WO2012068419A2

WO2012068419A2 - Heating and plastic forming of bulk metallic glass shells by inductive coupling

Info

Publication number: WO2012068419A2
Application number: PCT/US2011/061278
Authority: WO
Inventors: William L. Johnson; Marios D. Demetriou
Original assignee: California Institute Of Technology
Priority date: 2010-11-17
Filing date: 2011-11-17
Publication date: 2012-05-24
Also published as: WO2012068419A3

Abstract

An apparatus and method of inductively heating, rheologically softening, and thermoplastically forming metallic glasses rapidly into a net shape are provided. The inductive coupling method utilizes the discharge of electrical energy to parabolically heat a closed-loop, thin-walled sample of metallic glass to a predetermined "process temperature" 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 without a direct electrical connection. Once the sample is heated such that the heated portion of the block has a sufficiently low process viscosity it may be shaped via any number of techniques including, for example, injection molding, dynamic forging, stamp forging, and blow molding in a time frame of less than 1 second.

Description

HEATING AND PLASTIC FORMING OF BULK METALLIC GLASS SHELLS BY INDUCTIVE COUPLING

FI ELD OF TH E INVENTION

[0001] This invention relates generally to a novel method of forming metallic glass; and more particularly to a process for forming metallic glass shells using inductive coupling heating.

BACKGROUND OF THE INVENTION

[0002] 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.

[0003] 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 bulk objects and articles was limited. [0004] Over the years it was determined that the "critical cooling rate" depends strongly on the chemical composition of amorphous alloys. Accordingly, a great deal of research was focused on developing new alloy compositions with much lower critical cooling rates. Examples of these alloys are given in U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, each of which is incorporated herein by reference. These amorphous alloy systems, also called bulk- metallic glasses or BMGs, are characterized by critical cooling rates as low as a few °C/second, which allows the processing and forming of much larger bulk amorphous phase objects than were previously achievable.

[0005] With the availability of low "critical cooling rate" BMGs, it has become possible to apply conventional casting processes to form bulk articles having an amorphous phase. Over the past several years, a concerted effort has been under way to develop commercial manufacturing technologies for the production of net shape metallic parts fabricated from BMGs. For example, manufacturing methods such as permanent mold metal die-casting and injection casting into heated molds are currently being used to fabricate commercial hardware and components such as electronic casings for standard consumer electronic devices (e.g., cell phones and handheld wireless devices), hinges, fasteners, medical instruments and other high value added products. However, even though bulk-solidifying amorphous alloys provide some remedy to the fundamental deficiencies of solidification casting, and particularly to the die- casting and permanent mold casting processes, as discussed above, there are still issues which need to be addressed. First and foremost, there is a need to make these bulk objects from a broader range of alloy compositions. For example, presently available BMGs with large critical casting dimensions capable of making large bulk amorphous objects are limited to a few groups of alloy compositions based on a very narrow selection of metals, including Zr-based alloys with additions of Ti, Ni, Cu, Al and Be and Pd-based alloys with additions of Ni, Cu, and P, which are not necessarily optimized from either an engineering or cost perspective.

