Crystallography, twinning elements and interface structure of hierarchical lamellar γ type martensite in Ni-Mn-Ga alloys

Microstructural features of lamellar γ in as-cast Ni54Mn28+xGa18–x (0, 4, 7, 9, 11, and 15) alloys are investigated. With increases of Mn content, γ grain size increases from one to hundreds of micrometers. In small grain, {011} primary microtwins with tetragonal crystal structure are found. The twinning elements are K1 = (011), η1 = [011¯], K2 = (011¯), η2 = [011], and s = 0.169. In medium-sized grain, it comprises of six groups of self-accommodated variants, each of which contains {011} secondary nanotwins. Within a single group, there are four variants, designated as A, B, C and D. The A-C and A-B interfaces are nearly straight, the A-D interface is zigzag, and they have a few lattice dislocations. In the vicinity of γ grain, tertiary nanotwins of non-modulated (NM) martensite arise. In large grain, a series of {111} twin intersections occurs within γ micro-lamella, forming configurations resembling self-accommodated NM martensite. Orientation relationship between lamellar γ and NM martensite is determined. Owing to the microstructural similarity to NM martensite, the lamellar γ is defined as a new type of martensite. The variation in grain size affects the hierarchical twin microstructure of γ type and NM martensites. Occurrence of such phenomenon may mainly source from their different stresses required to activate higher-order twinning. Influence of γ on the mechanical property of Ni-Mn-Ga is discussed.