Crystallographic features of the structure of a martensite packet of the NiMn intermetallic compound

Optical microscopy, scanning electron microscopy, and X-ray diffraction are used to show that a pseudosingle crystal forms upon cooling of an alloy Ni$_{49}$Mn$_{51}$ single crystal below the temperature of the $\beta\to\theta$ (bcc $\to$ fct) transformation. At room temperature, this pseudosingle crystal has the structure of tetragonal L1$_{0}$ martensite with parameters $a$ = 0.3732 nm and $c$ = 0.3537 nm and a tetragonality $c/a$ = 0.94775. The temperatures of the forward and reverse B2 $\to$ L1$_{0}$ transformations are determined. The crystallographic features of martensite packet formation are analyzed. As shown by EBSD, neighboring martensite packets always have three kinds of tetragonal martensite plates, which are in a twin position and have different tetragonality axis directions. Repeated heating and quenching of the pseudosingle crystal result in recrystallization with the formation of coarse grains. The packet structure of the tetragonal martensite is retained in this case, and the sizes of the packets formed within a grain decrease by a factor of 2–3 as compared to the initial pseudosingle crystal.