Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy
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INTRODUCTION
DEFORMATION processing of g titanium aluminides constitutes a crucial step in the successful development and application of these materials.[1] High strength and resistance to plastic deformation exhibited by gamma titanium aluminides at high homologous temperatures make them very attractive for high temperature service, but drastically reduce their hot workability. In general, the hot working regime in which the material can be processed with adequate control over both microstructural evolution and nucleation and propagation of cracks is quite narrow. This is especially true for cast materials with relatively coarse grain sizes, which are prone to cracking and fracture under the influence of tensile stresses. Even though primary hot working processes such as forging or extrusion used for the breakdown of the ingot structure involve nominally compressive states of loading, secondary tensile stresses can be generated due to geometrical, frictional, or thermal effects. Examples of failures induced by tensile stresses include free surface bulging and fracture in open die forging, edge cracking during rolling, and nose fracture during canned extrusion.[1,2,3] Hot workability is generally evaluated through the use of compression, tension, or torsion tests. Of these, the hot compression test is useful for determining the resistance of a given material to plastic deformation at different temperatures and strain rates without the complication of concurrent fracture related events. By contrast, torsion and tension tests provide valuable information on the hot ductility of V. SEETHARAMAN, Senior Scientist, is with the Materials and Processes Division, UES, Inc., Dayton, OH 45432-1894. S.L. SEMIATIN, Senior Scientist, is with the Metals and Ceramics Division, Wright Laboratory, Materials Directorate, WL/MLLM, Wright-Patterson Air Force Base, OH 45433-7817. Manuscript submitted October 11, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
the alloy. Hot ductility is a reliable and accurate measure of the intrinsic hot workability and is affected by dynamic structural changes and by the occurrence of cavitation and wedge cracking phenomena. Tensile deformation and fracture of near gamma titanium aluminide alloys at elevated temperatures have been investigated extensively.[1,2,4–18] These investigations can be broadly classified into two groups: (a) studies devoted to the evaluation of tensile properties of different alloys containing a variety of microstructures under potential service conditions, i.e., at temperatures below 1000 7C and at a fixed strain rate of ;1024 s21;[4–11] and (b) studies dealing with superplasticity and related phenomena in wrought alloys containing fine, equiaxed aggregates of g, a2, and b2 phases.[12–18] Accordingly, the latter group of studies includes tension tests conducted at temperatures ranging from 900 7C to 1250 7C and strain rates varying from 1025 to 1022 s21. In contrast, only a few investigations[2,15,19,20] have focused on the tensile flow and fracture of gamma titanium alu
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