ROD-LAMELLA TRANSITION IN DIRECTIONALLY S OLIDIFIED NI-W EUTECTIC ALLOY

The mechanism of rod to lamella transition with increasing growth rate was studied. The relationship between gamma phase [100] direction and the alpha-phase microstructure was examined using the back-reflection Laue method. The influence of growth rate on the transition was also estimated by increasing the rate in the middle of directional solidifi cation. The experimental results revealed that both the growth rate an d the deviation of gamma [100] from the growth direction amplified the aspect ratio of the transverse alpha-phase microstructure. A mechanis m of the transition was proposed and summed up as follows. At first, w e assume that the following two control factors exist for the decision of the microstructure: (1) tendency to organize low energy habit plan es. (2) eutectic second phase tends to grow perpendicular to the S/L i nterface (parallel to the growth direction). If no confliction between the two tendencies occurs at the S/L interface, a rod structure will be formed. However, when the derivation of gamma [100] from the growth direction exists, the rod structure will induce the confliction betwe en the two tendencies proportional to the growth rate. In such a case, in order to avoid the confliction, the lamella structure will become metastable with an increase in alpha/gamma interfacial energy and unde rcooling.