Shape memory effect of dual-phase NiMnGaTb ferromagnetic shape memory alloys

The evolution of microstructure, reverse martensitic transformation and the correlated influence on shape memory effect was investigated in as-cast and directionally solidified dual-phase NiMnGaTb alloys. The directionally solidified alloys exhibit single-crystal microstructure, preferred dendrite microstructure, and mussy dendrite microstructure in the specimens grown at a withdrawal rate (v) of 10, 50 and 200, and 1000 μm/s, respectively. The precipitates dispersively distribute in the martensite matrix for the directionally solidified alloys. With the refined grains and particle precipitates, the reverse martensitic transformation gradually shifts to lower temperatures and the temperature span is significantly broadened. The directional solidification technology can effectively enhance the strains recovered due to shape memory effect (εsme) and decrease the compressive stress required to trigger the reorientation of twins (σ) via the realization of preferred orientation, while the maximal εsme and minimum σ can reach 4.96% and 14 MPa in v = 10 μm/s specimens, respectively. The formation of dendrite morphology degrades the shape memory strain, and εsme decreases with the growth of secondary dendritic arms.