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 1 h 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.