Optimization of the strain rate to achieve exceptional mechanical properties of 304 stainless steel using high speed ultrasonic surface mechanical attrition treatment

Surface mechanical attrition treatment (SMAT), an efficient way to create nanostructured surface/subsurface layers, has been extensively exploited in the last decade. However, the impact velocity of the balls in the treatment has not yet been measured in detail. The motivation of the present paper was to investigate the ball velocity and the effect of the number of balls on the resulting mechanical properties and the associated microstructures. Employing a high-speed camera, the maximum impact velocities of balls were quantified. This velocity is affected by the density and size of the ball. In the present paper an optimum number of balls for SMAT was also identified. With a detailed knowledge of the ball velocity we were able to accurately estimate the strain rate at different depths by analytical modeling and to study the correlation between the resulting microstructures and the strain/strain rate history of the material.