1997 TMS Annual Meeting: Monday Abstracts

FUNDAMENTALS OF GAMMA TITANIUM ALUMINIDES: Session I: Phase Equilibria and Transformations

Session Chairpersons: Michael J. Kaufman, Dept. of Materials Science and Engineering, Univ. of Florida, 201 Rhines Hall, Gainesville, FL 32611; Bruce A. MacDonald, National Science Foundation, 4201 Wilson Blvd., Arlington, VA 22203-9966

8:25 am

OPENING REMARKS: Kwai S. Chan, Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510

8:30 am INVITED

KINETIC AND MECHANISTIC ASPECTS OF THE m MASSIVE TRANSFORMATION IN TIAL ALLOYS: Ping Wang, D. Veeraraghavan, Vijay K. Vasudevan, Dept. of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012

The kinetics and temperature dependence of the transformation in Ti-(46-48) at.% Al alloys was studied using a novel computer-controlled temperature and electric resistivity acquisition system. Samples of the alloys were heated by controlled direct resistance heating and cooled at various rates by a helium jet quench. In situ, real time, high speed, simultaneous measurements of electrical resistivity and temperature were made during both heating and cooling. Using the resistivity and thermal arrest data, the start and finish temperatures of the various transformation modes, viz., lamellar, Widmanstatten/feathery and massive were determined as a function of cooling rate. The data obtained was correlated with light and electron microscope observations to establish transformation diagrams, and to determine the growth rate of the massive phase as a function of undercooling. The experimental data was fitted into physical models and thermodynamic quantities such as the enthalpies/driving forces of formation of the massive were determined. The activation enthalpy for boundary diffusion estimated from this data indicates that the massive transformation is controlled by interfacial (rather than volume) diffusion. Defect structures in the massive phase and the massive -parent interface were characterized in detail by TEM. The implications of these studies on the mechanism of the massive transformation will be discussed. This research is supported by grants from the National Science Foundation (Dr. Bruce MacDonald, Program Monitor) and UES/Wright Laboratory (Dr. Madan Mendiratta, Program Monitor).

9:00 am

CALCULATION OF THE TI-AL PHASE DIAGRAM: F. Zhang, S.L. Chen, Y.A. Chang, Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706; U.R. Kattner, National Institute of Standards and Technology, Gaithersburg, MD 20899

A thermodynamic description of the Ti Al system is developed in this study. Nine phases were considered in this system. They are disordered solution phases: liquid, ­(Ti, Al), ­(Ti, Al), (Al); ordered intermetallic phases: 2­Ti3Al, ­TiAl, TiAl3 and stoichiometric compounds: TiAl2, Ti2Al5. The Redlich Kister equation was used to describe the excess Gibbs energy for the disordered solution phases; the generalized quasi-chemical model recently developed in our research group was used to describe the ordered intermetallic phases. The parameters describing these models were obtained by optimization using the experimental phase equilibrium and thermodynamic data available in the literature. The quasi-chemical model is an extension of the Bragg Williams model similar to the extension of the regular solution model for disordered solution phases. Thermodynamic values obtained from the model parameters as well as the calculated phase diagram are in good agreement with the experimental data. The intrinsic defect concentrations calculated from the model parameters for ­TiAl are compared with the predictions of the first principle calculation, the semi empirical relationship and the available experimental data, reasonable agreement is obtained. The model parameters for the ordered intermetallic phases can be converted to the format of sublattice model and Wagner Schottky model.

9:20 am

PHASE DIAGRAM MODELLING OF TiAl ALLOYS: Nigel Saunders, Thermotech Ltd, The Surrey Research Park, Guildford GU2 5YG, UK; IRC in Materials for High Performance Applications, University of Birmingham, Birmingham B15 2TT, UK

The phase relations in Ti-Al based systems are by no means well established except for the case of a few specific ternary systems. This lack of information is now being exacerbated as new generation alloys are commonly multi-component in nature. Thermodynamic phase diagram modeling for Ti-Al-X systems has been presented previously which can now form the basis for the extension of the modeling technique to multi-component alloys. Examples of calculations for a variety of multi-component alloys will be shown and the effect of light elements such as O will also be modeled. An important factor in the success of the calculation method is that the ordering of the phase to 2 can now be incorporated. 1. N. Saunders, to be published in "Titanium '95: Science and Technology," eds. P. Bleckinsop et al (London: Inst. Materials, 1996).

9:40 am

EFFECTS OF SUBLATTICE ORDERING AND COMPOSITIONAL UNCERTAINITIES ON THE DETERMINATION OF ACCURATE AND PRECISE DEBYE-WALLER AND STRUCTURE FACTORS IN TIAL: S. Jayanthi, S. Swaminathan, H.L. Fraser, Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210; I.P. Jones, School of Metallurgy and Materials, University of Birmingham, Edgbaston B15 2TT, UK; D.M. Maher, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC

There has been significant interest aimed at determining the anisotropy of charge densities in the intermetallic compound TiAl. Such determinations require the accurate measurement of low order structure factors, which in turn requires an accurate knowledge of the Debye-Waller factors of the given alloy/compound. Hence, the quest for assessments of charge densities in TiAl has involved determinations of the Debye-Waller factors and structure factors. The first of these are conveniently extracted from single crystal x-ray diffraction experiments, and it has been found that in samples whose compositions are Al rich sub-lattice ordering occurs, such that there are unequal values of the Debye-Waller factors for the two Ti sites. The effect of this sub-lattice ordering on the determination of the Debye-Waller factors has been assessed and will be discussed. Furthermore, the effect of compositional uncertainties on the determination of both Debye-Waller factors and structure factors has been investigated. It has been shown that sample compositions must be known to within ±0.15at.% for accurate determination of charge densities. These limitations will be discussed, and an experimental approach to overcome these will be detailed. This work has been supported by the National Science Foundation, Dr. Bruce MacDonald as Program Manager.

