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

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.