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 μm independently of the prior austenite grain site.