The defining role of phosphorous on microstructure, nanohardness and thermal stability of pulsed electrodeposited nanocrystalline nickel-phosphorous alloys

Abstract

Free-standing nanocrystalline (NC) nickel-phosphorous (Ni–P) alloy foils with P content in the range of 0.2–5.0 wt percent are synthesized by pulsed electrodeposition technique. The aim of the present study is to evaluate the effect of P content on the hardness and microstructural evolution in Ni–P alloys during both electrodeposition and thermal treatment. The microstructure, hardness and thermal analysis of the alloys are investigated using electron microscopy, nanoindentation and differential scanning calorimetry (DSC), respectively. This study demonstrates that there is localized structural heterogeneity in conjunction with significant grain refinement in the alloys containing excessive P content beyond its solid solubility. Nanohardness measurements indicate that both the grain refinement and internal strain induced by codeposition of P solute atoms affect the strengthening of Ni matrix. The exothermic peaks observed in the DSC thermograms are found to be strongly dependent on P content in the alloy and these are associated with structural relaxation, formation of metastable and stable nickel phosphide phases along with concurrent grain growth and particle coarsening. The grain growth kinetics study showed that in Ni–P alloys Smith-Zener drag mechanism involving control of grain boundary mobility by Ni3P nanoparticles overtakes the solute drag effect of P with increasing temperature.