Rod-lamella transition in directionally solidified Ni-W eutectic alloy

The mechanism of rod to lamella transition with increasing growth rate was studied. The relationship between γ phase 〈100〉 direction and the α-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 solidification. The experimental results revealed that both the growth rate and the deviation of γ 〈100〉 from the growth direction amplified the aspect ratio of the transverse α-phase microstructure. A mechanism of the transition was proposed and summed up as follows. At first, we assume that the following two control factors exist for the decision of the microstructure: (1) tendency to organize low energy habit planes. (2) eutectic second phase tends to grow perpendicular to the S/L interface (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 γ 〈100〉 from the growth direction exists, the rod structure will induce the confliction between 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 α/γ interfacial energy and undercooling.