Production of bulk metallic glass sheet and its hybrid laminates
thesisposted on 2017-03-03, 00:48 authored by East, Daniel Raymond
Bulk metallic glasses (BMGs) are a class of alloys that can retain an amorphous structure when cast at moderate cooling rates. This allows castings to be produced that have an amorphous structure at cross-sections that can be measured in the millimetre length-scale. The transfer of BMGs from laboratory settings to that of commercially produced alloys has been limited to date, with the lack of a production process that can produce commercial volumes of material being a limiting factor. This project aims to investigate the production of BMG sheet and BMG/Al laminates using processes that are readily scalable for transfer to an industrial setting. It investigates the use of twin roll casting (TRC) and warm rolling to produce BMG and BMG/Al hybrid in sheet form. The Mg₆₀Cu₂₉Gd₁₁ alloy is chosen due to its low processing temperatures and its tolerance to oxygen allowing the processing to be conducted in air. It is found that it is possible to produce 100% amorphous sheet at thicknesses between 1 and 3 mm from the Mg₆₀Cu₂₉Gd₁₁ alloy using TRC. The effects of casting conditions, including contact length, roll speed and sheet thickness, are investigated and the relationship of these variables to casting parameters are established. Bulk metallic glass of the Mg₆₀Cu₂₉Gd₁₁ alloy possesses a supercooled liquid region (SCLR) where it flows and can be formed similar to a plastic. Rolling trials within the SCLR are under taken and the effects of rolling parameters on the structure of the glass are examined using differential thermal analysis, transmission electron microscopy and X-ray absorption fine structure analysis. It is found that, when rolling at a low strain rate, it is possible to roll the alloy without inducing crystallisation. However, dynamic crystallisation occurs when the alloy is rolled to large reductions at large stain rates. The crystallised phase that is dynamically formed has a quasicrystalline structure with a composition similar to that of the amorphous matrix. The formation of this quasicystalline phase in the Mg-Cu-Gd system has not been reported previously. The formation of chemically similar quasicrystals from an amorphous matrix has been shown in other glass forming systems to be an indication of icosohedral short-range atomic ordering within the matrix. Roll bonding is used to produce laminates from BMG and aluminium interlayers. Rolling is conducted within the SCLR of the glass. In addition to the temperature regime being within the SCLR the reduction applied to the laminate is in excess of 50%. This resulted in an improvement of over 90% in the flexural stress, flexural strain and Charpy impact energy over that of the monolithic glass.