Ambient-temperature nanoindentation creep in ultrafine-grained titanium processed by ECAP

Ultrafine-grained (UFG) Ti was prepared by equal channel angular pressing (ECAP) through 1 and 4 passes at room temperature. The ambient-temperature creep behavior and mechanism of UFG Ti were studied by nanoindentation creep tests at the loading strain rate of 0.005 and 0.1 s−1. The effect of grain size on creep behavior and relevant creep parameters such as steady creep strain rate and creep stress exponent (n) was estimated for coarse-grained (CG) and UFG Ti. The results show that the creep resistance of UFG Ti is enhanced with respect to CG Ti. UFG Ti after 1 pass of ECAP exhibits the highest creep resistance. The creep resistance of UFG Ti decreases with increasing the number of ECAP passes. The creep stress exponents of UFG Ti are much higher than those of CG Ti while steady creep strain rates do not vary much with grain size. The creep stress exponents of CG and UFG Ti are dependent on the loading strain rate and increase with increasing the loading strain rate. The power-law creep with a stress exponent of 18.1-24.6 is consistent with dislocation process, especially for dislocation emission and annihilation at grain boundaries, which is a key mechanism in creep deformation of UFG Ti. Grain boundaries may play an important role in the creep behavior of UFG Ti.