Effect of melting and electron-phonon coupling on the collapse of depleted zones in copper, nickel, and alpha -iron.

Abstract

Previous work has shown that the nucleation of a vacancy loop by collapse of a displacement cascade is possible when the depleted zone (DZ) can melt and crystallize during the thermal spike stage. In this paper calculations based on molecular-dynamics (MD) simulations of Cu, Ni, and Fe have been performed to investigate the influence on the melting and loop formation of the average vacancy concentration 〈${\mathit{C}}_{\mathit{v}}$〉 and number of vacancies, ${\mathit{N}}_{\mathit{v}}$, in the DZ. It is demonstrated that collapse of the DZ is possible at a fixed value of 〈${\mathit{C}}_{\mathit{v}}$〉 only for conditions where ${\mathit{N}}_{\mathit{v}}$ exceeds a critical number ${\mathit{N}}_{\mathit{v}}^{\mathrm{cr}}$. Values of ${\mathit{N}}_{\mathit{v}}^{\mathrm{cr}}$ have been estimated and found to be in good agreement with ones obtained from fitting our previous theoretical calculations to the experiments. The strong electron-phonon coupling (EPC) in Ni influences the vacancy loop yield through the formation of an amorphouslike core in the DZ. The influence of the size of the DZ and 〈${\mathit{C}}_{\mathit{v}}$〉 on the amorphization process has been investigated. The dependence of the amorphization temperature on the size of the DZ has been obtained. It has been shown that a DZ with small diameter (less than 2${\mathit{a}}_{0}$, where ${\mathit{a}}_{0}$ is the lattice parameter) cannot become amorphous in Ni even for 〈${\mathit{C}}_{\mathit{v}}$〉 more than 20\char21{}30 at. % and initial temperatures above 5000 K. The DZ in iron does not melt and produce a vacancy loop if the EPC is taken to have the same value as Ni. A reduction by a factor of 2 in the strength of the EPC leads to the formation of melted regions and amorphous cores in the DZ in this metal. In reality, however, the EPC strength for Fe is believed to be higher than for Ni, which means that the very low yield of vacancy loops in Fe compared to Ni can be explained by the low probability for the DZ to melt in the thermal spike. \textcopyright{} 1996 The American Physical Society.