Crystallographic and Morphological Studies on the FCC to FCT Transformation in Mn–Cu Alloys

The FCC to FCT transformation in Mn–22 at% Cu and –26 at% Cu alloys has been examined by means of X-ray diffraction and optical and electron microscopies with a cooling stage. The c parameter and the c⁄a ratio of the FCT martensite decrease with decreasing temperature, abruptly below the transformation temperature and then very slowly, showing a characteristic of the second order type phase transformation. On the other hand, the optical microscopy shows a coexistence of untransformed and transformed regions, exhibiting a characteristic of the first order type phase transformation. Banded relief is observed in the transformed region, which corresponds to the formation of tetragonal twins. No internal structure as lattice invariant shear is observed in the banded twins even by electron microscopy. The banded region expands and the relief contrast increases with decreasing temperature. Correspondingly, the electron microscopy exhibits lamellar contrast below the transformation temperature. However, very fine striations appear prior to the lamellar appearance. Both the lamellar and striations are parallel to {011} planes of the FCT martensite, and are supposed to be tetragonal twins introduced to reliese a tetragonal distortion due to the FCC to FCT transformation and the magnetic transformation, respectively. The temperature difference between the structural and magnetic transformations is about a few of ten degrees, being in agreement with that in a previous work.