Twinning-dominated scratch mechanism transitions in commercial pure Titanium from micro to macro scales

Scratch-induced subsurface microstructure evolution and intrinsic deformation mechanisms play a decisive role in determining the scratch resistance of metallic materials. In hexagonal close-packed (HCP) metals, known for their complex twinning behavior, the microstructural origins governing scratch processes remain poorly understood. Here, we report a three-stage relationship between the wear rate and loading scale in commercial pure titanium (Ti) under single-pass scratching. Cross-sectional microstructure characterization and in-situ strain field analysis reveal dual transitions in the twinning-dominated scratch mechanisms that underpin this relationship. In the load range of 0.05 N-10 N (micro-scale), the wear rate initially declines in stage Ⅰ and then stabilizes in stage Ⅱ with increasing load, marking the first transition from periodic shear driven by surface-nucleated twinning to stable material removal facilitated by a topmost nanograined layer formed through subsurface deformation twins. In the load range of 10 N-50 N (macro-scale), the wear rate rises in stage Ⅲ, corresponding to the second transition in scratch mechanisms, where microstructure heterogeneity ahead of the indenter leads to massive delamination, associated with local nanocrystallization within the twin lamellae. Our findings offer insights into designing scratch-resistant Ti and other HCP alloys through tailoring the twin structure, with implications for improving their surface quality in response to various machining and forming processes.