Plastic flow of martensitic model alloys

Modeling of the constitutive behavior was undertaken for Fe-Cr-C tempered martensitic alloys. Tensile tests were carried out at room temperature and constant nominal strain rate. In order to obtain a fully martensitic structure with different prior austenite grain size, two heat-treatments, consisting of an austenization followed by a tempering, were applied to the alloys. Two tempered martensitic structures were so produced with two different prior austenite grain sizes. The "as-heat-treated" microstructures were well characterized by electron transmission microscopy and metallographic observations. The effect of the prior austenite grain size on the yield stress and on the strain-hardening was investigated. It was found that a moderate increase of the yield stress exists, which is certainly not consistent with a Hall-Petch relationship between the yield stress and the prior austenite grain size. The post-yield behavior was found completely unaffected by the prior austenite grain size. The stress dependence of the strain-hardening was shown to be consistent with a simplified phenomenological description of the strain-hardening based on one structural parameter, the total dislocation density. An effective mean displacement distance of the dislocations was estimated form the strain-hardening law, equal to about 10 mum independently of the prior austenite grain site. (C) 2004 Elsevier B.V. All rights reserved.