Crystallographic Origins of Interfacial Incompatibility and Failure Mechanisms in Ultrasonic Additive Manufactured Ti/Al Laminated Composites

Premature interfacial failure limits the mechanical integrity of Ti/Al laminates fabricated by ultrasonic additive manufacturing. To clarify its crystallographic origin, in situ scanning electron microscopy and electron backscatter diffraction were used to track deformation and damage evolution. The Al layer accommodates most plastic strain through lattice rotation, subgrain fragmentation, and recovery, whereas the Ti layer shows limited rotation and stronger interfacial constraint. Slip transfer analysis based on the Luster Morris parameter reveals that the fraction of highly compatible interfacial segments with m′ ≥ 0.77 decreases to ~15% at high strain. Accordingly, the Ti‐side of the interface develops elevated lattice curvature and geometrically necessary dislocation (GND) density, indicating compatibility induced lattice bending and constrained deformation within the Ti boundary layer rather than conventional dislocation pile up in the softer Al. The resulting GND associated hardening increment exceeds 300 MPa locally in Ti but remains below ~70 MPa in Al. Interfacial microcracks initiate at ~20% strain, evolve into delamination by ~30%, and are followed by Al necking and final failure at ~40%. These findings show that interfacial weakness and crystallographic incompatibility jointly govern fracture by amplifying local strain gradients and Ti‐side hardening near the interface.