Gradient nanostructure, phase transformation, amorphization and enhanced strength-plasticity synergy of pure titanium manufactured by ultrasonic surface rolling

In this study, ultrasonic surface rolling process (USRP) was used to fabricate a gradient nanostructured commercial pure titanium. The site-specific microstructure, refining mechanisms and mechanical properties were investigated by high-resolution transmission electron microscopy, electron backscatter diffraction and tensile test. The results show that the surface layer exhibited multi-gradient features along the depth, including grain size gradient, deformation twin gradient, strain gradient, orientation gradient and hardness gradient. The gradient structure consisting of equiaxed nanograin layer, elongated lamellar layer, deformation twinning profuse layer, and coarse-grained layer along the depth direction. The formation mechanisms of gradient nanostructure are thoroughly elucidated, and the synergistic effect of twin and dislocation plays an important role in grain refinement. In addition, amorphous bands (generated by localized dislocations or phase transformation triggered crystalline to amorphous transition) and nano-thick lamellar with face-centered cubic (FCC) structure (<0001>_HCP//<001>_FCC, {1 1¯ 00}_HCP//{2 2¯ 0}_FCC and {11 2¯ 0}_HCP//{220}_FCC) were formed in the nanograin layer, and the phase transformation mechanisms were elucidated. The HCP to FCC phase transformation and amorphization play a role in refining grain. Tensile test results suggested that the gradient nanostructure improved the strength (yield strength and ultimate tensile strength improved from 342 MPa and 526 MPa to 412 MPa and 598 MPa, respectively) while maintaining enough plasticity. The strength-plasticity synergy was attributed to the coordinated deformation of the gradient nanostructure and the coarse-grained core.