Unraveling dislocation-type evolution dominated strain hardening in laminated Ti/Nb composites

Dislocation behavior in laminated metallic composites (LMCs) plays a pivotal role in strength and ductility of bulk materials. Here, we studied evolutions of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) as well as their effects on strain hardening in the uniaxially deformed Ti/Nb LMCs fabricated by accumulative roll bonding plus the subsequent annealing. By combining in-situ neutron diffraction and ex-situ electron backscattered diffraction (EBSD) techniques, for the first time, we quantitatively reveal that dislocation evolutions in the laminates are independent of initial layer thickness within the micron scale. As the applied strain reaches the uniform elongation stage, GND density in Ti increases remarkably by an order of magnitude in all laminates, while that in Nb almost remains unchanged. Besides, the total dislocation density in Ti develops rapidly with deformation, whereas the SSD density does not increase monotonically. In Nb, both total dislocation density and SSD density increase significantly below 0.02 true strain, and gradually saturate in the late stage of strain hardening. During the whole plastic deformation, Nb bears more stress and dominates the global hardening because of the significant development of SSDs in this metal. Importantly, the strengthening induced by heterophase interfaces is more significant in the softer Ti in which GNDs are extensively activated, and SSDs only marginally contribute to the heterophase interface strengthening. These findings provide insights into the exploration of deformation mechanisms in materials with laminated and gradient structures, and also guide the development of LMCs with advanced mechanical properties.