[0006] In addition, the incumbent processing technology requires a great deal of expensive machinery to ensure appropriate processing conditions are created. For example, most shaping processes require a high vacuum or controlled inert gas environment, induction melting of material in a crucible, pouring of metal to a shot sleeve, and pneumatic injection through a shot sleeve into gating and cavities of a rather elaborate mold assembly. These modified die-casting machines can cost several hundreds of thousands of dollars per machine. Moreover, because heating a BMG has to date been accomplished via these traditional, slow thermal processes that commence with the liquid being in equilibrium above its melting point, the prior art of processing and forming bulk-solidifying amorphous alloys has always been focused on cooling the molten alloy from above the thermodynamic melting temperature to below the glass transition temperature. This cooling has either been realized using a single-step monotonous cooling operation or a multi-step process. For example, metallic molds (made of copper, steel, tungsten, molybdenum, composites thereof, or other high conductivity materials) at ambient temperatures are utilized to facilitate and expedite heat extraction from the molten alloy. Because the "critical casting dimension" is related to the critical cooling rate, these conventional processes are not suitable for forming larger bulk objects and articles of a broader range of bulk-solidifying amorphous alloys. In addition, it is often necessary to inject the molten alloy into the dies at high-speed, and under high-pressure, to ensure sufficient alloy material is introduced into the die prior to the crystallization of the liquid, and before the liquid loses its fluidity (due to increase in viscosity), particularly in the manufacture of complex and high-precision parts. Because the liquid is fed into the die under high pressure and at high velocities, such as in high-pressure die-casting operation, the flow of the molten metal becomes prone to developing flow-front instabilities. Flow-front instabilities are characterized by a high Weber and a high Reynolds number, and are associated with the break-up of the flow front causing the formation of protruded seams and cells, which appear as cosmetic and structural micro-defects in cast parts. Also, there is a tendency to form a shrinkage cavity or porosity around the center of the cast part when unvitrified liquid is trapped inside a solid shell of vitrified metal. Additionally, the high temperatures and pressures associated with these die casting methods would accelerate tool wear and dramatically reduce tool life, and thus substantially increase the manufacturing cost. [0007] Attempts to remedy the problems associated with rapidly cooling the material from above the equilibrium melting point to below the glass transition were mostly focused on utilizing the kinetic stability and viscous flow characteristics of the supercooled liquid. Methods have been proposed that involve heating glassy feedstock above the glass transition where the glass relaxes to a viscous supercooled liquid, applying pressure to form the supercooled liquid, and subsequently cooling to below glass transition prior to crystallizing. These attractive methods are essentially very similar to those used to process plastics. In contrast to plastics however, which remain stable against crystallization above the softening transition for extremely long periods of time, metallic supercooled liquids crystallize rather rapidly once relaxed at the glass transition temperature or above. Consequently, the temperature range over which metallic glasses are stable against crystallization when heated at conventional heating rates (20 °C/min) are rather small (50 - 100 °C above glass transition), and the liquid viscosity within that range is rather high (10⁹ - 10⁷ Pa s). Owing to these high viscosities, the pressures required to form these liquids into desirable shapes are enormous, and for many metallic glass alloys could exceed the limiting stresses afforded by conventional high strength tooling (<1 GPa). Metallic glass alloys have recently been developed that are stable against crystallization when heated at conventional heating rates up to considerably high temperatures (165 °C above glass transition). Examples of these alloys are given in U.S. Pat. Appl. 20080135138 and articles to G. Duan et al. (Advanced Materials, 19 (2007) 4272) and A. Wiest (Acta Materialia, 56 (2008) 2525-2630), each of which is incorporated herein by reference. Owing to their high stability against crystallization, process viscosities as low as 10⁵ Pa-s become accessible, which suggests that these alloys are more suitable for processing in the supercooled liquid state than traditional metallic glasses. These viscosities however are still substantially higher than the processing viscosities of plastics, which typically range between 10 and 1000 Pa-s. In order to attain such low viscosities, the metallic glass alloy should either exhibit an even higher stability against crystallization when heated by conventional heating, or be heated at an unconventionally high heating rate which would extend the temperature range of stability and lower the process viscosity to values typical of those used in processing thermoplastics.

[0008] Recently, a novel method for rapidly and uniformly heating a bulk metallic glass using a capacitive discharge and Ohmic heating of a metallic glass charge of uniform cross section by an electrical current pulse has been developed. The energy pulse is coupled to the sample through use of conducting electrodes, which make electrical contact with the sample to deliver a current. The rapidly heated sample can be subsequently formed by injection molding, blow molding, compression molding, or other thermoplastic process to produce a high quality net-shaped metallic glass component. The method, called "Forming of Metallic Glass by Rapid Capacitor Discharge", is described in U.S. Patent Pub. No. 2009/0236017, and is incorporated by reference herein. The method allows a wide range of bulk metallic glass alloys to be rapidly heated to process temperatures between the glass transition and the alloy melting point, and then be processed thermoplastically to form net shapes in millisecond time scales while avoiding the crystallization of the metallic glass. However, this system requires a direct electrical contact between the metallic glass sample and a highly conductive electrode.

[0009] Accordingly, a need exists to find a novel approach to instantaneously and uniformly heat a BMG specimen volume without requiring such a direct electrically interconnection.

BRIEF SUMMARY OF THE INVENTION

[0010] In accordance with the current invention, there are provided a method of heating and shaping bulk metallic glasses (BMG) shells where the heating current is inductively coupled to the metallic glass sample to achieve rapid and substantially uniform heating across the shell thickness, and the surface of the shell.

