1998 MRS Fall Meeting & Exhibit

Chairs

Jean Marie Dubois, CNRS 
Patricia Thiel, Iowa State Univ
An-Pang Tsai, Natl Research Inst for Metals
Knut Urban, Forschungszentrum Juelich GmbH

Symposium Support 

• Ford Motor Company
• NORAN Instruments Japan/JEOL 

1998 Fall Exhibitor 

* Invited paper

SESSION LL1: PHASE STABILITY AND GROWTH 
Chairs: Thomas A. Lograsso and 
Srinivasa Ranganathan 
Monday Morning, November 30, 1998 
Gardner A/B (S)

8:30 AM *LL1.1 PHASE DIAGRAM DETERMINATION FOR MODIFICATIONS OF THE D PHASE IN THE Al-AlCo-AlNi SYSTEM. Reinhard Lü ck, Max Scheffer, Tilo Gödecke, Max-Planck-Institut für Metallforschung, Stuttgart, GERMANY; Stefan Ritsch, Tohoku Univ, JST, Institute for Materials Research, Sendai, JAPAN; Conradin Beeli, ETH Zürich, Lab for Electron Microscopy, SWITZERLAND. 
The Al-AlCo-AlNi ternary subsystem has been investigated extensively. The data have been obtained by differential thermal analysis, magnetothermal analysis, dilatometry, metallography, X-ray investigations, transmission electron microscopy, and high-resolution electron microscopy. Liquidus projection surface, several isopleths and isothermal sections, polythermic projections of four phase reactions, and the Scheil reaction scheme have been determined. 11 phases from the binaries Al-Co and Al-Ni and the three ternary phases Y₂ (Co₂NiAl₉), X and the decagonal phase D were found at room temperature. The decagonal phase is formed from the melt peritectically via critical tie line and its primary formation area dominates at the liquidus projection surface. 45 three-phase regions are present according to the reaction scheme. Structural investigations of the decagonal phase resulted in the detection of several different modifications. From thermodynamic considerations, the question arises, which of the found modifications are Gibbs phases, and if so, which are stable. If some of these modifications are really stable Gibbs phases, further phase diagram determination is required, i.e. a subdivision of the phase field of the D phase. It is important for phase diagram delineation to know the order of transformation between these modifications, and consideration of Lifshitz's theory on second order transformation is necessary. First results on the phase diagram of the modifications will be discussed. The existence regions of modifications are influenced by the composition parameters / and  and by the temperature. 

9:00 AM *LL1.2 
PHASE DIAGRAM AND APPROXIMANTS. M. Quiquandon , Y. Calvayrac, A. Quivy, F. Faudot, CECM/CNRS,Vitry, FRANCE; D. Gratias, LEM-CNRS/ONERA, Chatillon, FRANCE. 

We present the phase diagram delineation for the two ternary systems AlCuFe and AlPdMn in the vicinity of the icosahedral phase. In the both cases, the quasicrystalline state has a tiny region of stability surrounded by numerous adjacent domains corresponding to crystalline structures with large unit cells. The diffraction patterns of these crystalline phases are close in peak locations and intensities to that of the parent quasicrystalline phase and they are called ``approximant'' phases. We shall discuss the geometric process for derivating these structures from the parent quasicrystal in the framework of the N dimensional crystallography and discuss the main features that validate them for being called ``approximant''. Substituting a few percent of aluminum by silicon in both systems lead to destabilizing the quasicrystalline phase towards cubic approximants of low orders. Their localization in the phase diagram and their structural model will be presented. 

9:30 AM LL1.3 
MELTING AND SOLIDIFICATION BEHAVIOUR OF NANOPARTICLES OF LEAD EMBEDDED IN QUASICRYSTALLINE MATRICES. Alok Singh , A.P. Tsai, National Research Institute for Metals, Tsukuba, JAPAN. 

