Martensitic transformation, shape memory effect and mechanical properties of dual-phase Ni50−xTbxMn30Ga20 (x=0–1) alloys

Abstract

The addition of rare earth elements (RE) is an effective way to improve the mechanical properties of Ni–Mn–Ga alloys. Unlike previous investigations which were completely focused on the N–Mn–(Ga,RE) alloys, in this work Ni50−xTbxMn30Ga20 (x=0–1) alloys in the form of (Ni,RE)–Mn–Ga were systematically studied with the microstructure, martensitic transformation, shape memory effect and mechanical properties. Dual-phase microstructure containing body-centered tetragonal martensite and Tb-rich hexagonal precipitates were formed. With the increasing Tb content, the volume fraction of the precipitates was also increased. Yet stable martensitic transformation occurring at about 80°C was observed over the whole composition range. Both the transformation temperatures and hysteresis (∼11°C) were quite insensitive to the composition variation. Sizable shape memory effect is obtained in all the polycrytalline alloys. Shape memory effect of 2.68% was detected for the alloy x=1 with large amount of precipitate. With the increasing volume fraction of precipitates both the compressive strength and strain were well increased for x≤0.2 and gradually decreased for 0.2≤x≤1. The transition of fracture mode from intergranular fracture to transgranular fracture and then to interphase fracture was revealed to contribute to the evolution of the mechanical properties.