Dynamic strain ageing of AISI 316L during cyclic loading at 300 °C: Mechanism, evolution, and its effects

Under the influence of dynamic strain ageing (DSA), at 300 degrees C 316L stainless steel exhibits: primary cyclic hardening due to a significant increase in dislocation density and the incidence of profuse point defects, then cyclic softening due to dislocation re-arrangement, and finally an almost stabilised response stage or secondary cyclic hardening. Flow stress serration is examined in every cycle along with a comprehensive microstructural investigation in order to study DSA and its relationship with cyclic deformation response. During primary cyclic hardening, stress serrations are initially pronounced, before gradually disappearing. There is then almost no incidence of stress serration during cyclic softening, in particular for low strain amplitude tests. However, serrated flow stress reoccurs towards the end of fatigue life. An analysis of potential dislocation locking mechanisms shows that Suzuki atmospheres of solute atoms forming in faulted areas of partial dislocations are responsible for DSA during the first two cyclic response stages. Upon further loading, Snoek atmospheres form to further strengthen the locking effect of solute atoms on dislocations, resulting in the re-occurrence of stress serration. Snoek atmospheres and the formation of a corduroy structure are responsible for secondary cyclic hardening. Finally, the influence of hydrostatic pressure on the mobility of vacancies induces a slight difference in DSA characteristics with respect to the loading direction, i.e. tensile direction or compressive direction transients. (c) 2012 Elsevier B.V. All rights reserved.