Investigation of dislocation structures in predeformed AISI 4340 steel

Abstract

It has been previously established that the presence of mobile dislocations due to small amounts of prestraining in AISI 4340 steels contribute to a significant increase in the toughness of this material. This process is in the present paper further analysed by examination of the development of dislocation structures from various cross sections along the gauge length of fractured specimens which were subjected to prestraining ranging from 0% to 15%. It is shown that the dislocation cell size decreases with the increase in local reduction in area until the local reduction in area reaches a critical value. The cell size then remains constant. In contrast, the critical value of the reduction in area decreases with the increase in initial prestraining. It is shown that the principle of similitude, which is part of the mesh length theory of strain hardening, is valid for this steel. Mobile dislocations, on prestraining less than 5%, are shown to play a major role in contributing to the process of similitude and also to the reported increase in fracture toughness.