Microstructural stability and superplasticity in an electrodeposited nanocrystalline Ni–P alloy

Stabilization of nanocrystalline grain sizes by second phase particles can facilitate superplasticity at high strain rates and/or low temperatures. A metastable single phase nano-Ni–P alloy prepared by electrodeposition, with a grain size of ∼6 nm, transforms to a nanoduplex structure at T > 673 K, with ∼4 vol.% Ni3P particles at triple junctions and within Ni grains. The nanoduplex microstructure is reasonably stable up to 777 K, and the growth of Ni grains occurs in a coupled manner with the growth of Ni3P particles such that the ratio of the two mean sizes (Z) is essentially constant. High temperature tests for a grain size of 290 nm reveal superplastic behavior with an optimum elongation to failure of 810% at a strain rate of 7 × 10−4 s−1 and a relatively low temperature of 777 K. Superplastic deformation enhances both grain growth and the ratio Z, implying that grain boundary sliding (GBS) significantly influences the microstructural dynamics. Analysis of the deformation processes suggests that superplasticity is associated with GBS controlled by the overcoming of intragranular particles by dislocations, so that deformation is independent of the grain size. The nano-Ni–P alloy exhibits lower ductility than nano-Ni due to concurrent cavitation caused by higher stresses.