Influence of Fe Addition on Phase Transformation, Microstructure and Mechanical Property of Equiatomic NiTi Shape Memory Alloy

Abstract

Based on equiatomic nickel and titanium, three kinds of NiTiFe alloys with a nominal chemical composition of Ni49Ti49Fe2, Ni48Ti48Fe4 and Ni47Ti47Fe6 (at.%), respectively, have been designed to investigate the influence of the addition of Fe element on phase transformation, microstructure and mechanical property of equiatomic NiTi shape memory alloy. The microstructures of three kinds of NiTiFe alloys are characterized by the equiaxed grains instead of the dendrites. Consequently, some Ti2Ni precipitates are found to distribute in the grains interior and at the grain boundaries. The content of Fe element has an important influence on mechanical property of NiTiFe alloy. With increasing content of Fe element, the strength of NiTiFe alloy increases substantially, but the plasticity decreases sharply. It can be concluded that precipitation strengthening and solution strengthening play a significant role in enhancing the strength of NiTiFe alloy. In the case of three NiTiFe alloys, neither martensitic transformation nor reverse transformation can be observed in the range from −150 to 150 °C. On the one hand, the phase transformation temperature is probably out of the scope of the present experimental temperature. On the other hand, the addition of Fe element probably suppresses first-order martensitic transformation or reverse transformation, and consequently the second-order-like phase transformation from an incommensurate stage to a commensurate stage can probably take place.