Autowave Aspects of Plastic Deformation Instabilities

Abstract

This paper examines the nature of h- and S-type plastic flow instabilities within the concept of localized plasticity au-towaves. It is shown that both of these instabilities can be observed in the same ARMCO iron material in the form of switching (h-type instability) or excitation (S-type instability) autowaves. The switching autowave represents the localized deformation front uniformly moving under a constant stress, and the excitation autowave represents the same front, but moving with a constantly decreasing velocity with reducing stress. The switching autowave passes continuously through the object, but the excitation autowave propagates intermittently. The manifestation of one or the other wave is deter-mined by the temperature-strain rate conditions. There is an interval of low temperatures where, regardless of the strain rate, only a switching autowave is generated, and the deformation front velocity increases exponentially with increasing stress. An excitation autowave can generate at high temperatures, when the deformation front moves abruptly during stress drops. This phenomenon can be interpreted in terms of dynamic strain aging. Under such conditions, the front velocity depends linearly on the stress. It is shown that the deformation front velocity is always determined by local strain rates at the front. Using the dislocation approach to dynamic strain aging and by analyzing the dependences of local strain rates on the effective stress, it is established that the switching autowave (h-type instability) is controlled by thermally activated motion of dislocations, and the excitation autowave (S-type instability) is governed by their viscous overbarrier motion.