Application of Electropulsing Treatment on the Rapid Thermoplastic Forming and Stress Relaxation of Bulk Metallic Glasses
Date Issued
2012
Date
2012
Author(s)
Yiu, Pak-Man
Abstract
The properties of bulk metallic glasses (BMGs), which are fabricated from rapid quenching of molten metal alloy, have been researched for decades. Being structurally amorphous, BMGs process high strength and high corrosion resistance over crystalline alloys. However due to its poor plasticity, BMGs’ engineering application is limited especially for structural purpose. Researchers have been seeking for the mechanism of plastic deformation in BMGs and thus ways to improve. BMGs undertake plastic deformation in a characteristic way: initiating shear bands. It has been commonly agreed that formation of shear bands of metallic glasses is a result of shearing motion between shear transformation zones (STZs), which could be described as core-shell structure of atoms. Despite the intrinsic plasticity is given by STZs, localized and inhomogeneous shear banding always causes weakness and brittle failure in BMGs.
On the other hand, due to good glass forming ability and fairly good ductility, Zr-Cu-Al has been extensively studied among existing metallic glass systems. Various ways such as controlling solidification condition and composite were used to improve the plasticity of Zr-based metallic glass. Among many of them, recent research discovers that electrical pulses are effective in improving the intrinsic plasticity of metallic glasses. In this thesis, the effect of applying electropulsing on the plasticity of metallic glasses was investigated systematically. It is concluded that electropulsing does not always improve plasticity of metallic glasses but only when thermal effect brought by Joule heating is small, else it only serve as an accelerated relaxation and embrittlement process. By using different pulsing frequencies, treated metallic glasses showed varying mechanical property. Therefore, the treatment temperature and amount of relaxation can be precisely controlled in this way. Making use of EPT will be a much rapid and accurate technique compared to conventional heating. These phenomena could potentially developed into a new, rapid thermoplastic forming technique.
Subjects
nanoindentation
metalic glass
termoplastic forming
electropulsing
Type
thesis
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