ELECTRON-MICROSCOPY OF NEW MARTENSITES WITH LONG-PERIOD STACKING ORDER STRUCTURES IN NI50ALXMN40-X ALLOYS .1. STRUCTURES AND MORPHOLOGIES

The martensitic transformations in Ni50AlxMn50-x (16.0 less than or eq ual to x less than or equal to 20.5) alloys were extensively studied b y electron microscopy observations at ambient temperature and differen tial scanning calorimetry measurements. As a result, a new martensite with 10M (15R, (<3(2)over bar>)(3)) long period stacking order structu re was found for x greater than or equal to 19.0. For x less than or e qual to 17.0, the martensite was invariably 14M (7R, (<5(2)over bar>)) . For 17.0 less than or equal to x less than or equal to 19.0, various martensites were found, and the situation was in between the above tw o cases. Based on these results, a phase diagram to explain various ma rtensitic transformations was proposed.