Optimizing strength and ductility synergy achieved by multistage strain hardening in gradient recrystallized pure titanium

Metallic materials with gradient nanostructure (GNS) have attracted much attention for their enhanced strength and ductility synergy. However, GNS materials still suffer from poor strain hardening, especially in extremely refined grains. In this work, we performed an ultrasonic surface rolling process (USRP) and post-deformed annealing for fabricating a gradient recrystallized (GRS) pure titanium. A gradient of grain size and recrystallization is presented, resulting in a new hardness gradient with a maximum value at the subsurface layer. The sequential activation of hetero-deformation-induced (HDI) hardening and mechanical-induced grain growth synergistically contributed to the multistage strain hardening behavior, leading to a considerable strength and ductility synergy. This work may expand the possibilities for optimizing the mechanical properties in the GNS materials by tailoring the recrystallized structure.