Composites of nanometric lead particles embedded in icosahedral Al-Cu-Fe, Al-Mn-Pd and Al-Cu-V and decagonal Al-Cu-Co quasicrystalline matrices have been prepared by melt-spinning route. The melting and solidification behaviour of the lead particles in these quasicrystalline matrices has been studied by transmission microscopy and differential scanning calorimetry (DSC). The solidified lead particles show well defined orientation relationships (OR) with the matrix, but which is not unique in case of the icosahedral matrix. DSC cooling curves in Al-Cu-Fe icosahedral phase suggest several distinguished nucleation sites for nucleation of solid lead, at undercooling levels of about 20K. The OR are decided by a local atomic match at the interface. In this alloy the particles are bounded by the major symmetry planes of the icosahedral matrix and three dominant OR were determined. A tendency for {211111} fivefold and {221001} twofold planes to align with lead {022} planes was observed in all these OR. The Al-Mn-Pd icosahedral phase does not show any facets with the particles and the DSC cooling curves suggest many more well distinguished nucleation sites. The solidification/melting behaviour of the lead particles in the Al-Cu-V icosahedral phase is found to be similar to that in an amorphous matrix. No OR could be observed and a depression in the melting temperature occured. The particles in the Al-Cu-Co decagonal matrix are flattened in the quasiperiodic plane and show a single OR with the matrix, with features similar to those in the icosahedral Al-Cu-Fe matrix. Solidification parameters are calculated from the DSC peaks and compared to those available for crystalline matrices. 

9:45 AM LL1.4 
OXIDATION OF Al-Cu-Fe QUASICRYSTALS AND RELATED CRYSTALLINE PHASES. Uwe Koster , Barbara I. Wehner, Dept of Chemical Engineering, University of Dortmund, Dortmund, GERMANY. 

Oxidation behavior of Al-Cu-Fe quasicrystals as compared to related crystalline phases was studied in air and synthetic air at high temperatures by means of XRD, SEM and TEM as well as thermobalance measurements. The oxidation behavior is characterized by a sequence of transient oxides proceeding the formation of the stable -Al₂O₃. The initial formation of a homogeneous layer of -Al₂O₃ crystals with a certain orientational relationship to the substrate is followed by the growth of highly defect -Al₂O₃ needles. Nucleation of the stable -Al₂O₃ is observed at the interface metal/oxide with arbitrary orientation to the substrate leading to the formation of -Al₂O₃ oxide nodules which overgrow the needle oxides. Humidity seems to favor the nucleation of the stable Al₂O₃. All the differences in oxidation observed between the quasicrystalline and the crystalline phases can be understood just due to the change in composition. The quasicrystalline surfaces, however, undergo significant surface rearrangements prior to the oxide formation. Formation of an aluminium oxide layer should result in a loss of aluminum in a surface near layer. In icosahedral quasicrystals at 820C this loss of aluminum was probably compensated by Al diffusion from the bulk. At lower temperatures, however, evidence is building up for a phase transformation of the i-phase into a bcc-phase with a lower Al content. 

10:30 AM *LL1.5 
PROCESSING OF AL-CU-FE SINGLE GRAINS. Amy R. Ross, Thomas. A Lograsso , Ames Laboratory - US-DOE. Materials Science and Engineering Department, Iowa State University, Ames, IA; Drew W. Delaney, Motorola Corporation Phoenix, AZ. 

The phase equilibria of the Al-Cu-Fe quasicrystalline phase is complex and conventional crystal growth techniques like the Bridgman or Czochralski methods are not applicable in preparation of large crystals. The icosahedral phase  forms via a peritectic reaction involving liquid and two crystalline solids at 860 C: L +  (FeCuAl) +  (Al₁3Fe₄) #61614; . The  phase is centered at a composition of approximately Al₆3Cu₂5Fe₁2 exhibits a limited range of solubility of several atomic % Cu and Fe between 700 and 800 C. The solubility range must necessarily decrease with increasing temperature from 800 C up to its melting temperature, where a unique composition is required for the four-phase equilibrium. Large single grains of the  phase, 10 mm x 5 mm x 5mm, can be prepared by either slow cooling or isothermal anneals. Arc melted ingots, roughly 12 mm in diameter and 80-100 mm in length were subjected to multiple heat treatments between 825 and 840 C. Following heat treatment, several grains exhibiting five-fold faceting and up to 0.3 cm³ in size were harvested from the ingot. Chemical analyses of the crystals indicate a slight shift in composition to Al₆3.4Cu₂4Fe₁2.6. Optical examination of the crystals indicated the existence of a minor quantity (on the order of 5-10%) of a second phase and porosity in the as-grown state. Post-growth solution annealing at 800 C for four hours, following by water quenching, completely dissolved the second phase into the icosahedral matrix, yielding a single phase quasicrystal grain. The lack of phase purity following crystal growth apparently results from the lower solubility range at the high heat treatment temperatures required for crystal growth. Porosity levels were found to be on the order of 2.5% with a bimodal size distribution. The pores are typically circular, with one set averaging 86 in diameter and a more populous set in the range of 5-10 diameters. Reduction of porosity of the single grains has been accomplished through hot isostatic pressing. 

