Discontinuous yielding in ultrafine-grained austenitic stainless steels

Abstract

The effects of structure, strain rate and temperature on the discontinuous yielding of ultrafine-grained 316 and 304 austenitic stainless steels have been studied. The stress-strain curves for these materials do not exhibit a yield drop but they contain a well-defined plateau corresponding to the propagation of Lüders bands. Transmission electron micrographs of dislocation structure seem to indicate that discontinuous yielding occurs by formation of several small Lüders bands within the gauge length of the tensile specimen. The magnitude of the Lüders strain also increases with increasing number of carbide particles per grain boundary unit area. The test temperature has a dramatic effect on the magnitude of the Lüders strain of type 316, which increases with decreasing temperature to about 10–12% at 77 K from approximately 1.5−2% at room temperature. At about 670 K, deformation becomes homogeneous. The structure of type 316 steel deformed within the Lüders strain at 77 K consists of well-defined overlapping stacking faults and/or microtwins. The Lüders strain does not exhibit any systematic dependence on crosshead speed (initial strain rate). However, the plateau stress of type 316 increases only slightly with increasing crosshead speed but quite drastically with decreasing temperature, approaching about 900–1000 MPa at 77 K from 500–600 MPa at room temperature. These observations have been qualitatively analysed on the basis of the Lüders deformation relation to the band-front velocity and the contribution of the mobile dislocation density provided by the grain boundary sources and their velocity to the strain rate.