Effects on plastic deformation by high-frequency vibrations on metals

Abstract

The effect of softening due to vibrations induced on metals has been used in many industrial processes such as forming, machining and joining. These industrial applications utilize ultrasonic vibrations in addition to quasi-static stresses in order to deform metals more easily. The phenomenon of ultrasonic softening is also called the Blaha effect or acoustoplastic effect. Besides the macro-scale softening due to ultrasonic vibrations imposed on quasi-static deformation stress, sub-micron level softening due to vibrations was also observed in nanoindentation experiments in recent years. These experiments made use of the oscillatory stresses of the vibrations provided by the continuous stiffness measurement (CSM) mode of nanoindentation. Lowering of loading and hardness data has been observed at shallow indent depths where the amplitude of vibration is relatively large. Despite the common industrial usages of acoustoplastic effect and the observation of softening in CSM mode nanoindentation, the physical principle underlying is still not well understood. For acoustoplastic effect the existing understanding is usually one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal. For the softening observed in CSM mode nanoindentation, the effect is either attributed to instrumental errors or enhancement of nucleation of dislocations which makes them move faster. Investigations on the link between microscopical changes and the softening have been rare. In this thesis, indentation experiments in both macro and micro scales were performed on aluminium, copper and molybdenum samples with and without the simultaneously application of oscillatory stresses. Significant softening was observed, and the amount of softening from macro to micro scale indentation of similar displacement/amplitude ratios is similar. 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. Electron microscopy analyses reveal subgrain formation under the vibrated indents, which implies intrinsic changes. To further give physical insight into the phenomenon, dislocation dynamics simulations were carried out to investigate the interactions of dislocations under the combined influence of quasi-static and oscillatory stresses. Under a combined stress state, dislocation annihilation is found to be enhanced leading to larger strains at the same load history. The simulated strain evolution under different stress schemes also resembles closely certain experimental observations previously obtained. The discovery here goes far beyond the simple picture that the effect of vibration is merely an added-stress one, since here, the intrinsic strain-hardening potency of the material is found to be reduced by the oscillatory stress, through its effect on enhancing dislocation annihilation. The experimental and simulation results collectively suggest that simultaneous application of oscillatory stress has the ability to enhance dipole annihilation and cause subgrain formation. The superimposed oscillatory stress causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation. In addition, microscopic observations showed that subgrain formation and reduction in dislocation density generally occurred in different metals when stress oscillations were applied. These suggest that the intrinsic oscillation-induced effects of softening and dislocation annihilation are a rather general phenomenon occurring in metals with different stacking fault energies and crystal structures.