Characteristics of ion-induced supersaturated Au-Ni alloy films

Abstract

Supersaturated metastable Au-Ni solid solutions have been obtained by ion-beam mixing of thin deposited Au and Ni layers of compositions Au75Ni25,Au50Ni50, and Au25Ni75. The alloys so obtained were studied by MeV 4He+ backscattering, x-ray diffraction, electrical resistivity, and transmission electron microscope measurements. X-ray diffraction analysis showed that the lattice parameters of the ion-induced alloys varied almost linearly with the composition with a slight positive deviation from Vegard’s law for ideal solid solutions. A pronounced increase of grain size in the films was observed as a result of ion-beam mixing along with a change of microstructure from a weak (111) texture to a strong (111) texture. The stability of those ion-induced Au-Ni alloy films has been examined by isothermal annealing at temperatures up to 550 °C. Decomposition of the alloys occurred at temperatures above 300 °C as indicated by resistivity and x-ray diffraction measurements. For dilute alloys (Au75Ni25 and Au25Ni75), decomposition proceeded by a discontinuous reaction where simultaneous presence of the equilibrium Au-rich phase, Ni-rich phase, and the original metastable phase was found. The composition of the metastable phase remained nearly constant during the decomposition. However, the concentrated alloy (Au50Ni50) displayed a continuous precipitation mechanism where a concentration gradient varying from Au50Ni50 to a Au-rich composition was present in the parent phase during decomposition. The precipitation kinetics of Au50Ni50 alloys have been studied by transmission electron microscopy. The possible origins for the occurrence of continuous precipitation phenomenon in the ion-induced Au50Ni50 alloy are discussed.