Forming of metallic glass by rapid capacitor discharge

#This invention relates generally to a novel method of forming metallic glass; and more particularly to a process for forming metallic glass using rapid capacitor discharge heating.

#Amorphous materials are a new class of engineering material, which have a unique combination of high strength, elasticity, corrosion resistance and processability from the molten state. Amorphous materials differ from conventional crystalline alloys in that their atomic structure lacks the typical long-range ordered patterns of the atomic structure of conventional crystalline alloys. Amorphous materials are generally processed and formed by cooling a molten alloy from above the melting temperature of the crystalline phase (or the thermodynamic melting temperature) to below the “glass transition temperature” of the amorphous phase at “sufficiently fast” cooling rates, such that the nucleation and growth of alloy crystals is avoided. As such, the processing methods for amorphous alloys have always been concerned with quantifying the “sufficiently fast cooling rate”, which is also referred to as “critical cooling rate”, to ensure formation of the amorphous phase.

#The “critical cooling rates” for early amorphous materials were extremely high, on the order of 106° C./sec. As such, conventional casting processes were not suitable for such high cooling rates, and special casting processes such as melt spinning and planar flow casting were developed. Due to the crystallization kinetics of those early alloys being substantially fast, extremely short time (on the order of 10−3 seconds or less) for heat extraction from the molten alloy were required to bypass crystallization, and thus early amorphous alloys were also limited in size in at least one dimension. For example, only very thin foils and ribbons (order of 25 microns in thickness) were successfully produced using these conventional techniques. Because the critical cooling rate requirements for these amorphous alloys severely limited the size of parts made from amorphous alloys, the use of early amorphous alloys as hulk objects and articles was limited.

#The description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:

#FIG. 1, provides a flow chart of an exemplary rapid capacitor discharge forming method in accordance with the current invention;

#FIG. 2, provides a schematic of an exemplary embodiment of a rapid capacitor discharge forming method in accordance with the current invention;

#The current invention is directed to a method of uniformly heating, rheologically softening, and thermoplastically forming metallic glasses rapidly (typically with processing times of less than 1 second into a net shape article using an extrusion or mold tool by Joule healing. More specifically, the method utilizes the discharge of electrical energy (typically 100 Joules to 100 KJoules) stored in a capacitor to uniformly and rapidly heat a sample or charge of metallic glass alloy to a predetermined “process temperature” about half-way between the glass transition temperature of the amorphous material and the equilibrium melting point of the alloy in a time scale of several milliseconds or less, and is referred to hereinafter as rapid capacitor discharge forming (RCDF). The RCDF process of the current invention proceeds from the observation that metallic glass, by its virtue of being a frozen liquid, has a relatively low electrical resistivity, which can result in high dissipation and efficient, uniform heating of the material at rate such that the sample is adiabatically heated with the proper application of an electrical discharge.

#By rapidly and uniformly heating a BMG, the RCDF method extends the stability of the supercooled liquid against crystallization to temperatures substantially higher than the glass transition temperature, thereby bringing the entire sample volume to a state associated with a processing viscosity that is optimal for forming. The RCDF process also provides access to the entire range of viscosities offered by the metastable supercooled liquid, as this range is no longer limited by the formation of the stable crystalline phase. In sum, this process allows for the enhancement of the quality of parts formed, an increase yield of usable parts, a reduction in material and processing costs, a widening of the range of usable BMG materials, improved energy efficiency, and tower capital cost of manufacturing machines. In addition, owing to the instantaneous and uniform heating that can be attained in the RCDF method, the thermodynamic end transport properties throughout the entire range of the liquid metastability become accessible for measurement. Therefore by incorporating additional standard instrumentation to a Rapid Capacitor Discharge set up such as temperature and strain measurement instrumentation, properties such as viscosity, heat capacity and enthalpy can be measured in the entire temperature range between glass transition and melting point.

#A simple flow chart of the RCDF technique of the current invention is provided in FIG. 1. As shown, the process begins with the discharge of electrical energy (typically 100 Joules to 100 KJoules) stored in a capacitor into a sample block or charge of metallic glass alloy, in accordance with the current invention, the application of the electrical energy may be used to rapidly and uniformly heat the sample to a predetermined “process temperature” above the glass transition temperature of the alloy, and more specifically to a processing temperature about half-way between the glass transition temperature of the amorphous material and the equilibrium melting point of the alloy (˜200-300 K above Tg), on a time scale of several microseconds to several milliseconds or less, such that the amorphous material has a process viscosity sufficient to allow facile shaping (˜1 to 104 Pas-s or less).