1997 TMS Annual Meeting: Tuesday Abstracts

FUNDAMENTALS OF GAMMA TITANIUM ALUMINIDES: Session IV: Microstructure/Property Relationships--Strength, Plasticity, and Toughness

Session Chairpersons: Chain T. Liu, Oak Ridge National Laboratory, Metals & Ceramics Div., PO Box 2008, Oak Ridge, TN 37831-6115; G.T. (Rusty) Gray, III, Los Alamos National Laboratory, Los Alamos, NM 87545

1:55 pm OPENING REMARKS

2:00 pm INVITED

THE ROLE OF GRAIN SIZE AND SELECTED MICROSTRUCTURAL PARAMETERS IN STRENGTHENING FULLY-LAMELLAR TiAl ALLOYS: D.M. Dimiduk, Wright Laboratories Materials Directorate, WL/MLLM, WPAFB, Dayton, OH 45433; P.M. Hazzledine, T.A. Parthasarathy, UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432; S. Seshagiri, SYSTRAN, Inc., 4126 Linden Ave., Dayton, OH 45432

More than five years ago wrought processing was first used to produce fully-lamellar microstructures in TiAl alloys having grain sizes less than ~400mm. These alloys exhibit an improvement in overall balance of properties, especially at high temperatures. More recently, such microstructural forms led to exceptional yield strengths (>1000Mpa at low temperatures), while maintaining attractive high-temperature properties. The improvements appeared to be attained principally through an unusually high apparent sensitivity of strength on grain size. Studies reported an apparent value for the slope of the "Hall-Petch plot" approaching 5 Mpa for fully lamellar gamma alloys, while that for single phase or duplex microstructures is near unity. The present investigations examine the slope of the Hall-Petch plot for fully lamellar microstructures, paying particular attention to the lamellar microstructural variables. Results show that 2-lamella thickness and spacing, and -lamella thickness can vary over more than two orders-of-magnitude. These in turn influence the value of o in the Hall Petch plot, and often change concomitantly with grain size in processing. Nonetheless, the corrected value of the Hall-Petch constant exceeds a magnitude of 2 Mpa; and it is strain dependent. The investigations also examine dislocation activity, flow behavior, glide barriers and slip multiplicity for polysynthetically twinned or PST crystals (single-grain analogue of fully lamellar material), then map this behavior into an explanation of the yield behavior of high strength fully lamellar gamma alloys.

2:30 pm INVITED

BULK, DEFECT, AND INTERFACIAL PROPERTIES OF TiAl and Ti3Al: M.H. Yoo, C.L. Fu, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831

Available theoretical and experimental data on the elastic constants, point defect properties, shear fault energies, and ideal cleavage energies of TiAl and Ti3Al are reviewed, including the recently calculated / and 2/ interfacial energies. Enhanced slip by ordinary dislocations along pseudo-twin and rotational / and the 2/ interfaces is a contributing factor to the strong plastic anisotropy of a fully lamellar microstructure. According to the calculated interfacial fracture energies, cleavage cracking is to occur on 2/ boundaries and the least likely on true-twin boundaries. The roles of misfit dislocations, kinetics of dislocation-interface interactions, and hydrogen embrittlement in deformation and fracture of two-phase TiAl alloys are discussed. This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract number DE-AC05- 96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation.

3:00 pm

INTERNAL STRESSES IN LAMELLAR TiAl: P.M. Hazzledine, UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432; M.A. Grinfeld, Department of Mechanical and Aerospace Engineering, Rutgers University, P.O. Box 909, Piscataway, NJ 08855; D.M. Dimiduk, Wright Laboratories Materials Directorate, WL/MLLM, WPAFB, Dayton, OH 45433

