Order degree and self-accommodation of γ phases in Ni-Mn-Ga alloys

Abstract

High temperature shape memory alloys Ni-Mn-Ga have high martensitic transformation temperature, good thermal stability and moderate shape memory performance. The second phase γ significantly affects the alloys’ macroscopic performances, but its atomic structure and microstructural feature still remain almost unknown. Microstructure and degree of atomic ordering of γ precipitate are uncovered in Ni54Mn30+xGa16–x (x = 0, 2, 5, 9, 11) and Ni57+yMn25Ga18–y (y = 0, 1, 2, 3, 4) alloys. For as-cast Mn-rich alloys, a hierarchical “nano-lamellae within micro-lamellae” microstructure is found. Nano-lamellae are paired to form a variant, two variants constitute a micro-lamella, a pair of micro-lamellae makes up single group, and there are six groups in a grain. Single nano-lamella is a partially ordered phase with tetragonal crystal structure. All nano-lamellae are of nearly identical crystallographic orientation. This arrangement of the 24 variants is self-accommodated. In annealed state, the lamellae in large γ precipitates still exist due to the constraint from martensites, but in small grains partly or completely disappear and further evolve into nanotwins. For as-cast Ni-rich alloys, the paired nano-lamellae are disordered and another partially ordered phases, respectively. After annealing, they become single crystal with ordered structure. Models describing atomic positions of γ phases with different orderings are built. For Mn-rich alloys, the martensitic transformation temperature reaches about 600 °C, shape memory strain is up to 5%, and compressive stress and strain are higher than 2100 MPa, and 26%, respectively. Relative to Ni-rich samples, Mn-rich alloys have very high martensitic transformation temperature, moderately enhanced ductility and reasonably good shape memory strain. They are closely related to martensite’s electron concentration and γ’s atomic ordering, crystal structure, and lamellar microstructure.