11:00 AM *LL1.6 
NANO-QUASICRYSTALLINE PARTICLES IN Mn-ION IMPLANTED GaAs. K. Sun, K.H. Kuo , Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, Beijing, CHINA. 

In Mn-ion implanted GaAs, nanometer-size icosahedral and decagonal quasicrystals were found in the surface zone by high-resolution electron microscopy. Nanoparticles of the Ga-Mn (about 50 at.% each) icosahedral quasicrystal form during the implantation. After a flash heating to 800 C, they grow to micron-size. The orientation relationship between it and the GaAs matrix is: i-5 // [110], i-3 // [11-1], and i-2 // [12-1]. This is the same orientation relation found between other icosahedral quasicrystals and their cubic approximant.  Nanoparticles of the Ga-Mn decagonal quasicrystal with a periodicity of 1.25 nm along its tenfold axis form also during implantation and its orientation relation with the GaAs matrix is: d-10 // [110], d-2 // [-110]. It is of interest to note that the distance of 3r[1/2,1/2,0] = 1.2 nm is about equal to the periodicity of this decagonal quasicrystal. In fact, large, bright image points with a distance of about 1.2 nm was observed continuously from the decagonal quasicrystal to the GaAs matrix, implying a direct growth of the former into the latter. newline In addition, several crystalline approximants have also been found to coexist with the quasicrystalline phases. 

11:30 AM LL1.7 
THERMODYNAMIC AND KINETIC ANALYSIS OF THE GROWTH OF DECAGONAL Al-Co THIN FILMS. Elisabeth Emeric, Claire Bergman , Centre de Thermodynamique et Microcalorimétrie, CNRS, Marseille, FRANCE; Gérard Clugnet, Patrick Gas, Laboratoire de Métallurgie, CNRS, Marseille, FRANCE. 

Quasicrystalline phases have among other, specific mechanical and electronic properties. Potential applications as coatings, optical or electronic parts of devices give interest to their preparation as thin films. Solid state reaction is known to lead to the formation of intermetallic phases (stable or metastable) in thin films without the necessity of high-temperature processing which is an important point from a technological point of view. In this paper, we report on a thermodynamic and kinetic analysis of the conditions of formation of decagonal thin films in the binary Al/Co system. From the initial state constituted by pure elements, the formation of the decagonal phase caracterised by TEM and X-Ray diffraction experiments proceeds in three steps; the third step begins at 450C and corresponds to the growth of the decagonal phase from pure Co and stable Al₉Co₂. Once formed, the quasicrystalline film is stable upon annealing from room temperature up to 710C. Different parameters such as the global atomic ratio Al/Co, the multilayer period and the heating rate are analysed by differential scanning calorimetry and in situ ramped resistance measurements carried out on bi- or multilayers of the same total thickness 300nm. In both cases, samples are heat treated at a constant heating rate. Quantitative analysis of the DSC measurements allows the determination of the enthalpy of formation of the decagonal phase and of the activation energy for its growth. 

11:45 AM LL1.8 
FABRICATION OF HIGH QUALITY QC FILMS VIA THE ROUTE OF THE AMORPHOUS PHASE. Roland Haberkern , Caroline Roth, Peter Haeussler, TU Chemnitz, Institut f. Physik, Chemnitz, GERMANY. 

