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 degrees C temperature range. In the 10(-3)-10(-5) s(-1) strain rate range, the maximum strain-rate sensitivity exponent in 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 (T-d) and the higher the strain rate ((epsilon) over dot) leading to superplasticity (m=0.5): for d=250 nm, T-d=400 degrees C and ((epsilon) over dot) is an element of [0.4-2.5] x 10(-5) s(-1), while for d=500 nm, T-d=500 degrees C and ((epsilon) over dot) is an element of [0.1-3] x 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. (C) 2014 Elsevier B.V. All rights reserved.