The Flow Stress of High-Purity Refractory Body-Centred Cubic Metals and its Modification by Atomic Defects

Abstract
The strong temperature and strain-rate dependence of the flow stress of high-purity refractory body-centred cubic metals has been shown to be an intrinsic property and is usually ascribed to a high Peierls barrier of a₀<111>/2 screw dislocations. These barriers are overcome by the formation of kink pairs on the screw dislocations. The paper reports on recent, very complete flow-stress data on ultra-high purity Mo crystals obtained by two different experimental techniques and covering the temperature range 4 K to 460 K. The results are in accord with earlier work of Brunner and Diehl on α-Fe, who showed that below the so-called knee temperature, T_K, three regimes in the temperature variation of the flow-stress should be distinguished. Two of them are fully accounted for by the same glide mechanism, namely elementary glide steps on {211} planes. The so-called upper bend separating these two regimes is an inherent feature of the theory of kink-pair formation and does not indicate a change in the glide mechanism. There is, however, strong evidence that the so-called lower bend, separating the range of {211} elementary glide steps from the low-temperature flow-stress regime, is due to a change in the glide mechanism. It is argued that at the lower bend the screw-dislocation cores undergo a "first-order phase transition" from a low-temperature configuration that allows glide of a given screw dislocation on any of its three {110} glide planes to a high-temperature configuration that can glide only on one definite {211} plane. Between T_K and the lower-bend temperature, T~, bcc metals may show the unique phenomena of alloy and irradiation softening. With regard to the latter phenomenon, Brunner and Diehl distinguish between "primary" and "secondary" softening. It is shown that alloy softening and the "secondary irradiation softening" of bcc metals may be explained by an "overheating" of the phase transition in the dislocation core. Although the underlying mechanism is not yet fully understood, there is fairly strong evidence that the "primary irradiation softening" is due to the interaction between screw dislocation cores and crowdion self-interstitials.