THE MECHANISM OF NUCLEATION OF VACANCY LOOPS WITH BURGERS VECTORS (100) IN BCC METALS

The mechanism of nucleation of unfaulted vacancy loops (VL) with Burgers vector b = 〈100〉 in bcc-metals is suggested. The mechanism takes into account the experimental fact that the nucleation of such loops takes place exclusively in the cascade region of damage. In substantiating the mechanism, use is made of data on simulation of the kinetics of vacancy clusters (VC) nucleation in depleted cascade zones in alpha-iron. When the VC contains a plane vacancy fragment {100} of or above a critical size in the depleted zone, the formation of an unfaulted loop nucleus takes place via the Eyre-Bullough shear reaction. In this case, the shift direction and, correspondingly, the Burgers loop vector depends on the configuration of vacancies in the VC surrounding the loop nucleus. Computer simulation has shown that the shear leading to the VL formation with b = 〈100〉 is the most probable for a dense three-dimensional VC which has over 20-35% of vacancies not in the original vacancy platelet of {100} critical size. With nucleation the VC is simply an isolated platelet of Nv > 46 vacancies and the spontaneous unfaulting is always realized, resulting in the formation of an unfaulted loop with b = 1 2〈111〉. © 1990.