From Strain Softening to Strain Hardening in Submicron Scale Ti2448 Associated with Slip Mode Transition Mediated by Nanoscale Martensitic Transformation

Effects of the stress-induced nanoscale α″ martensitic transformation (SINMT) on the deformation behavior of Ti-24Nb-4Zr-8Sn (Ti2448) alloy at sub-micrometer dimension were investigated by in situ tension under transmission electron microscope (TEM) and ex situ compression. A unique transition in deformation behavior from strain softening to strain hardening was observed by decreasing the external sample size of Ti2448 alloy, which is independent of loading type. It is found that the avalanche-like dislocation motion dominates the deformation of samples with large nominal diameter (D > 600 nm), leading to concentrated deformation and strain softening. By decreasing the sample size (D) to around a critical value (~530 nm), the yield strength of samples increased to 1.3 GPa. At this stress, nanoscale martensitic transformation was observed to be induced in both compression and tension deformation. In the presence of such nanoscale martensite during deformation, dislocation avalanche is efficiently hindered and the corresponding deformation behavior transforms from localized planar slip with strain softening in samples larger than 530 nm to uniform deformation with apparent strain hardening in samples smaller than 530 nm.