The Formation and Hardening Mechanisms of Multiscale Gradient Structures Tailoring by Explosive Hardening and Annealing in Commercially Pure Titanium

Gradient structures (GS) introduced by plastic deformation have been shown to effectively enhance the surface properties of metallic materials. However, the GS fabricated by existing methods often suffers from limited hardening depth and limited control over the GS layer, which constrain their engineering applications. In this study, a combined approach of explosive hardening (EH) followed by subsequent annealing was employed to fabricate and tailor multiscale GS in commercially pure titanium (CP Ti), aiming to improve its surface hardness. By systematically varying the annealing temperature, gradient microstructures with depths ranging from 1.4 to 3 mm can be achieved with maximum surface microhardness values between 1.99 and 3.3 GPa. This EH + annealing method demonstrates competitive advantages in both achievable hardness and the depth of the GS layer. Microstructural characterization along the depth reveals gradients in grain size, twin density, dislocation density, and recrystallization degree. Notably, a unique bimodal structure consisting of coarse grains surrounded by nanograins is formed in localized regions due to partial recrystallization. The recrystallization extent, which decreases with depth due to a decreased density of microdefects, can be controlled by the annealing temperature. A quantitative model correlating the GS parameters with microhardness was established, and its predictions showed good agreement with experimental data. The analysis indicates that twins and grain refinement are the primary hardening mechanisms near the EH surface layer, whereas grain refinement combined with dislocations contributes to the hardening in deeper regions.