The annealing of neutron irradiation hardening in copper crystals

Abstract

Abstract The thermal annealing of the principal parameters of the stress-strain curve of neutron irradiated copper crystals is described. The parameter most sensitive to annealing is the Lüders extension at 20°c, which is substantially reduced by anneals which have little effect on the critical shear stress. The other two parameters which recover more rapidly than the critical shear stress at 20°c are the critical shear stress at −195°c and the stress at which stage III commences at 20°c. The rate of hardening in stage II at 20°c is reduced by irradiation and is unusual in that it recovers only after the critical shear stress at 20°c has been reduced to approximately half the irradiated value. Activation energies have been measured for the recovery of the Lüders extension (1·5 ev increasing to 2·3 ev as recovery proceeds) and the critical shear stresses at −195°c (1·9 ev increasing to 2·5 ev) and 20°c (2·2 ev). These results can only be satisfactorily interpreted in terms of a spectrum of irradiation-produced defect clusters. Very small clusters appear to have a very important effect on both the critical shear stress at −195°c and, surprisingly, the Lüders extension at 20°c; larger clusters control the critical shear stress at 20°c. For this reason empirical relationships between the various parameters are unlikely to have any general application. A model is proposed to reconcile the apparent anomaly of easy cross slip in stage I with an increased stress for the onset of stage III.