Tailoring the microstructure, phase transition characteristics and one-way shape memory effect of Ni-Mn-Ga alloys by dual treatment of annealing and directional solidification

Improving both shape memory effect (SME) and mechanical properties is a major challenge of Ni-Mn-Ga high temperature shape memory alloy. In this work, the Ni₅₇Mn₂₅Ga₁₈ alloy system with the coexistence of martensite phase and γ phase at room temperature is selected. A regulation strategy involving annealing and directional solidification (DS) is adopted to control the phase transition characteristics and crystallographic texture of the alloy. Annealing treatment can reduce the elastic strain energy in the alloy and reduce the macroscopic energy barrier between the martensite phase and the austenite phase. As a result, the martensitic phase transition temperatures of the alloy shift towards the high-temperature zone, and the transition width decreases. DS can eliminate transverse grain boundaries and reduce orientation difference. By this composite method, the one-way shape memory effect (OWSME) and mechanical properties are improved, the strain generated by OWSME and the fracture stress of the annealed DS alloy reaches 4.62 % and 1922 MPa, respectively. In annealed DS samples, stacking faults are accumulated in martensite phases and the atomic arrangement of the γ phases becomes ordered, which can provide many slip systems, they are account for the enhanced mechanical properties. The columnar grains obtained by DS have the consistent deformation direction same and small internal stress, thus their deformation is easy to recover. The martensitic texture, particularly with the (001)_NM orientation, is parallel to the DS direction and conducive to the OWSME.