Relationship between grain boundary relaxation strengthening and orientation in electrodeposited bulk nanocrystalline Ni alloys

Many studies have been performed to better understand the Hall-Petch effect at the nanometer scale. Hardening can be caused not only by a reduction of the grain size, but also through the relaxation of the nonequilibrium grain boundary structure. Despite considerable effort, there is still a large discrepancy among the available data for the strength values of nanocrystalline metals because of the difficulty in quantitatively evaluating the state of grain boundary relaxation. In this study, we used electrodeposited bulk nanocrystalline Ni-Fe and Ni-W alloys to develop a better predictive method of the grain boundary relaxation behavior. Relatively low-temperature thermal treatment resulted in grain boundary relaxation and thus increased the hardness by 0.07-0.74 GPa. We found that the increase in hardness decreased with increasing orientation index for the (2 0 0) plane. Specifically, we concluded that electrodeposited Ni alloys with an orientation index for the (2 0 0) plane greater than 3.0 do not exhibit grain boundary relaxation strengthening, because these alloys do not have a nonequilibrium grain boundary structure even in the as-deposited state. The relationship also enables the prediction of the grain boundary relaxation state of electrodeposited bulk nanocrystalline Ni alloys.