The Effect of Atomic Lattice Defects on the Softening Phenomena of High-Purity α-Iron

To investigate softening in b.c.c. metals by atomic lattice defects, α-iron single crystals of high purity and with different orientations were irradiated at 78 K with different doses of 2.5 MeV electrons and then tested in tension between 5 and 185 K. In as-irradiated crystals the flow stress at 78 K decreases rapidly with increasing electron dose (so-called primary irradiation softening (PIS)), but soon reaches saturation without further change. With decreasing deformation temperature PIS becomes smaller. Within saturation, the orientation dependence of the flow stress seems to disappear. With annealing above recovery stage I, PIS is extinguished but another type of softening, called secondary irradiation softening (SIS), is revealed. It is similar to alloy softening, occurs only at deformation temperatures above ≈︂100 K, and anneals out in recovery stage III. The results can be interpreted in terms of thermally activated kink-pair formation in screw dislocations, which is influenced in PIS and SIS by two different interstitial-atom configurations (crowdions or dumbbells) in accordance with the two-interstitial annealing model.