Grain Size Governs Mechanical Properties of Roll-Bonded C7701/Ti/C7701 (Cu–Ni–Zn Alloy) Composite Foils via a Bonding–Diffusion–Intermetallic Cascade

Grain size plays a decisive role in governing the interface evolution and mechanical properties of ultra-thin metal composite foils. This study systematically investigates this relationship in roll-bonded C7701/Ti/C7701 (Cu-Ni-Zn alloy) composite foils. By controlling the initial grain size via pre-annealing, we demonstrate that a moderate grain size (~7–8 μm) optimally regulates a sequential “bonding–diffusion–intermetallic compound (IMC) formation” process at the interface. This results in a continuous, thin IMC layer and the best strength–ductility synergy (e.g., UTS ~217.5 MPa, elongation ~4.15%). In contrast, excessively fine or coarse grains lead to thick, brittle IMCs or interfacial defects, respectively, degrading performance. The mechanism by which grain size influences performance is revealed through a sequential mechanism of “bonding–diffusion–intermetallic compound formation.”