The transition mode of dilute binary alloys during directional solidification near to the absolute stability limit

Abstract

The morphological evolution near the absolute stability limit during directional solidification has been studied systematically on dilute Al–Mn alloys. It is found that the interfacial morphology of Al–0.52wt%Mn and Al–1.2wt%Mn alloys changes from coarse cellular structure to fine cells, and then again to be coarsened with the increase of velocity to near the absolute stability limit. This indicates that there exists a minimum cell spacing corresponding to the maximum effective constitutional supercooling. As the growth rate approximates to or exceeds the critical velocity of absolute stability by calculation according to M–S theory, the interfacial morphology of Al–0.52wt%Mn alloy may still retain a cellular structure. For Al–1.2wt%Mn alloy, when the growth velocity is near the absolute stability limit, the fine cells may change to a band or grain-like structure which in some cases takes an oscillating manner, which possibly implies the existence of a non-linear effect during high growth rate.