In situ mapping of interfacial strain and crystallographic evolution in SS316L-IN625 wire-DED bimetals

Wire-laser directed energy deposition enables the fabrication of heterostructured dissimilar-metal architectures with spatially tailored properties, enabling structural components that integrate varied performance requirements within a single assembly. In such bimetallic structures, global mechanical behavior is strongly influenced by the chemically and microstructurally heterogeneous transition zone. However, conventional ex-situ approaches cannot directly capture the coupled evolution of macroscopic response, local strain partitioning, and crystallographic change during loading. This study investigated SS316L-IN625 wire- directed energy deposition (DED) bimetals, selected as a widely used dissimilar-metal combination for balancing material economy and performance, using a correlative in-situ approach combining scanning electron microscopy (SEM) tensile testing, digital image correlation (DIC), and staged electron backscatter diffraction (EBSD). During tensile deformation, the bimetal exhibited a lower flow stress than both monolithic counterparts, indicating that the global response is governed by deformation interactions within the heterogeneous transition region rather than by a simple average of the constituent responses. Full-field strain mapping revealed pronounced strain partitioning, with preferential accumulation on the more compliant SS316L side and increasingly steep gradients toward the interface during deformation. Crystallographic analysis further showed progressive increases in the low-angle grain boundary (LAGB) fraction and mean kernel average misorientation (KAM), consistent with grain subdivision and increasing local deformation storage. Together, these observations indicate that the transition zone is a mechanically active region that redistributes plastic strain toward the SS316L side during loading. These results clarify how a heterogeneous transition zone accommodates strain, providing a mechanistic basis for the design and deployment of advanced structural multimaterial metallic systems.