1998 MRS Fall Meeting & Exhibit

Chairs

Easo George, Oak Ridge National Laboratory 
Michael Mills, Ohio State Univ
Masaharu Yamaguchi, Kyoto Univ

Symposium Support 

• Oak Ridge National Laboratory 
  
  

* Invited paper

SESSION KK1: TITANIUM ALUMINIDES I 
Chairs: David P. Pope and Tresa M. Pollock 
Monday Morning, November 30, 1998 
Commonwealth (S)

8:30 AM *KK1.1 CREEP OF FULLY LAMELLAR NEAR GAMMA TiAl INTERMETALLICS. J. Beddoes , Dept. of Mech. & Aero. Engineering, Carleton Univ, Ottawa, CANADA, L. Zhao, Structures, Materials & Propulsion Lab., National Research Council of Canada, Ottawa, CANADA. 

The envisaged -TiAl applications are within a temperature/stress domain consistent with creep deformation. Although several complex multi-component -TiAl compositions have been proposed, an understanding of the microstructural and compositional requirements for creep resistance is not well developed. Therefore, this paper presents an overview of an experimental program, involving binary and ternary gamma TiAl compositions, aimed at elucidating the factors responsible for the creep resistance of near -TiAl intermetallics. The influence of several factors, such as: lamellar grain size and volume fraction, lamellar interface spacing, grain boundary morphology and precipitation of third phase  along lamellar interfaces, on the primary creep, the minimum strain rate and tertiary creep are discussed. Narrow interface spacing leads to improved creep resistance for a binary Ti-48%Al. However, this effect is reduced substantially by the addition of ternary tungsten, highlighting the requirement for greater concentration on the interactive nature of microstructural and compositional changes. The significance of the knowledge gained through investigation of the relatively simple binary and ternary -TiAl compositions on the future development of complex  alloys is highlighted. The eventual goal of the work presented is the development of guidelines for the microstructural design of creep resistant near -TiAl intermetallics for specific gas turbine applications. To achieve this goal, the influence of the application creep failure criterion on the microstructure and composition selected is needed. For example, failure criterion of time to 1% strain versus time to rupture may demand different microstructures and compositions. Both these failure criteria are relevant for specific gas turbine components. 

9:00 AM KK1.2 
MICROSTRUCTURAL EFFECTS ON THE CREEP AND CRACK PROPAGATION BEHAVIOURS OF A GAMMA TITANIUM ALUMINIDE ALLOY. Valentino Lupinc , Giovanni Onofrio, CNR-TEMPE, Milan, ITALY; Mohamed Nazmy, Marc Staubli, ABB Power Generation, Baden, SWITZERLAND. 

Ganmma titanium aluminides class of materials possess several unique physical and mechanical properties. These characteristics can be attractive for specific industrial applications. By applying different heat treatment schedules one can change the microstructural features of this class of materials. In the present investigation, two heat treatment schedules were used to produce two different types of microstructures in the cast and HIP'd Ti-47Al-2W-0.5 Si alloy. Specifically, these microstructures were of the duplex and the nearly lamellar types. The tensile strength and creep behavior of this alloy, in the temperature range of 700-850C, have been determined and corelated to the corresponding microstructures. In addition, the fatigue crack propagation behavior in this alloy has been studied at different temperatures. The results on the creep behavior showed that the alloy with lamellar microstructure has an improved creep strength as compared with that of the duplex microstructure. The static and dynamic mechanical properties of this alloy were also compared with those of nickel base superalloys. 

9:15 AM KK1.3 
CREEP MECHANISMS OF FULLY-LAMELLAR TiAl ALLOYS BASED UPON INTERFACE SLIDING. Luke M. Hsiung and T.G. Nieh, Materials Science and Technology Division, Lawrence Livermore National Laboratory, Livermore, CA. 

