Ab Initio Study of Ternary W₅Si₃ Type TM₅Sn₂X Compounds (TM = Nb, Ti and X = Al, Si)

The adhesion of the scale formed on Nb-silicide based alloys at 1473 K improves when Al and Sn are in synergy with Si and Ti. This improvement is observed when there is segregation of Sn in the microstructure below the alloy/scale interface and a layer rich in intermetallics that include TM₅Sn₂X compounds is formed at the interface. Data for the ternary compounds is scarce. In this paper elastic and thermodynamic properties of the Nb₅Sn₂Al, Ti₅Sn₂Si, Ti₅Sn₂Al and Nb₅Sn₂Si compounds were studied using the first-principles, pseudopotential plane-wave method based on density functional theory. The enthalpy of formation of the ternary intermetallics was calculated using the quasi-harmonic approximation. The calculations suggest that the Nb₅Sn₂Si is the stiffest; that the Nb₅Sn₂Al and Ti₅Sn₂Si are the most and less ductile phases respectively; and that Nb significantly increases the bulk, shear and elastic moduli of the ternary compound compared with Ti.