Molding of fine surface features into bulk metallic glass
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Molding of fine surface features into bulk metallic glass Ivan McCracken and Ralf Busch Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331, USA ABSTRACT Molding of fine surface features into a bulk metallic glass (BMG) from the molten state was investigated. It was studied down to which a feature size in a mold could be reproduced in the BMG. A new casting apparatus was designed and built to carry out the experiments. The BMG used was Vitreloy 1, Zr41.24Ti13.75Cu12.5Ni10Be22.5, and it was observed to replicate mold features down to several nanometers. This has been verified with SEM photographs of both the mold surfaces and the BMG parts. Processing variables, including the injection temperature of the molten BMG and the atmosphere, in which it is molded, appear to have a great effect on the ability to reproduce the fine features on the mold surface. INTRODUCTION The random atomic arrangement of a bulk metallic glass (BMG) in the solid state lends itself to exploration of fine feature replication. The feature size created during a molding process should only be limited by the viscosity of the molten BMG and its surface energy relative to surface of replication. If the molten alloy were to crystallize during cooling, there would be an abrupt reduction in volume and features inherent to the crystal structure would emerge at the interface of the solidified alloy and the mold surface. The premise then, is that if the molten BMG could be made to flow into a mold feature, that feature should be reproducible almost to the atomic level. EXPERIMANTAL DETAILS A machine has been designed to address the two areas of concern; namely the viscosity of the molten BMG and the ability to overcome the surface energy at the flow front leading into the mold features to be replicated. These issues are addressed by moving the molten BMG from the heating zone and into the mold quickly, using a pneumatic injection cylinder, in order to minimize the temperature loss and resulting viscosity increase. After injection into the mold, a positive holding pressure is applied by the injection plunger used to force the melt into the mold in an effort to overcome the surface energy at the molten flow front (at the end of fill). Figure 1 shows the components of the machine. A copper mold is used because of its high thermal conductivity and acts as a heat sink for the rapid cooling of the BMG. The cavity dimensions are 3.18 x 12.70 x 19.05 mm (thick region) and 1.59 x 12.70 x 0.19.05 mm (thin region). The cavity size was influenced both by the coupling requirements of the induction heater used to melt the BMG and the size of the pre-fabricated BMG samples available for use in the test. The BMG used was Vitreloy1, Zr41.24Ti13.75Cu12.5Ni10Be22.5 [1] provided by Liquid Metal Technology. Mold inserts (not shown) were created to fit into the mold surface. These inserts were polished and then scribed or indented using a diamond tip, which created the features to be replicated, as well as a grid for location purposes
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