[0011] In some embodiments, the invention is directed to methods of rapidly heating and shaping an amorphous metal using inductive coupling including:

• providing a closed-loop, thin-walled sample of amorphous material, the sample having a substantially uniform wall thickness and prismatic shell shape; • discharging a transient current pulse of electrical energy and inductively coupling said discharge energy into the sample using a time varying magnetic field produced and directed along the axis of the sample, wherein the time variation of the magnetic field should be configured such that the electromagnetic penetration depth of this dynamic magnetic field is large by comparison to the thickness of the sample wall such that the magnetic field is substantially homogeneous both in the interior of the sample as well as across the thickness of the sample walls, to produce a flowing current that dissipates a uniform power density within the entire sample, which produces a substantially uniform heating of the entire sample via Ohmic dissipation to a processing temperature between the glass transition temperature and the equilibrium melting point of the amorphous material;

• applying a deformational force simultaneously with or just after said discharging and heating to plastically form the heated sample into an final net shape amorphous article; and

• cooling the article to a temperature below the glass transition temperature of the amorphous material sufficiently rapidly to avoid the crystallization of the amorphous sample.

[0012] In one such embodiment, the sample is a cylindrical tube.

[0013] In another such embodiment, the amorphous material has an electrical resistivity, (T) that varies slowly with temperature such that -0.0003 < S < +0.0003 where S=(l/ )d dT and S is the relative change in resistivity per unit of temperature change.

[0014] In still another embodiment, the temperature of the sample is increased at a rate of at least 500 K/sec.

[0015] In yet another embodiment, the amorphous material has a relative change of resistivity per unit of temperature change (S) of no greater than about 1 x 10^"4 °C ^_1, and a resistivity at room temperature (p) between about 80 to 300 μΩ-cm.

[0016] In still yet another embodiment, the dynamic magnetic field will induce an emf and associated circular electric field, E, along with an electric current density, J = E (Ohms Law where is the electrical conductivity of the sample), which flows circularly around the sample shell (Lenz's Law) If the walls of said sample are sufficiently thin in comparison to the outer radius of the sample, then this electric field and the induced current will be very nearly uniform both over the sample wall thickness and over the surface area of the shell.

[0017] In still yet another embodiment, the quantum of electrical energy is delivered by a pulsed current waveform of typical pulse duration between 0.1 ms and Is.

[0018] In still yet another embodiment, the processing temperature is such that the viscosity of the heated amorphous metal prismatic sample is from about 1 to 10⁴ Pas-sec.

[0019] In still yet another embodiment, the sample is substantially defect free.

[0020] In still yet another embodiment, the amorphous material is an alloy based on an elemental metal selected from the group consisting of Zr, Pd, Pt, Au, Fe, Co, Ti, Al, Mg, Ni and

Cu.

[0021] In still yet another embodiment, the step of discharging the quantum of electrical energy is produced by a time varying current in a cylindrically wound inductive coil within which the sample prism is symmetrically disposed with its axis aligned with that of the solenoid coil and which generates a substantially uniform time varying magnetic field within the solenoid coil, where the magnetic field is aligned with the axis of the sample, the magnetic field is uniform around the sample surface and across the sample wall thickness, and wherein the electromagnetic skin depth of the time varying magnetic field is large compared to the wall thickness of the sample. In one such embodiment, the sample is a cylindrical tube and the coil is cylindrical. In another such embodiment, the ratio of the thickness of the sample wall to the outer (and inner) radius of the sample is selected such that the relative temperature difference generated across the thickness of said sample is less than 10%.

[0022] In still yet another embodiment, the sample is a closed-loop, thin-walled shape where the wall thickness of the sample is uniform, the shape being selected from the group consisting of a hollow circular cylinder, a hollow cylinder of elliptical cross-section, a square prism, or other symmetric shape. [0023] In still yet another embodiment, the step of shaping uses a shaping tool selected from the group consisting of injection molding, dynamic forging tool, stamping tool, or blow molding tool.

[0024] In still yet another embodiment, the shaping tool is heated to a temperature preferably around the glass transition temperature of the amorphous material.

[0025] In still yet another embodiment, the deformational force is applied such that the heated sample is deformed at a rate sufficiently slow to avoid flow-front breakup (splashing) or shear tearing of the sample shell.

[0026] In still yet another embodiment, the heating and shaping of the sample are complete in a time of between about 100 μ≤ to 1 s.