Quasicrystals are assumed to be dominated by an extreme strong interrelation between the atomic structure and its electronic system. This causes peculiar mechanical and (thermo-) electrical properties combined with high corrosion resistance, which makes quasicrystals interesting as a material for applications. Especially the coating of metals may be of technical interest as the brittleness of quasicrystals is irrelevant for thin films. First we present the fabrication of thin films via the route of the amorphous state. Hereby, homogeneous amorphous films are deposited at room temperature by the technique of sequential flash evaporation for the systems Al-Cu-Fe, Al-Pd-Mn and Al-Ni-Co and by sputtering for Al-Pd-Re. All systems mentioned can be transformed to the quasicrystalline state by an annealing treatment which is characterized by surprisingly low temperatures and short times. Amorphous Al-Cu-Fe, e.g., shows for a particular range of composition a direct transition to the icosahedral phase at 430C on a time scale of minutes. The qc films fabricated by this technique show a smooth surface and are of good quasicrystalline quality. The temperature coefficient of resistivity, sensitive for the sample quality, is in the range of bulk samples. The low transition temperatures and the short transition times may be of practical interest. The reason for them is discussed in the context of the non-equilibrium state of the amorphous phase and the similarity of the amorphous to the qc phase, with regard to their interrelation between structure and electronic system (Hume-Rothery systems). In situ measurements of electronic transport properties (electrical conductivity, thermopower) were used to optimize the annealing procedures and to investigate the occurrence of crystalline intermediate phases as a function of composition. The composition-dependent investigations of transport properties give information on the interrelation between structure and electronic system, which are helpful to find optimal materials for technical applications. 

SESSION LL2: MECHANICAL PROPERTIES 
Chairs: Conradin M.H. Beeli and Reinhard V. Lueck 
Monday Afternoon, November 30, 1998 
Gardner A/B (S)

1:30 PM *LL2.1 
PLASTICITY OF QUASICRYSTALS. Michael Feuerbacher , Institut fuer Festkoerperforschung, Juelich, GERMANY. 

Quasicrystalline materials show a plastic behavior which distinctly differs from that of crystals, particularly of metals. For example, they show a brittle to ductile transition at very high temperatures and a complete lack of work hardening. A review of the experimental work, mainly performed on icosahedral Al-Pd-Mn will be presented. This includes results of macroscopic deformation experiments as well as microstructural characterization of deformed samples and in-situ tensile tests in a transmission electron microscope. The interpretation of the results will be presented in terms of a cluster-friction model. This is a qualitative model based on the assumption that the elementary Mackay-type clusters act as rate controlling obstacles on dislocation motion. In addition, recent results of deformation tests on decagonal Al-Ni-Co and on crystalline approximants in the Al-Pd-Mn system will be presented. 

2:00 PM LL2.2 
DEFORMATION CREEP CHARACTERISTICS OF A SINTERED Al-Cu-Fe APPROXIMANT WITH B2 STRUCTURE. L.M. Zhang , Q.G. Zhou, Dalian University of Technology, Department of Materials Engineering, C. Dong, Dalian University of Technology, The State Key Laboratory of Modification by Laser, Electron and Ion Beams, Dalian, P.R. CHINA. 

The deformation creep characteristics of a sintered B2 intermetallic alloy Al62.5Cu25Fe12.5 as an approximant were determined by high temperature creep experiments between 680-720C and 15-40Mpa. Structural features of specimens were characterized by means of X-ray diffractometer and transmission electron microscope. Chemical compositions were obtained by means of electron probe microanalysis. The structural relationship between the icosahedral quasicryctal and the B2-based approximant phase were also discussed. Compered with the classical creep theories of order alloys, the creep characteristics of the sintered B2 approximant were mainly controlled by compatibility of dislocation viscous glide mechanism and dislocation climb mechanism in the range of test temperature and stress. Microstructural analysis supported this conclution. Key words: quasicrystals, B2 approximant, sintering, deformation creep 

2:15 PM LL2.3 
THERMAL EXPANSION AND GRUENEISEN PARAMETER OF QUASICRYSTALS. Keiichi Edagawa , Kazuhiko Kajiyama, Univ. of Tokyo, Institute of Industrial Science, Tokyo, JAPAN; Shin Takeuchi, Science Univ. of Tokyo, Dept. of Materials Science and Technology, Chiba-ken, JAPAN. 

