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. • Unexpected weakened annealing-hardening effect of nanocrystalline NiFe alloys occurred below a critical grain size. • The apparent inverse Hall-Petch trend and the elevated m value after for annealed NiFe alloys evidenced the more pronounced GB-mediated mechanisms. • The relatively weak grain boundary pinning effect of Fe solutes, accompanied with the more pronounced grain boundary mediated activities after annealing, should be responsible for the weakened annealing-hardening effect. [ABSTRACT FROM AUTHOR]