Interface assisted formation of a metastable hcp phase by ion mixing inan immiscible Ag-Ni system

Abstract

A metastable Ag-Ni phase was formed by 200 keV xenon ion mixing at 77 K in theAg80Ni20 multilayered films, in which the excess interfacial freeenergy provided partial driving force for alloying in an equilibrium immiscibleAg-Ni system characterized by a large positive heat of formation. Themetastable phase was identified to be of D019 hcp structure with astoichiometry of Ag3Ni and can therefore be considered as a Hume-Rothery 7/4electron compound. A thermodynamic calculation showed that when the multilayersconsisted of 8 or more bilayers, the interfacial free energy could elevate theirinitial energy levels up to a state higher than that of the hcp phase, and thatwhile the multilayers composed of 6 or less bilayers, it was thermodynamicallynot favoured to form the hcp phase. Kinetically, the metastable hcp phase wasgrown from the fcc Ag lattice through a fast sliding mechanism. Furthermore, anab initio calculation showed that a minimum total energy of the Ag3Niphase did correspond to the above observed metastable state, indicating that thestability of the metastable Ag-Ni hcp phase was correlated with its electronicstructure.