[0027] In still yet another embodiment, the characteristic pulse frequency is in the range of 30 Hz to 3 kHz. In one such embodiment, the skin depth of the inductive coupling is determined by the pulse frequency, and is less than the thickness of the sample wall thickness.

[0028] In other embodiments, the invention is directed to a rapid capacitor discharge apparatus for shaping an amorphous material using inductive coupling including:

• a closed-loop, thin-walled sample of an amorphous material, the sample having a substantially uniform cross-section;

• a source of stored electrical energy, such as for example a capacitor;

• an inductive coil interconnected in a series to the source of electrical energy wherein the coil and sample preferably have the same shape, the sample of amorphous metal being disposed within the coil;

• a shaping tool disposed in forming relation to the sample;

• wherein the source of electrical energy is capable of producing a transient current pulse of electrical energy, and the coil is capable of inductively coupling the current pulse into the sample via a magnetic field having a penetration depth sufficient to ensure said magnetic field is homogenous across the sample thickness and over the sample surface such that a uniform eddy current density is established within the sample, and wherein the walls of the sample are sufficiently thin in comparison to the outer dimension of the sample cross section such that the temperature gradient across the sample is sufficiently small such that the sample is thereby rapidly and substantially uniformly heated via Ohmic dissipation to a processing temperature between the glass transition temperature and the equilibrium melting point of the amorphous material; and

• wherein the shaping tool is capable of applying a deformational force sufficient to form the heated sample to a net shape article.

[0029] In one such embodiment, the shaping tool is selected from the group consisting of injection molding, dynamic forging, stamp forging and blow molding.

[0030] In another such embodiment, the shaping tool further comprises a temperature- controlled heating element for heating the tool to a temperature preferably around the glass transition temperature of the amorphous material.

[0031] In still another such embodiment, the apparatus further includes one of either a pneumatic or magnetic drive system in operative relation to the shaping tool for applying the deformational force to the sample.

[0032] In yet another such embodiment, the amorphous metal has a resistivity that does not increase with temperature.

[0033] In still yet another such embodiment, the temperature of the sample is increased at a rate of at least 500 K/sec.

[0034] In still yet another such embodiment, the amorphous material has a relative change of resistivity per unit of temperature change (S) whose absolute vale is no greater than about 3 x 10^{"4 o}C ¹ and a resistivity at room temperature (p₀) between about 80 and 300 μΩ-cm.

[0035] In still yet another such embodiment, the quantum of electrical energy is at least about 100 J, the maximum amplitude of the time varying magnetic field in the solenoid is at least 1 Tesla, and a time constant for variation of the field is between about 10 μ≤ and 10 ms.

[0036] In still yet another such embodiment, the processing temperature is about half-way between the glass transition temperature of the amorphous material and the equilibrium melting point of the alloy. [0037] In still yet another such embodiment, the processing temperature is such that the viscosity of the heated amorphous material is from about 1 to 10⁴ Pa-sec.

[0038] In still yet another such embodiment, the sample is substantially defect free.

[0039] In still yet another such embodiment, the ratio of the thickness of the sample wall to the radial dimension of the sample is selected such that the temperature variation across the sample thickness and over its area is less than 10% of the total induced average sample temperature.

[0040] In still yet another such embodiment, the amorphous material is an alloy based on an elemental metal selected from the group consisting of Zr, Pd, Pt, Au, Fe, Co, Al, Mg, Ti, Ni and Cu.

[0041] In still yet another such embodiment, the apparatus is capable of forming the article from the room temperature sample in a time of from about 100 μ≤ to about 1 s.

[0042] In still yet another such embodiment, the coil generates a varying magnetic field that induces eddy currents within said sample, and wherein the magnetic skin depth of the varying magnetic field generated is large compared to the wall thickness of the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The description will be more fully understood with reference to the following figures, 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:

[0044] FIG. 1 provides a flow chart of an inductive heating method in accordance with an embodiment of the current invention.

[0045] FIG. 2 provides a schematic showing the operation of inductive heating on a solid rod.

[0046] FIG. 3 presents a data plot showing the variation of the electromagnetic penetration depth of the inductive heating method vs. the typical frequency of the current pulse and dynamic magnetic field in accordance with an embodiment of the current invention. [0047] FIG. 4 provides a schematic of an inductive heating and shaping tool in accordance with an embodiment of the current invention.

[0048] FIGs. 5a & b present data plots showing results of transient response studies in accordance with an embodiment of the current invention where the time scale (time units) are ~lms and the peak current is about 2xl0⁴ amps.

