The low-temperature plastic deformation of α-titanium and the core structure of a-type screw dislocations

Abstract

α-titanium single crystals, containing three different amounts of interstitial impurities, have been tested in compression at temperatures between 77 and 700 K. The mechanical behaviour is characterized by a strong temperature dependence of the yield stress for slip on the primary prismatic slip plane (1010)[1210], a strong dependence of the critical resolved shear stress on the orientation of crystal axis and the failure of the Schmid law, an anomaly at temperatures between 300 and 500 K which is associated with cross-slip into a first-order pyramidal plane, and the occurrence of two hardening stages on the deformation curves at low temperatures. Both conventional and in situ transmission electron microscopy examinations revealed that a large lattice friction opposes the motion of a-type screw dislocations at temperatures below 550 K. Since edge dislocations are found to be highly mobile, it appears that, at low temperatures, the yield stress of pure α-titanium is governed by a Peierls force acting on screw dislocations. These results are interpreted by analogy with a similar situation met in b.c.c. metals. It is assumed that the core configuration of a-type screw dislocations is non-planar, so that they move by a succession of thermally activated sessile-glissile transitions. A model is proposed for this core structure in terms of dissociations and partial dislocations, which qualitatively accounts for our experimental observations. The hardening due to interstitial impurities is found not to be consistent with Fleisher-type models for the elastic interaction between dislocations and interstitial atoms. This hardening is rather interpreted as resulting from a chemical interaction, namely a modification of the core structure of screw dislocations in the presence of impurity clusters.