Features of thermoelastic martensitic transformations, structure and properties in ternary B2-alloys based on NiMn – NiTi, NiMn – NiAl, NiMn – NiGa, Ni2MnGa – Ni3Ga quasi-binary systems

Abstract

Structural phase transformations and properties of B2-alloys based on NiMn – NiTi, NiMn – NiAl, NiMn – NiGa, Ni2MnGa – Ni3Ga systems were studied. The effect of titanium, aluminum and gallium alloying on the structure, the phase composition, and the properties of quasi-binary alloys was 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 was analyzed. A general phase diagrams of the structural and magnetic transformations in the alloys were plotted. It is shown, that alloing by third component reduces transformations temperatures. The temperature concentration boundaries of austenite and martensite phases were 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}<011¯>B2 “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses. Investigations were carried out by methods of scanning and transmission electron microscopy, diffraction of x-rays and electrons, resistometry and magnetometry.