Twin formation and its strengthening mechanism of pure titanium processed by cryogenic laser peening

•Cryogenic laser peening (CLP) could improve higher microhardness.•CLP induced higher-density of dislocations and deformation twins.•More beneficial microstructure induced by CLP contributed to higher microhardness. The aim of this present study was to explore the effects of room temperature laser peening (RT-LP) and cryogenic laser peening (CLP) on the microstructural characteristics and microhardness of pure titanium (Ti). The in-depth microhardness of the specimens subjected to different treatments were measured. The microstructural evolution was characterized by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Experimental results showed that compared with RT-LP, the microhardness improvement caused by CLP was higher. Moreover, CLP induced higher-density of mechanical twins and dislocation structures. Compared with RT-LP specimen, the proportion of the near-surface twin boundary of the CLP specimen increased by 133.3%. During the process of CLP, the ultra-low temperature can inhibit the dynamic recovery and annihilation of the dislocation structures induced by laser shock wave, thus leading to higher dislocation density. Additionally, cryogenic temperature can also suppress the motion and slip of dislocation structures, which is more conducive to the formation of mechanical twins.