing grain growth include the introduction of second phases or pores to pin the grain boundaries (Zener drag) or segregants to reduce their mobility (solute drag). We have prepared nanocrystalline alloys of Zr in Pd to study the influence of solute (Zr) drag on the thermal stability of such materials. Zirconium segregation is expected to be effective at reducing the rate of grain growth in Pd for both kinetic and thermodynamic reasons. In samples with the largest amount of Zr segregation, the average grain size according to x-ray diffraction remains below 40 nm even after a 24-hour anneal at 1100 C--a heretofore unattained degree of thermal stability for a metallic nanocrystalline system. Transmission electron microscopy reveals a heterogeneous microstructure containing a large number of grains of the as-prepared size ( 10 nm), along with regions yielding the distinct diffraction spots of a single crystal. In dark-field images these regions appear to be composed of nanocrystalline (<20 nm) grains oriented nearly parallel to one another, suggesting that grain growth occurred at least partly by grain rotation rather than solely by the usual mechanism of boundary migration. We discuss the implications of these results for the preparation of thermally stable bulk nanocrystalline metals by powder consolidation.
10:45 AM *B1.8
SOLIDIFICATION AND SOLID STATE TRANSFORMATION IN PERITECTIC Fe-Ni ALLOYS. W. Kurz and M. Vandyoussefi, Swiss Federal Institute of Technology Lausanne, Lausanne, SWITZERLAND.
During solidification of peritectic alloys various phase transformations may be observed. Depending on composition i) two phases crystallize successively at different temperatures, leading, in a positive temperature gradient, to two distinct interfaces, ii) two phases, one stable, the other metastable, compete. In the latter case the metastable phase often wins at high solidification velocities due to its smaller solute rejection. Experimental evidence for these phenomena in Fe - Ni alloys will be presented. The results are discussed in the light of modern solidification theory.
2:30 PM B10.4
PHASE STABILITY IN THE NANOCRYSTALLINE TiO2 SYSTEM. Hengzhong Zhang, Jillian F. Banfield, Dept of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI.
Our initial thermodynamic analysis predicts that surface tension of a fine solid particle decreases with the decrease of the particle size. Both the surface phase and the bulk phase of the particle contribute to its thermodynamic stability. In nanocrystalline TiO2 system, the surface tensions and the surface energies of both anatase and rutile were estimated by modeling available thermochemical data and kinetic data. The particle size-temperature phase diagram of the nanocrystalline TiO2 system was calculated. It is found that 1-8 nm anatase is more stable than rutile of the same size at a certain temperature. Such calculations, showing size dependent stability fields, are quite unique and have important implications for nanocrystalline materials analysis.
3:00 PM B10.5
POINT DEFECTS AND DISORDER IN MECHANICALLY MILLED FeRh STUDIED BY MOESSBAUER EFFECT. Luke S.-J. Peng and Gary S. Collins, Dept of Physics, Washington State Univ, Pullman, WA.
Moessbauer and x-ray measurements were made on annealed samples of FeRh (50-56 at.% Fe) and on a 52 at.% Fe sample after milling under argon using a high-energy SPEX 8000 vibrator mill. Hyperfine fields at room temperature were used to identify ferromagnetic (F) and antiferromagnetic (AF) B2 phases and a metastable fcc phase produced during milling that is non-magnetic. X-ray diffractograms indicated that the fcc phase is accompanied by a distribution of L10 distortions. Excellent ordering was indicated by narrow Moessbauer linewidths for annealed AF and F samples. Satellite fields observed for annealed, ferromagnetic FeRh having 52, 54 and 56 at.% Fe are attributed to Fe-antisite atoms on the Rh-sublattice (FeRh) in the first neighbor shell of Fe probe atoms, with a +8% increase in field magnitude due to one antisite atom. Milling leads to progressive transformation from B2 to fcc phase, with the tranformation half complete after 8 minutes. At the same time, additional point defects are formed in the remaining B2 phase. Spectral components with a field shifted by -30% were also observed in milled FeRh. Many-spectra fits under various defect models lead to the conclusion that the -30% shift is most probably due to Fe-vacancies (VFe) in the second-neighbor shell of Fe probes. Simultaneously, about half as many FeRh atoms were formed, indicating that plastic deformation of B2 FeRh produces point defects in the triple-defect combination (2VFe + FeRh). After 10 minutes of milling, fractional defect concentrations induced by deformation were about 3 and 1.5 at.%, respectively, for VFe and FeRh.
3:15 PM B10.6
STRUCTURAL EVOLUTION OF AG-FE MIXTURES DURING HIGH ENERGY BALL MILLING. Massimo Angiolini, Giorgio Mazzone, Amelia Montone, Marco Vittori-Antisari, ENEA,INN-NNUMA, Roma, ITALY; Antonio Deriu, INFM and Dept. of Physics, University of Parma, ITALY; Franco Ronconi,Giovanni Bevilacqua,INFM and Dept. of Physics, University of Ferrara, ITALY; Federica Malizia, Dept. of Physics, University of Ferrara, ITALY.
High energy ball milling of powder blends made of immiscible elements has been deeply studied in the last years in order to elucidate the observed behaviour. In fact while systems like Cu-Co and Cu-Fe give rise, after prolonged milling, to a metastable solid solution, Ag-Fe, which has a higher value of the positive heat of mixing do not form any intermediate phase, and the milling process produces a fine dispersion of Fe particles in the Ag matrix. The process of structural refinement in Ag-10wt%Fe blends has been studied with several experimental methods. The evolution of Fe particle size has been measured by TEM and by SANS, while the local >environnement of the Fe atoms has been studied by Mossbauer spectroscopy. SANS results show a very broad log-normal distribution function ranging from a few nm to several hundred nm. The average particle size increases with the milling time after 10 h of milling, when the maximum Fe dispersion is obtained. TEM microdiffraction shows that Fe and Ag have often well defined orientation relationships suggesting some precipitation from a solid solution whose presence should be then hypothetized in some step of the process. The microstructure appears to be quite complex when >the results of Mossbauer spectroscopy are considered. In fact, the spectra of the samples milled up to 10 h reveal the existence of bcc-Fe whose structural desorder increases with the milling time. At 50 h, two components of comparable volume fraction appear.One corresponds to deformed bcc-Fe and the other may be ascribed to bcc-Fe incorporating Ag atoms.
