CHARACTERISTIC SUBSTRUCTURE IN DIRECTIONALLY SOLIDIFIED DILUTE Al-Cu ALLOYS

摘要

Al-0.2 wt.% Cu alloy samples were directionally solidified under similar thermal gradient conditions. As the growth velocity was increased, the transition among successive stages from planar, nodes, bidimensional cells, regular cells and irregular cells was characterized through careful met-allographic analysis of the microsegregation. During the different transitions, a nodal precipitation mechanism seems to be responsible for the evolution of the substructure, where the eutectic precipitation at the nodes plays a fundamental role in the fixation of the substructure. During planar to bands transition, the nodes slow down the interface at certain critical points of it, and the primary spacing can be well defined. This phenomenon seems to be similar during the transition of all the substructure stages under study. The primary spacing λ_1 fits the order of magnitude of maximum rate wavelength predicted by Morphological stability theory. In several cases, a second order microsegregation pattern at cellular wall level has been observed. This seems to be due to small variation in the microsegregation during the lateral growth of the cells in an enriched intercellular liquid. The secondary microsegregation pattern detected has a small periodicity λ_(min) close to the minimum unstable wavelength predicted by the Morphological stability theory.