The effect of tensile deformation by in situ ultrasonic treatment on the microstructure of low-carbon steel

Abstract The softening effect in metals due to ultrasonic vibration is used in many industrial applications. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic treatment either imposes additional stress waves to supplement the quasi-static applied load or causes heating of the metal. In both cases the intrinsic deformation resistance and/or mechanisms of the metal are assumed to be unaltered by the ultrasound. In this study, the effect of an in situ ultrasonic treatment on the microstructure of low-carbon steel (Fe–0.051C–0.002Si–0.224Mn–0.045Al (wt.%)) under tensile deformation is reported. Detailed microstructural analyses reveal that the ultrasonic treatment intrinsically alters the deformation characteristics of the metal. The deformation microstructure underneath the area of treatment in the deformed samples was investigated by a combination of optical microscopy, scanning electron microscopy, crystal orientation mapping by electron backscattered diffraction and X-ray diffraction. The results show that the dislocation density and the fraction of low-angle grain boundaries decrease significantly, accompanied by preferential grain rotation. The softening effect of the ultrasound is found to drive recovery associated with a significant reduction in subgrain formation during deformation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, the reduction in subgrain formation is shown to occur due to the combined application of the quasi-static loading and the ultrasound, but is not a simple addition of the two factors acting separately. The effect of the ultrasound can be attributed to its ability to enhance dislocation dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances, thereby increasing the probability of annihilation.