1997 TMS Annual Meeting: Monday Abstracts

FUNDAMENTALS OF GAMMA TITANIUM ALUMINIDES: Session II: Phase Transformations and Microstructure Evolution

Session Chairpersons: Hamish L. Fraser, Dept. of Materials Science and Engineering, Ohio State University, Columbus, OH 43210; Hubert I. Aaronson, Dept. of Materials Science & Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

1:55 pm OPENING REMARKS

2:00 pm INVITED

PHASE TRANSFORMATION BEHAVIOR OF GAMMA TITANIUM ALUMINIDE ALLOYS DURING SUPERTRANSUS HEAT TREATMENT: S.L. Semiatin*, V. Seetharamann, D.M. Dimiduk*, Y-W. Kim, K.H.G. Ashbee* *Wright Laboratory Materials Directorate, WL/MLLM, Wright-Patterson AFB OH 45433; UES, Inc., 4401 Dayton-Xenia Rd., Dayton OH 45432

Recent work has suggested that near-fully lamellar or fully-lamellar microstructures may provide attractive combinations of room and elevated temperature properties in near-gamma titanium aluminide alloys. The development of such microstructures via thermal processing high in the two-phase (alpha+gamma) field or in the single-phase (alpha) field is described. In particular, the interaction of the dissolution of gamma grains and the growth of alpha grains during isothermal and transient heat treatment processes will be summarized. Models for the kinetics of gamma grain dissolution and alpha grain growth will be presented. The broad application of such models for the design of heat treatments to obtain fully lamellar microstructures will be illustrated for several forged gamma components.

2:30 pm

THE ROLE OF THE 2 PHASE IN ULTRAFINE LAMELLAR MICROSTRUCTURES DEVELOPED IN TWO-PHASE -TiAl ALLOYS: P.J. Maziasz, C.T. Liu, Metals & Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831- 6115

-TiAl alloys (i.e., Ti-47Al-2Cr-2Nb(at.%)) have ultrafine fully-lamellar structures after processing or heat-treatment above the -transus temperature; such structures produce outstanding high-temperature strength. The lamellar colonies consist of fine laths of 2 and phases, with 100-200 nm average lamellar spacings and 200-500 nm 2-2 spacings. Generally these structures are dominated by /2 interfaces rather than / interfaces, and they are relatively free of various structural defects often found in fully-lamellar structures. Aging studies of different alloys at 800-1000°C indicates that dissolution of the fine 2 lamellae is one of the critical first steps that triggers instability and continuous coarsening of the overall lamellar structure during aging or creep. This paper focuses on detailed TEM/AEM characterization of the 2 component of the microstructure and how that information feeds into designing better TiAl alloys. Research supported by the U. S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Industrial Technologies, Advanced Industrial Materials (AIM) Program, and Assistant Secretary for Defense Programs, Technology Management Group, Technology Transfer Initiative, under contract DE-AC05-96OR22464 with Lockheed-Martin Energy Research Corp.

2:50 pm

MICROSTRUCTURE EVOLUTION THROUGH THE PHASE TRANSFORMATION IN A TI-48 AT.% AL ALLOY: T. Kumagai, E. Abe, M. Nakamura, National Research Institute for Metals, Tsukuba-shi, Ibaraki 305, Japan

The (disordered h.c.p.) (TiAl; ordered L10 structure) massive transformation is partially suppressed even in a Ti-48 at.%Al alloy, when the alloy is quenched rapidly from the high temperature a phase filed. The untransformed (meaning 'not massively transformed') regions show an extremely fine 2 (Ti3Al; ordered DO19 structure) / lamellar structure rather than an 2 single phase structure, which is commonly observed in the quenched alloys with Al concentration of less than 47 at.%Al. By the subsequent aging treatment this fine 2/ lamellar structure changes easily to the fine grain structure, which is quite similar to the massively transformed grain structure. The microstructural development of the extremely fine 2/ lamellae during the isothermal aging treatments is presented and the phase transformation through the 2/ lamellar structure will be discussed.

3:10 pm

THE GAMMA TO ALPHA TRANSFORMATION IN A TI-48AL ALLOY: K. Muraleedharan, T.M. Pollock, Dept. of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213; P. Wang, V.K. Vasudevan, Dept. of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221

The transformation from to in a Ti-48Al alloy during aging in the + phase field between 1275-1350°C is reported using microhardness, optical, scanning and transmission electron microscopy. The results indicate that on heating a primary structure to temperatures in the two-phase + phase field, packets of a nucleate within the grains in four orientations parallel to the four {111}g planes. The a platelets generally nucleate at grain boundaries and stacking faults on {111} planes bounded by 1/6<112] Shockley partial dislocations appear to serve as nuclei for them. The a precipitation kinetics, volume fraction and packet thickness depend strongly on the aging temperature, generally increasing with increase in temperature. These changes are also accompanied by significant hardening with time at temperature, from the initial value to a maximum, followed by a decrease at longer times. The morphology of the resulting microstructures, nucleation mechanisms, orientation relationship between the phases, sub-structure development and kinetics of precipitation during the transition will be discussed.

