Microstructural evolution and phase stability in semi-Heusler NiMnSb and vanadium added equi-atomic NiMnSbV alloys

Abstract

Phase and microstructure evolution in semi-Heusler NiMnSb and vanadium added quaternary equi-atomic NiMnSbV alloys has been studied in the as solidified state. In the as solidified NiMnSb alloy, cubic semi-Heusler phase and in the as solidified NiMnSbV alloy, Frank-Kasper type cubic SbV3 phase along with the cubic semi-Heusler NiMnSb phase are observed. Nucleation and growth of SbV3 phase takes place in the alloy through solute rejection mechanism, and at the interface of SbV3 and NiMnSb semi-Heusler phase, lamellar structure is observed. While SbV3 phase is stable upto 700 °C, the semi-Heusler NiMnSb phase is not stable beyond 400 °C and it transforms to hexagonal (Ni/Mn)Sb phase. Line defects are observed in the NiMnSb semi-Heusler phase. However, extensive twin like defects are observed in the cubic SbV3 phase. In the SbV3 phase edge sharing 12-fold coordinated icosahedral clusters and 14-fold coordinated clusters are present. Atomic size difference of Sb and V indicates that the clusters are distorted, which might be the viable cause behind the formation of such kind of compound deformation twins. The phase formation and stability in these two alloys has been studied through Miedema’s model. It is observed that apart from atomic size difference, enthalpy of mixing; the ternary and higher order self-interactions, electronic contribution to enthalpy also play an important role. Systematic substitution in the lattice of semi-Heusler NiMnSb phase based on enthalpy of mixing and atomic radius may not be the design strategy for development of semi-Heusler NiMnSb based single phase multicomponent/high entropy alloys.