Tailoring tensile ductility of thin film by grain size graded substrates

Extensive experiments have shown that thin metal or metallic glass films usually rupture under a small tensile strain, which is extremely unfavorable for their engineering applications. The introduction of a highly deformable substrate with homogeneous microstructures can enhance the ductility of the films. However, the strong mechanical contrast between the substrate and the film usually induces interface debonding and premature cracks at the interface. Another disadvantage of the above film/substrate system is its significantly reduced strength due to the low strength of the homogeneous substrate. Recently, experiments have shown that a heterogeneous substrate with a grain size graded microstructure is able to provide an exceptional strength-ductility synergy in the film/substrate system. However, why does the graded substrate render such outstanding properties remains unsolved due to its complex microstructure. In order to answer this question, here a dislocation density-based constitutive model was incorporated into a finite element scheme to investigate the deformation of a thin film attached to a graded substrate. The deformation of the film on a normal substrate with a homogeneous grain microstructure was also analyzed for comparison. A V-shaped notch was adopted in the film to model its necking behavior based on a necking criterion. The necking strain is adopted as the tensile ductility. The results show that three typical deformation behaviors can be triggered by tailoring the microstructure of the graded substrate including the grain size of the topmost surface layer and the thickness of the grain size graded region. The three modes correspond to very different necking strains. Interestingly, the graded substrate with an optimal microstructure can provide the film with much higher necking strain than the homogeneous coarse-grained substrate could although the former itself is less ductile than the latter. In addition, the film/graded-substrate system possesses much better strength-ductility synergy than the film/homogeneous-substrate system. The enhancement of the tensile ductility results from the strong strain delocalization effect induced by the graded substrate. The predicted stress-strain responses are also compared with available experimental measurements.