Understanding rapid solidification behavior at velocities relevant to additive manufacturing (AM) is critical to controlling microstructure selection. Although in-situ visualization of solidification dynamics is now possible, systematic studies under AM conditions with microstructural outcomes compared to solidification theory remain lacking. Here we measure solid-liquid interface velocities of Ni-Mo-Al alloy single crystals under AM conditions with synchrotron X-ray imaging, characterize the microstructures, and show discrepancies with classical theories regarding the onset velocity for absolute stability of a planar solid-liquid interface. Experimental observations reveal cellular/dendritic microstructures can persist at velocities larger than the expected absolute stability limit, where banded structure formation should theoretically appear. We show that theory and experimental observations can be reconciled by properly accounting for the effect of solute trapping and kinetic undercooling on the velocity-dependent solidus and liquidus temperatures of the alloy. Further theoretical developments and accurate assessments of key thermophysical parameters – like liquid diffusivities, solid-liquid interface excess free energies, and kinetic coefficients – remain needed to quantitatively investigate such discrepancies and pave the way for the prediction and control of microstructure selection under rapid solidification conditions.

了解与增材制造 (AM) 相关的速度下的快速凝固行为对于控制微观结构选择至关重要。虽然就地凝固动力学的可视化现在是可能的，与凝固理论相比，在 AM 条件下具有微观结构结果的系统研究仍然缺乏。在这里，我们使用同步加速器 X 射线成像测量 AM 条件下 Ni-Mo-Al 合金单晶的固液界面速度，表征微观结构，并显示与关于平面固液绝对稳定性的起始速度的经典理论的差异界面。实验观察表明，细胞/树枝状微结构可以在大于预期绝对稳定极限的速度下持续存在，理论上应该出现带状结构形成。我们表明，通过适当考虑溶质捕获和动力学过冷对合金的速度相关固相线和液相线温度的影响，可以协调理论和实验观察。仍然需要对关键热物理参数（如液体扩散系数、固液界面过剩自由能和动力学系数）进行进一步的理论发展和准确评估，以定量研究这些差异，并为快速凝固条件下微观结构选择的预测和控制铺平道路.