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Interparticle spacing in materials science is a key parameter controlling ductile fracture. It is generally evaluated as the distance between particle centers of mass. High strength medium carbon martensitic steels contain a high density of carbides, whose size is non-negligible with respect to the distance between them. Current microstructural quantification methods do not take the shape of analyzed particles into account, thus being not necessarily relevant to materials containing a high density of particles that trigger void nucleation and ductile fracture. This work reports on the potency of a simple full field particle quantification method based on a watershed algorithm that takes the shape and spatial distribution of particles into account. Three microstructures were produced by heat treating a hot-rolled 40CrMo4 steel bar. Cementite carbide populations of each microstructure were quantified using carbon extractive replicas, conventional image processing and application of the developed algorithm. A comparison of results from a full field Voronoi tiling algorithm revealed pronounced differences in the shape of distributions. The watershed-based method was shown to allow efficient interpretation of the relationships between carbide distribution and ductile fracture features.

Colloque avec actes et comité de lecture. Internationale.; International audience; Nous étudions le fluage d'un acier 9%Cr-1%Mo modifié (de type P91) en fonction de la contrainte et de la température. Parmi les nombreux essais menés entre 475 et 600°C, l'un a donné lieu à rupture à 160000h à 500°C et un autre à 94000h à 600°C. Le modèle d'instabilité viscoplastique de Hart couplé à la loi de Norton permet de décrire l'évolution temporelle de la section en striction jusqu'à rupture. Le modèle prédit correctement les durées de vie pour les essais moins longs à basse température et les surestime pour les hautes températures.

International audience; The effect of three-dimensional grain morphology on the deformation at a free surface in polycrystalline aggregates is investigated by means of a large scale finite element and statistical approach. For a given 2D surface at z=0 containing 39 grains with given lattice orientations, 17 random 3D polycrystalline aggregates are constructed having different 3D grain shapes and orientations except at z=0, based on an original 3D image analysis procedure. They are subjected to overall tensile loading conditions. The resulting stress-strain fields at the free surface z=0 are analysed. Ensemble average and variance maps of the stress field at the observed surface are computed. In the case of an anisotropic elastic behaviour of the grains, fluctuations ranging between 5% and 60% are found in the equivalent stress level at a given material point of the observed surface from one realization of the microstructure to another. The obtained fields are compared to the prediction based on the associated columnar grain microstructure, often used in literature. These results have important implications in the way of comparing finite element simulations and surface strain field measurements in metal polycrystals.