Anisotropy of defect creation in electron-irradiated iron crystals

Abstract

Single crystals of \ensuremath{\alpha}-iron were irradiated perpendicularly to the (100), (110), and (111) planes with electrons in the range 0.35-1.7 MeV and their electrical resistivity change rates were measured. A geometrical model of the threshold-energy surface for atomic displacement in a bcc lattice produces a fit to the experimental data leading to the following values for the threshold energies in the principal crystal directions: ${T}_{d}^{〈100〉}=17\ifmmode\pm\else\textpm\fi{}1$ eV, ${T}_{d}^{〈111〉}=20\ifmmode\pm\else\textpm\fi{}1.5$ eV, and ${T}_{d}^{〈110〉}\ensuremath{\gtrsim}30$ eV. The specific resistivity of a Frenkel pair is deduced to $\ensuremath{\rho}_{F}^{\mathrm{Fe}}=(30\ifmmode\pm\else\textpm\fi{}5)$ \ensuremath{\mu}\ensuremath{\Omega}cm/at.%. From the obtained ${T}_{d}'\mathrm{s}$ we derived an interatomic potential of the Born-Mayer type, valid in the range $1.2\ensuremath{\le}r\ensuremath{\le}2.5$ \AA{}. We propose as a good choice: $V(r)=8900{e}^{\ensuremath{-}4.5r}$ eV. The recovery due to isochronal annealing during stage I, after irradiation at different electron energies, was measured and related to specific recovery mechanisms. Thus, the first important substage, ${I}_{B}$ (\ensuremath{\sim} 66 K), is due to the recovery of close Frenkel pairs created in the $〈100〉$ direction, while a comparison of calculated cross sections suggests that ${I}_{C}$ (\ensuremath{\sim} 87 K) possibly stems from $〈111〉$ close pairs. Substage ${I}_{D}$ (90-110 K) is complex; its first part, below 100 K, originates mostly from defects produced in the $〈100〉$ direction and the second part, above 100 K, together with ${I}_{E}$, principally originates from defects produced in the $〈111〉$ direction.