Ascertaining the microstructural evolution and strengthening mechanisms of the gradient nanostructured pure titanium fabricated by ultrasonic surface rolling process

This work utilized the ultrasonic surface rolling process (USRP) to construct a gradient nanostructured (GNS) layer in pure titanium. The microstructural characterization at different depths was conducted using a transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) to elucidate the grain refinement mechanisms. Results showed that the {10−12} extension twins (ETs) and {11−22} compression twins (CTs) and their intersections accommodate the strain at the early deformation stage. As the strain increased, the dense dislocation walls (DDWs) and dislocation cells (DCs) gradually evolved into laminated structures with low-angle grain boundaries (LAGBs). Compared to conventional severe plastic deformation (SPD) technology, the LAGB-dominated grain refinement mechanism delays the onset of the transformation process from LAGBs to high-angle grain boundaries (HAGBs). With the coaction of discontinuous dynamic recrystallization (dDRX) and amorphization, the grains are extremely refined to 11.7 nm. Tensile test results suggested that a considerable strength-ductility synergy is achieved. The enhanced strength was attributed to grain size, dislocation, and synergetic strengthening mechanisms, accounting for 55.5 %, 20.7 %, and 23.8 %, respectively. This work deepens the current understanding of the formation mechanism of the GNS layer in pure titanium processed by USRP and the relationship between microstructure and mechanical properties.