C. Savall

Impact of coherent and incoherent twin boundaries on the microhardness of annealed nanocrystalline Ni–W alloys

Abstract The microstructural evolution of nanocrystalline Ni–W alloys with annealing temperature and more specifically grain boundary (GB) character is investigated through several techniques and correlated with the hardening behaviour. It is shown that two distinct regions can be identified in relation to the annealing temperature and the microstructural evolution. At temperatures below 550 °C (Regime I), a small increase in grain size is observed and is accompanied by a significant hardening and an increase in the fraction of Σ3 incoherent twin boundaries. At temperatures above 550 °C (Regime II), the thermal stability is overcome and important grain growth occurs with a decrease in both the volumic fraction of GBs and the microhardness. It is suggested that the microhardness evolution during heat treatment is influenced by two opposing processes: an increase in the fraction of incoherent twin boundaries (hardening effect) and grain growth (softening effect). Both aspects are directly associated with the mean free path of mobile dislocations.

Dislocations effect on kinetic of passivation of polycrystalline nickel in H2SO4 medium

Abstract Metallurgical state is an important parameter which affects the passivation process. In this study, the first experimental results on the effect of dislocation density and their distribution on the surface on the process of oxide growth are reported for polycrystalline nickel in H 2 SO 4 media at 291 K. It is demonstrated that this effect can not be investigated without tacking into account the grain size and composition of medium. Dislocation density promotes the adsorption of hydroxide and modifies the properties of the oxide layer.