STRESSES AND DEFECTS IN THE FORMATION OF A CELLULAR-PATTERN IN DIRECTIONAL SOLIDIFICATION - REAL-TIME OBSERVATION BY SYNCHROTRON X-RAY TOPOGRAPHY ON A AL-0.73WT-PERCENT CU ALLOY

In situ observations by the synchrotron white beam X-ray topographic m ethod are carried out during the directional solidification of thin sa mples (0.2-0.3 mm) of a Al-0.73 wt% Cu alloy. Stresses associated to m orphological instability and pattern formation are analysed. Due to co mpressions and dilatations induced by the local curvature of the solid ification front (Laplace law), strain contrasts in crystal parts close to the interface appear in the early stages of amplification of pertu rbations. 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 microstruct ure, related to nearly periodic emission of droplets and high gradient s of solute, which locally create regions with important parametric mi smatch. The time evolution of average cell spacing and amplitude is st udied. It follows from the variation of cell spacing with growth speed that the present series of experiments are straddling the finite ampl itude cellular and deep cellular regimes.