Metallic glass formation reactions and interfaces

Abstract

With the discovery of a number of new classes of bulk glass forming alloys that can achieve full vitrification in sizes of the order of millimeters to centimeters, there is a need to identify methods for the fabrication and joining of components. From the thermodynamics and kinetics of glass formation it is known that processing either to yield the suppression of crystal nucleation or crystal growth inhibition can yield amorphous alloys. In the latter case, usually rapid melt quenching is required and this results in thin ribbons or powders. For the bulk glass forming alloys, the kinetics are represented by suppressed nucleation so that cooling rates as low as 1 K/s may be sufficient to achieve glass formation. While the cooling rate and associated undercooling required for glass formation are largely estimated from calculation, they can be accurately monitored by using a wedge casting method which moreover allows for the identification and documentation of the kinetic competition that occurs at the onset of glass formation. Because of the high stability of bulk glass forming alloys, a clear glass transition signal and a region of undercooled liquid are characteristic features. Within this region liquid state joining operations may be performed to fabricate various bulk amorphous alloy components. In an alternate approach, joining may be accomplished by deformation flow in metallic glasses to achieve solid-state bonding. In this case, interface reactions between the amorphous layer and adjacent crystalline layers are important as well as the development of mixing at the joint interfaces where the high viscosity of the undercooled liquid inhibits crystallization from the adjoining crystalline surfaces. These developments represent promising opportunities for metallic glass alloys in joining applications.