Deformation mechanisms of fully-lamellar (FL) TiAl alloys with a refined microstructure ( lamellae: 100 - 300 nm thick,  lamellae: 10 - 50 nm thick) crept at temperatures between 650 and 810C have been investigated. A nearly linear creep behavior (i.e. the steady-sate creep rate is linearly proportional to the applied stress) was observed when the alloys were creep deformed at low applied stresses (< 400 MPa). When the alloys were crept at high stresses (> 400 MPa), deformation twinning was found to become an important deformation mode. Since the operation and multiplication of lattice dislocations within both  and  lamellae are very limited in a low stress level as a result of a refined lamellar microstructure, creep mechanisms based upon the glide and/or climb of lattice dislocations become insignificant. Instead, a cooperative glide of preexisting interfacial dislocations on both / and / interfaces (i.e interface sliding) has found to be a predominant deformation mechanism. It is accordingly proposed that the viscous glide of interfacial dislocations controls the creep deformation of refined FL-TiAI in the low-stress regime. During the gliding of interfacial dislocations along the lamellar interfaces, their mobility can be impeded by obstacles such as impinged lattice dislocations. This event increases as applied stress or strain increases and subsequently resulting in the pile-up of interfacial dislocations. It is accordingly suggested that deformation twinning in lamellar TiAI is a stress relaxation process for dissipating stress concentration at the head of each dislocation pile-up. A twinning formation mechanism driven by the pile-up of interfacial dislocations is proposed and verified. 

9:30 AM KK1.4 
HIGH-TEMPERATURE DEFORMATION OF UNIAXIALLY ALIGNED LAMELLAR TiAl/Ti₃Al. Helge Heinrich , Donny J. Wilkins, Gernot Kostorz, ETH Zürich, Institute of Applied Physics, Zürich, SWITZERLAND. 

Uniaxially aligned polysynthetically twinned two-phase TiA/Ti₃Al material is produced by induction zone melting and deformed in uniaxial compression tests. Above 900 K the strain rate sensitivity increases strongly with increasing deformation temperature. If the lamellae are oriented parallel or perpendicular to the deformation axis, strain softening occurs after a few percent compressive strain. After plastic deformation the dislocation density inside the lamellae is remarkably low indicating recovery processes. At the lamellar boundaries misfit dislocations and dislocations with a Burgers vector inclined to the lamellar interfaces are found. At some pseudo-twin boundaries of TiAl lamellae, a periodic arrangement of dislocations appear with a Burgers vector inclined to the interface. It results from ordinary dislocations moving on planes inclined to the lamellar interfaces. This dislocation arrangement is not found in undeformed samples. Only a few interfacial dislocations have a Burgers vector inclined to the lamellar interfaces in undeformed material. In contrast to plastic deformation at lower temperatures, deformation twinning is rare. 

9:45 AM KK1.5 
TITANIUM ALUMINIDES WITH NANOMETER LAMELLAR MICROSTRUCUTRE: FORMATION MECHANISMS AND PROPERTIES. Y.Q. Sun , Department of Materials Science and Engineering, University of Illinois, Urbana, IL. 

Refinement of the fully-lamellar microstructure into the nanometer scale has been achieved in titanium aluminides by exploiting the phase transformation mechanism for the lamellar microstructure. This paper explores the phase transformation mechanism of lamellar titanium alumindes, explains how the mechanism is exploited for microstructure refinement and the effects of microstructure refinement on mechanical properties. 

10:15 AM *KK1.6 
TEXTURES AND YIELD BEHAVIOUR OF TiAl WITH LAMELLAR MICROSTRUCTURES. Arno Bartels , Henning Uhlenhut, Technical University of Hamburg-Harburg, Dept of Physics and Technology of Materials, Hamburg, GERMANY. 