3:30 PM B10.7
MECHANICAL ALLOYING OF Cu and Fe AND THE EFFECT OF MILLING TEMPERATURE. J.-H. He, P.J. Schilling and E. Ma, Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA.
Solid-state alloying on the atomic level is induced between Cu and Fe, a binary system exhibiting positive heat of mixing, by mechanical milling of elemental powders. This phenomenon is explained based on a model for a dynamic system, considering the effects of externally driven mixing as well as thermodynamic driving forces. Besides single-phase supersaturated fcc and bcc solid solutions, a two-phase (fcc-bcc) region exists after room temperature milling. The possibility to enhance alloying by lowering the milling temperature is explored by milling powder blends with compositions in this two-phase region at the liquid nitrogen temperature. In addition to routine techniques, X-ray absorption near-edge structure (XANES) was used to characterize the product phases and their fractions. No notable change has been observed in the composition range of the two-phase region at the two milling temperatures. This observation may be understood as a result of the presence of milling enhanced diffusion mediated by nonequilibrium vacancies.
3:45 PM B10.8
PHASE TRANSFORMATIONS IN AL-AG ALLOYS UNDER STRONG PLASTIC DEFORMATION. Ferdinand Haider, Inst. f. Physik, Univ. Augsburg, GERMANY; Holger Voss, Inst. f. Metallphysik, Univ. Goettingen, GERMANY.
Supersaturated single crystals were plastically deformed in fatigue at temperatures between 100 C and 190 C. The specimens were afterwards characterized by conventional and high resolution TEM. Compared to only thermally treated material, we found (depending on the deformation temperature and velocity) (a) an accelerated formation of spherical Guinier-Preston zones ( -phase) due to deformation induced surplus vacancies, (b) a transformation of the spherical eta-particles into extremely small gamma' plates with hexagonal structure. The latter formed along Shockley partials, which facilitate the transformation from the fcc to the hexagonal structure.
4:00 PM B10.9
DEFORMATION-ENHANCED DECOMPOSITION IN BALL-MILLED METASTABLE ALLOY POWDERS. J. Xu* and M. Atzmon*,**, * Department of Nuclear Engineering and Radiological Sciences** Department of Materials Science and Engineering University of Michigan, Ann Arbor, MI.
During ball milling of two-phase powder mixtures one often observes homogenization, resulting in the formation of a single-phase amorphous or crystalline metastable alloy.+ It has long been suggested that the deformation process enhances the rate of atomic transport by creating a supersaturation of vacancies. With increasing milling temperature, a single-phase alloy may become unstable. In this case, the alloy in steady state may consist of two nonequilibrium phases.
A less-explored phenomenon is the behavior of an unstable single-phase alloy under milling conditions for which the steady state corresponds to a mixture of two or more phases. In such a regime, the enhancement of atomic transport kinetics dominates over the mixing effect due to milling. There is a driving force for decomposition, and enhanced kinetics are expected. We have studied the decomposition kinetics in Cu-Fe supersaturated solutions milled in a low-energy ball mill at temperatures ranging from room temperature to 250 C. X-ray diffraction, calorimetry and transmission electron microscopy were used to characterize the samples. We observe that decomposition is significantly faster than at the same temperature without milling. Since the temperature of the powder during milling is well known, this confirms that the decomposition rate is enhanced by deformation. We also observe that the steady state is a function of the milling temperature, but the rate at which the steady state is approached is insensitive to the temperature. This observation is explained in terms of conventional defect-reaction theories developed for irradiated solids.
4:15 PM B10.10
EFFECT OF PLASTIC DEFORMATION ON THE KINETICS OF SOLID STATE REACTION BETWEEN CU AND ZN. Massimo Angiolini, Amelia Montone, Giorgio Mazzone, Marco Vittori-Antisari ,ENEA, INN-NUMA, Roma, ITALY.
The kinetics of phase formation during high energy ball milling is the less investigated feature of the process, owing to the intrinsic difficulty to determine, in a quantitative way, the thermo-mechanical hystory axperienced by the powder particles during the ball impacts. In the present work we report measurements on the kinetics of phase transformation induced by compressive plastic deformation of bulk Cu-Zn diffusion couples. The formation of the Zn rich epsilon phase, sufficiently ductile to avoid fracture at the etherogeneous interface during the codeformation process, has been followed as a function of the macroscopic couple strain and of the strain rate. Our results show that the interdiffusion rate during plastic deformation is about three orders of magnitude larger than in thermally activated conditions at the same temperature and that the interdiffusion reaction appears to be interface controlled with a reaction constant depending linearly on the strain rate. Finally, quantitative fitting of the experimental results shows that the only parameter controlling the reaction kinetics is the plastic strain experienced by the diffusion couple.
4:30 PM B10.11
MECHANISTIC STUDY OF THE FORMATION OF ALUMINUM NITRIDE BY BALL MILLING AND LOW TEMPERATURE ANNEALING. J.I. Nikolov, D. Llewellyn, A. Calka* and J.S. Williams, Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, AUSTRALIA. *Advanced Materials Processing Research Centre, University of Wollongong, Wollongong, AUSTRALIA.
We have previously shown that AIN is extremely difficult to form by ball milling at room temperature. Milling of Al in N2 gas results only in microstructural and topographical changes to Al powders whereas milling in NH3 for an extended period results in selective nitrogen adsorption and an almost complete loss of structure as observed by XRD. In the latter case, XRD revealed AIN formation only on annealing of milled powders at 800- l,000 C. In this study we have examined the mechanism for phase formation in some detail, using a range of techniques including thermal analysis, XRD, combustion analysis, SEM and TEM. We find that an amorphous AlNX phase forms directly during milling and that this metastable phase transforms into crystalline AlN during annealing. The nitridation reaction requires the availability of atomic nitrogen (formed by decomposition of absorbed NH3 during milling) and the reaction with Al appears to be diffusion limited during room temperature milling.
4:45 PM B10.12
METASTABLE PHASE FORMATION IN MECHANICALLY ALLOYED Sm-Co-Fe MATERIALS. P.A.I. Smith*, J. Ding+, P.G. McCormick and R. Street, Special Research Centre for Advanced Mineral and Materials Processing, The University of Western Australia, Nedlands, AUSTRALIA. * now at Department of Physics, Trinity College, Dublin, IRELAND; + now at Department of Materials Science, National University of Singapore, SINGAPORE.