3:30 pm BREAK

3:50 pm

THE ORDERING TIE LINES AND TIE TRIANGLES IN TITANIUM ALUMINIDES: D.-H. Hou, H.L. Fraser, Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210

The site occupancies of alloying elements in TiAl and the ordering states in orthorhombic titanium aluminides are investigated using the newly developed Ordering Tie Line (OTL) analysis. The OTL, which can represent the atom configuration in an ordered alloy in a graphical and intuitive way, is a parameter that is unique to ordered alloys. There are two properties of an OTL, one being its slope, indicating the trend for sublattice occupancy, and the other the compositional endpoints, corresponding to the compositions of the individual sublattices. The slope can be determined conveniently by Atom Location by Channeling Enhanced Microanalysis (Alchemi) experiments, whereas the compositional endpoints may be determined either from additional information concerning the ordering scheme or determined by other experiments/simulations. In the present study, compositional endpoints are determined by simulation using the dynamical theory of electron diffraction. The OTL analysis which was originally developed for ternary compounds has been further extended to quaternary systems such as TiAl with two alloying elements and ternary system with three sublattice sites such as the orthorhombic alloys based on Ti2AlNb. For the TiAl alloys, the effect of alloying elements of Nb, Cr, Mo and Mn on the OTL are determined and discussed. For the orthorhombic alloy, it will be shown how the ordering state can be described by the Ordering Tie Triangle (OTT), and how to measure the OTT by the Alchemi experiments. Since the OTL can provide a direct measure of the ordering state, it will be an important parameter for alloy design. This work has been supported by the US ONR, Dr. George Yoder as Program Manager.

4:10 pm

DECOMPOSITION OF -TiAl/2-Ti3Al LAMELLAR STRUCTURE: J. Zhang, Z.H. Zhang, D.X. Zou, Z. Y. Zhong, Central Iron and Steel Research Institute, Beijing 100081, China

It's found in the cast Ti-46.5Al-2.5V-1.0Cr (at%) ingot, that the discontinuous coarsening (DC) dominates the decomposition of the lamellar structure when annealing at temperatures lower than 1273K while the continuous coarsening (CC) does at the temperature between 1373K and 1473K. In the case of CC, the lamellae coarsened segmentally and then the FL microstructure decomposed to a finer equiaxed near gamma (NG) microstructure as annealing time went on. The TEM observation showed that the prior lamellae are quite neat and perfect and the annealed lamellae have inner terminations. Thus, the Rayleigh's perturbation and breakdown of lamellae was believed to have occurred along with the CC. Furthermore, a cycle heat treatment of 1173K-1423K has been designed to increase the density of the inner terminations of lamellae. After that, the FL microstructure has been decomposed to a more homogeneous and even finer equiaxed NG microstructure.

4:30 pm

MICROSTRUCTURE EVOLUTION DURING POSTWELD HEAT TREATMENT OF GAS TUNGSTEN-ARC AND ELECTRON BEAM WELDS IN CAST Ti-48Al-2Cr-2Nb ALLOY: W.A. Baeslack III, C.M. Jensen, H. Zheng, Department of Industrial, Welding and Systems Engineering, Ohio State University , Columbus, OH 43210; T.J. Kelly, GE Aircraft Engines, 1 Neumann Way, Evendale, OH 45215

The microstructures of multi-pass gas tungsten-arc (GTA) welds produced in cast Ti-48Al-2Cr-2Nb (at.%) have been evaluated in the as-welded condition and following postweld heat treatment over a range of temperatures from 1000 to 1300°C. Although postweld heat treatment did not significantly affect the cast, HIP'ed and heat-treated base metal microstructure, it did promote transformation of a predominantly lamellar 2+ microstructure in the as-welded fusion zone, to microstructures comprised principally of equiaxed grains. An increase in the heat treatment temperature resulted in an increased proportion of 2 located principally at grain boundaries and grain boundary triple points. These microstructural changes were associated with softening and toughening of the fusion zone, both of which increased with an increase in postweld heat treatment temperature. Featureless, hard bands (>400 DPH as-welded) observed to parallel the fusion boundaries, which exhibited an extremely fine lamellar microstructure, were also softened by postweld heat treatment, although they remained harder than the surrounding weld metal microstructure. This work was supported by a grant to the Carnegie Mellon University from the Air Force Office of Scientific Research.

4:50 pm

INTERACTION BETWEEN TIAL AND ALN AT HIGH TEMPERATURES: Y. Paransky, E.Y. Gutmanas, Department of Materials Engineering, Technion, Haifa 32000, Israel

In this work, interfacial reactions between TiAl intermetallic and aluminum nitride have been studied in the 800-1200°C temperature range. Titanium aluminides are developing as a new group of materials for high temperature applications. Their load bearing capacity can be considerably improved by introducing high strength ceramic fibers, such as SiC or boron. Due to the high reactivity of Ti-containing materials, chemical interaction between fibers and TiAl matrix takes place at the processing and/or service temperatures resulting in deterioration of mechanical properties. Fibers can be protected by a thin coating layer which dissolves slowly enough to prevent the matrix from attacking the fiber during the composite lifetime. AlN is a possible choice for such a coating in Ti-containing matrices. Interfacial reactions between AlN and TiAl matrix have been studied using the diffusion couple approach. Phases growing at the interface between TiAl and AlN have been identified by XRD, SEM/EDS, AES and TEM; the kinetics of reaction layer growth has also been investigated. It has been found that AlN is more stable in TiAl than in pure Ti. In the latter case, the dissolution of AlN is faster due to the higher activity of Ti and high diffusivity or nitrogen in Ti.