ELECTRON-MICROSCOPY STUDY OF THE FORMATION OF NI5AL3 IN A NI62.5AL37.5 B2 ALLOY .1. PRECIPITATION AND GROWTH

The formation and growth of Ni5Al3 precipitates found inside an anneal ed B2 matrix in Ni62.5Al37.5 samples is described on the basis of conv entional and high resolution electron microscopy images and electron d iffraction patterns. Short anneals introduce three-pointed star shaped precipitates consisting of twin related segments with different varia nts of the Ni5Al3 structure. Longer anneals result in separate plates growing from these wings and developing microtwinning in order to acco mmodate stress built-up at the interfaces with the surrounding matrix. Two different growth mechanisms were observed and are discussed. The lattice distortions are found to be much higher in the small precipita tes than in the large microtwinned plates.