[0049] FIG. 6 provides a schematic of a sample in accordance with an embodiment of the current invention.

DETAILED DESCRIPTION OF THE INVENTION

[0050] The current invention is directed to a method of heating and shaping bulk metallic glasses (BMG) shells where the heating current is inductively coupled to the metallic glass sample to achieve rapid and substantially uniform heating across the shell thickness. Using such an inductive coupling method it is possible to eliminate the need for making a direct electrical contact between the metallic glass sample and a highly conductive electrode thereby decoupling the heating of the sample from the shaping of the heated sample. This is accomplished by the current invention using inductive coupling of a capacitive discharge (or other suitable current pulse source) to a metallic glass sample that is both closed-loop and thin- walled located within an induction coil of symmetrical shape. For some shaping techniques, such as, for example, "injection molding", the method of the invention enables the separation of the current coupling function and mechanical injection function of the conducting electrode- plunger.

[0051] The inductive heating 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. Inductive heating of bulk metallic glasses has been known for some time, however to date such inductive heating is typically performed at high frequencies and hence has relied on heating the outer skin of a sample, while the interior heats by conduction from the outer shell (thermal relaxation). Such a mechanism is inherently slow, as thermal relaxation times for millimeter thick samples are on the order of seconds, and so limits the types of bulk metallic glasses that can be used, and the types of forming processes that can be employed. Uniformly heating and forming metallic glass shells is of particular interest, as it can be applied in processes like thermoplastic blow molding. However, typical inductive heating methods would fail to rapidly and uniformly heat a metallic glass shell, and consequently forming would either be limited, or be interrupted by crystallization. The current invention presents the discovery of a method of using inductive heating with BMG shells to rapidly and uniformly heat a shell thereby expanding the types of forming processes and BMG materials accessible by such an inductive heating method. Specifically, by rapidly and uniformly heating the BMG shell, the method of the invention extends the stability of the supercooled liquid against crystallization to temperatures substantially higher than the glass transition temperature, thereby bringing the heated volume to a state associated with a processing viscosity that is optimal for forming.

[0052] A simple flow chart of the inductive heating and forming technique of the current invention is provided in FIG. 1. As shown, the process begins with the discharge of electrical energy (typically 100 J to 100 KJ), i.e., such as a charge that has been stored in a capacitor, and inductively coupling that discharge of energy into a sample shell of metallic glass alloy. More particularly, a transient current pulse is utilized to produce a time varying magnetic field. This time varying field is configured to induce "eddy currents" within the sample, which in turn heats the sample by Ohmic dissipation. In accordance with the current invention, the application of the electrical energy may be used to rapidly and uniformly heat the inductively coupled sample shell 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 T_g), 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). Moreover, the current method accomplishes such heating without requiring direct contact between the sample and the electrical source, thereby avoiding the need for any interconnection between the sample and the heating apparatus.

[0053] Once the sample shell is uniformly heated such that it has a sufficiently low process viscosity, it may be shaped into a high quality amorphous bulk article via any number of techniques including, for example, injection molding, dynamic forging, stamp forging, blow molding, etc. However, the ability to shape a sample of metallic glass depends entirely on ensuring that the heating of the sample is both rapid and substantially uniform. In particular, if bulk shaping of samples is desired then the entire area of the sample must inductively coupled to ensure uniform heating of the entire sample. Likewise, if the sample heating is not sufficiently rapid (typically on the order of 500 - 10⁵ K/s) then either the material being formed will lose its amorphous character, or the shaping technique will be limited to those amorphous materials having superior processability characteristics (i.e., high stability of the supercooled liquid against crystallization), again reducing the utility of the process.

[0054] Although the above has described an embodiment of the process of inductive heating in accordance with the current invention, to make such a method operational it is necessary to understand the nature of inductive heating, and the constraints and requirements for performing inductive heating.

[0055] As discussed in the Background, prior art systems have recognized the inherent conductive properties of amorphous materials, and that to ensure uniform heating of the entire sample it is also necessary to avoid the dynamic development of spatial inhomogeneity in the energy dissipation within the heating sample. For example, the prior art rapid capacitive discharge forming (RCDF) method disclosed in U.S. Pub. No. 2009/0236017 (previously disclosed) sets forth two broad criteria, which must be met to prevent the development of such inhomogeneity and to ensure uniform heating of the sample:

• Uniformity of the current within the sample; and

• Stability of the sample with respect to development of inhomogeneity in power dissipation during dynamic heating. [0056] Although these criteria seem relatively straightforward, they place a number of physical and technical constraints on the electrical charge used during heating. With regard to uniformity of current, one of the features of inductive heating that makes it uniquely challenging to implement for BMG shaping is that it is inherently non-uniform. As will be discussed, this fundamental difference has implications for both the types of forming that are suitable for use with the inductive heating method of the invention, and the constraints that need to be placed on the nature of the BMG sample.

