Supersaturated vacancies and vacancy complexes in rapidly solidified B2 aluminide ribbons

The formation of vacancy complexes through aging heat treatments between 800 and 1200 K was studied for rapidly solidified NiAl and CoAl ribbons using TEM. In the case of the NiAl ribbons, the density of dislocations decreased with increasing annealing temperature, while voids with tens to a hundred nanometers in size were formed in a low density. In the CoAl ribbons, noteworthy microstructures of dislocations and voids appeared after annealing. A zone consisting of dislocation loops, in which prismatic loops 20–50 nm in diameter had formed at high density, was found in the CoAl ribbons annealed at 800 K for 360 ks. On the other hand, voids with a size of several nanometers and a remarkably high density were formed by annealing not only at 800 K but also at 1200 K, suggesting that the growth rate of voids is extremely low because of low diffusivity in CoAl.