Deformation-Induced Formation of Stray Grains in Additive Manufacturing of Single Crystals.

The formation of stray grains (SGs) remains a critical and pervasive challenge hindering the additive manufacturing (AM) of single crystals for high-temperature aerospace applications. Here, we elucidate the mechanism underlying SG formation during the AM of Ni-based single-crystal alloys, through integrating in situ synchrotron imaging/diffraction, ex situ characterization, and multi-physics modeling. In contrast to the conventional understanding that attributes SG formation solely to thermal effects, we demonstrate that SG originates from subgrain rotation driven by heterogeneous dislocation activity. We further reveal that dislocation-induced SG formation can be regulated by substrate orientations, in which the Gini coefficient derived from dislocation distributions is proposed to serve as the physics-based predictive metric for SG susceptibility. Specifically, high-symmetry orientations exhibiting low Gini coefficients suppress SGs via more uniform dislocation distribution. This study advances the understanding of SG formation under extreme nonequilibrium solidification processes, thereby guiding the fabrication of high-quality AM single-crystal components for aerospace applications.