CYCLIC STRESS-STRAIN RESPONSE AND DISLOCATION SUBSTRUCTURE EVOLUTION OF A FERRITE-AUSTENITE STAINLESS-STEEL

The hardening-softening response, the cyclic stress-strain behavior an d the evolution of dislocation structures of an AISI 329 ferrite-auste nite stainless steel have been studied. Fatigue testing has been condu cted under fully reversed total strain control and constant total stra in rate. Detailed transmission electron microscopy studies have been c arried out in order to determine the individual substructure evolution , as a function of increasing imposed strain amplitude, in each consti tutive phase. In general, the cyclic response of the studied material may be described in terms of three different regimes within the plasti c strain amplitude (epsilon(pl)) range investigated, i.e. from 2 x 10 (-5) to 6 x 10(-3): at epsilon(pl) below 10(-4) the dominant cyclic de formation mechanisms are those correlated to planar glide of dislocati ons 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 epsilon(pl) higher than 6 x 10(-4) the dominant substructure evolutio n is observed inside the ferritic matrix. In this case, strain localiz ation 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-str ain curve. At epsilon(pl) in-between these values, the cyclic behavior is characterized by a relatively high strain hardening rate and may b e 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 inf luenced 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 constituti ve phases.