On interface and twin boundaries in a forged alloy

Abstract

This paper discusses XD (exothermic dispersion) processed TiAl/TiB{sub 2} alloys which are a new class of intermetallic composites with improved toughness, creep resistance and oxidation resistance at high temperatures. They generally have a two-phase {alpha}{sub 2} + {gamma} matrix (Ti{sub 3}Al (DO{sub 19}) and TiAl (L1{sub 0} structures, respectively), which contains monocrystalline TiB{sub 2} particles. Although the deformation mechanisms of these alloys are essentially the same as those of conventional TiAl alloys, dislocation substructures, particularly dislocations at {alpha}{sub 2}/{gamma} interfaces, remain to be unambiguously identified. In a recent paper on a Ti-45Al/TiB{sub 2} alloy, these interfacial dislocations were assumed to be misfit dislocations with 1/6 {l angle}112{r angle}{sub {gamma}}. Burgers vectors and their presence was attributed to boron interstitials in the {gamma} matrix, However, the {alpha}{sub 2} phase has been shown to getter interstitial impurities from the {gamma} phase, thus increasing the ductility of the two phase alloy. Moreover, 1/6 {l angle}112{r angle}{sub {gamma}} partial dislocations generally result from the dissociation of glissile {gamma} dislocations during deformation and cannot be resolved by conventional weak beam electron microscopy because of their small separation distances. Their assumed presence is also unusual, in that lattice mismatch is not normally accommodated by partial dislocations.