[0057] To understand why inductive heating is inherently non-uniform across a solid sample it is necessary to observe the physics of an exemplary heating process. I n one example, take a long solenoid coil having length L, inner diameter D, and with N turns of conductor per meter of length. Such a coil produces a uniform magnetic field, H, along the z-axis of the coil. The field is uniform over most of the length of the coil (end effects are only important within a distance of order D from the ends. Accordingly, if L » D, then the field is uniform over most of coil winding. If a solid cylindrical rod of BMG of radius R is symmetrically positioned within the coil and a time varying current, l(t), used to energize the coil it will produce a time varying uniform magnetic field within the coil. This time varying field penetrates the sample and produces a time varying electric field E within the sample such that:

curlE = dH/dt = constant (Eq. 1)

where both E and H are aligned along the coil (and sample rod) axis. For simplicity, a two dimensional cross-section of the rod is shown in FIG. 2.

[0058] I n such an example, a "ring" of radius r and thickness dr will be intersected by a time varying flux given by:

Οφ/dt = Tir² dH/dt (Eq. 2)

which, according to Lenz's Law produces an induced "angular" electric field of magnitude E along the ring (two horizontal arrows) such that:

2RTE = Tir² dH/dt (Eq. 3)

or

E = (r/2) dH/dt (Eq. 4)

- U- which varies in strength linearly with r.

[0059] This E-field will induce a current density, J(r) within the conducting sample where

J(r)= σΕ =(or/2) dH/dt (Eq. 5)

where σ is the electrical conductivity of the sample. This induced current density will dissipate power within the sample given by:

Power = J(r)²/o = pj(r)² = (r²/4p) (dH/dt)² (Eq. 6)

[0060] From this it is imminently evident that the power dissipation will produce heating in the solid sample that is "parabolic" along the rod radius. Accordingly, the heating rate within the solid rod will be dependent on the square of the radial distance from its centerline and is given by:

dT(r)/dt = (r²/4pc) (dH/dt)² (Eq. 7)

where c is the heat capacity per unit volume of the rod material.

[0061] In other words, heating via inductive coupling is inherently non-uniform if the sample is a solid cylinder. As such, if a solid sample is used the heating will occur to a greater extent around the outer perimeter of the rod, which might be useful for surface shaping and forming, or welding and joining. However, if uniform heating is desired (as would be the case if one wishes to form the overall amorphous sample) it is necessary to replace the solid sample with a shell-shaped sample, such as, for example, a thin-walled tube. For example, by replacing the solid rod of this example with a cylindrical "shell" or tube of where the thickness (s) is much smaller than the radius (R) of the sample (s « R), then heating within this shell would be substantially uniform since the radial variation of the power density would be negligible across this "thin shell". Moreover, the heating would be much more efficient in the shell sample compared to the solid sample, as the overall average heat generated per unit volume of sample would be considerably higher.

[0062] Using the above calculation, it is possible to quantify the heating gradient across any thin-walled sample, and the constraints of wall-thickness and radius required to ensure substantially uniform heating of the BMG sample. In particular, based on the equations provided above it is possible to provide a calculation of the percent heating gradient (φ = (Toutside-Tinside)/(Toutside+Tinside)) as related to the ratio of the wall thickness (s) and radius of the sample cylinder (R) as follows:

cJ) = s/R (Eq. 8)

Based on this it is possible to calculate that a sample having a wall-thickness of 1mm and a radius of 10mm would have a relative temperature gradient of 10%; preferably the temperature gradient is held below this threshold to ensure that the heating of the sample is substantially uniform. The method can thus be used to produce a substantially uniform heating of any closed-loop thin walled prismatic BMG sample, such as, for example, hollow cylinders, square prisms, or prisms of elliptical cross section, etc. as long as the magnetic field to which the sample is exposed is uniform symmetric with the sample itself, such as, for example, a cylindrical shell sample exposed to a magnetic field formed from a cylindrical coil. Accordingly, in a preferred embodiment, the sample is a thin-walled object of uniform closed-loop cross- section where the wall thickness of the object is very small in comparison with the cross sectional size.

