A review on the influences of high speed impact surface treatments on mechanical properties and microstructures of metallic materials

Abstract

The strain rate during the process of high speed impact surface treatments has a significant effect on the mechanical properties as well as the microstructures of metallic materials. In this paper, the effects of strain rate during the process of high speed impact surface treatments on the variation of both strength and ductility of metallic materials are reviewed from macroscopic and microscopic prospective based on the current research achievements. The emphases are concentrated on the microstructural evolution under various strain rates, including grain structures, adiabatic shear bands, phases, dislocation structures, precipitates and deformation twins. At relatively low strain rates, grains tend to be elongated with respect to the loading direction, and they may be refined when the strain increases to a certain extent. In comparison, with the increment of strain rates, the free path of dislocation motion is remarkably reduced so that grains can be further refined to consume the impact energy and dislocations are multiplied significantly. However, the relatively high strain rates may also bring about adiabatic temperature rise and frictional heat, which may give rise to dynamic recovery and recrystallization in some materials so that the dislocation density would in turn be reduced. Moreover, precipitates can be formed and they may interact with dislocations owing to the combined effects of high strain rates and temperature rise. When the strain rates increase to the extremely high level, the movement of dislocations may be inhibited and deformation twins can be triggered to coordinate the deformation. As a result, the strain rate effects are complicated phenomena which comprehensively affect the microstructural strengthening and softening effects. Based on these, the influences of both microstructural evolution and the transition of microscopic deformation mechanisms with strain rates on the enhancement and deterioration of mechanical properties are analyzed. Finally, the characteristics of deformation mechanisms of the gradient microstructures derived from high velocity impact surface treatments are concluded. Furthermore, a comprehensive model embodying the influences of different microstructures is proposed, which can provide a foundation for the further researches of strain rate effects.