After casting TiAl shows a lamellar microstructure with a strong texture. The lamellae are preferentially aligned with the normal direction parallel to the direction of the heat flow during solidification. Like in PST crystals the alignment of lamellae leads to extremly anisotropic yield behaviour. The highest yield stresses are observed perpendicular and parallel to the lamellae, the lowest under an angle of 45. Compression parallel to the lamellae results in a plain strain deformation and ends with a brass texture. The results of compression tests with PST crystals performed in <112>-direction parallel to the lamellae confirm, that the brass position is a stable one. 
After a heat treatment in the -phase field still the casting texture is observed but much weaker. After forging combined with a final heat treatment in the -phase field the texture of the lamellar microstructure is randomized and no anisotropy of yield is observed. The texture after further deformation at room temperature shows clearly the strong influence of mechanical twinning. 

10:45 AM KK1.7 
WORK HARDENING CHARACTERISTICS AND RECOVERY OF GAMMA BASE TITANIUM ALUMINIDES. Fritz Appel , Ulf Sparka, GKSS-Research Centre, Institute for Materials Research, Geesthacht, GERMANY. 

Micromechanisms of deformation contributing to work hardening of gamma-base titanium aluminides at room temperature have been investigated. Deformation has been considered as stress driven thermally activated process and the experiments described were designed to identify the nature of glide obstacles generated during deformation. The investigations involve mechanical testing, electron microscope observations of the defect structure and recovery experiments. The analysis of the experimental data gives supporting evidence that work hardening is derived from long-range elastic interactions between dislocations on parallel and oblique slip planes. Another source of work hardening arises from dislocations dipoles and debris defects, which were trailed and terminated at jogs in screw dislocations and can be overcome with the aid of thermal activation. These defects probably give rise to a significant recovery of the work hardening upon annealing at moderate temperatures. The activation energy of the recovery process is close to the self-diffusion energy. Thus, the recovery of the work hardening has been attributed to diffusion assisted climb. 

11:00 AM KK1.8 
PROPERTIES OF <011] DISLOCATIONS IN Al-RICH GAMMA-TiAl SINGLE CRYSTALS DEFORMED OVER THE DOMAIN OF FLOW STRESS ANOMALIES. Fabienne Gregori, Patrick Veyssiere , LEM, CNRS-ONERA, Chatillon, FRANCE. 

Al-rich gamma-TiAl single crystals deform essentially by motion of 011] dislocations and show a peak of flow stress located between about 700C and 950C, depending on load orientation (Inui et al., Phil Mag. A, 75 (1998) 395). We present mechanical data of such alloys oriented so as to deform by single slip, complemented by TEM observations in samples strained at room temperature, 400C, 600C and 800C. Our study addresses (i) the properties of dislocation organization, (ii) the dissociation mode of 011] dislocations in their slip plane, (iii) the locking properties of 011] dislocations and (iv) the conditions of formation of stacking fault dipoles. These findings are discussed in the light of results available in the literature. 

11:15 AM KK1.9 
YIELD STRESS ANOMALY FOR <112] SLIP IN GAMMA TITANIUM ALUMINIDE. S. Jiao, N. Bird, P.B. Hirsch, G. Taylor , Oxford University, Dept of Materials, Oxford, UK. 

Transmission electron microscopy studies of dislocation structures in single crystals of gamma titanium aluminide deformed at different temperatures revealed the occurrence of slip on octahedral planes by the passage of 1/2<112] dislocations at room temperature and near the peak of the yield stress anomaly (YSA). Measurements of the corresponding yield stresses revealed the existance of a YSA for this type of slip. Weak-beam TEM showed the presence of locks at room temperature for the 30 deg dislocations and at higher temperatures for edge dislocations. Both types of lock involve dissociation on two intersecting {111} planes, driven by reduction in elastic strain energy. The edge dislocation dissociation at high temperatures involves both climb and glide. 

11:30 AM KK1.10 
GLIDE AND CROSS-SLIP OF ORDINARY DISLOCATIONS IN SINGLE CRYSTAL -Ti-56Al. Qiang Feng , Sung H. Whang, Polytechnic University, Six MetroTech Center, Brooklyn, NY. 

