RATE PROCESS OF YIELDING IN SOME BODY-CENTERED CUBIC ALKALI METALS

Temperature dependence of the critical resolved shear stress (CRSS) of 5N pure body-centered cubic (bcc) alkali metals K, Li, and Na has been critically examined within the framework of the Feltham–Butt model of flow stress in crystals with high intrinsic lattice friction. The model is based on the concept of the Peierls mechanism, i.e., the kink-pair mode of escape of screw dislocations trapped in Peierls valleys. It predicts that the square–root of CRSS, τ1/2, varies with the temperature, T, in accord with the relation τ1/2 = A – BT, where A and B are positive constants. Evaluation of the microscopic parameters of the model from the experimental τ – T data shows that the Peierls mechanism on (321) plane is responsible for the initial flow stress in K single crystal at rather low temperatures between 1.5 and 25 K. However, the rate–controlling process of yielding in Li (90–300 K) and Na (50–150 K) single crystals is not Peierls mechanism but stress-assisted, thermally activated, breakaway of edge-dislocation segments from short rows of pinning points due to localized defects, by cooperative unzipping and at the same time expanding in the shape of shallow bulge to the saddle point.