[0063] Another constraint that must be taken into account in inductive heating is the penetration depth of the magnetic field. In particular, it is possible to vary the penetration the cross-section of the parabolic heating by varying the electromagnetic penetration depth, or skin depth, of the dynamic magnetic field in the sample. This penetration depth may be controlled by the time rate of change of the magnetic field or equivalently by the typical frequency of the Fourier components of the dynamic current l(t) in the circuit. For example, FIG. 3 shows a plot of the electromagnetic penetration depth Λ=[ρ/πμ₀ί]^{1 2} for a typical metallic glass (e.g. Vitreloy 1) with resistivity p~200 μΩ-m, vs. frequency f. For a frequency of f~lkHz, or a typical current pulse time of ~1 ms, a penetration depth of 2-3 cm may be obtained. This is roughly an order of magnitude larger than the radius of a typical metallic glass sample, R~2-4 mm, which will be inductively heated by a current pulse in the solenoid. In turn, this ensures that the magnetic field will penetrate homogeneously across the entire volume of the sample. For smaller areas, or surface heating, the frequency can be increased as required altering the area effect by the magnetic field and allowing for selective heating of the sample. However, in preferred embodiments of the invention the penetration or skin depth of the magnetic field is configured such that it is at least as large as the dimension of the sample perpendicular to the eddy currents, thereby ensuring a homogenous magnetic field across the entire sample, and, in turn, rapid and uniform heating along any radial band of the sample.

[0064] To understand the necessary criteria for obtaining rapid heating of a metallic glass sample using inductive coupling it is also necessary to understand how the nature of the metal materials being heated can effect the process. For example, the temperature dependence of the electrical resistivity of a metal can be quantified in terms of a relative change of resistivity per unit of temperature change, S, where S is defined as:

S = (l/p₀)[dp(T)/dT]_To (Eq. 9)

where S is in units of (1/degrees-C), p₀ is the resistivity (in Ohm-cm) of the metal at room temperature T₀, and [dp/dT]_To is the temperature derivative of the resistivity at room temperature (in Ohm-cm/C) taken to be linear. A typical amorphous material has a large p₀ (80 μΩ-cm < po < 300 μΩ-cm), but a very small (and frequently negative) value of S (-1 x 10^"4 < S < +1 x 10^"4).

[0065] By contrast, common crystalline metals have much lower p₀ (1- 30 μΩ-cm) and much greater values of S ~ 0.01 - 0.1. This leads to significant differences in behavior. For example, for common crystalline metals such as copper alloys, aluminum, or steel alloys, p₀ is much smaller (1-20 μΩ-cm) while S is much larger, typically S~ 0.01 - 0.1. The smaller p₀ values in crystalline metals will lead to smaller dissipation in the sample and make the coupling of the energy of the discharge to the sample less efficient. Furthermore, when a crystalline metal melts, p(T) generally increases by a factor of 2 or more on going from the solid metal to the molten metal. The large S values along with increase of resistivity on melting of common crystalline metals leads to extreme non-uniform Ohmic heating in a uniform current density. The crystalline sample will invariably melt locally. In turn, a discharge of energy through a crystalline rod leads to spatial localization of heating and localized melting wherever the initial resistance was greatest (typically around defects such as pores). [0066] Turning now to the issue of stability, for a thin-walled tube sample of length L, wall- thickness s, and cross-sectional area A= R² (R = sample radius), stability of the sample with respect to development of inhomogeneity in power dissipation during dynamic heating can be understood by carrying out stability analysis which includes Ohmic "Joule" heating by the current and heat flow governed by the Fourier equation. For a sample with a resistivity that increases with temperature (i.e. positive S), a local temperature variation along the axis of the sample tube will increase local heating, which will further increase the local resistance and heat dissipation. For sufficiently high power input, this leads to "localization" of heating along the tube. For crystalline materials, it results in localized melting. This behavior is extremely undesirable if one wishes to uniformly heat an amorphous material. The present invention provides a critical criterion to ensure uniform heating. Using S as defined above, we find heating should be uniform when the g inequalities are satisfied:

<™

where D is the thermal diffusivity (m²/s) of the amorphous material, C_s is the total heat capacity of the sample, and R₀ is the total resistance of the sample. Using values of D and C_s typical of metallic glass, and assuming a dimension (L or R ~lcm) , and an input power l²R₀ ~ 10⁶ Watts, typically required for the present invention, it is possible to obtain a S ~ 10^"4 - 10^"5. This criterion for uniform heating should be satisfied for many metallic glasses (see above S values). In particular, many metallic glasses have S < 0. Such materials (i.e., with S < 0) will always satisfy this requirement for heating uniformity. Exemplary materials that meet this criterion are set forth in U.S. Patent Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, the disclosures of which are incorporated herein by reference.

