Sheet Processing of Bulk Metallic Glass

Bulk metallic glasses are a class of material that combine high strength with high formability when processed under correct conditions. The lack of an industrial-scale production method that can fabricate sheet limits the applications available to bulk metallic glass. Twin roll casting is a technology that can cast sheet continuously and has been used to produce sheet of bulk metallic glass several meters in length at gauges between 1 and 4 mm. To produce high quality bulk metallic glass sheet the casting must exit the roll bite within the supercooled liquid region of the alloy. The supercooled liquid region lies between the glass transition temperature and the crystallization temperature. Within the supercooled liquid region a bulk metallic glass is soft and malleable yet does not crystallize. To prove the operational widow casting trials at various thicknesses and various contact lengths were undertaken. The as cast sheet was examined by X-ray diffraction (XRD) to confirm whether the cast sheet was amorphous.

The supercooled liquid region has also been exploited to super plastically roll bulk metallic glass sheet. By controlling the temperature and strain rate of the rolling it is possible to roll bulk metallic glass sheet and preserve the amorphous structure of the alloy. Rolling trails were conducted with various strain and strain rate values to produce processing maps for bulk metallic glass. The as rolled sheet was examined with XRD and transmission electron microscopy. It was found that at high strains and strain rates that strain induced crystallization occurs with the bulk metallic glass sheet.  

Combining twin roll casting and super plastic rolling gives a combination of technologies that allow bulk metallic glass to be produced as sheet in various gauges.