Microstructure, mechanical and shape memory properties of polycrystalline Ni-Mn-Ga High temperature shape memory alloys

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni_57+xMn₂₅Ga_18-x (x=0, 0.5, 1, 1.5, 2, 3) high-temperature shape memory alloys were investigated. A single martensite phase with tetragonal structure is present for x=0, and dual phases composed of tetragonal martensite and face-centered cubic γ phase are observed for x≥0.5. The martensite variants are all twinned and well self-accommodated. The volume fraction of γ phase increases from 4.2% to 35.1% with increasing Ni content. Martensitic transformation temperature (M_s) increases from 437°C for x=0 to 469°C for x=0.5 and then remains almost unchangeable for x≥1. Ductile γ phase dispersed on the grain boundaries and martensite can apparently raise the strength and plasticity. However, the maximum shape memory strain decreases from 4.7% for x=0 to 1.0% for x=3, which means γ phase reduces the shape memory properties. The hampering effect of the γ phase particles on shape recovery of the alloys where x≥1.5 is more remarkable than on the alloys (x=0.5 and x=1) due to the larger γ phase fraction and the unfavorable γ phase distribution.