ELECTRON-MICROSCOPY STUDY OF THE FORMATION OF NI5AL3 IN A NI62.5AL37.5 B2 ALLOY .2. PLATE CRYSTALLOGRAPHY

Different twinning arrangements in Ni5Al3 plates grown inside the B2 p hase in Ni62.5Al37.5 samples are described on the basis of conventiona l and high resolution electron microscopy images and electron diffract ion. The plate morphology is comparable with that of pure 2M martensit e plates. The choice of ordered variants within a plate is dictated by the ordering coherency of the twin plane. No dislocations were observ ed at the primary plate-matrix interfaces. An explanation using the st reaking due to aperiodic twinning is given for unexpected reflections.