Mechanical alloying has been used to form a wide range of metastable Sm-Co, Sm-Fe and Sm-Co-Fe materials which are then heat treated to form nanocrystalline intermetallic compounds which are useful as permanent magnet materials. It has also been shown that the metastable structure formed in certain Sm-Co and Sm-Co-Fe materials allows the formation of metastable nanocomposites of intermetallics with -(Fe-Co) which exhibit useful hard magnetic properties. We have undertaken a detailed investigation of phase formation in the Sm-Co and Sm-Fe binary systems, and the Sm-Co-Fe ternary system, using X-ray diffraction and Mossbauer spectroscopy. Phase formation is shown to be very different in the two binary systems, with complete amorphisation for Sm-Co and the formation of a mixture of an amorphous phase and nanocrystalline -Fe for Sm-Fe. In the ternary system, a gradual increase of the Fe content leads to an increasing proportion of a nanocrystalline Fe-Co solid solution. We compare experimental results for phase formation with predictions based on the Miedema model, and some conclusions are drawn as to the importance of the chemical free energy of mixing in phase formation during mechanical alloying. The implications of the phase formation process for the formation of technologically useful hard magnetic nanocomposites are also discussed.
SESSION B11: POSTER SESSION:
NONEQUILIBRIUM AND THIN FILM TRANSFORMATIONS
Chairs: Michael Atzmon, Pascal Bellon, Evan Ma and Rohit Trivedi
Thursday Evening, December 4, 1997
America Ballroom (W)
Strontium and barium titanate (SrTiO3 and BaTiO3) undergo cubic-to-tetragonal displacive phase transitions at 110 and 390 K, respectively. TEM samples were irradiated in situ in the microscope by 800 keV Kr+ ions using the HVEM-Tandem Facility at Argonne National Laboratory. The microstructural development was monitored as a function of sample temperature and ion dose. The critical amorphization dose was determined at temperatures ranging from 20 to 600 K, and the effects of the phase transitions were investigated. Samples were also irradiated to intermediate doses and subsequently heated or cooled through the transition temperatures, and the microstructural changes were documented by electron diffraction and conventional bright-field imaging. The initial response to heavy-ion-irradiation was to produce unusual microstructural features whose orientation and symmetry depended on the crystal structure (and hence on temperature), and which could be used as a guide for the occurrence of the phase transitions. With increasing ion dose, these features faded, and the materials gradually became amorphous. The critical amorphization dose was in the range of 0.1 to 4 dpa for the temperature range investigated, and showed a significant decrease across the transition temperature for BaTiO3. This effect was not observed for SrTiO3, possibly because at the low transition temperature (110 K), any change in the critical amorphization dose is within experimental error. The critical amorphization temperature was approximately 390 K for SrTiO3 and 550 K for BaTiO3. Samples irradiated to an intermediate dose and cooled through the transition temperatures showed the development of antiphase boundaries, and defect structures were observed to migrate and coalesce at these boundaries. The observed microstructural features shifted at the phase transition, and their symmetry was observed to change in conjunction with the changes in crystal structure
MECHANISM OF PHASE TRANSFORMATIONS INDUCED BY LASER BEAMS. Ailun Rong, Rui Wang, Zhonglin Zhang, Luqing Shi, Amorphous and Opto-Information Laboratory, Beijing Univ of Aeronautics and Astronautics, Beijing, CHINA; Yanwu Lu, Department of Physics, Univ of Massachusetts, Boston, MA.
In this contribution, the mechanism of phase transformations induced by laser beams in different materials, or by laser pulses in different wavelength ranges with varied pulse widths in a material, are reviewed. In infrared region when photon energy ( ) is equal to the energy barrier between the glassy and crystalline states of a MCS (metal semiconductor-alloy) thin film (TF), phase transformations due to the thermal effects of laser beam dominate. TF is amorphised by melting and liquid-quenching while crystallization via nucleation and growth. The thermaldynamic activation energy of crystallization can be obtained by J-M-A equation. In visible region when Eg of a MSC TF at a pulse width 10-50 ns, phase transformations of nonequilibrium processes hot carrier thermalization and recombination take place. New points of view and new models are proposed. In UV region or Nd: YAG laser acted at its second harmonics, 532 nm, and the pulse width in ps scale, phase transformations based on optical nonlinearity occur. The third order optical nonlinearities are monitored by pump-probe beam spectroscopy. The photon excited electron-hole-pair generation and recombination processes in MSC TF, and the donor-acceptor substituted model in metal-phthalocyanines, are discussed experimentally as well as theoretically. Those media with ultra fast response to laser pulse are promising materials for future photonic switching devices or optical memory.
PHASE TRANSFORMATION AND ION BEAM INDUCED CRYSTALLIZATION IN SiC LAYERS FORMED BY MEVVA IMPLANTATION OF CARBON INTO SILICON. Dihu Chen, S.P. Wong, L.C. Ho, H. Yan*, Dept of Electronic Engr & Matls Technology Research Centre, The Chinese Univ of Hong Kong, HONG KONG; R.W.M. Kwok, Department of Chemistry, The Chinese Univ of Hong Kong, HONG KONG.
Buried SiC layers were synthesized by carbon implantation into silicon with a metal vapor vacuum arc (MEVVA) ion source at energies ranging from 30 to 60 keV to doses from 3 1017 to 1.6 1018 cm-2. Annealing was performed in nitrogen at temperatures from 700 to 1200 C for various time intervals. It was found that the Fourier transform infrared absorption (FTIR) spectra of these samples could be consistently decomposed into two or three gaussian components depending on the preparation conditions. One component peaked at around 700 cm-1 was assigned to amorphous SiC. The other two components, both peaked at 795 cm-1 but with different values of full width at half maximum (FWHM), were attributed to -SiC. The one with a larger (smaller) FWHM corresponds to -SiC of smaller (larger) grains. With this decomposition scheme, the fraction of various SiC phases in these samples were determined. Hence, the phase formation characteristics and ion beam induced crystallization (IBIC) effects were studied. It was found that for the as-implanted samples, at a fixed implant dose (energy), there was a critical energy (dose) at which the fraction of the crystalline SiC phase increased abruptly. This was attributed to the IBIC effect. It was also shown that in samples synthesized by double energy implantation, the IBIC effect depends strongly on the order of implantation. Analysis of the evolution of the fraction of the crystalline phase with annealing temperature and annealing time indicated that the crystallization process and phase transformation characteristics in these SiC layers could well be described by the classical random nucleation and growth (RNG) theory. The dependence of the crystallization and phase transformation on the initial nucleation density will also be discussed.
