Electron microscopy study of the formation of Ni5Al3 in a Ni62.5Al37.5 B2 alloy—I. Precipitation and growth

The formation and growth of Ni₅Al₃ precipitates found inside an annealed B2 matrix in Ni_62.5Al_37.5 samples is described on the basis of conventional and high resolution electron microscopy images and electron diffraction patterns. Short anneals introduce three-pointed star shaped precipitates consisting of twin related segments with different variants of the Ni₅Al₃ structure. Longer anneals result in separate plates growing from these wings and developing microtwinning in order to accommodate 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 precipitates than in the large microtwinned plates.