Formation and stability of nanocrystalline Nb-Cu alloys

This paper reports on nanocrystalline Nb100-xCux (0 ≤ x ≤ 30) alloys of 8-25 nm grain size which are synthesized with the mechanical alloying technique. Different scanning calorimetry reveals two separate stages of grain growth. In the first stage, the grain growth is associated with migration of solute Cu atoms to grain boundaries. Grain growth stops as the grain boundaries are saturated with Cu. The second reaction takes place either when the threshold for the nucleation of an fcc Cu phase in the grain boundary is overcome by thermal activation, or when segregating Cu atoms in the grain boundary are driven by diffusion to the growing Cu grains, and the Cu concentration in the grain boundary drops. This increases the grain boundary energy, and initiates a second stage of rapid grain growth