Study Of Electroplastic Deformation Behavior In Zr-based Bulk Metallic Glass

The brittleness at room temperature and unworkability of bulk metallic glasses(BMGs) has been the main problem that limits their industrial applica tion. How to resolve the brittleness and improve the plastic deformation ability of BMGs has always been the hot problem in the research field of BMGs. In recent years, pulse current has been introduced as a new technology in BMGs. It has been reported tha t the obvious heating effect caused by pulse current could be used to realize the fast thermoplastic forming of BMGs. Compared to traditional thermoplastic forming technology, this method just improves the heating efficiency, while it's still hard to avoid the crystallization of BMGs at the process of high temperature forming. Literatures have shown that pulse current obtains an electroplasticity effect, which could improve the plastic deformation ability of crystalline metals. Whether a similar electroplasticity effect exists in BMGs has not been reported yet. Based on this, this work applied a pulse current on BMGs during its tensile deformation test. Using this approach, we could research the change of deformation behavior of BMGs and explore the possibility of electroplasticity effect in BMGs at a relative low temperature.The thermal effect of pulse current in materials is highly affected by the resistivity, which highly relied on the temperature and the materials' structure. So, it's necessary to measure and research the resistivity of BMGs. Firstly, the resistivity of three Zr-based BMGs, Zr55Cu30Al10Ni5 ? Zr61Ti2Cu25Al12 and Zr52.5Cu17.9Ni14.6Ti5Al10, was measured during the continuous heating process. The result showed that the change of slope in resistivity-temperature curve is able to characterize the process of structure relaxation, crystallization and crystal phase transition of BMGs, while the glass transition is unable to be characterized clearly. The local atom clusters in amorphous alloys will experience the transformation of order-disorder with the increase of temperature, which could be used to explain how the resistivity could be affected by the structure change. The characteristic temperatures defined by resistivity method are much lower than that by DSC.Secondly, the tensile deformation behavior of Zr55Cu30Al10Ni5 BMG under pulse current was researched. The result showed that the samples obtained an enhanced plastic deformation with an elongation of 15%~20%. The tensile strength and elongation highly rely on the current density, pulse width and strain rate. The measured lowest temperature when a plastic deformation happened was about 280?, which was far lower than Tg. All the stress-strain curves showed an inhomogeneous deformation, which was similar to the viscosity rheology of BMGs in high temperature, indicating the temperature played the main role in this plastic deformation, and the electronic effect of pulse current could accelerate this process.Finally, the XRD, DSC and microhardness test were conducted on the deformed samples. The results showed that the deformed samples still kept an amorphous structure. The Tg and Tx showed a certain reduction, and the DSC test also showed that the region of the sample closed to the fracture contained more free volume. Along the Axis of the sample, with the decrease of plastic deformation, the microhardness increased gradually. Combining the “shear transformation zone”(STZ) model, the electroplastic effect in BMGs was explained qualitatively.