Thermal stability of nanogradient microstructure produced by surface mechanical rolling treatment in Zircaloy-4

A nano-/ultrafine grain gradient microstructure, which is composed of high-angle grain boundaries (HAGBs) and low-angle grain boundaries or subgrains of dislocation–twin, was fabricated in Zircaloy-4 using surface mechanical rolling treatment (SMRT). Thermal stability of gradient microstructure has been investigated through characterizing the evolution of microstructure during post-SMRT annealing treatment from 400 to 600 °C using optical microscopy and transmission electron microscopy. Experimental results show that the gradient microstructure exhibits a good thermal stability at 400 °C, since the overall grain size remains similar, except a decrease in dislocation density due to recovery. In comparison, a hierarchical microstructure is formed after annealing at 600 °C. An obvious grain growth was observed at the depth of 50 μm. The activation energy for grain growth of nanograined Zircaloy-4 is estimated to be ~ 161 kJ/mol between 400 and 600 °C. Nano-/ultrafine grains predominantly consisting of HAGBs have the highest thermal stability. Both yield strength and ultimate tensile strength of Zircaloy-4 decrease due to anneal, specifically at 600 °C.