DEFECT KINETIC IN CRYSTAL UNDER STOCHASTIC DISTURBANCE OF IRRADIATION. P.A. Selischev and V.V. Nikiforov, Kiev University, Kiev, UKRAINE.
Dynamics of defect concentration and crystal temperature change is studied according for mean, variance about a mean (s) and correlation time (t) of point defect production rate, environment temperature (Tc) and crystal properties. Kinetic of defect and temperature is described by stochastic nonlinear differential equations. The kinetic becomes unusual with increasing intensity of disturbance (s2t>10-10s). At the high environment temperatur stationary stochastic process goes after transient period. The means and the variances of defect concentration and crystal temperature become constant in this case. At low Tc means of defect concentrations and crystal temperature oscillate. Their period is of order several tens of seconds. Variance change has shape of pulses with increasing amplitude. Pulse period and mean oscillation period are the same. Pulse duration is about 0.1-0.7 of pulse period. These conditions are randomly alternated with range (about tens of seconds) of intricate behavior of mean and variance. In so doing the latter rises steeply. In the third field of parameters the means are constant. And the variances rise. This stochastic process looks like the Wiener¹s process
ION IRRADIATION OF GLASS BONDED ZEOLITE TO SIMULATE STRUCTURAL AND CHEMICAL CHANGES RESULTING FROM ALPHA DECAY IN NUCLEAR WASTE FORMS. B.G. Storey and T.R. Allen, Engineering Division, Argonne National Lab - West, Idaho Falls, ID.
Nuclear waste disposal requires knowledge of the waste form behavior in the radiation field caused by alpha and beta decay of fission products and actinides within the waste form, which could potentially alter the structural and corrosion resistance properties in an unforeseen manner. Glass bonded zeolite is currently being investigated for use as a waste form in the electrometallurgical treatment process for DOE spent nuclear fuel. The effects of alpha decay in a glass bonded zeolite are simulated with 30 keV He particles for the alpha damage and 100 keV Pb ions for a typical actinide recoil event. (TRIM calulations show 100 keV Pb ions are similar to 90 keV U in range, total displacement, and peak damage.) The waste form studied is a 1:1 wt.% ratio of glass and zeolite 5A with surrogate fission products absorbed ( 2 wt.%) into the zeolite prior to hot-isostatic-pressing with the glass frit. This loading enables the fission product behavior to be observed under irradiation. To determine the waste form durability, both amorphization behavior and phase stability are studied using TEM and associated techniques. The irradiations were performed on TEM samples in-situ using the HVEM-Tandem User Facility at ANL.
THERMALLY DRIVEN CLUSTERING OF VACANCIES AS A MECHANISM OF VOID FORMATION IN ION IMPLANTED SEMICONDUCTORS. J. Jasinski, Inst of Experimental Physics, Warsaw Univ, Warsaw, POLAND; Z. Lillental-Weber, Lawrence Berkeley National Laboratory, Berkeley, CA.
Ion implantation being a well established technological method has also stimulated more fundamental research activity concerning beam induced damage formation and its thermal recovery. In spite of numerous studies especially, the last problem remains unclear. Number of different microstructures and defects have been observed in implanted semiconductors after thermal treatment. On the other hand, no satisfactory mechanisms has been provide to explain formation of most of these defects. In this paper we will propose formation mechanism of voids which are defects commonly present after annealing of heavily implanted semiconductors. Transmission electron microscopy (TEM) performed on heavily implanted gallium arsenide samples showed that high-temperature treatment of this material lead to the formation of high density of voids. They were observed whenever amorphous layer was present in the as-implanted sample. These voids were arranged in the form of layers located roughly at the depths initially occupied by amorphous crystalline (a-c) interfaces. Such phenomenon was observed for different implants and ion energies. We postulate that voids are formed by clustering of high concentration of vacancies produced during implantation in crystalline areas adjacent to the a-c interfaces. Additionally observed in experiment increase in the average void size and in the layer thickness with annealing temperature was a result of increase in the diffusion length which controlled vacancy clustering efficiency.
EVOLUTION OF METASTABLE INTERMEDIATE PHASES UNDER STIMULATED FORMATION OF ULTRA-THIN BURIED OXIDE IN SILICON. Alexey Efremov, Vladimir Litovchenko, Boris Romanyuk, Inst of Semiconductor Physics NAS of Ukraine, Kiev, UKRAINE.
In order to obtain ultra-thin buried oxide under low dose ion implantation and at moderate temperatures or subsequent annealing it is necessary to accumulate oxygen in very thin region and to suppress SiO2 precipitation in silicon outside this region. The necessary direction of the process may be provided if the respective self-organization takes place, namely due to self consistent generation of point defects. One or the most effective way to this aim is to introduce into silicon such impurities which (i) form temporary complexes with oxygen or silicon in respective region of matrix; (ii) provide addition free volume for SiO2 inclusions accommodation in silicon matrix (iii) change of an equilibrium shape of SiO2 nuclei.
Experimental data show that after preliminary implantation of foreign atoms as carbon or hydrogen metastable intermediate phases are created which are transformed finely into SiO2 seed burled layer. Respective computer code is developed for simulation of such processes. The code calculates the solution of a set of differential equations describing the diffusion, drift and reactions for all components of a system which behaviour is important for the process studied. Calculated final phase distribution proved to be in a good accordance with the experimental one. Detail evolution of such intermediate phases as SIC, SiOC, and hydrogen containing nanobubles is obtained.
NON-LINEAR PHASE TRANSFORMATIONS IN SiO2 FILMS INDUCED BY O2 ION IMPLANTATION. Alexey Efremov, Galina Romanova, Institute of Semiconductor Physics NAS of Ukraine, Kiev, UKRAINE.
