The aim of this paper is to report the very first in situ observations of the deformation behaviour of an Al-Cu alloy in the semisolid state by using ultrafast, high-resolution X-ray microtomography. It is shown that this deformation is non-homogeneous and involves an accumulation of liquid at an intergranular surface nearly perpendicular to the strain axis. Once the liquid is no longer able to feed such a region, micropores form and grow at this surface, finally leading to a crack.

The microstructural evolution of an Al-10 wt.% Cu alloy was investigated during solidification at constant cooling rate by in situ synchrotron X-ray microtomography with a resolution of 2.8 lm. Solidification of this alloy leads to a coarse dendritic microstructure which was fully characterized in terms of variation with temperature of the solid fraction, the specific surface area of the solid-liquid interface and the local curvatures of the solid phase. By analysing the evolution with solid fraction of individual dendrites, at least two coarsening mechanisms were clearly identified in addition to solidification growth. The first mechanism involves remelting of small secondary dendrite arms to the benefit of bigger adjacent arms. The second is the coalescence of adjacent secondary arms, with progressive filling of the inter-arm spacing and coalescence at the tips. Although this mechanism preferentially occurs at high solid fractions, these results show that the evolution of the dendritic microstructure during solidification is complex and involves the occurrence of various mechanisms operating concurrently. In situ X-ray tomography thus allows revisiting the various models which have been proposed to account for dendrite coarsening during solidification.

Micropores formed in Al-Cu alloys cast under controlled conditions have been analyzed using high-resolution X-ray tomography. The influence of inoculation conditions, copper content, cooling rate and initial hydrogen content on the morphology of pores has been investigated. Based on the three-dimensional reconstructed shape of the pores, the distribution of curvature was estimated. It is shown that the mean curvature of pores in either non-inoculated or inoculated Al-4.5 wt.%Cu alloys can be as large as 0.35 lm À1 near the end of solidification and can be fairly well approximated by a set of interconnected cylinders growing in between the primary phase dendrites. The so-called "pinching" effect, i.e. the restriction of the pore curvature by the solid network, is a function of the volume fraction of the primary phase and of the secondary dendrite arm spacing. If the fraction of porosity is highly dependent on the initial hydrogen content, the curvature itself is only weakly influenced by this parameter. Based on these results, it is concluded that curvature plays a major role in porosity models and that the analytical pinching model developed by Couturier et al.[1] offers a fairly good and simple approximation of this contribution.

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