Study of grain growth in electrodeposited nanocrystalline nickel-1.2 wt% phosphorus alloy

The paper describes a transmission electron microscope study of the microstructural evolution of electrodeposited nanocrystalline Ni-1.2wt.%P and pure nickel during heating experiments performed in situ. The grain structure of the Ni-1.2wt.%P alloy was found to be stable up to a temperature of 360 degrees C. The grain growth was concurrent with Ni3P precipitation, suggesting the important role of phosphorus in supersaturated solid solution in imparting the thermal stability to the microstructure. Normal grain growth was observed in Ni-1.2wt.%P at annealing temperatures up to 480 degrees C. Subsequent analysis revealed pinning of grain boundaries by Ni3P precipitates. The value of activation energy for grain growth in Ni-P alloy (2.25 eV), obtained from continuous scan rate DSC experiments, is consistent with the above observation. Electrodeposited nanocrystalline nickel, on the other hand, showed abnormal grain growth at temperatures as low as 260 degrees C. At 320 degrees C, the microstructure had largely transformed from the nanocrystalline to the microcrystalline state.