Grain growth and mechanical properties of electrodeposited nanocrystalline nickel–4.4mass% phosphorus alloy

The relationship between microstructural evolution and mechanical properties in Ni–4.4mass% P alloy was systematically examined. As-electrodeposited Ni–4.4mass% P alloy consists of grains having an average grain size of 37 nm and texture strongly oriented to (1 1 1). Phosphorus is entirely in solid solution. Heat treatment induces precipitation of Ni3P phase. Rapid grain growth occurs only at the initial stage of heat treatment, and saturated at all the annealing temperatures, even at temperatures higher than 873 K (0.51Tm). Grain growth exponents n in Ni and Ni3P phases were found to be 4.5–5.9 and 4.9–5.4, respectively. These values are close to those for grain boundary diffusion-controlled growth. Activation energies for grain growth in Ni and Ni3P phases were estimated as 210 and 180 kJ mol−1, respectively. The nanocrystalline and ultrafine-grained specimens (about 40–400 nm) fractured prior to yielding, while the larger grained specimens (>400 nm) showed plastic deformation. Fracture strength and fracture strain depend not only on grain size but also on connectivity of Ni3P grains, where cracks were preferentially nucleated.