Nathalie Limodin

Digital volume correlation analyses of synchrotron tomographic images

Three-dimensional (3D) images of nodular graphite cast-iron samples obtained with synchrotron tomography are analysed by resorting to digital volume correlation. This technique uses the material microstructure to measure displacement fields within a sample submitted to mechanical loading. Compared to classical two-dimensional (2D) digital image correlation, the reconstructed volumes used by DVC tend to increase the uncertainty in the correlation calculations, yet elastic strains of the order of 10−3 can be measured. Displacement fields around the front of a fatigue crack have also been measured by an extended version of DVC, and are used to extract stress intensity factors (in modes I, II, and III) along the crack front. The values are in good agreement with finite element calculations when the experimentally measured displacements prescribed on the sample boundaries are considered as input in the numerical simulations.

Capillary equilibrium in a semi-solid Al-Cu slurry

Abstract The microstructural evolution of an Al-15.8u2009wt.% Cu slurry held for 80u2009min at 555 °C has been characterised in-situ by X-ray microtomography. The different parameters measured by segmentation of the 3D images are analysed by reference to a model describing capillary equilibrium in an aggregate of solid particles with uniform solid/liquid interface curvature. Excellent agreement is found between model predictions and experimental data, except for the fact that the solid/liquid interface area predicted by the model is lower by about 12.5 % than that measured experimentally. This suggests that, owing to the progressive grain coarsening, the slurry does not achieve uniform interface curvature. The model makes possible the conversion of experimental data into evolutions of the dihedral angle, average surface tensions ratio, and capillary pressure in the liquid.

X-Ray Microtomography Investigation of the Effect of Ba Additions on the Microstructure of Partially Remelted Al-4%Cu Alloys

The aim of this work is to study by X-Ray microtomography carried out at ESRF Grenoble the microstructure of an Al-4wt%Cu alloy which was previously cold rolled to obtain globules of the solid phase upon heating in the semi-solid range. Since this process produced entrapped liquid in the globules, 3D quantification of this liquid was performed. Moreover, the influence of the addition to the alloy of a small amount of Ba, which has been shown to decrease the contiguity between the solid globules as a consequence of the decrease of the solid-liquid interfacial energy σsl, was investigated. It is in particular shown that the amount of entrapped liquid is much larger in the Ba-containing alloy in agreement with the reduction of σsl, whereas the size of the liquid pockets is similar. In addition characterization of the interglobular liquid shows that the interface area between this liquid and the solid per unit volume is larger for the alloys containing Ba in agreement with previous observations carried out on 2D sections. The influence of strain during cold rolling is also reported but it is shown to have a quite limited influence on the previous parameters.

Influence of the Lost Foam Casting Microstructure on Low Cycle Fatigue Damage of A319 Aluminum Alloy

In cast aluminum alloys used in the automotive industry the microstructure inherited from the foundry process has a strong influence upon the fatigue behavior. In the cylinder heads produced by the Lost Foam Casting process, the microstructure consists of hard intermetallic phases and large gas and microshrinkage pores. In order to study the influence of this complex 3D microstructure on fatigue crack initiation and propagation, an experimental protocol using laboratory and synchrotron tomography, Finite Element simulation and 3D Digital Volume Correlation has been used. Full field measurements at the microstructure scale were performed during a low cycle fatigue test at room temperature performed in situ under synchrotron X-ray tomography (TOMCAT beamline, SLS). Synchrotron tomography allowed characterizing the eutectic Al-Al2Cu, iron based intermetallics phases and above all eutectic Si, which could not be distinguished with laboratory tomography; these constituents were proved a suitable natural speckle for Digital Volume Correlation.