Grain boundary status dependent mechanical property of annealed nanocrystalline Ni(Fe) alloy

The strength and hardness of nanocrystalline (NC) metals and alloys have been often correlated with their grain size. We have synthesized NC Ni(Fe) alloys by electrodeposition and studied the evolution of microstructure and microhardness with annealing. By minimizing the interval of annealing temperature from previously utilized 50–150 °C to 10 °C, we have found unique step-type changes, instead of previously observed continuous changes, in the microhardness of annealed NC Ni alloys. The microhardness decreases slightly within two wide temperature ranges and largely drop within two narrow temperature ranges (20–40 °C). Our experimental results and analysis suggest that the microhardness of annealed NC Ni(Fe) alloy is dependent not only on the grain size but also largely on the grain boundary status, i.e., the segregation and desegregation of solute and impurity atoms at grain boundaries (GBs), the grain boundary (GB) relaxation, and the GB nanoprecipitation. Our studies also suggest that the change of mechanical property with annealing in NC metals and alloys should be examined within a narrow range of annealing temperature, e.g., 10 °C, to observe and study their unique, or even abnormal, structure–property relations, which may have been neglected in previous studies.