PREFERRED-PATTERN FORMATION DURING THE INITIAL TRANSIENT IN CELLULAR SOLIDIFICATION

Critical information on the pending problem of preferred-pattern forma tion and wavelength selection in cellular directional solidification i s currently expected from the study of initial transients. Therefore, experiments were carried out in lead-thallium alloys to analyse the dy namical process by which a periodic array of cells is formed by using two classical but different experimental procedures, one violent and o ne soft. The statistical analysis of pattern characteristics (average wavelength, level of disorder) shows that both procedures converge tow ards a common asymptotic state, which means that a preferred pattern d oes exist. Although different, the initial transients are qualitativel y similar. It is found that in both cases, the solid-liquid interface is first dendritic and then restabilizes into a cellular array. Oscill ations, in periodicity and in the sense of disorder, are observed, tha t are associated to the repetition of phases of cell divisions, probab ly by tip-splitting, separated by rearrangement periods.