Internal stresses necessarily develop in multilayered coherent materials with mismatched lattice parameters as the individual layers adjust to their neighbors. In Ti-Al the layers consist of 2 plates and six orientation variants of gamma plates which occur in an apparently random sequence. The internal stresses in any one layer are affected by the orientations of all its neighbors. We have calculated, in closed form, the stresses present in one layer of a stack of gamma layers in which the sequence of variants is in any order and of any length and in which the thicknesses of each layer may take any value. In addition we have calculated the stresses in a stack of alternating 2 and layers of arbitrary thicknesses. The results are applicable directly to an idealized form of PST TiAl in which the domain size is large and the lamellae thickness is small. The relaxation of internal stresses which occurs in thick lamellae and by the formation of domains is discussed. The consequences of both these relaxations to the plastic strength of PST TiAl are examined.

3:20 pm

RELATIONSHIPS BETWEEN MICROSTRUCTURE/COMPOSITION AND MECHANICAL PROPERTIES OF GAMMA TITANIUM ALUMINIDES: W.O. Soboyejo, Y. Ni, C. Mercer, Dept. of Materials Science and Engineering, Ohio State University, 2041 College Road, Columbus, OH 43210; A.B.O. Soboyejo. Dept. of Aerospace Engineering, Applied Mechanics and Aviation, Ohio State University, 155 W. Woodruff Ave., Columbus, OH 43210; R. Armstrong, Dept. of Mechanical Engineering, University of Maryland, College Park, MD 20742

Quantitative relationships will be presented for the prediction of basic mechanical properties (yield/ultimate tensile strength, fracture toughness and plastic elongation to failure) at room- and elevated- temperature. These include Hall-Petch relationships between basic mechanical properties and the average equiaxed grain size/lamellar packet size. Multiple linear regression expressions are also presented for the prediction of the effects of grain/packet size, lamellar volume fraction and composition. The observed grain size dependence of mechanical properties is shown to be consistent with Hall-Petch exponents predicted for dislocation/dislocation and dislocation/grain boundary interactions in duplex 2 + alloys. The implications of the empirical relationships are assessed for the engineering of balanced mechanical properties in gamma titanium aluminides.

3:40 pm BREAK

3:50 pm INVITED

TEMPERATURE DEPENDENT DEFORMATION IN GAMMA TITANIUM ALUMINIDES: S.H. Whang, Z.M. Wang, Q. Feng, C. Wei, Polytechnic University, Six Metrotech Center, Brooklyn, NY 11201

Deformation of gamma titanium aluminides may be best described by the lack of sufficient ductility at RT as well as intermediate temperatures, and the anomalous yielding at high temperatures. Such deformation behavior is mainly attributed to unusual behavior of two independent slip systems: superdislocation slip and ordinary dislocation slip, both of which simultaneously operate over the entire temperature range in the polycrystalline alloys. For this reason, the study on the role of individual slip system is required to carry out deformation experiments with single crystals of a -TiAl alloy. In -Ti-56Al, the yield stress curves of superdislocation slip show positive temperature dependence over two distinct temperature regimes while those for ordinary dislocation slip exhibit a single temperature range. These distinct temperature regimes can be explained with the characteristic features of the dislocation structures which were obtained from the postmortem examinations of the deformed specimens conducted by TEM. The thermodynamic aspects of both cross-slip configurations of superdislocations will be presented based on anisotropy strain energy calculations. Finally, the CRSS values depend on deformation orientation in superdislocation slip, in consistent with the cross-slip models. The details of the orientation dependence will be discussed.

4:20 pm

PLASTIC INSTABILITIES IN -TiAl AND THEIR RELATION TO WORK HARDENING AND TOUGHNESS: François Louchet, Groupe Physique du Metal, LTPCM- UMR.CNRS/INPG/UJF, B.P. 75, Domaine Universitaire, 38402 - St Martin d'H'eres, France