The results of SIMS depth profiling of SiO2 films implanted with 0+1 and O2+ ions are presented. The total dose of implantation was the same in both cases. Sufficient distinguishes in depth profiles of Si+, SiO+ and SiOH+ secondary ions have been revealed. In the case of O+ implantation a layer with modified structure is located near the external surface of the film. It's location and width correspond to the depth distribution of primary defects generated by ions. In the case of O2+ implantation the modified layer is much more pronounced, very thin (10 nm ) and is located at some distance from the surface of the film. The model of the process in the latter case have been developed and respective simulation was carried out. The model include such points. (i) Just after incoming molecular ion is dissociated. (ii) After this two particles move in correlated manner while electronic energy losses remain dominating and angles of scattering are small. (iii) At some depth nuclear energy transfer increase and the subcascades proved to be overlapped with very high density of defects. Due to this the angles of scattering of moving particles increase also. (iv)As a result correlativity in their trajectories is vanished and abrupt decrease both of the cascade overlapping and density of defects takes place at the rest of their path. (v) As a result of this nonlinear and self regulated process a very high density of defects prove to be localized in thin buried layer. (vi)Strongly modified structure is formed after rapid rel
RESPONSE OF ZIRCON TO ELECTRON AND Ne+ IRRADIATION. R. Devanathan and W.J. Weber, Pacific Northwest National Laboratory, Richland, WA; L.A. Boatner, Oak Ridge National Laboratory, Oak Ridge, TN.
Zircon (ZrSiO4) is an actinide host phase in vitreous ceramic nuclear waste forms and a potential host phase for the disposition of excess weapons plutonium. Previous studies have extensively examined the response of zircon to heavy ion irradiation. However the response to highly ionizing radiation such as high-energy light ions and electrons is not well understood. In the present work, the effects of 800 and 900 keV electron, and 1 MeV Ne+ irradiations on the structure of single crystals of ZrSiO4 have been investigated. The microstructural evolution during the irradiations was studied in situ using a high-voltage electron microscope interfaced to an ion accelerator at Argonne National Laboratory. The results indicate that electron irradiation at 15 K cannot amorphize ZrSiO4 even at fluences an order of magnitude higher than that required for amorphization by 1.5 MeV Kr+ ions. However, the material is readily amorphized by I MeV Ne+ irradiation at 1 5 K as well as room temperature. These results will be compared to previous observations of radiation damage in ZrSiO4.
PHASE TRANSFORMATIONS DURING ION BEAM SYNTHESIS: COMPUTATIONAL APPROACHES TO THE KINETICS OF NUCLEATION, GROWTH, OSTWALD RIPENING AND COALESCENCE. K.H. Heinig, M. Strobel and B. Schmidt, Forschungszentrum Rossendorf, Dresden, GERMANY.
Ion implantation is an unique method to drive systems far from their thermodynamic equilibrium. This technique offers possibilities to synthesize metastable (e.g. amorphous) phases and new materials like layers of nanocrystals which have a high excess of interfacial energy. During the (thermally activated) relaxation towards their thermal equilibrium, such systems have a great potential for self-organization. In the present paper the kinetics of nucleation and growth of nanocrystals during high-temperature ion implantation or during annealing following low-temperature implantation will be described by kinetic 3D lattice Monte Carlo simulations. The tendency of self-ordering of nanoclusters during nucleation and the initial stage of growth will be discussed. Special attention will be paid to the influence of concentration gradients (the implantation depth profile) and layer/bulk interfaces on this ordering process.
Starting from the spatial and size distribution of nanocrystals obtained by Monte Carlo calculations, the kinetics of the Ostwald ripening process has been studied by direct numerical integration of rate equations. Spatial inhomogenieties of the initial nanocrystal distribution and/or a strong influence of boundary conditions can initiate the self-organization of spatial structures which have characteristic lengths of the order of the diffusional screening length of the nanocrystal ensemble. The predictions of our computational approaches to phase transformations during ion beam synthesis will be compared with experimental studies of the formation of Ge nanocrystal in SiO2.
ELECTRON-BEAM-INDUCED CRYSTALLIZATION OF TRANSITION METAL OXIDE AMORPHOUS FILMS. Oksana V.Leontieva, Vladimir P.Poliakov, Univ.Estadual do Norte Fluminense, Lab. de Materiais Avancados,Campos,RJ,Brazil; Liudmila M.Kriukova Moscow Inst. for Steel and Alloys,RUSSIA.
Transmission electron microscopy was used to investigate electron-induced crystallization of thermally evaporated films of several transition metal oxides such as Nb2O5, V2O5, MoO3. All films (with thickness 50 nm), deposited onto TEM copper grids, were initially amorphous. It was observed that electron irradiation caused phase transformations from an amorphous to a crystalline structure in all films. It was noticed that depending on a beam irradiation density, various phases can be formed. When irradiation with low beam current density was used, formation of periodic strain-contrast striations in amorphous film was detected during initial stages of crystallization. For these composition modulations in a solid solution the mechanism of spinodal decomposition is most probable. Later the formation of separate monocrystals of intermediate oxide (Nb6O13,Mo4O11, V6O13 was observed. With higher beam current density irradiation crystallization of a film with formation of a maximally valent oxide on the nucleation and growth mechanism was noticed. In case of molybdenum trioxide films, simultaneous crystallization of both phases can be obtained.
The data obtained shows that crystallization process of metastable amorphous films under electron irradiation consists of two competitive processes: (i) formation of a new phase ( lower oxide) at the expense of changing the concentration of the solid solution; (ii) crystallization of the maximally valent oxide. If the first process prevails, formation of a new phase under the spinodal mechanism occurs. Thus, directed propagation of a new phase formation front precedes the propagation of a crystallization front. In case of crystallization of initial phase outstripping the process of formation of a new phase associated with solid solution composition changes, the latter is suppressed and formation of an initial phase by the nucleation and growth mechanism takes place.
PHENOMENOLOGICAL MODEL OF ION-BEAM MIXING IN ENERGETIC COLLISION CASCADES. Byungwoo Park, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA.
In a dense collision cascade of energetic ions with a solid, energy is deposited instantaneously in a very small region, leading to a disordered liquid-like state. An activated process for diffusion of energized atoms is assumed with a temperature distribution T(r,t), considering the effects of thermal conduction into a substrate and temperature-dependent thermal conductivity. A phenomenological model is developed that the mixing rate Dt depends linearly on nuclear stopping power (instead of power-law dependence), and is correlated with a heat of mixing (analogous to Darken's relation). This resolves the problem of Seitz and Koehler's 1956 model, and many succeeding publications. Previous ion-beam mixing experiments agree well with the model's predictions: mixing rates (Dt)/(ion-dose) 1 nm4, and an activation enthalpy of approximately 1 eV for atomic diffusion in liquid-like cascades.