To investigate anharmonic phonon properties, thermal expansion measurements have been performed for icosahedral Al-Pd-Mn and decagonal Al-Cu-Co quasicrystals by X-ray diffractometry in the temperature range between 77 and 800K. Using the data of lattice specific heat and elastic modulii previously reported, Grueneisen parameters have been estimated for these quasicrystals. For the decagonal quasicrystal, the anisotropy in the thermal expansion measured is discussed in terms of anisotropic phonon properties originating in its structural feature. 

2:30 PM LL2.4 
GAMMA-SURFACES IN QUASICRYSTALS. R. Mikulla Los Alamos National Laboratory, Los Alamos, NM; G. Schaaf, H.-R. Trebin, Universität Stuttgart, Inst. für Theoretische und Angewandte Physik, Stuttgart, GERMANY. 

The concept of generalized stacking faults and the associated gamma-surfaces provides a useful tool for the analysis of dislocation core structures in crystalline materials. Applied to quasicrystals, suitable Burgers vectors for a given slip plane can be identified by calculating the minima of the gamma-surface. Instead of using a specific interaction model, we motivate a relation between structural properties and a subset of the minima of the gamma surface. The results agree with calculations based on pair potentials. As a consequence the continuum of possible Burgers vectors in physical space is reduced to a quasiperiodic set, described in terms of a strip-projection method. Further conclusions for the plasticity of quasicrystals are discussed. 

3:15 PM *LL2.5 
BULK MECHANICAL PROPERTIES OF QUASICRYSTALS. Shin Takeuchi , Dept. of Materials Science and Technology, Science University of Tokyo, Noda, Chiba, JAPAN. 

Quasicrystals are hard and brittle at room temperature; the hardness values being about 5% of the Young's modulus, almost the same as those of metallic glasses. At high temperatures above 0.7T_m, quasicrystals undergo plastic deformation by a dislocation process. A review is given of some characteristic features of plastic behavior at high temperatures of quasicrystals, both of icosahedral and decagonal phases; in some cases the conventional Arrhenius type strain-rate equation fails to apply. The features of the mechanical properties of quasicrystals are interpreted based on the characteristics of the quasicrystalline dislocation which is accompanied not only by a phonon strain field as in crystalline dislocation but also by a phason strain field. The complete phason strain relaxation with the dislocation glide is difficult since it requires medium range atomic shufflings in a wide range, and thus the quasicrystalline dislocation glide is exerted more or less by a dragging stress due to the production of a defect field around the dislocation. There is an indication that the Peierls potential is high for quasicrystalline dislocation. 

3:45 PM *LL2.6 
NUMERICAL SIMULATIONS OF PLASTICITY AND FRACTURE IN QUASICRYSTALS. Hans-Rainer Trebin , Inst for Theoretical and Applied Physics, Stuttgart Univ, Stuttgart, GERMANY. 

Numerous stress experiments have proven that quasicrystals are brittle at low temperatures, with cleavage planes rough on a nanometer scale, and become ductile without hardening at 80 of the melting temperature. Tiling models and extensive numerical simulations show that such mechanical behaviour is governed by the phason modes. The mechanism of plasticity is motion of dislocations which drag along phason walls. At high temperatures the walls are broadened by stress-induced diffusion. They weaken atomic bonds and initiate slip bands. At low temperatures, crack tips emit dislocations with sharp phason walls along which the material is opening. dynamics simulations were performed on two- and three-dimensional model quasicrystals with two-body Lennard-Jones potentials. Gamma surfaces were calculated to obtain preferred Burgers vectors. The results are presented in video clips. Temperature was varied up to melting to study the development of phason walls and the dynamics of crack propagation under different loads. 

