Impact of B alloying on ductility and phase transition in the Ni–Mn-based magnetic shape memory alloys: Insights from first-principles calculation

Abstract

Brittleness is a bottleneck hindering the applications of fruitful functional properties of Ni–Mn-based multiferroic alloys. Recently, experimental studies on B alloying shed new light on this issue. However, the knowledge related to B alloying is limited until now. More importantly, the mechanism of the improved ductility, which is intrinsically related to the chemical bond that is difficult to reveal by routine experiments, is still unclear. In this context, by first-principles calculations, the impact and the correlated mechanism of B alloying were systemically studied by investigating four alloying systems, i.e., (Ni2-xBx)MnGa, Ni2(Mn1-xBx)Ga, Ni2Mn(Ga1-xBx) and (Ni2MnGa)1-xBx. Results show that B prefers the direct occupation manner when it replaces Ni, Mn and Ga. For interstitial doping, B tends to locate at octahedral rather than tetrahedral interstice. Calculations show that the replacement of B for Ga can effectively improve (reduce) the inherent ductility (inherent strength) due to the weaker covalent strength of Ni(Mn)–B compared with Ni(Mn)–Ga. In contrast, B staying at octahedral interstice will lead to the formation of new chemical bonds between Ni(Mn) and B, bringing about a significantly improved strength and a greatly reduced ductility. Upon the substitutions for Ni and Mn, they affect both the inherent ductility and strength insignificantly. For phase transition, the replacement of B for Ga tends to destabilize the austenite, which can be understood in the picture of the band Jahn–Teller effect. Besides, the substitution for Ga would not lead to an obvious reduction of magnetization.