1997 TMS Annual Meeting: Tuesday Abstracts

FUNDAMENTALS OF GAMMA TITANIUM ALUMINIDES: Session III: Processing, Microstructure and Properties

Session Chairpersons: James C. Williams, GE Aircraft Engines, 1 Neumann Way, Evendale, OH 45215; Young-Won Kim, UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432

8:25 am OPENING REMARKS

8:30 am INVITED

PROCESSING-PROPERTY-MICROSTRUCTURE RELATIONSHIPS IN TiAl-BASED ALLOYS: M H. Loretto, D. Hu, A. Godfrey, T.T. Cheng, I.P. Jones, P.A. Blenkinsop, IRC in Materials for High Performance Applications, The University of Birmingham, Edgbaston B15 2TT, UK

A range of TiAl-based alloys has been produced by plasma melting either small buttons (1kg samples) or ingots (up to 50kg). These alloys have been subsequently processed using isothermal forging, extrusion and/or HIPping. Some problems associated with the melting and subsequent processing of these alloys will be discussed and the ways found to overcome these problems outlined. Microstructural data obtained from some of these alloys will be presented which show the relationship between processing route and microstructure, and these observations will be correlated with mechanical properties. It will be shown that the microstructure can be closely controlled by appropriate processing and further that for a range of alloy compositions the properties can be controlled more by processing than by composition. For other alloys the composition is more important.

9:00 am

DEFORMATION OF NANOCRYSTALLINE Ti-48Al: L.S. Kim, T. Klassen, C.J. Altstetter, R.S. Averback, Materials Science and Engineering Department, University of Illinois, 1304 W. Green St., Urbana, IL 61801

When the grain size is a few tens of nanometers, TiAl is ductile at moderately elevated temperatures. Such grain sizes were produced by ball milling of titanium and aluminum powders, which must be consolidated without excessive grain growth. Green compacts (80% dense) were sinterforged at temperatures near 650°C, achieving relative densities of 99%. The final grain size was roughly 60 nm. The deformation temperature is several hundred degrees below those used for sintering microcrystalline powders. Pore elimination is promoted during sinterforging by the deviatoric stress components due to the absence of die- wall constraints. The resultant shear deformation is undoubtedly enhanced by grain boundary deformation modes in nanocrystalline material. Hardness measurements at moderately elevated temperatures are used to develop the constitutive relation for deformation of TiAl. Densification and deformation have been studied as a function of temperature, load and grain size.

9:20 am

ROLE OF MICROSTRUCTURE ON SUPERPLASTICITY IN -TiAl ALLOYS: R.S. Mishra, A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616

A number of -TiAl alloys exhibit superplasticity. The optimum superplastic temperature is strongly dependent on the grain size. For materials with grain sizes >10 µm, the superplastic temperatures are above 2 transition temperature. On the other hand, superplastic temperatures for materials with grain sizes <5 µm is below 1373K. Surprisingly, however, the state of second phase, i.e., ordered 2 or disordered a does not influence the kinetics of superplastic deformation and the parametric dependencies remain similar. These observations suggest that the slip accommodation and dislocation climb in g phase as the rate controlling mechanism. Some new results on nanocrystalline TiAl are presented, which shows the possibility of low temperature superplasticity. The flow stresses in submicrocrystalline and nanocrystalline -TiAl alloys are consistent with a slip accommodation model.

9:40 am

MICROSTRUCTURE AND TENSILE PROPERTIES OF ROLLED GAMMA TITANIUM ALUMINIDE: Gopal Das, Pratt and Whitney, P.O. Box 109600, West Palm Beach, FL 33410-9600; Helmut Clemens, Plansee AG, Technology Center, A-6600 Reutte, Austria

Gamma titanium aluminide (Ti-47Al-2Cr-2Nb-0.2B, Ti-47Al-2Cr-0.2Si at. %) sheets were produced by near-isothermal rolling of forged material. The microstructures resulting from annealing of the rolled sheets below and above the eutectoid temperatures were studied by a combination of optical, X-ray, SEM, and TEM methods. Transition temperatures including the alpha transus temperature were determined by DTA and annealing experiments. Textures were studied on as-rolled and annealed sheets. Tensile properties of annealed specimens were determined at RT-800°C. Deformation microstructure was analyzed by TEM and fractographs were studied by SEM. In the case of Ti-47Al-2Cr-0.2Si at. % the influence of strain rate on the ductile-to-brittle transition temperature was studied for different microstructures. Additionally, the superplastic behavior was investigated for Ti-47Al-2Cr-0.2Si (at. %) sheet in the temperature range of 1000-1100°C. The results will be presented and the underlying deformation mechanisms discussed. The Ti-47Al-2Cr-2Nb-0.2B sheet material was provided by McDonnell Douglas and AFWL/MLLM and the portion of the work based on Ti-47Al-2Cr-2Nb-0.2B sheet material was supported by NASA LeRC, Cleveland, OH, under Contract No. NAS3-26385.

