Converging competitive growth in bicrystal of Ni-based superalloy during directional solidification

Ni-based single crystal superalloy has been widely used in turbine blades due to its excellent high temperature mechanical behavior. In order to completely exhibit high temperature mechanical properties, the seed method has been used to produce Ni-based single crystal components for [001] orientation paralleling to main force direction. Stray crystals, which unexpectedly nucleate in the melt-back region, will competitively grow with seed during directional solidification. It is important to profoundly understand the mechanism of competitive growth to find ways of overgrowing stray crystal during producing Ni-based single crystal components. However, within the published research there are conflicting views on the mechanism of competitive growth at converging case. Bi-crystal converging competitive growth was investigated in Ni-based single crystal superalloy with different pulling rates using seed technology. A series of polishing and imaging quenching interface were done for the positional relationship of dendrites near grain boundary in 3D reference. It was found that solidification microstruc tures were different with different crystal orientations. Unfavorable oriented dendrite tilting to heat flux restrained favorable oriented dendrite aligning to heat flux mainly thought inserting into the favorable oriented dendrites channel, and this resulted in unfavorable oriented dendrite overgrowing favorable oriented dendrite at low pulling rate. However, at high pulling rate the unfavorable oriented dendrites mainly blocked by grain boundary favorable oriented dendrite and the grain boundary grew paralleling to favorable oriented dendrite core. Favorable oriented dendrite being depressed and vanished, owning to that unfavorable oriented dendrite inserting into favorable oriented dendrites channel result in adjusting primary dendrite spacing, is the main factor to favorable oriented grain overgrew by unfavorable oriented grain. According to above mechanism, effect of pulling rate on competitive growth at converging case was interpreted. This could broaden our understanding of competitive growth at converging case in 3D reference.