An evidence of stress-induced α2 → γ transformation in a γ-TiAl-based alloy

Two phase {gamma}-TiAl-based alloys composed of ({gamma} + {alpha}{sub 2}-Ti{sub 3}Al) are currently under extensive investigation because their mechanical properties are superior to either single phase {alpha}{sub 2} or {gamma} alloys. The alloys usually have a duplex microstructure consisting of equiaxed {gamma}-grains with various volume fractions of lamellar colonies in them. The lamellar colony, which consists of alternating layers of ordered {gamma} and {alpha}{sub 2} plates with the orientation relationship {l_brace}111{r_brace}{sub {gamma}}//{l_brace}0001{r_brace}{sub {alpha}{sub 2}}, {gamma}// {sub {alpha}{sub 2}}, is formed either by a phase transformation {alpha} {r_arrow} {alpha}{sub 2} and then {gamma} precipitates from the {alpha}{sub 2} matrix, or by {alpha} {r_arrow} {alpha}/{gamma} {r_arrow} {alpha}{sub 2}/{gamma} upon cooling. The microstructure of the colony has been studied in detail by many investigators. It has been suggested that the precipitation of the {gamma} phase essentially involves a HCP {r_arrow} FCC structure change together with long-range transport of atoms to obtain the composition of the two phases. The structure change can be brought about if a/3 Shockley partials travel on alternate basal planes of the HCP phase. Thus, the {gamma} nuclei are essentially basal stacking faults in the {alpha}{sub 2} phase. This means that gliding of a/3 partial appears tomore » play an important role in the transformation. If it is so, it would be expected that an external stress would probably enhance the process. More recently, Gao et al. have claimed that stress-induced phase transformation may occur in two-phase TiAl-based alloys during deformation at room temperature, which is related to a stage on stress-strain curves. In this note, the authors are briefly reporting on evidence of the stress-induced {alpha}{sub 2} {r_arrow} {gamma} transformation in a {gamma}-TiAl-based alloy.« less