Formation mechanism and mechanical properties of surface nanocrystallized Ti-6Al-4V alloy processed by surface mechanical attrition treatment

In the present work,surface mechanical attrition treatment(SMAT)was proposed to achieve surface nanocrystallization on Ti-6Al-4V surface via high-energy planetary ball milling method using a planetary ball mill.The characteristics of microstructure were studied using different methods.Surface nanocrystallization is achieved on Ti-6Al-4V substrate.The process of refinement could be summarized into four steps.During the refinement process,the reticular continuous beta phase performs a significant role,it cracked,broke up and moved to each side along the grain boundaries.The movement of beta grains has the capabilities of effectively optimizing the grain orientation and accelerating the further refinement of alpha grains.Twinning also plays an important role during the refinement.The grain orientation between different types of grains seems to be larger than that of same type grains.The interface will be divided into smaller nanocrystalline grains once the dislocation density breaks the threshold.Then,the balance will be achieved again and owns a higher critical value which cannot be broken,then a stable grain size can be achieved ultimately.The results of microhardness,friction coefficient and wear mass loss tests of SMAT samples indicate that the mechanical behaviors of substrate are greatly enhanced after this novel SMAT treatment.