Formation of an Face-Centered Cubic Ta-Rich Solid Solution by Ion Beam Mixing in the Immiscible Ag-Ta System

Ta-rich and a Ag-rich solid solutions, both of face-centered cubic (fcc) structures, are observed in the Ag(14)Ta(86) multilayered films on 200-keV xenon ion beam mixing, and the formation of two co-existing solid solutions is the result of a process of spinodal decomposition. The abnormal fcc Ta-rich solid solution has a Ta concentration larger than 86 at. pct and a lattice constant determined to be around 4.28 . Besides, first-principles calculations were employed to understand the possibility of a structural phase transition from body-centered cubic to fcc in Ta. Comparing the heat of formation (Delta H (f) ) differences between fcc and bcc structures, Ta has the smallest Delta H (f) difference among the four refractory metals (Ta, Mo, Nb, and W). It is concluded that the interfacial and irradiation energies play important roles in overcoming the energy barrier for the structural phase transition to take place.