THERMODYNAMICS OF MECHANICAL ALLOYED Fe-Ni. T.J. Zhang, J.Y. Yang, J. Zhang, K. Cui, Dept. of Materials Sci & Eng., Hauzhong Univ. of Sci & Tech., Wuhan, CHINA.
Fe60Ni40 powders were prepared by mechanical alloying (MA) of the elements in a planetary ball milling in argon atmorsphere. XRD analysis points out that He Fe-Ni MA powders consist of (bcc) and (fcc) nanoclystalline supersaturated solid solutions. And the two phases transformed each other during milling. In initial stage, the phase was dominant, then it decomposed and transformed into phase gradully with milling. Some structural parameters of the MA powders, such as lattice parameter, grain size and lattice strain and so on, were determined by analysing the XRD patterns. A thermodynamic model of MA of Fe-Ni system was developed with refrence to Miedema semi-experimental theory to explaine the phases transformation of Fe-Ni system during milling. The thermodynamic analysis shows there is no chemical drying force to form amorphous alloy. And what type phase or phases presented in the MA powders during milling was controlled by structural contribution of the forming enthalpy of the alloy. All of the results are coincident with the experiment.
METASTABLE TO STABLE TRANSFORMATION IN MECHANICALLY ALLOYED IRON-ZINC-SILICON MATERIALS. A. Jordan and O.N.C. Uwakweh, Materials Science and Engineering Department, University of Cincinnati, Cincinnati, OH.
Mechanical alloying of the zinc-rich Fe-Zn binary alloys with the addition of 0.12 wt % Si from elemental powders was performed through the use of high energy ballmilling. These correspond to the binary and intermetallics, along with + 0.12 wt % Si, + 0.12 wt % Si, and the intermediate + + 0.12 wt % Si compositions. The asball-milled materials were determined to be homogeneous, and exist in metastable crystalline states. Thermal analyses based on DSC measurements show that they evolve to stable equilibrium states through characteristic stages with distinct activation energies. These are 131 kJ/mole, 167 kJ/mole and 244 kJ/mole, respectively for + 0.12 wt % Si. Two stages with activation energies of 96 kJ/mole and 641 kJ/mole were identified for the + 0.12 wt % Si material, together with an invariant reaction at 420 C 3 C. The eutectic reaction in the binary Zn-Si system, coupled with the melting of Zn in the Fe-Zn system, are used to explain the formation of FeSi as observed during the zinc coating of Si bearing steels.
AMORPHIZATION OF Cu50Nb50 BY MECHANICAL ALLOYING. Junyou Yang, Jiansheng Wu, Shanghai Jiao Tong University, Department of Materials Science, Shanghai, CHINA.
The mechanical alloying process of elemental powder mixtures of Cu50Nb50 was performed with a planetary ball mill in this paper. XRD,SEM,TEM and microhardness measurement were used to characterized powders at different milling time. The results showed that Cu50Nb50 powders could be fully amorphized by high energy ball milling; atomic ordinal number contrast could be seen at the beginning stage of milling, and it disappeared after milling for 60h because of the accomplishment of the mechanic
8:30 AM *B12.1
IN-SITU X-RAY STUDIES OF MOCVD GROWTH. G. B. Stephenson, Argonne National Lab, Argonne, IL; D. W. Kisker, IBM Research Div, Yorktown Hgts, NY; P. H. Fuoss, AT&T Research, Murray Hill, NJ; S. M. Brennan, Stanford Synchrotron Radiation Lab, Stanford, CA.
The evolution of atomic-scale surface morphology during crystal growth is determined by a complex interplay between factors such as deposition rate, surface transport, and the crystallographic orientation of the surface, which together determine the growth mechanism. This talk will involve examples from our in-situ x-ray scattering studies of surface morphology during metallo-organic chemical vapor deposition (MOCVD) of GaAs. Our observations include the transition between nucleation/coalescence and step-flow growth modes, kinetic ordering of steps during step-flow growth, and a facetting instability during Si-doped growth. The latest results from recent studies of MOCVD growth of GaN will also be presented.
9:00 AM B12.2
REAL-TIME X-RAY DIFFRACTION MEASUREMENTS OF STRAIN RELAXATION DURING GAN GROWTH ON SAPPHIRE(0001). Arthur R. Woll, Joel D. Brock, Dept of Applied and Engineering Physics, Cornell University, Ithaca, NY; Randy L. Headrick, Stefan Kycia, Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY.
Real-time x-ray surface diffraction techniques have been used to study the nucleation and growth of GaN on Sapphire(0001) by Metal-Organic Molecular Beam Epitaxy (MOMBE). Prior to GaN growth, an AlN layer is formed by exposing the sapphire substrate to NH3 at high temperature. The formation of this nitridation layer is indicated, at sim 900 C, both by a sudden change in the specular reflectivity and by the appearance of the AlN (10 0) Bragg peak. The thickness of the nitridation layer is self-limiting at 12,Å. The GaN film growth is performed using triethylgallium and 30 eV NHx+ ions as precursors. The in-plane lattice parameter of the GaN film is initially strained to match that of the nitridation layer. As growth continues, the film begins to relax immediately, and is 70% relaxed when the calculated critical thickness is reached. Possible causes for the early relaxation of the GaN film, including the role of the nitridation layer as a compliant substrate, will be discussed.
9:15 AM B12.3
TEXTURE DEVELOPMENT DURING THE FORMATION OF SUBMICRON SILICIDE STRUCTURES. K.F. Ludwig, Jr., Y. Zheng, J. Mainville, Boston University, Dept. of Physics, Boston, MA; C. Lavoie, C. Cabral, Jr., J.L. Jordan-Sweet, IBM Research Division, T.J. Watson Research Center, Yorktown Heights, NY.
Understanding the growth of submicron titanium, cobalt and nickel silicide structures is of continuing importance as the semiconductor industry scales device sizes downwards. We have been using synchrotron-based in-situ time resolved x-ray scattering to investigate the kinetics of silicide formation in fine lines and blanket films. The evolution of the film texture is followed in real time with a CCD area detector as the samples are isothermally annealed or quickly ramped up in temperature to simulate industrial rapid thermal annealing procedures. In TiSi2, texturing of the final C54 phase exhibits a complex dependence on titanium deposition parameters, structure size, anneal temperature/rate and doping. Although crystallites are randomly oriented in thick films, the (040) orientation dominates in both deep submicron lines and in thin blanket films. Moreover the exact pole directions can evolve as the C54 formation progresses, and different thermal treatments (e.g. isothermal anneal vs. ramp anneal) often yield different final textures. The existence of this wide variation in texture suggests that a number of different physical mechanisms play an important role in C54 TiSi2 formation.
