Effects of grain size and temperature on mechanical properties of nano-polycrystalline Nickel-cobalt alloy

Influences of grain size and temperature on the mechanical properties of nano polycrystalline Ni-Co alloy were investigated with molecular dynamics simulations. It is found that the critical grain size of Hall-Petch relationship is 4.3 nm, at which the maximum flow stress of 4.83 GPa is obtained. In samples with the d 4.3 nm, the average flow stress increases with the decrease of d, according with the Hall-Petch relationship, caused by the dislocation glide and growth of deformation twin with the breaking of individual grain boundaries. For samples with the d < 4.3 nm, the evolution of flow stress and d is consistent with the inverse Hall-Petch relationship, attributed to the softening caused by the rotation of grain and migration of grain boundary, in which small grain merging can be observed, promoting grain boundary movement, and the extrusion of atoms leads to thinning of grain boundary. In addition, under high temperature loading, the fraction of grain boundary atoms increases, while the fraction of grain interior atoms decreases gradually. Also, the grain boundary will melt with the increase of the atomic disorder degree and the density of partial dislocation decreases. Simultaneously, the resistance to grain growth of samples at high temperature is lower than that of samples at low temperature. These conclusions have positive significance for the design of this fascinating nano-polycrystalline alloy. (C) 2020 The Author(s). Published by Elsevier B.V.