Thermal-solutal convection-induced low-angle grain boundaries in single-crystal nickel-based superalloy solidification

Low-angle grain boundaries (LAGBs) are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties. The formation of LAGBs is related to dendrite deformation, while the mechanism has not been fully understood at the mesoscale. In this work, a model coupling dendrite growth, thermal-solutal-fluid flow, thermal stress and flow-induced dendrite deformation via cellular automaton-finite volume method and finite element method is developed to study the formation of LAGBs in single crystal superalloys. Results reveal that the bending of dendrites is primarily attributed to the thermal-solutal convection-induced dendrite deformation. The mechanical stress of dendrite deformation develops and stabilises as solidification proceeds. As the width of the mushy zone gets stable, stresses are built up and then dendritic elastoplastic bending occurs at some thin primary dendrites with the wider inter-dendritic space. There are three characteristic zones of stress distribution along the solidification direction: (i) no stress concentration in the fully solidified regions; (ii) stress developing in the primary dendrite bridging region, and (iii) stress decrease in the inter-dendritic uncontacted zone. The stresses reach maximum near the initial dendrite bridging position. The lower temperature gradients, the finer primary dendritic trunks and sudden reductions in local dendritic trunk radius jointly promote the elastoplastic deformation of the dendrites. Corresponding measures are suggested to reduce LAGBs.