9:30 AM B12.4
IN-SITU UHV TEM INVESTIGATIONS OF THE INITIAL OXIDATION STAGE OF COPPER THIN FILMS. J. C. Yang, M. Yeadon, B. Kolasa, J. M. Gibson, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL.
It is of fundamental and practical interest to understand the oxidation process because a desirable property for any material is resistance to corrosion. However, there is a wide gap between information provided by surface science methods, which have examined the adsorption of up to a few monolayers of oxygen, and that provided by bulk oxidation studies, which have focused on the growth of an oxide layer at the later stages of oxidation. In order to bridge this gap, we are visualizing the initial oxidation stage of Cu(001) by in-situ ultra-high vacuum (UHV) transmission electron microscopy (TEM). In-situ UHV-TEM experiments permit visualization of oxidation processes in real time and provide information on buried interfaces. The nucleation and growth of Cu2O due to oxidation of Cu(001) films were monitored at various temperatures and oxygen partial pressures. For all examined temperatures and pressures, Cu2O islands were observed to form epitaxially with respect to the copper film. The nucleation of these oxide islands was homogeneous -no clear evidence was observed for either steps or dislocations being preferential nucleation sites. Based on this data, we have developed a semi-quantitative model of the initial oxidation stage where the dominant mechanism for transport, nucleation and growth of oxide islands is oxygen diffusion on the Cu surface. The initial nucleation and growth rates of Cu2O formation on Cu(111) are presently being studied.
9:45 AM B12.5
COMPARISON OF METHODS FOR LOWERING THE C49-C54 PHASE TRANSFORMATION TEMPERATURE OF TITANIUM SILICIDE. James M.E. Harper, Larry A. Clevenger, Cyril Cabral Jr., Christian Lavoie, Francois M. d'Heurle, Ronnen A. Roy, Katherine L. Saenger, IBM T.J. Watson Research Center, Yorktown Heights, NY; Glen L. Miles, Randy W. Mann, James S. Nakos, IBM Microelectronics, Essex Junction, VT.
The phase transformation of titanium disilicide from the C49 to the C54 structure has been shown to be weakly driven and sparsely nucleated, resulting in difficulty forming the C54 phase in submicrometer dimensions used in microelectronics. We demonstrate that the transformation temperature can be lowered by more than 100 degrees C by alloying titanium with small amounts of the refractory metals Mo, Ta or Nb, prior to reacting with Si. This method for lowering the transformation temperature is compared with the method of ion implantation into Si prior to Ti deposition and the method of thin interlayer deposition on Si prior to Ti deposition. It is shown that these three approaches all increase the density of nucleation sites for the C49 to C54 phase transformation. The role of the C40 phase as a template site is also addressed.
10:30 AM B12.6
NUMERICAL MODELING OF SELF-PROPAGATING EXOTHERMIC REACTIONS IN MULTILAYER SYSTEMS. S. Jayaraman*, A.B. Mann*, O.M. Knio**, G. Bao**, and T.P. Weihs*, *Department of Materials Science and Engineering, **Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD.
Self-propagating reactions in free standing multilayer foils provide a unique opportunity to study very rapid, diffusion-based transformations in non-equilibrium materials systems. During these reactions, atoms diffuse normal to the interfaces and generate large quantities of heat as they mix exotherimcally. The heat from this mixing diffuses along the foil, parallel to the interfaces, and thereby generates a self-propagating reaction. These reactions can be ignited by a spark and propagate with speeds greater than 1 m/s along the foil. To fully understand the coupling between mass and thermal diffusion, to characterize the effects of metastable phase formation, and to optimize the commercial use of reactive foils in self propagating reactions, we have undertaken analytical and numerical modeling. Analytical models have been successful in characterizing the reaction velocities and the final temperatures of these reactions in which two solids diffuse to give a solid solution. Our model predicts an increase in the reaction velocities with decreasing bilayer thickness down to a critical bilayer thickness. Below this thickness, the trend reverses as verified using experimental result. While analytical models are useful, they cannot account for intermediate phase transformations, changes of state such as melting, and variations in material properties with temperature. Furthermore, they cannot predict the composition and temperature profiles ahead of a reaction front. To circumvent these limitations, we have used numerical methods to study the self propagating reactions. We have included variations in the degree of premixing, changes in material properties, and the formation of metastable phases in the determination of composition and the temperature profiles ahead of the reaction front. These theoretical results are compared with experimental values where possible.
10:45 AM B12.7
THE SELECTIVE SYNTHESIS OF MOLYBDENUM SILICIDES FROM MODULATED ELEMENTAL REACTANTS. Christopher D. Johnson and David C. Johnson, Materials Science Institute and Department of Chemistry, University of Oregon, Eugene, OR.
Controlled crystallization of elementally modulated reactants with repeat layer thicknesses of less than 15 were found to crystallize various molybdenum silicides depending upon their compositions. Reactants with compositions less than 50% molybdenum were found to selectively nucleate -MoSi2 at 400 C with an activation energy of 2 eV even though -MoSi2 is metastable with respect to -MoSi2 below 1900 C. Reactants with compositions between 50% and 65% molybdenum were found to nucleate Mo5Si3 at 700 C with an activation energy of 2.5 eV. Reactants with compositions containing between 65% and 80% molybdenum were found to selectively nucleate Mo3Si between 750 and 800 C with an activation energy of 2.5 eV. This ability to control the crystalline product via the initial composition is distinctly different from the behavior of elementally modulated reactants with larger repeat spacings, which were observed to initially form -MoSi2 at the reacting interfaces in agreement with previously reported studies. The measured nucleation energy as a function of composition suggests that the phase selectivity is nucleation controlled. The selectivity depends upon the elimination of large composition gradients at the reacting interfaces before nucleation of crystalline products.
11:00 AM B12.8
LASER INDUCED FORWARD TRANSFER OF PECVD AMORPHOUS SILICON ASSISTED BY HYDROGEN EVOLUTION. Michael O. Thompson, Cornell University, Dept. of Materials Science, Ithaca, NY; Patrick M. Smith, P.G. Carey, M.J. Bennahmia, and T.W. Sigmon, Lawrence Livermore National Laboratory, Livermore, CA.
