New insight on acoustoplasticity - Ultrasonic irradiation enhances subgrain formation during deformation

Many industrial applications make use of ultrasonic vibration to soften metals. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deformation resistance or mechanism is assumed to be unaltered by the ultrasound. In this study, indentation experiments performed on aluminum samples simultaneously excited by ultrasound reveal that the latter intrinsically alters the deformation characteristics of the metal. The deformation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. The softening effect of the ultrasound is found to constitute recovery associated with extensive enhancement of subgrain formation during deformation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deformation, the enhanced subgrain formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the enhanced subgrain formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Such effects of the ultrasound are interpreted by its ability to enhance dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation.