Insights into the abnormal flow softening of lamellar γ-TiAl alloys during hot-working: Experimental analysis and numerical simulation

In comparison with ordinary alloys, it is well-known that the high temperature flow curves of lamellar TiAl alloys generally exhibit pronounced flow softening and sharp stress peak. In order to clarify the origin of the abnormal flow behavior, in this study a Ti-43.5Al–8Nb-0.2W-0.2B alloy with refined fully lamellar microstructure was isothermally compressed at (α+γ) phase field. The deformation pattern as well as the mechanical response was studied by means of experimental analysis and finite-element modelling. Microstructural observations revealed that the lamellar colony exhibited significantly plastic anisotropy due to the preferential activity of the longitudinal slip systems with the slip plane/direction parallel to the lamellar interface. Consequently, lamellar kinking/bending predominated the deformation, and the colonies showed rather retarded recrystallization kinetics as well as significant crystalline rotation toward the hard orientation. The numerical simulation demonstrated that the mechanical response of individual lamellar colonies was strongly dependent on the prior orientation. When the interface aligned in about 30–60° with respect to the compression axis, the colony exhibited continuous flow hardening. Otherwise evident flow softening occurred at the beginning of straining. Analysis of the deformation pattern revealed that the mechanical response of the colony was attributed to the peculiar crystalline rotation pathway of the lamellae, which was fundamentally determined by the preferential activity of the longitudinal slip. The proposed numerical model reproduced the experimental phenomena quite well, and provided insights into the flow softening mechanism of lamellar TiAl alloys.