Engineering bi-directional microstructural heterogeneity through mechanical surface treatment

By integrating regions with distinct mechanical properties, heterogeneous materials can offer a wide range of mechanical properties and may even surpass the mechanical and functional properties observed in their homogeneous counterparts. Impact-based mechanical surface treatments have proven highly effective in generating microstructural heterogeneity by introducing severe plastic deformation at the surface and subsurface layers, resulting in gradient grain refinement and microstructural modifications while preserving the original properties of the core material. Herein, we employed a strategically modulated ultrasonic shot peening (USP) technique to engineer materials exhibiting bi-directional heterogeneity, i.e., both planar (across the specimen surface) and transversal (in-depth), specifically by creating gradients in both grain size and geometrically necessary dislocation (GND) density. The innovative localized treatment uses masks of various geometric patterns to selectively shield or expose designated regions of the specimens during USP treatment. Tensile tests performed on the USPed specimens evidenced modulated material strengthening, supported by site-specific variations in microstructural features, microhardness, and residual stress distribution. The findings indicate that this novel USP strategy can be implemented to enable engineering materials with heterogeneous structures and tailored mechanical characteristics.