Electron microscopy of crystalline and amorphous Ni-P electrodeposited films: In-situ crystallization of an amorphous solid

Abstract

Electron microscopy has been used to study the structure of Ni-P electrodeposited thin films with 7, 12, 20 and 22 at. pct P. For the crystalline as-deposited films with 7 and 12 at. pct P (low P films), the crystal structure is fcc and the grains are a supersaturated solid solution of P in Ni. Grain size decreases with increasing P content; the present findings agree with previous ones. For “amorphous” as-deposited films with 20 and 22 at. pet P (high P content films) the amorphous phase is not completely homogeneous and there are regions in which small microcrystals exist. Electron beam heating a low P con-tent film causes the crystalline array of supersaturated Ni grains to decompose to an equilibrium mixture of Ni and Ni₃P; both types of grains are randomly oriented. Electron beam heating a high P content film causes the amorphous regions to undergo several complex transformations. The first reaction to occur is: Amorphous (Ni-P) -Ni_xP_y + Ni (random) where Ni_xP_y is a newly discovered phase with a variable composition. Further beam heating causes a second transformation to equilibrium phases: Ni_xP_y + Ni → Ni₃P + Ni (random). The microstructure resulting from the above transformations depends on variations in composition of the as-deposited specimens, rates of heating and temperature gradients. The mode of phase transformation in microcrystalline regions and amorphous regions is distinctly different. Crystallization in amorphous regions occurs by a nucleation and growth of Ni_xP_y, and Ni; a crystallization front is seen to advance into the amorphous re-gion. Crystallization in microcrystalline regions occurs by nucleation and growth of the Ni₃P phase and grain coarsening of the Ni phase. No distinct crystallization front is ob-served.