Quantitative mechanism of abnormal hardening behavior of Ti6Al4V alloy strengthened by ultrasonic surface rolling

In general, the outermost region of a metallic material has the highest hardness after surface mechanical strengthening. However, an abnormal surface hardening behavior was observed in Ti6Al4V alloy after the ultrasonic surface rolling process (USRP) in this work. The region with the highest surface hardening was not at the top surface but at the subsurface. By analyzing the distribution of grain size, dislocation density, texture, and kernel average misorientation (KAM) at different depths from the surface, and combining these findings with finite element analysis (FEA), the microstructural evolution underlying this abnormal surface hardening was elucidated. The microstructural characterization and FEA results indicate that the subsurface region underwent the most significant deformation. Subsequently, through the application of theoretical analysis, the potential mechanism of abnormal hardening of USRP treatment is described quantitatively for the first time. The results demonstrated that the hardening effect resulting from grain refinement was less pronounced, whereas the hardening effect resulting from dislocation pile-up was more prevalent. At the subsurface region of the USRP sample, a large number of interfaces resulted in the highest accumulation of dislocations in this area. Consequently, the subsurface region exhibited the highest microhardness, leading to the abnormal surface hardening phenomenon.