Stresses and defects in the formation of a cellular pattern in directional solidification. Real-time observation by synchrotron X-ray topography on a Al-0.73 wt% Cu alloy

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Abstract
In situ observations by the synchrotron white beam X-ray topographic method are carried out during the directional solidification of thin samples (0.2-0.3 mm) of a Al-0.73 wt% Cu alloy. Stresses associated to morphological instability and pattern formation are analysed. Due to compressions and dilatations induced by the local curvature of the solidification front (Laplace law), strain contrasts in crystal parts close to the interface appear in the early stages of amplification of perturbations. Advection of dislocations into the grooves and subboundaries outcropping are evidenced for a cellular solid-liquid interface. Two types of strains are left in the grown solid by a cellular microstructure, related to nearly periodic emission of droplets and high gradients of solute, which locally create regions with important parametric mismatch. The time evolution of average cell spacing and amplitude is studied. It follows from the variation of cell spacing with growth speed that the present series of experiments are straddling the finite amplitude cellular and deep cellular regimes

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