Effects of annealing on the structure and microhardness of nanocrystalline Ni–Mn electrodeposits

Abstract

Nanocrystalline pure nickel and Ni-Mn alloy with 1.2 at% Mn deposits were obtained via pulse current electrodeposition. To understand the effects of annealing on their structure and microhardness, the surface morphology, crystal structure, grain size, microhardness and microstructure of the obtained Ni-Mn and pure Ni deposits were investigated. The results indicated that Ni-Mn alloy exhibited a similar face-centered cubic structure with a strong (111) preferred orientation to that of Ni deposits, but as-deposited Ni–Mn had a smaller crystallite size and substantially higher microhardness compared to as-deposited Ni deposit. During annealing, the crystal grains of Ni deposits grew sharply, resulting in a great decrease in their microhardness, while Ni-Mn deposits showed good thermal stability and annealing induced hardening at 300 °C and 350 °C. Neither precipitate nor clear segregation of Mn was observed on annealed deposits; however, a number of twin boundaries were found. It is suggested that the thermal stability of Ni-Mn was mainly derived from the formation of twin boundaries and the annealing-induced hardening could be ascribed to grain boundary relaxation and annealing twin strengthening.