The deformation and fracture of Ti 3 Al at elevated temperatures
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THE titanium aluminides, Ti3A1 and TiA1, have properties which make them desirable candidates for applications in aircraft turbine engines. 1-3 These ordered intermetallic compounds are lighter and stiffer than conventional titanium alloys. Their room temperature strength is not particularly high; but since they are ordered, their static strength and stiffness do not degrade very rapidly with increasing temperature. Also, they have improved oxidation resistance compared to conventional titanium alloys. The unalloyed aluminides have only very limited ductility; however, since their other properties are sufficiently attractive, it is valuable to study the behavior of the stoichiometric compounds to determine the possibility of changing their ductilitytemperature relationships. The objective of this paper is to report the tensile and fracture properties of unalloyed, single phase, Ti3A1, over a broad temperature range; to characterize the deformation modes; and to relate the fracture modes, dislocation structures, and tensile properties at each temperature. The compound Ti3A1 is known to order as the DO19 superlattice. Marcinkowski4 first speculated that superdislocations of the type a 'o (1120) would form, each consisting of four partial dislocations of the type 1/2 a 'o (1010) (Fig. 1). His analysis of the motion of the same type dislocation (1/2 a 'o (1120)) on prism planes showed that no net disorder should occur with respect to first nearer neighbor atoms. On this basis alone he predicted that neither superlattice dislocations nor antiphase boundaries should occur for slip on prism planes. Williams and Blackburn 5 examined various Ti-A1 compositions and analyzed the dislocation character in the a 2 (Ti3A1) phase. They found 1/3 a 'o (1120) dislocations HARRY A. LIPSITT is a Senior Scientist in the Processing and High Temperature Materials Branch, Metals and Ceramics Division, Materials Laboratory, Air Force Wright Aeronautical Laboratories, Wright-Patterson AFB, OH 45433. DAN SHECHTMAN, formerly with the Materials Laboratory, is now a Lecturer in the Department of Materials Engineering, Technion, Haifa, Israel. ROBERT E. SCHAFRIK, Major, USAF, formerly in the Processing and High Temperature Materials Branch, is now in the Computer Integrated Manufacturing Branch, Manufacturing Technology Division, Materials Laboratory, Air Force Wright Aeronautical Laboratories, Wright-Patterson AFB, OH 45433. Manuscript submitted August 29, 1979.
existing as superdislocations on the prism, pyramidal, and basal planes. They also reported seeing very few 1/3 a 'o (1123) dislocations compared to the density of such dislocations typically found in deformed a titanium alloys. EXPERIMENTAL PROCEDURE Two sources of Ti3A1 were used for this investigation. Early in the study, extruded bar was manufactured from chips machined from an ingot. The procedure for homogenizing the ingot; producing, canning, and extruding the chips; and machining and grinding the resultant bar stock has been described. 2 Later in the program, bar was extruded f
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