Dendrite growth and defects formation with increasing withdrawal rates in the rejoined platforms of Ni-based single crystal superalloys

The effect of withdrawal rates on the dendrite growth and defects formation in the rejoined platforms of a Ni-based single crystal superalloy during directional solidification was studied by experimental investigations and ProCAST simulations. The results showed that the dendrite pattern in the rejoined platforms mainly comprised two regions: original primary dendrite region and lateral growth region. With increasing withdrawal rate, the lateral growth was more severe and the tendency of low angle grain boundaries (LABs) was greater because of dendrite deformation. Once the withdrawal rate exceeded 100 μm/s, local fragmentation appeared in the middle of the platforms and some ternary dendrites were detached from their parent dendrites. When the withdrawal rate was increased to 150 μm/s, lots of dendrite fragments formed and the integrity of single crystal was completely destroyed, which might be caused by the solute enrichment and was further promoted by the unstable growth conditions.