Effect of the grain/subgrain size on the strain-rate sensitivity and deformability of Ti–50 at%Ni alloy

The strain-rate sensitivity of coarse-grained (CG) and ultrafine-grained (UFG) Ti–50 at%Ni alloys was studied in the 20–500 °C temperature range. In the 10−3–10−5 s−1 strain rate range, the maximum strain-rate sensitivity exponent m was measured by the strain-rate-jump test, while in the 10−5–10−6 s−1 strain rate range, m was measured by the stress-jump creep test. It was determined that the smaller the initial grain/subgrain size (d), the lower the deformation temperature (Td) and the higher the strain rate ( ε ̇ ) leading to superplasticity (m=0.5): for d=250 nm, Td=400 °C and ε ̇ ∈[0.4–2.5]×10−5 s−1, while for d=500 nm, Td=500 °C and ε ̇ ∈[0.1–3]×10−5 s−1. The low-temperature deformability of the CG and UFG Ti–Ni alloys have also been compared under conditions of severe cold rolling (accumulated strain of 2) by measuring the length and concentration of the processing-induced microcracks. The results of this study showed an increase in the low-temperature deformability of UFG Ti–Ni alloy compared to CG Ti–Ni alloy.