The Effects of Large Electric Current Pulses on Plastic Deformation and Atomic Scale Structure of Metallic Glasses

Bulk metallic glasses (BMG) exhibit peculiar combinations of properties which make them ideal materials for some niche applications as functional or structural materials, such as diaphragms for pressure sensors in automobiles, precision microgears, soft magnetic materials, etc. The development of various net-shape or near-net-shape forming technology, including different casting techniques and thermoplastic forming for producing BMG components with intricate geometries, also contributes to the realization of the applications.Thermoplastic forming involves heating the BMG to temperatures above the glass transition temperature so that the material can deform like a viscous fluid. The principal investigator (PI) of this proposed research noticed that a less well known forming process, viz. electroplastic forming or electropulsing treatment (EPT), can be a potentially better processing route for shaping BMG. According to our experience on the use of EPT on crystalline engineering alloys, EPT is a much more rapid and energy efficient method. The material can be heated by the very short electric current pulses uniformly and exhibits drastically accelerated structural changes within seconds even when the temperature of the material is only moderately increased due to the small total energy input. The accelerated migration of atoms are usually explained in terms of an 'electron wind force' on top of the joule heating effect. However, there is no detailed investigation of the effects of EPT on plastic deformation of BMG so far.The PI therefore proposes to carry out detailed investigation of the effects of in-situ EPT during deformation of BMG. Technologically, this will develop practical processing schemes for electroplastic forming of BMG. We will also characterize the microstructural changes of BMG after the treatment in order to gain better insight into the mechanisms of the coupled effects of 'electron wind force' and joule heating. In order to probe the structural changes down to atomic- or cluster-scale, experiments of extended x-ray absorption fine structure spectroscopy and small-angle x-ray scattering will be arranged in Shanghai Synchrotron Radiation Facility. These results may provide valuable information relating to the nature and size of the deforming/migrating units in the material during electroplastic forming.Apart from bulk samples, the PI also plans to carry out EPT on thin film metallic glass samples. With carefully designed patterns on the thin film samples, it may be able to visualize the material migration due to the 'electron wind force' under controlled stress applied through bending of the substrate. The electroplastic forming data of such thin film samples, apart from providing quantitative information relating to the deformation process, can be useful for designing micro- or even nano-imprinting processes of thin film metallic glass using electroplastic forming. This can be a promising route for producing MEMS.When the project is completed, it is anticipated that the general processing parameters for electroplastic forming of bulk and thin film metallic glasses can be clearly defined and the practical processing windows can be identified. Furthermore, the microstructural investigation will provide useful clue for understanding the deformation mechanism of metallic glasses.