镍基单晶高温合金经激光熔凝后，熔池底部易出现杂晶。根据杂晶形成理论，杂晶形成能力反比于柱状晶前沿的温度梯度和柱状晶生长速率的比值，熔池底部因具有高温度梯度和低凝固速率，理论上不利于杂晶形成。熔池底部碳化物提供形核点是杂晶形成的关键因素，但具体机制尚不清楚。为了明确熔池底部碳化物诱导杂晶形成的具体机制，本工作选用C含量高的镍基单晶高温合金DD32为对象，结合组织表征和数值计算研究了碳化物对激光重熔熔池底部杂晶形成的作用机制。结果表明，熔池底部杂晶的形成与碳化物(TaNb)C相关，该碳化物来源于基体本身且在重熔过程中难以完全熔化和溶解，在重力作用下会沉降到熔池底部，增大底部形核点密度。将形核点密度变化与杂晶体积分数计算模型相结合，定量阐明了碳化物导致熔池底部形核点密度增大从而促进杂晶形成的作用机制，为单晶修复过程中的杂晶控制提供了理论指导。

Nickel-based single-crystal superalloys are widely utilized for manufacturing turbine blades in aerospace engines owing to their excellent oxidation and hot-corrosion resistance. However, these components are susceptible to various types of damage under high-temperature conditions, and their high replacement cost makes repair essential. Laser additive manufacturing (LAM) is commonly used for repairing single-crystal blades; however, it often introduces stray grains, which form grain boundaries that degrade the alloy's properties. Therefore, suppressing stray grain formation is a critical objective in the laser repair process. During laser melting, nickel-based single-crystal superalloys are highly prone to stray grain formation at the bottom of the melt pool. According to the stray grain formation theory, the propensity for stray grain formation is inversely proportional to the ratio of the temperature gradient to the growth rate at the columnar dendrite front. As the melt-pool bottom has a high thermal gradient and low solidification rate, stray grain formation should theoretically be suppressed in this region; however, experimental observations yet contradict this prediction. Carbides may act as potent nucleation sites and play a critical role in facilitating stray grain formation at the melt-pool bottom; however, the detailed mechanism remains elusive. To clarify this mechanism, laser remelting was performed on DD32, a high-carbon nickel-based single-crystal superalloy produced in China. Particulate inclusions within the stray grain area at the melt-pool bottom were detected via optical microscopy and subsequently identified as MC-type carbides (TaNb)C via scanning electron microscopy and energy-dispersive X-ray spectroscopy. Their considerable size difference from that of carbides reprecipitated after remelting indicates that they originate from the alloy matrix, and they exhibit resistance to complete melting or dissolution during the remelting process. As these carbides have a considerably higher density than the alloy melt, they settle to the melt-pool bottom due to gravity, thereby increasing the nucleation site density and promoting the formation of stray grains. By integrating electron probe microanalysis (EPMA) with numerical simulation, this study quantitatively elucidated how carbide-induced nucleation sites increase nucleation density at the melt-pool bottom and promote stray grain formation. These findings explain the phenomenon of stray grain formation at the bottom of the laser-melted pool in nickel-based single-crystal superalloys and provide a theoretical basis for controlling stray grains in future single-crystal repair processes.