Thermal stability of Ni–Mn electrodeposits

The effect of Mn additions on the structural stability of electrodeposited Ni is investigated by comparing the microstructure evolution of Ni and Ni–Mn specimens with similar crystallographic initial textures. As deposited, Ni–Mn electrodeposits have a smaller crystallite size and substantially higher yield strength than Ni deposits, in agreement with the Hall–Petch relationship. Moreover, dilute Ni–Mn electrodeposits exhibit a thermal stability that significantly exceeds that of pure Ni. Indeed, Ni–Mn retains its texture, fine-grain microstructure, and strength above 500°C (for 1h anneal), and does not recrystallize up to 800°C. In contrast, pure Ni with larger average grain size and similar preferred orientation shows abnormal grain growth at 300°C and recrystallization at 600°C. This study suggests two distinct temperature regimes. Below 600°C, grain boundary segregation appears as a plausible mechanism for the thermal stability of Ni–Mn electrodeposits, whereas grain boundary pinning by precipitation contributes to the improved microstructural stability of Ni–Mn above 600°C.