Direct printing of silicon onto low temperature substrates has the potential to dramatically reduce the cost of large area electronics. Laser induced forward transfer printing of Si thin films (30-100 nm) from low temperature PECVD hydrogenated amorphous Si onto glass and plastic substrates has been demonstrated using a 30 ns pulsed excimer laser. Hydrogen gas evolved during melting of an amorphous Si film on a transparent target acts as a propellant to transfer the film across a 30-500 m gap to the substrate with a single laser pulse. In addition, since vaporization of the Si is not required, low temperature substrates can be used without thermal damage from the enthalpy of condensation.
Experiments were performed using both Ge and Si films with varying hydrogen concentrations. The dynamics of the forward transfer were followed using both in situ diagnostic probes and post-transfer analysis by RBS and AFM. In our model, hydrogen diffuses out of the molten Si or Ge, forms a high pressure gas film between the target and film, and accelerates the film toward the substrate. Once fully molten, the film becomes unstable breaking into submicron droplets that travel to the substrate at 50-500 m/s. A shock wave vapor component, traveling at 700 m/s, is also formed and precoats the substrate prior to arrival of the droplets. A numerical model for this process exhibits quantitative agreement with our experiments. Initial results of thin-film transistors fabricated by conventional lithography on these films will also be presented.
11:15 AM B12.9
FORMATION OF METALLIC SYSTEMS FAR FROM EQUILIBRIUM BY PULSED LASER DEPOSITION. H.U. Krebs, O. Bremert, S. Fähler, M. Hamp, M. Störmer, K. Sturm and M. Weisheit, Institut für Metallphysik, Universität Göettingen, Göettingen, GERMANY.
The properties of metallic thin films (Fe-Nb, Fe-Zr, Cu-Co, Fe-Ag) prepared by pulsed laser deposition (PLD) strongly differ from samples grown by conventional thin film techniques like sputtering and evaporation . Strongly supersaturated solid solutions with enlarged lattice parameters are formed, a broader formation range occurs for amorphous samples and a metastable phase formation with larger thickness is observed at the interfaces of multilayers. Reasons for these behaviors are discussed with respect to atomic mixing and implantation effects due to the high kinetic energy of the deposited ions of more than 100 eV.
11:30 AM B12.10
THE DECOUPLING OF THE MAGNETIC AND STRUCTURAL PHASE TRANSFORMATIONS IN MAGNETO-OPTIC MnBi THIN FILMS. Prabhakar R. Bandaru, Timothy D. Sands, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA.
MnBi thin films are potential candidates for ultra - high density magneto- optical recording media due to their large Kerr rotation in the blue wavelength regime and perpendicular anisotropy. However, coincident structural and magnetic phase transformations near the Curie temperature (Tc) causes poor thermal cycling behavior on thermomagnetic (Curie point) writing. It is the aim of this study to decouple the structural and magnetic phase transformations so as to improve the cycling characteristics. Disorder - induced first order phase transitions, as in the case of the phase transformation in MnBi can be ``forced'' to be second order thus avoiding the hysteretic structural transformations. In line with the ideas of Bean and Rodbell1 this can be done by clamping the system, either structurally or magnetically. In the present study, the effects of alloying additions and film thickness are explored as means for decoupling the two kinds of phase transformations. The addition of 10% Cr was found to be effective in lowering the Tc from 630 K to 520 K. This is postulated as due to the Cr mediating the magnetic superexchange interactions in the MnBi lattice so as to introduce a net antiferromagnetic effect. It was also observed that the Tc could be varied with film thickness from the bulk value of 630 K (80 nm) to 510 K (58 nm), presumably due to different degrees of relaxation of strain arising from the difference in thermal expansion coefficients of the film and substrate. Furthermore, cyclic annealing experiments suggest greater thermal reversibility in (Mn0.9Cr0.1) Bi films, implying a suppression of the deleterious first order hysteretic characteristics. The effect of the substrate in the regulation of the phase transformation characteristics of thin films will be discussed.
11:45 AM B12.11
TRANSMISSION ELECTRON MICROSCOPY OF THE TRANSFORMATION BEHAVIOR OF TETRAGONAL ZIRCONIA CRYSTALLITES IN ZIRCONIA-ALUMINA NANOLAMINATES. M. A. Schofield, M. Gajdardziska-Josifovska, Univ Wisconsin-Milwaukee, Dept of Physics and Lab for Surface Studies, Milwaukee, WI; C. R. Aita, Univ Wisconsin-Milwaukee, Materials Dept and Lab for Surface Studies, Milwaukee, WI; P. M. Rice, Oak Ridge National Lab, Metals and Ceramics Div, Oak Ridge, TN.
Zirconia-alumina multilayer films consisting of polycrystalline tetragonal zirconia and amorphous alumina were grown by reactive sputter deposition to study the transformation behavior of tetragonal zirconia nanocrystallites both during growth and during in-situ electron beam irradiation and cooling experiments in a transmission electron microscope. The zirconia crystallites initially grow in the tetragonal phase to a critical size of 6.0 0.2 nm, in agreement with a value of 6.2 nm predicted by end-point thermodynamics. Past the critical size, incorporation of additional zirconia molecules into the zirconia layers is accomplished predominantly by transformation of the growing crystallites to the monoclinic phase, and less frequently by deposition of amorphous zirconia. Transformation to the monoclinic phase is accompanied by a highly faulted intermediary phase. The subsequent growth behavior of monoclinic crystallites is consistent with a 3-dimensional interface-controlled, diffusion-limited growth process with a growth exponent between 3 and 4. During irradiation and cooling it was found that the constraint provided by the alumina layers in the nanolaminate was important in the stabilization of the tetragonal phase of the zirconia, and overrides the thermodynamic predictions which govern the phase composition of zirconia crystallites during growth. A partial transformation of the tetragonal zirconia crystallites to the monoclinic phase was observed in cases where the alumina constraint is greatly relaxed due to knock-on damage to the alumina layers by the electron beam of the microscope. In extreme cases of alumina loss, re-crystallization of the zirconia occurred producing larger monoclinic zirconia crystallites. Fundamentally, the nano-sized zirconia crystallites present in the films under investigation were found to have a qualitatively different transformation behavior compared to micron-sized dopant-stabilized tetragonal zirconia crystallites. Consequently, the zirconia-alumina nanolaminates studied here do not appear to function as a classical transformation toughening system.