Effect of gallium alloying on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary Ni–Mn–Ga alloys

The effect of gallium alloying on the structure, the phase composition, and the properties of quasibinary Ni$_{50}$Mn$_{50-z}$Ga$_{z}$ (0 $\le z\le$ 25 at.%) alloys is studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures is analyzed. A general phase diagram of the magnetic and structural transformations in the alloys is plotted. The temperature–concentration boundaries of the $B2$ and $L2_{1}$ superstructures in the austenite field, the tetragonal $L1_{0}(2M)$ martensite, and the 10$M$ and 14$M$ martensite phases with complex multilayer crystal lattices are found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to $\{011\}_{B2}$. Martensite crystals are twinned along one of the 24 $\{011\}\langle 01\bar1\rangle_{B2}$ “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses.