Cyclic stress-strain response and dislocation substructure evolution of a ferrite-austenite stainless steel

Abstract

The hardening-softening response, the cyclic stress-strain behavior and the evolution of dislocation structures of an AISI-329 ferrite-austenite stainless steel have been studied. Fatigue testing has been conducted under fully reversed total strain control and constant total strain rate. Detailed transmission electron microscopy studies have been carried out in order to determine the individual substructure evolution, as a function of increasing imposed strain amplitude, in each constitutive phase. In general, the cyclic response of the studied material may be described in terms of three different regimes within the plastic strain amplitude ( ϵ pl) range investigated, i.e. from 2 × 10 −5 to 6 × 10 −3:at ϵ pl below 10 −4 the dominant cyclic deformation mechanisms are those correlated to planar glide of dislocations within the austenite which is the phase which carries a large part of the macroscopic strain in this first regime. On the other hand, at ϵ pl higher than 6 × 10 −4 the dominant substructure evolution is observed inside the ferritic matrix. In this case, strain localization is enhanced, within the ferritic grains, through the development of veins into the wall structure. Such evolution induces a pronounced decrease of the cyclic strain hardening rate in the cyclic stress-strain curve. At ϵ pl in-between these values, the cyclic behavior is characterized by a relatively high strain hardening rate and may be classified as a mixed “ferritic/austenitic-like” behavior. In this intermediate regime substructural changes are observed in both phases and the dislocation activity in each of them seems to be strongly influenced by their particular cyclic strain hardening behaviors. Finally, the results are analyzed and compared with data from the literature in terms of volume fraction and chemical composition of the constitutive phases.