STRAIN VISUALIZATION OF THE ONSET OF MORPHOLOGICAL INSTABILITY AND DEFECT FORMATION IN CELLULAR SOLIDIFICATION OF A DILUTE AL-CU ALLOY

The evolution of a cellular interface from the threshold of instabilit y until the formation of deep cells is studied in situ during the dire ctional solidification of thin samples of a dilute Al-Cu alloy by usin g the real-time white-beam synchrotron X-ray topography method. This m eans that strain visualization provides a powerful tool to access the very birth of morphological instability, even at times before classica l optical methods become applicable. Indeed. elastic strains that give specific contrasts on the topographs precede visible interface destab ilization. These contrasts can be attributed to the strains induced by the selective enhancement of fluctuations at the solid-liquid interfa ce. Then, the strain contrasts and the evolution of the dislocations b ound to the cellular microstructure are analysed for the different cel lular patterns obtained near the threshold and somewhat above. In part icular, it is shown that the cellular bodies are always strained due t o the non-uniform composition field in the solid generated by the cell ular shape. (C) 1997 Acta Metallurgica Inc.