4:15 PM LL2.7 
PLASTIC DEFORMATION OF Al-Cu-Fe ICOSAHEDRAL QUASICRYSTALS. E. Giacometti , N. Baluc and J. Bonneville, Ecole Polytechnique Fédérale de Lausanne, Lausanne, SWITZERLAND. 

Icosahedral quasicrystals are well known to be extremely brittle at ambient and intermediate temperatures and plastically deformable at temperatures higher than about 0.7 Tm. However, the mechanisms that control the high-temperature ductility are still matter to controversial discussions. While dislocation motion appears to be responsible for plastic deformation of icoshedral Al-Pd-Mn, ductility of Al-Cu-Fe seems to be governed by other mechanisms that still have to be identified. This research is aimed at providing information about the mechanical behavior and deformation mechanism(s) of icosahedral Al-Cu-Fe by combining conventional mechanical testing with internal friction measurements and transmission electron microscopy observations. 
Poly-quasicrystalline samples of an icosahedral Al-Cu-Fe alloy have been deformed in compression under constant strain-rate conditions at temperatures ranging from 300 K to 1020 K (Tm1150 K). Deformation experiments revealed that no plastic deformation takes place at ambient temperature. Stress-strain curves of samples deformed at T  0.7 Tm exhibit an elastic stage followed by a marked yield and a stage of plastic deformation with a negative slope representative of strain softening. The strain-rate sensitivity of the stress was investigated by performing load relaxation experiments. Values of activation enthalpy H and activation Gibbs free energy G have been determined. 
TEM observations were performed on as-cast and deformed specimens in order to characterize the microstructure of the samples and to identify the operating deformation mechanisms. 

4:30 PM LL2.8 
THE ACTIVATION PARAMETERS AND THE MECHANISMS OF PLASTIC DEFORMATION OF Al-Pd-Mn SINGLE QUASICRYSTALS. Ulrich Messerschmidt , Bert Geyer, Martin Bartsch, Max Planck Institute of Microstructure Physics, Halle S., GERMANY; Michael Feuerbacher, Knut Urban, Jülich Research Centre, Jülich, GERMANY. 

Icosahedral Al-Pd-Mn single quasicrystals were deformed in compression along fivefold and twofold axes. At a strain rate of 10^-5 s^-1, specimens in fivefold orientation deformed plastically including upper and lower yield points from 820 C down to 635 C. Down to 595 C, plastic deformation was achieved during stress relaxation tests before the yield point. Samples compressed along a twofold orientation are brittle below 680 C. The flow stress and its strain rate sensitivity are independent of the orientation but increase strongly with decreasing temperature. The strain rate sensitivity may have values either constant or even decreasing below 670 C. Stress relaxation curves following an original relaxation test but starting at a slightly reduced stress do not coincide, pointing at recovery of the mobile dislocations. The degree of recovery increases with increasing temperature. The Helmholtz free energy H of deformation increases from about 2 eV at 635 C up to about 5.5 eV at 820 C. The experimental activation volume is a unique function of stress. These observations are interpreted together with results of in situ straining experiments in a high-voltage electron microscope in the following way. The dynamic behaviour of dislocations is controlled by cutting the Mackay-type clusters of the quasicrystal structure, a process Feuerbacher et al. (1997) termed cluster friction. Since the experimental activation volume is larger than that following from cutting the clusters individually, it is concluded that the clusters act like extended obstacles which are overcome collectively. During the first loading before the yield point, rapid dislocation multiplication leads to a strong work-hardening. Further straining and the yield point are governed by a dynamic equilibrium between hardening and recovery, where recovery causes the strong decrease of the flow stress at high temperatures. 

SESSION LL3: POSTER SESSION 
Chair: An-Pang Tsai 
Monday Evening, November 30, 1998 
8:00 P.M. 
America Ballroom (W)

LL3.1 
MONTE CARLO SIMULATIONS OF ICOSAHEDRAL QUASICRYSTAL GROWTH AND MELTING. Vladimir E. Dmitrienko , Sergey B. Astaf'ev, Inst of Crystallography, Crystallophysics Lab, Moscow, RUSSIA.

Abstract not available.