The plastic deformation of gamma titanium aluminides is largely attributed to ordinary dislocation slip and cross-slip. In order to have an in-depth understanding of the dynamic behavior of ordinary dislocations, single slip of ordinary dislocations was generated in single crystals of -Ti-56Al by compression deformation under varying temperature and deformation orientation. The yield stress and CRSS show an anomalous hardening occurring from 673K to 1073K. The hardening also exhibits deformation orientation dependence. The dislocation structures in the postmortem specimens from three different orientations as well as temperatures between 673K and 1073K were investigated by two-beam and weak-beam method using 200kV TEM. The ordinary dislocations observed were screw and aligned in the moving direction. Some of them possessed a number of bow-out segments and pinning points along the dislocation lines. The majority of dislocations glide in the primary slip plane, but some portions of the gliding dislocations were found to cross-slip onto other octahedral planes or (110) type planes. In addition to the cross-slipped dislocations, dislocation dipoles, dislocation loops, jogs and other debris defects were found in the vicinity of the pinning points. These unusual structures appear to be correlated with deformation temperature, strain, and deformation orientation. Possible structures responsible for anomalous hardening will be discussed. 

11:45 AM KK1.11 
TEXTURE DEPENDENCE ON THE SLIP SYSTEMS ACTIVATED DURING TENSILE DEFORMATION OF Ti-48Al-2Cr ROLLED SHEET. M.A. Morris , M. Leboeuf, Institute of Structural Metallurgy, University of Neuchâtel, Neuchâtel, SWITZERLAND; H. Clemens, Institut für Metallkunde, Universität Stuttgart, Stuttgart, GERMANY; S.M. Schlögl, Laboratory for Mechanics, Delft University of Technology, Delft, NETHERLANDS. 

Previous studies of mechanical properties during tensile deformation of TiAl rolled sheet have suggested that the anisotropy of deformation between samples loaded along the transverse and rolling directions should be explained using macroscopic considerations of Schmid factor calculations. In this way the activation of superdislocations is the only possible slip system in grains oriented with the c-axis in the transverse direction while ordinary dislocations are possible in grains oriented with the a-axes along the rolling direction. Mechanical twinning has been considered as having a negligible influence on the mechanical anisotropy at room temperature. In the present study the deformed tensile samples of the sheet material have been examined by electron microscopy to analyse and quantify the deformation systems activated in the materials when deformed along the transverse or rolling directions. SEM observations have confirmed that both when the sheet is loaded along the transverse or rolling directions a similar fraction of grains deform by mechanical twinning. TEM observations have confirmed the absence of superdislocations as a major deformation mechanism. Instead twinning, and ordinary dislocations are active. The sample loaded along the transverse direction, with the majority of grains oriented along the [001] axis, exhibit high densities of ordinary dislocations even though such slip systems are macroscopically unfavourable (m=0). Initially, before the onset of strain incompatibilities between adjacent grains, the crystal responds to the macroscopic stress and the deformation begins by activating the systems with the highest Schmid factor. In the case of the transverse direction the twinning systems 1/6[112]{111} are thus activated on loading with ordinary dislocations being emitted at twin/twin and twin/boundary intersections at a later stage. In the material loaded along the rolling direction only ordinary dislocations are initially activated with twinning occurring afterlocal stresses are built up. 

SESSION KK2: TITANIUM ALUMINIDES II 
Chairs: C. T. Liu and Kevin J. Hemker 
Monday Afternoon, November 30, 1998 
Commonwealth (S)

1:30 PM *KK2.1 
THE DEPENDENCE OF TENSILE DUCTILITY ON INVESTMENT CASTING PARAMETERS IN GAMMA TITANIUM ALUMINIDES. R. Raban, L.L. Rishel and T.M. Pollock , Carnegie Mellon University, Pittsburgh, PA. 