10:00 am BREAK

10:20 am

EFFECT OF EXTRUSION TEMPERATURE ON THE MICROSTRUCTURE OF A POWDER METALLURGY TiAl-BASED ALLOY: L.M. Hsiung, T.G. Nieh, Lawrence Livermore National Laboratory, P. O. Box 808, L-370, Livermore, CA 94551-9900; D.R. Clemens, Advanced Engineering Operations, Pratt & Whitney, West Palm Beach, FL 33410-9600

The microstructure of a P/M Ti-47Al-2Cr-1Nb-1Ta (at.%) alloy powder-extruded at different temperatures has been studied. Three different temperatures: one near to the eutectoid temperature (T1), one within the (+) two phase field (T2), and one at a phase field (T3), were employed for the extrusion. The as-prepared powder consists of mainly phase and small amount of supercooled /B2 grains and a fine-grained (+2) duplex structure. The observation of stacking faults coexisted with thin plates within the 2 grains reveals that stacking faults are intimately related to the formation of the plates within the 2 phase. A nearly fully lamellar (FL) structure composed of alternating and 2 lamellae is developed for the alloy extruded at T3. Both lamellar grain size and lamellar interface spacing are finer than those in conventionally processed FL TiAl alloys.

10:40 am

SUPER TRANSUS PROCESSING OF Ti-48Al-2Nb-2Cr ALLOYS: G.E. Fuchs, Lockheed Martin Company, P.O. Box 1072, Schnectady, NY 12301-1072

Fine grained lamellar microstructures would be expected to exhibit high strength, high creep strength, high fracture toughness and moderate ductility. This presentation discusses the use of high temperature extrusion to produce fine grained lamellar microstructures in both ingot and powder metallurgy Ti-48Al-2Nb-2Cr alloys. The effect of processing parameters, such as extrusion temperature and cooling rate, on the microstructure and properties are examined. In addition, the thermal stability of the fine grained microstructures was examined by subsequent heat treatments. The results of this study indicate that fine grained lamellar microstructures can be generated in both ingot and powder metallurgy materials. However, the selection of the appropriate processing parameters is required to optimize the microstructure and properties.

11:00 am

DENSE IN SITU TIAL IMCS VIA PRESSURE-ASSISTED THERMAL EXPLOSION: I. Gotman, C. Zakine, E.Y. Gutmanas, Department of Materials Engineering, Technion, Haifa 32000, Israel

Dense -TiAl and in-situ TiAl based IMCs were fabricated via pressure-assisted thermal explosion. Fine Ti-Al powder blends with and without the addition of BN or NiB were used as the starting reagents. Rapid heating to the ignition temperature of thermal explosion was realized via resistive heating or by placing the reagent compacts into a preheated pressure die. The presence of an additional low temperature phase in the composition with NiB greatly assisted the process of consolidation. The application of a moderate external pressure (¾ 200 MPa) during processing was shown to be enough to accommodate negative volume changes associated with reactive synthesis and, thereby, to ensure full density of the final product. Microstructure and phase composition of the materials obtained were characterized employing x-ray diffraction and scanning and transmission electron microscopy (SEM and TEM) with energy dispersive analysis (EDS). The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. Rapid cooling due to heat transfer into the pressure die allowed to prevent the coarsening of microstructure in the materials synthesized. The formation of a finely distributed reinforcing phase resulted in high mechanical properties of the in-situ TiAl matrix composites.

11:20 am

PROCESSING AND PROPERTIES OF INVESTMENT CAST, NEAR-BINARY TIAL ENGINE EXHAUST VALVES: Steven G. Dettloff, Wesley R. Thayer, Walter W. Milligan, Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931

Near binary gamma TiAl engine valves were investment cast by the Hitchener process. Effects of HIP temperature, pressure and time on mechanical properties and microstructure were investigated, and the HIP process was optimized for ductility. A HIP + heat treat study was able to separate the beneficial contributions of porosity closure and microstructural development. Effects of iron additions on the optimum aluminum content were noted and will be discussed. We gratefully acknowledge the support of GM Powertrain, Ron Cafferty and Paul Mikkola, who provided the valves and financial support, as well as the National Science Foundation, under grant DMR-92-57465, which is monitored by Dr. Bruce MacDonald.

11:40 AM

AUTOGENOUS GAS TUNGSTEN ARC WELDABILITY OF CAST ALLOY Ti-48Al-2Cr- 2Nb VERSUS EXTRUDED ALLOY Ti-46Al-2Cr-2Nb-0.9Mo: V.L. Acoff, D. Bharani, Department of Metallurgical and Materials Engineering, Box 870202, The University of Alabama, Tuscaloosa, AL 35487-0202

In the majority of the engineering applications for which gamma based titanium aluminide alloys are being considered, fusion welding is the primary form of joining. Sound (crack and void free) welds using the gas tungsten arc welding (GTAW) process with matching, or near matching, filler metal have been accomplished. However, procedures for consistently producing crack-free autogenous (without filler metal) GTA welds have not been achieved. This paper will discuss the autogenous gas tungsten arc weldability of cast alloy Ti-48Al-2Cr- 2Nb (at.%) and of extruded alloy Ti-46Al-2Cr-2Nb-0.9Mo (at.%) and will compare their resultant fusion zone microstructure and properties. Samples were spot welded without any preheat. Microstructural characterization has been performed using optical microscopy and scanning and transmission electron microscopy equipped with an energy dispersive X-ray spectroscopy (EDS) system. Mechanical properties of the fusion zone will be discussed in terms of Vickers microhardness numbers and tensile testing.