Anomalous weakened annealing-induced hardening in extremely fine nanograined NiFe alloys

Annealing-induced hardening effect was studied in electro-deposited nanocrystalline Ni and NiFe thin films. Nanoindentation testing showed obvious hardening effect for all grain sizes in experiment, which were precisely controlled by adding Fe solutes. Contrast to the general trend in which the hardening effect monotonically increases with decreasing grain size, it enhanced firstly, peaking at a critical grain size (~ 16 nm) and then lowered. Based on the apparent inverse H-P trend and unusual enhancement of strain rate sensitivity after annealing, we attributed the weakened annealing-induced hardening phenomena, despite of the strongest grain boundary pinning effect resulted from the highest Fe solutes (up to 50 at%), mainly to the more pronounced grain-boundary mediated plasticity in extremely fine nanograins. This work contribute directly to understanding the distinct effect of alloying on mechanical properties of nanocrystalline metals.