Consideration of the Timing Effect of Dendrite Pinch-Off on Dendrite Fragmentation During Directional Solidification of Superalloys

Abstract

Single-crystal superalloys play a vital role in aerospace and power generation due to their superior mechanical properties at high temperatures. However, the formation of freckles during the casting process is highly detrimental. The freckle is characterized by a trace of small misoriented spurious (equiaxed) grains with a local accumulation of eutectics. The primary source for the spurious grains is considered to be dendrite fragmentation. It is known that the interdendritic flow in the dendrite growth direction promotes dendrite remelting, creating favourable conditions for dendrite fragmentation. Therefore, a flow-driven fragmentation model was primarily proposed, assuming that the fragmentation occurs when the following criterion,\(\vec{u}_l-\vec{u}_c\cdot\nabla c_l<0\), is fulfilled. Although the above model has considered global transport phenomena and their impacts on the local thermodynamic condition for the dendrite remelting, some other microscopic events influencing the fragmentation were ignored or simplified. One such event is the timing effect of dendrite pinch-off. In this conference contribution, a modification to the previous flow-driven fragmentation model was suggested. In addition to the above flow-driven remelting criterion, a second condition, i.e., the necessary time for the remelting(t_r) of the dendrite roots to allow the pinch-off (\(\tau\)) to occur, is applied to the model. An improved simulation-experiment agreement is achieved in terms of both the distribution of the segregation channel and the formation of spurious grains. The mechanisms for the onset of segregation channels and the production of spurious grains are studied.