Effect of repeated solid impact on bulk metallic materials

The controlled increase in dislocation density and grain refinement can improve hardness and retard the fatigue crack nucleation. Therefore, methods such as shot peening, waterjet peening, laser shock peening, rolling, and many others are used. Degrading processes such as fretting wear, sliding wear, or erosion cause similar material responses, including an increase in dislocation density, the creation of low-angle boundaries, and, in some cases, including this study, grain refinement. The refined grain layer is usually described by terms such as tribologically transformed structure, mechanically attrited structure, or mechanically modified superficial structures based on the studied materials and mechanisms. This paper aims to provide a detailed quantitative study of bulk material behavior during the repeated local impacts of solid indenter. Three distinct samples - Al-based, Ti-based, and Fe- based alloys were impacted with spherical tungsten carbide indenter by the impact load of 200 N. The number of impacts ranged from similar to 100 to similar to 105. The depth and volume of the craters, as well as the height and volume of the material pushed above the initial surface (pile-ups), were obtained using confocal microscopy. An energy-based approach was used to calculate dynamic impact resistance of materials. The surface changes were observed by SEM. The formation of nano-grain and transition layers and mechanisms of material removal were studied using TEM lamellae extracted from the specimens. The subsurface dislocation structure was compared and discussed in the three materials. The material removal mechanism during repeated impact was proposed based on TEM and SEM observations of the subsurface and surface.