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 Ni57+xMn25Ga18-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 gamma phase are observed for x >= 0.5. The martensite variants are all twinned and well self-accommodated. The volume fraction of gamma phase increases from 4.2% to 35.1% with increasing Ni content. Martensitic transformation temperature (MO increases from 437 degrees C for x=0 to 469 degrees C for x=0.5 and then remains almost unchangeable for x >= 1. Ductile gamma 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 gamma phase reduces the shape memory properties. The hampering effect of the gamma 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 gamma phase fraction and the unfavorable gamma phase distribution. (C) 2015 Elsevier B.V. All rights reserved.