THE STRUCTURE OF THE HIGH-TEMPERATURE PHASE MNAL(H) AND THE DISPLACIVE TRANSFORMATION FROM MNAL(H) INTO MN5AL8

Splat-cooling investigations, as well as heat treatment at temperatures corresponding to the stability range of the high-temperature phase MnAl(h), show that this phase cannot be retained by quenching. Therefore, high-temperature X-ray powder diffraction was used for the determination of the crystal structure. MnAl(h) is isotypic to tungsten, a l230K = 0.3063 nm, and Im-3m. The displacive transformation from MnAl(h) into Mn5Al8 (Cr5Al8 type) was studied by means of X-ray diffraction and optical and scanning electron microscopy (SEM). For the low-temperature phase Mn5Al8, the axial ratio c/a, as well as the volume of the unit cell, was investigated in dependence on mole fraction. Crystal chemical parameters of phases isotypic to Cr5Al8 are compared. © 1990 The Metallurgical of Society of AIME.