Ab Initio Study of Binary and Ternary Nb₃(X,Y) A15 Intermetallic Phases (X,Y = Al, Ge, Si, Sn)

Elastic and thermodynamic properties of binary and ternary A15 phases containing Al, Ge, Si, and Sn were studied using the first-principles pseudopotential plane-wave method based on density functional theory. The temperature dependence of the enthalpy of formation for the A15 intermetallics is reported using the quasiharmonic approximation. Elastic properties of the studied compounds were calculated at T = 0 K and were in agreement with the measured values reported in the literature. The elastic properties and thermodynamic data for the metastable A15-Nb₃Si are reported for the first time. The Nb₃Si has the highest bulk, shear, and Young’s modulus values and is predicted to be less ductile than the other three binary A15 intermetallics. The calculations suggest (i) that Al and Sn have a positive effect on the ductility of the A15 compounds of this study, (ii) that Ge as a ternary addition has a ductilizing effect only in the A15-Nb₃Si, and (iii) that Si as a ternary addition has a negative effect on the ductility of all the A15 compounds of the present study. The linear thermal expansion coefficients of the Nb, Al, the A15 Nb₃Al, Nb₃Ge, Nb₃Sn, and Nb₃Si (A15) phases are reported. The Sn and Al additions in the Nb₃Si stabilize the A15 structure, while the Ge addition has the opposite effect, stabilizing the tP32 Nb₃Si.