Intergranular fracture of gamma titanium aluminides under hot working conditions
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I.
INTRODUCTION
Gamma titanium aluminides are associated with limited (intrinsic) hot workability and relatively narrow processing windows. For example, isothermal forging of cast and homogenized preforms is generally conducted at temperatures between 1050 7C and 1200 7C and nominal strain rates between 1022 and 1023 s21.[1,2] Similarly, successful pack rolling of wrought preforms requires preheating of the packs in the a 1 g phase field at temperatures approximately 40 7C to 150 7C below the alpha transus of the alloy with reductions per pass restricted to 10 to 15 pct.[3,4] These processing windows have been expanded by means of concerted alloy development efforts and through the incorporation of extrinsic, process-related modifications. For example, conventional and hot-die forging of a two-phase gamma alloy Ti-45.5Al-2Nb-2Cr has been successfully demonstrated at strain rates of ;0.1 to 1.0 s21.[1] A major reason for the limited hot workability of titanium aluminides is their tendency to undergo cracking/fracture during hot deformation. Wedge cracking is commonly observed at high strain rates and intermediate temperatures.[5,6] Nucleation of wedge cracks occurs at very low levels of plastic strain and leads to brittle, intergranular fractures. Such fracture behavior is very crucial in the design of canned forging, extrusion, and rolling processes conducted at high strain rates (;1 s21). At higher temperatures and lower strain rates, these alloys may undergo a cavitation type of damage. In regions of the workpiece subjected to secondary tensile stresses (e.g., bulged free surfaces of pancake forgings), the cavities may grow and coalesce to produce macroscopic fracture.[7] Alternatively, nucleation of voids during isothermal forging can be supV. SEETHARAMAN, Senior Scientist, is with the Materials and Processes Division, UES, Inc., Dayton, OH 45432-1894. S.L. SEMIATIN, Senior Scientist, is with Materials Processing/Processing Science, Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/MLLM, Wright Patterson Air Force Base, OH 45433-7817. Manuscript submitted December 16, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
pressed or delayed by the application of superimposed hydrostatic pressures.[8] An understanding of the kinetics of the cavitation phenomenon is essential for the design of slow strain-rate processes such as isothermal forging and superplastic forming. The present work deals with the characterization of the hot workability of a near gamma titanium aluminide alloy using hot tension tests. The specific objectives of this work were two fold: (1) to determine the influence of the gamma grain size on the ductile-to-brittle (DB) transition in the mode of fracture during hot tension testing, and (2) to develop a simple model for the intergranular fracture process occurring during hot working of gamma titanium aluminides in order to predict the critical conditions for the onset of brittle fracture. II.
EXPERIMENTAL PROCEDURE
A gamma titanium aluminide alloy with a nominal comp
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