10:00 am BREAK

10:20 am

PHASE RELATIONS AND ALPHA DECOMPOSITION IN Ti-(25-50) Al ALLOYS: Y-W. Kim, UES, Inc., Dayton, OH 45432

The phase relations in Ti-(25-55) at.%Al involved in the decomposition and transformations of the high temperature alpha phase were investigated by conducting annealing/cooling experiments in a cooling/heating rate range of 0.2-100°C/min. X-ray and differential thermal analyses were conducted to determine phase relations, transformation temperatures and enthalpies involving various alpha-decompositions. Three types of decomposition reactions were found to exist, depending on composition and cooling rate, that is; I: 2; II: L(2 + ); and III: L(' + )L('2 + ). In Types II and III, the undercooling and the lamellar spacing (L) for the lamellar structure (L) were related to cooling rate and Al content. The ordering reaction in Type III, 2 took place at increasingly higher temperatures (with Al content) than the Type I extrapolation due to the compositional changes. Both Type I and II reactions are favored in low Al-content alloys; however, they (Type II in particular) could take place even in Ti-48Al. This and the other reactions will be analyzed in detail.

10:40 am

GRAIN REFINEMENT AND LAMELLAR FORMATION IN HOT-WORKED Ti-(42-47)Al-(0-0.5)B ALLOYS: Y-W. Kim, UES, Inc., 4401 Dayton-Xenia Rd., Dayton, OH 45432

Small amounts of boron additions are known to effectively refine the alpha grains in hot worked TiAl alloys when heat treated in the alpha field, resulting in the formation of fine lamellar colony microstructures. The boron additions raise significantly the critical cooling rate above which the formation of fully lamellar structures begins to be suppressed. Since this cooling rate allows the lamellar spacing to be controllable to a greater range, it is critical to understand the formation mechanism and kinetics of gamma laths in the presence of boron (in the form of borides). For this, annealing and cooling experiments and microstructural observations have been conducted on forged alloys of Ti-(42-47)Al-(0-0.5)B. DTA measurements are underway to detect the undercoolings before gamma precipitation in the alpha matrix as a function of cooling rate. The results will be presented and analyzed to explain the role of borides on the grain refinement, the formulation of gamma precipitates, and the retardation of nonlamellar microstructure formation.

11:00 am

THE 2 SHEAR TRANSFORMATION: P.M. Hazzledine, UES, Inc., 4401 Dayton-Xenia Rd. Dayton, OH 45432; B.K. Kad, Dept. of Applied Mechanics & Engineering Science, University of California-San Diego, La Jolla, CA 92093; V.K. Vasudevan, Dept. of Materials Science and Engineering, Univ. of Cincinnati, Cincinnati, OH 45221

When a hexagonal close packed crystal, with ideal c/ ratio, is sheared by Shockley dislocations on alternate basal planes the resulting crystal is face centered cubic. The six Shockley vectors generate cubic structures in two orientations which are twins of each other. When the DO19 structure is sheared by the same six Shockley vectors the result is three different 12H structures and their three twins. These 12H structures are composed of tetragonal unit cell building blocks, having axial ratios 1:1:/2. In this description, all of the {100} planes have alternate compositions Ti, TiAl and the unique c axes are in six different directions, one resulting from each of the six Shockley shears. The 'tetragonal' description of the structures is the same as the tetragonal structure L10 except that in L10 the {100} planes have alternate compositions Ti, Al. Thus, shearing the DO19 structure in one of the Shockley directions by 1/2:/2 creates a crystal structure which is as close to L10 as the composition allows. The six variants of gamma may therefore be formed from a single crystal of 2 by the operation of dislocations with the six available Shockley Burgers vectors so long as the composition may be corrected by diffusion.

11:20 am

A SYSTEMATIC STUDY OF THE ELECTRONIC STRUCTURE OF STABLE AND METASTABLE FORMS OF -TIAL: J.M.K. Wiezorek, X.D. Zhang, R. Banerjee, H.L. Fraser, Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210

Numerous theoretical, first principle calculations studies have shown that the interatomic bonding in TiAl is a mixture of covalent and metallic. There is a need to complement these studies with experimental observations. Electron energy loss spectroscopy (EELS) is capable of providing electronic structure information through the near edge fine structure in core loss peaks, since these reflect the combined effects of bonding parameters such as charge transfer, hybridization, and variation of the crystal and ligand field. In this paper, EELS has been used to study the electronic structure of stable and metastable forms of -TiAl. The near-edge EELS signatures of the equilibrium phases at Ti-25at.%Al, Ti-75at.%Al and Ti-52at.%Al have been used to assess the suitability of this experimental method for the study of the electronic structure in -TiAl compounds. Metastable phases of -Ti-50at.%Al and 2-Ti-48at.%Al have been generated by very rapid quenching from the -phase field, and thin amorphous films of -Ti-48at.%Al have been deposited by magnetron sputtering. The near edge structure of these phases have been studied using a field emission TEM equipped with an imaging parallel EELS. Furthermore, results for 2-Ti-36at.%Al and -Ti-52at.%Al in binary lamellar TiAl are presented. The results of this study are discussed with a view to the mechanical behavior and the phase stability of -TiAl based compounds. This work has been supported by a grant from the National Science Foundation with Dr. Bruce MacDonald as program manager.