Thermal stability and microstructure of Ni–Mn–Ga–Cu high temperature shape memory alloys

Abstract

The thermal stability and microstructure after ageing at 770 K for different times have been studied on six Ni–Mn–Ga–Cu polycrystalline high temperature shape memory alloys, in which Cu substituted Mn, Ga, or both simultaneously. Prior to ageing, those compositions with higher e/a ratio not only show higher martensitic transformation temperatures but also higher thermal hysteresis. The quaternary alloys that contain less Cu show the best thermal stability, almost independently from the element that is substituted by Cu. Nevertheless, all the quaternary alloys studied in this work show notably worse thermal stability than the most stable ternary Ni–Mn–Ga high temperature shape memory alloys reported so far. Increase in ageing time at 770 K promotes the decrease of the transformation temperatures and the c/a ratio of the non-modulated martensite as a consequence of the Cu depletion in the matrix induced by the precipitation of Cu-rich ordered f.c.c. γ′ particles. Although the γ′ precipitates are heterogeneously distributed, the unique alloy with transformation temperatures very close to 770 K also show a high concentration of particles located at some intervariant martensitic boundaries, attributed to an enhanced precipitation process on the habit plane of the retained martensitic planes.