The effect of temperature on strain rate sensitivity in a nanocrystalline Ni–Fe alloy

Abstract

The effect of strain rate on mechanical properties at room temperature (RT) and a cryogenic temperature (77K) was studied in a nanocrystalline Ni–20%Fe alloy (grain size: 22nm). The activation volume, strain rate sensitivity, strain hardening rate, and microstructural changes were also characterized. The results show that this nanocrystalline alloy has a combination of high strength and large plastic strain at both temperatures. At 77K, the strength increased dramatically without a loss in the plasticity. Calculations demonstrate that the effect of temperature on yield strength, activation volume, and strain rate sensitivity is strongly dependent on the grain size. The high strain rate sensitivity and strong yield strength dependence on temperature are due to small activation volumes at fine grain sizes. In addition, at 77K, both the activation volume and strain rate sensitivity exponent decreased significantly. The effect of temperature on the strain hardening capability in this nanograined alloy is opposite to what was observed for the large-grained metals. Comprehensive analysis suggests that the deformation at 20nm is still dominated by dislocations. However, the dislocation generation models may be different at RT and 77K.