Partially Ordered Hierarchical Substructure of As-Cast Γ Phase in Ni-Mn-Ga Alloys

As-cast Ni-Mn-Ga shape memory alloys have high thermal stability and twinning stress, but the second phase γ, which exerts significant effect on the alloys’ performance, remains almost unknown in terms of its microstructure. Here we unravel the microstructural characteristics of the γ phase in as-cast Ni-rich Ni57+xMn25Ga18–x (x = 0, 1, …, 4) and Mn-rich Ni54Mn32+xGa14–x (x = 0, 3, 7, 9) alloys. For the first time, a substructure of the γ phase is discovered to be composed of hierarchically arranged “nano-lamellae in micro-lamellae” with high-density dislocations. As building blocks of the γ phase, the nano-lamellae are not twin-related but form pairs with disordered and partially ordered phases alternating in the micro-lamellae. The partially ordered phase is presumably an intermediate of the disorder–order transition. A geometric model for describing the crystallographic orientation relationships among the micro-/nano-lamellae is established. This hierarchical substructure disappears and may evolve into twins of ordered phases after annealing. In addition, thermal/mechanical properties of the as-cast and annealed alloys are compared, and the effects of γ phase on the properties are discussed.