Flow stress anomaly in -TiAl is associated, as in many other intermetallics, to a strain rate sensitivity (SRS) close to zero, to plastic instabilities, and to a high work hardening rate (WHR). In situ TEM observations also show a jerky motion of groups of cusped ordinary screw dislocations (1), in agreement with macroscopic instabilities. The present paper investigates these instabilities in the framework of the Local Pinning Unzipping (LPU) model (2), in which the flow stress anomaly and the low value of the SRS are predicted as a consequence of a balance between dislocation exhaustion and multiplication rates. This balance is applied here to groups of N dislocations, and results in a non linear equation in N, from which the minimum stress required to deform the material at a given strain rate can be determined. The corresponding average value of N is obtained as a function of temperature. The consequences of the temperature dependence of N on both WHR and toughness are discussed. Possible consequences on the particular dramatic brittleness of PST materials loaded perpendicular to lamellar boundaries (3), in which slip in -TiAl takes place across lamellae, can also be contemplated as a function of these instabilities, and particularly in terms of the variations with temperature of the number N of dislocations in pile-ups.
(1). S. Farenc and A. Couret, Mat. Res. Soc. Symp., 1993, vol.288, p.465.
(2). F. Louchet et B. Viguier, Phil. Mag. A , 1995, vol.71, p.1313.
(3). M. Yamagushi, H. Inui, K. Kishida, M. Matsumoro and Y. Shirai, Mat. Res. Soc. Symp., 1995, vol.364, p.163.

4:30 pm

MORPHOLOGICAL ASPECTS OF 1/2<110] UNIT DISLOCATIONS IN AND (2 +) TiAl ALLOYS: Sriram Seshagiri, SYSTRAN Corporation, 4126, Linden Ave., Dayton, OH 45432; Dennis M. Dimiduk, Materials Directorate, WL/MLLM, WPAFB, OH 45433; Vijay K. Vasudevan, Dept. of Materials Sci. & Eng., Univ. of Cincinnati, Cincinnati, OH 45221

The b=1/2<110] unit dislocations are a dominant part of the deformation substructure in both single phase () and two-phase (2+) TiAl alloys, at ambient temperatures and above. These dislocations have been characterized in a coarse-grained (~300 mm) single-phase binary TiAl alloy. The results show that these dislocations undergo double cross-slip, resulting in numerous pinning points (jogs) and mixed character dislocation segments bowed about these pinning points, giving rise to a cusped morphology. The linear pinning-point density in single-phase alloys increases with deformation temperature from ~2-5 mm-1 at RT, to ~10 mm-1 at higher temperatures, with an attendant increase in the yield strength of the alloy. In two-phase (2+) fully-lamellar polycrystals and PST crystals the, lamellae thicknesses are typically ~1mm, which is approximately the same order as the free-segment length of the unit dislocations in the single-phase alloys. Hence, the morphological evolution of 1/2<110] unit dislocations in the fully lamellar material, wherein deformation propagation is constrained by the lamellae dimensions, is of particular interest. Transmission electron microscopy observations pertaining to these aspects, and their implications on strength and plasticity, will be discussed in the presentation.

4:50 pm

TENSILE/COMPRESSIVE PROPERTIES OF SINGLE CRYSTAL GAMMA Ti-55.5%Al ALLOY: Marc Zupan, Kevin J. Hemker, Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD 21218

Understanding of the deformation mechanisms that lead to the flow strength anomaly of Ni3Al has been greatly enhanced by the measurement of the orientation dependence of flow stress and tension/compression asymmetry. To date, lack of high quality single crystals sufficiently large to facilitate mechanical testing have precluded such studies of TiAl. High quality single crystals of -Ti(55.5%)Al have been grown using an optical float zone furnace, which provides a containerless growing environment. These crystals have been oriented and cut into microsample tension/compression specimens with a gage area of 250 mm x 250 mm and effective gage length of 300 mm. These specimens have been deformed using a microsample testing machine which applies loads on the order of 1 lb. and strain using an interferometric strain gage. Stress-strain curves will be reported for a variety of orientations, temperatures and as a function of the sense of applied load. Experimental results will be discussed within the current context of current dislocation theories.