Construction of free energy diagram and phase formation by ion mixing in the NiNb system

Abstract

In the NiNb system, eight amorphous alloys were formed by 200 keV xenon ion mixing at room temperature of multilayered films with various compositions covering a range of 20–85 at.% of Ni. In addition, a new metastable crystalline (MX) phase was observed and identified to be of hexagonal structure with a stoichiometry around 75 at.% of Ni. The MX phase transformed into an amorphous state upon thermal annealing at 400°C for two hours. The free energy diagram of the system was constructed by calculating the free energy curves of the solid solution, the amorphous phase, the equilibrium compound and the MX phase. In calculation of the MX phase the identified structural characteristics and the ferromagnetic effect from Ni atoms were considered. The calculated diagram can explain the amorphization range upon ion mixing as well as the unexpected phase transition from the MX to the amorphous phase upon heating.