Anisotropy of defect creation in electron-irradiated iron crystals

Abstract

Single crystals of α-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: $T_d^{〈100〉}=17\mathrm{}\pm 1$ eV, $T_d^{〈111〉}=20\mathrm{}\pm 1.5$ eV, and $T_d^{〈110〉}\gtrsim 30$ eV. The specific resistivity of a Frenkel pair is deduced to $\rho _F^{}{}_{}{}^{\mathrm{Fe}}=(30\mathrm{}\pm 5)$ μΩcm/at.%. From the obtained $T_d\text{'}\mathrm{s}$ we derived an interatomic potential of the Born-Mayer type, valid in the range $1.2\le r\le 2.5$ Å. We propose as a good choice: $V\left(r\right)=8900\mathrm{}{e}^{-4.5r}$ 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, $I_B$ (∼ 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 $I_C$ (∼ 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.