The main challenge associated with processing of bulk metallic glass (BMG) is a consequence of their metastable nature: fast cooling conditions are required to avoid crystallization during processing. This limits the range of geometries that can be cast since filling of the mold and fast cooling (which require opposite ideal conditions) must be carried out simultaneously. As an alternative to casting, BMGs can be thermoplastically formed (TPF) in their supercooled liquid region, where the required fast cooling to vitrify the BMG is decoupled from the mold filling operation. This is made possible by the unique crystallization behavior of BMGs which results in a supercooled liquid region. When an amorphous BMG sample is heated into the supercooled liquid region, the BMG first relaxes into a readily deformable supercooled liquid before it eventually crystallizes. Therefore, BMGs can be considered high strength metals that can be processed like plastics. A wide range of processing methods based on TPF have been developed in our lab that exploit the unique softening of some BMGs in the supercooled liquid region.
When metallic glasses are thermoplastically formed they combine two previously mutually exclusive attributes; the strength of high strength metals and the processibility of plastics.
Schematic of a time temperature transformation (TTT) diagram indicating that metallic glasses can be processed via direct casting (1) or thermoplastic forming (2). Whereas in direct casting (1) cooling and forming must be carried out simultaneously, during thermoplastic forming cooling (1) is decoupled from forming (2). Therefore, thermoplastic forming allows for more versatile and complex shapes.