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

The properties, the martensitic transformation, and the structure of Ni$_{50}$Mn$_{50-x}$Al$_x$ ($x$ = 5, 10, 18, 20, 22, 24, 25) alloys are studied by electrical resistivity measurements, transmission electron microscopy, scanning electron microscopy, and X-ray diffraction over wide temperature and composition ranges. It is found that, as the aluminum content increases, the martensite transformation temperature decreases significantly and the structure of martensite changes. Complex multilayer (10M, 14M) martensite phases are detected in the ternary alloys. Martensite is shown to have a predominant morphology in the form of hierarchic packets of thin coherent plates of nanoand submicrocrystalline crystallites, which have plane habit boundaries close to $\{110\}_{B2}$ and are pairwise twinned along one of the 24 equivalent $\{011\}\langle 01\bar1\rangle_{B2}$ systems of twinning shear.