The use of investment cast gamma titanium aluminides in aircraft engine applications requires that some degree of tensile ductility be achieved reproducibly. As with any casting process, a range of cooling rates are likely to be encountered. Thus a better understanding of the influence of casting parameters on microstructure development and tensile ductility is needed. In this study three gamma titanium aluminide alloys have been subjected to varying cooling rates during investment casting. These alloys include Ti-48Al-2Cr-2Nb, Ti-47Al-2Cr-2Nb+0.5at%B and Ti-45Al-2Cr-2Nb+0.9at%B. Cooling rates have been estimated with the use of thermal data from casting experiments, along with UES ProCast solidification simulations. The influence of cooling rate on microstructure development and tensile ductility of each of these alloys will be discussed. 

2:00 PM KK2.2 
Abstract Withdrawn. 

2:15 PM KK2.3 
CONTRASTING THE FATIGUE-CRACK GROWTH RESISTANCE OF GAMMA-BASED TITANIUM ALUMINIDE ALLOYS AT LARGE AND SMALL CRACK SIZES. J.J. Kruzic , J.P. Campbell and R.O. Ritchie, University of California, Dept. of Materials Science and Mineral Engineering, Berkeley, CA. 

To date, most evaluations of the fracture and fatigue-crack propagation properties of  + ₂ titanium aluminide alloys have been performed using standard ``large-crack'' sample geometries, e.g., compact-tension specimens containing crack sizes which are large (on the order of a few millimeters) compared to microstructural dimensions. However, these alloys have been targeted for applications, such as blades in gas-turbine engines, where critical crack sizes will be much smaller (< 500 m) and where the small-crack fatigue threshold may be the most relevant design parameter. In this study, we compare and contrast the cyclic crack-growth behavior of both large ( > 3 mm) and small (  25 - 300 m) cracks in a -TiAl based alloy, of composition Ti-47Al-2Nb-2Cr-0.2B (at.), specifically for duplex (average grain size  17 m) and refined lamellar (average colony size  150 m) microstructures. It is found that, whereas the lamellar microstructure displays far superior fracture toughness and fatigue-crack growth resistance in the presence of large cracks, in small-crack testing the duplex microstructure exhibits better properties. The reasons for such contrasting behavior are examined in terms of the intrinsic and extrinsic (i.e., crack bridging) contributions to cyclic crack advance. 

2:30 PM KK2.4 
APPARENT SIZE EFFECTS IN NOTCHED TITANIUM ALUMINIDES UNDER MONOTONIC LOADING. Jack Beuth , Jorge Milke, David Knaul, Carnegie Mellon University, Department of Mechanical Engineering, Pittsburgh, PA. 

Because of their comparatively low density, gamma titanium aluminides are candidate materials for replacing nickel superalloys in aircraft engine components. Substantial concerns with these materials are their limited ductility and variations seen in ductility within cast slabs. In particular, the consequences of ductility limits and variability are of concern with regard to the strength of components containing stress concentrators. In this study, the notched strength of a cast near-gamma Ti­47.9Al-2.0Cr-1.9Nb alloy and a cast fully lamellar Ti­47.3Al­2.2Nb­0.5Mn­0.4W­0.4Mo-0.23Si alloy are are considered under monotonic tensile loading. Efforts are further focused on behavior under conditions of plane stress and on cases where notch radii are large relative to grain size. Results from testing of both alloys suggest that they exhibit size effects under monotonic loading. Notch root strains at failure become larger as the volume of highly strained material near the notch is reduced. Variability in notch root strains at failure is also increased as the highly strained volume is decreased. This suggests that specimen size can be an issue in quantifying ductility limits and variability. It also suggests that some conservativism results in designing notched components based on failure strains measured in large unnotched specimens. 

2:45 PM KK2.5 
PROCESSING AND CHARACTERIZATION OF AN Al₂Ti/Al₃Ti TWO-PHASE ALLOY. Michael J. Lukitsch and John E. Benci, Wayne State University, Materials Science and Engineering, Detroit, MI. 

Previous work in this laboratory has demonstrated that single-phase stoichiometric Al₂Ti has relatively high strength at room and elevated temperatures and good resistance to oxidation in air. This study was undertaken with the goal of improving the ductility of Al₂Ti through control of the microstructure in a two-phase Al₂Ti/Al₃Ti alloy. Several investigators have claimed that there is a single-phase region in the binary Ti-Al system corresponding to an Al content of about 71 at. occurring at a temperature greater than 1000C. Through appropriate heat treatment, it may be possible to achieve a microstructure conducive to good ductility analogous to ₂/ two-phase alloys. In this study, a binary alloy containing approximately 71 at. Al was prepared from elemental Ti and Al through plasma arc-melting. This alloy was then solution heat treated in the single-phase region and subsequently heat treated to produce a two-phase microstructure. The two-phase material was characterized using SEM/EDS, X-ray diffraction, DTA and microhardness testing. The results will be compared to previous results from stoichiometric Al₂Ti. 

3:00 PM KK2.6 
ELASTIC AND PLASTIC PROPERTIES OF GAMMA + LAVES PHASE IN-SITU COMPOSITE ALLOYS USING NANOINDENTATION TECHNIQUES. Mark L. Weaver , Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL; Ronald D. Ott, Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL; Michael P. Brady, Oak Ridge National Laboratory, Oak Ridge, TN. 

A novel new class of titanium aluminides, Laves phase reinforced gamma titanium aluminides, offers the potential for oxidation-resistant coating and/or structural applications use at temperatures up to 1000C in oxidizing environments. These  + Laves Ti-Al-Cr alloys are lightweight, oxidation-resistant, and potentially quite strong. However, microhardness measurements have revealed that these alloys are plagued by poor ambient temperature fracture toughness. In the present study, the local elastic and plastic properties of the  and the Laves phases have been investigated in a series of  + Laves alloys by hardness measurements using a nanoindenter with an emphasis on elucidating the local property changes in  + Laves alloys deformed at room temperature. This study shows that nanoindentation can be used to provide useful information on plastic flow in multiphase intermetallic alloys. 

3:30 PM *KK2.7 
THE EFFECT OF BORON ADDITION ON BRITTLE-TO-DUCTILE TRANSITION TEMPERATURE AND ITS STRAIN RATE SENSITIVITY IN GAMMA TITANIUM ALUMINIDE. Dongliang Lin (T.L. Lin) , Yu Wang, Deparatment of Materials Science, Shanghai Jiaotong University, Open Laboratory of Education Ministry for High-Temperature Materials and Tests, Shanghai, CHINA; Chi C. Law, Materials & Mechanics Engineering, United Technologies-Pratt & Whitney, East Hartford, CT. 

Tensile properties and fracture mode of two gamma titanium aluminides, Ti-47Al-2Mn-2Nb and Ti-47Al-2Mn-2Nb-0.8TiB₂, were investigated in a temperature range from 77 to 1373K and strain rate range from 10^-5 to 10^-1s^-1. Brittle-to-ductile transition (BDT), which was accompanied by a transition in fracture mode, was manifested in the investigated alloys. BrittIe-to-ductile transition temperatures (BDTT) of both alloys were determined under different strain rates and, based on the strain rate dependence of the determined BDTTs, apparent BDT activation energies were determined using Zener-Hollomon factor. It was found that the BDTT of either alloy increases sharply with the strain rate and that the minor addition of 0.8TiB₂ reduced BDTT by about 100K under the same strain rate. The TiB₂ addition also decreases apparent BDT activation energy from 324 to 256 kJ/mol. But BDT activation energies of the two alloys approximate to self- and inter-diffusion of Ti and Al atoms in TiAl phase.