Aging effects on the cyclic deformation mechanisms of a duplex stainless steel

Abstract

Aging effects on the cyclic deformation mechanisms of an AISI-329 duplex stainless steel have been studied on the basis of the cyclic hardening-softening response, cyclic stress-strain curve and substructure evolution within the individual phases. The cyclic behavior of an unaged and two aged materials shows, in terms of plastic strain amplitude (ε pl), three well-defined stages. In the first regime, at low ε pl, no differences are observed among the response of the three materials as a consequence of the dominance of “austenitic-like” deformation mechanisms for all the materials. In the second regime, at intermediate ε pl, the cyclic behavior of unaged material is associated with a mixed “austenitic/ferritic-like” character, mainly due to plastic activity of both phases. On the other hand, the cyclic response of aged material within this intermediate ε pl range is rather correlated to “austenitic-like” cyclic deformation mechanisms because of the intrinsic brittleness of the ferritic matrix. A third regime, at relatively large ε pl, suggests a synergetic phenomenon of dislocation activity, deformation twinning and demodulation of spinodal microstructure in ferrite that enables this phase to sustain plastic deformation. Thus, in this ε pl interval, the observed mechanical and substructural behavior within ferrite may be considered as relatively similar to that observed in unaged material at much lower stress levels; and therefore is amenable to be associated with “ferritic-like” cyclic deformation mechanisms. Finally, based on the results presented, the prevalence of “austenitic-like” or “ferritic-like” cyclic deformation mechanisms, for a given plastic strain range, is discussed in terms of the different role played by the ferritic matrix in each material investigated, depending upon its embrittlement degree.