Growth directions of microstructures in directional solidification of crystalline materials.

In directional solidification, as the solidification velocity increases, the growth direction of cells or dendrites rotates from the direction of the thermal gradient to that of a preferred cristalline orientation. Meanwhile, their morphology varies with important implications for microsegregation. Here, we experimentally document the growth directions of these microstructures in a succinonitrile alloy in the whole accessible range of directions, velocities, and spacings. For this, we use a thin sample made of a single crystal on which the direction of the thermal gradient can be changed. This allows a fine monitoring of the misorientation angle between thermal gradient and preferred crystalline orientation. Data analysis shows evidence of an internal symmetry which traces back to a scale invariance of growth directions with respect to a Péclet number. This enables the identification of the relationship between growth directions and relevant variables, in fair agreement with experiment. Noticeable variations of growth directions with misorientation angles are evidenced and linked to a single parameter.

Shape of growth cells in directional solidification.

The purpose of this study is to characterize experimentally the whole shape of the growth cells displayed in directional solidification and its evolution with respect to control parameters. A library of cells is first built up from observation of directional solidification of a succinonitrile alloy in a large range of pulling velocity, cell spacing, and thermal gradient. Cell boundaries are then extracted from these images and fitted by trial functions on their whole profile, from cell tip to cell grooves. A coherent evolution of the fit parameters with the control parameters is evidenced. It enables us to characterize the whole cell shape by a single function involving only two parameters which vary smoothly in the control parameter space. This, in particular, evidences a continuous evolution of the cell geometry at the cell to dendrite transition which denies the existence of a change of branch of solutions at the occurrence of sidebranching. More generally, this global determination of cell shape complemented with a previous determination of the position of cells in the thermal field (the cell tip undercooling) provides a complete characterization of growth solutions and of their evolutions in this system. It thus brings about a relevant framework for testing and improving theoretical and numerical understanding of cell shapes and cell stability in directional solidification.

[Intra-hepatic biliary cystadenocarcinoma]. / Cystadénocarcinome biliaire intra-hépatique.

Biliary cystadenocarcinoma is a rare tumor of the intrahepatic biliary tract, which frequently develops in a preexisting benign biliary cystadenoma. In the present case, diagnosis was difficult because of the lack of specificity of clinical, biological and radiological findings. The correct diagnosis was only achieved by histological examination of the resected lesion. Macroscopically, the right lobe of the liver showed evidence of a whitish, multilolobed, malignant mass of about 6 cm in diameter. Upon light microscopic analysis, cysts were found to be lined with papillary forms. In some areas, epithelial cells were clearly malignant contrasting with persistent non dysplasic areas, suggesting the presence of underlying cystadenomas. Eleven months after complete surgical resection, the patient is in good condition with no evidence of recurrence.

Maximal curvature and crystal orientation on directionally solidified dendrites.

We experimentally address the locations of maximal curvature on crystalline cellular or dendritic interfaces that directionally grow in a thin sample of a transparent material. Local curvatures are determined on the whole dendrite tips by considering the intersection of nearby normals. It is found that, at the location of the curvature maximum, the interface normal points toward a particular direction solely set by the crystal lattice and equal in practice to the dendrite growth direction at large pulling velocity. This property is independent of the growth conditions (thermal gradient, velocity, dendrite spacing, and crystal orientation). It enables crystal orientations to be recovered from dendrite shapes and provides a bridge to understand the implications of anisotropy on the forms and orientations of directionally solidified dendrites.