[0067] Moreover, beyond the fundamental physical criteria of the applied magnetic field and the amorphous materials used, there are also technical requirements to ensure that the charge is applied as evenly as possible to the sample. Some of these constraints include, the following: • It is important that the sample be of a closed-loop, i.e., a continuous conductive path such that the eddy currents established by the magnetic field circulate around the entire sample, that the sample be substantially free of defects, and that the sample be formed with thin-walls and a uniform cross-section along its length. If these conditions are not met the heat will not dissipate evenly across the sample and uncontrolled localized heating will occur. Specifically, if there is a discontinuity or defect in the sample block then the physical constants (i.e., D and C_s) discussed above will be different at those points leading to differential heating rates. In addition, because the thermal properties of the sample also are dependent on the dimensions of the item (i.e., L and R) if the cross-section of the item changes then there will be localized hot spots along the sample block. Accordingly, in one embodiment the sample block is formed such that it is substantially free of defects and has a substantially uniform cross-section. It should be understood that though the cross-section of the sample block should be uniform, as long as this requirement is met there are no inherent constraints placed on the shape of the block. For example, the block may take any suitable geometrically-uniform closed-loop shape, such as a hollow sphere, hollow cylinder or tube, etc. as long as the magnetic field imposed on the sample is configured to penetrate homogeneously across the sample.

• In addition, provided that the electric energy is successfully discharged homogeneously into the sample, the sample will heat up uniformly if heat transport towards the cooler surrounding is effectively evaded, i.e., if adiabatic heating is achieved. To generate adiabatic heating conditions, dT/dt has to be high enough, or the pulse duration small enough, to ensure that thermal gradients due to thermal transport do not develop in the sample. To quantify this criterion, the magnitude of τ should be considerably smaller than the thermal relaxation time of the amorphous metal sample, ι¾, given by the following equation:

T_th = c_ss²/k_s (Eq. ll). where k_s and c_s are the thermal conductivity and specific heat capacity of the amorphous metal, and s is the characteristic wall thickness of the amorphous metal sample (e.g. the thickness of a shell sample). Taking k_s ~ 10 W/(m K) and c_s ~ 5xl0⁶ J/(m³ K) representing approximate values for Zr-based glasses, and s ~ lxlO^"3 m, we obtain τ¾ ~ 0.5 s. Therefore, pulse durations τ considerably smaller than 0.5 s should be used to ensure uniform heating.

[0068] One significant advance of the current inductive system over the prior art RCDF system is that in the RCDF method the electrical current must be directly and physically in contact with the BMG sample. Hence, there is a further limitation that the sample contact surfaces must be adequately flat and parallel otherwise an interfacial contact resistance will exist at the electrode/sample interface. To prevent this, RCDF requires that the electrode/sample interface be designed to ensure that the electrical charge is applied evenly, i.e., with uniform density, such that no "hot points" develop at the interface. This places significant design limitations on the shaping tools used with such a system. The inductive method of the instant application is, in contrast, contactless allowing shaping to be completely decouple from the heating of the sample.

[0069] Although the above discussion is focused on the method of heating using inductive coupling, and the design constraints required for such a process, the current invention is also directed to an inductive-heating shaping tool. A schematic of an exemplary shaping tool in accordance with the inductive method of the current invention is provided in FIG. 4. As shown, in some embodiments the invention consists of an "RLC" circuit (10) comprising a capacitor (12) (having capacitance C) used to store electrical energy E=(1/2)CV², and an inductor (14) comprising a long conducting cylindrical heating coil (16) wound from a highly conducting material (such as, for example, copper), a barrel (18) consisting of an electrically insulating concentric cylindrical "tube" located within the coil, and a metallic glass sample of BMG (20) (in this case a hollow-walled cylinder) located within the barrel and situated within the induction coil. A suitable controller (22), such as, for example, a silicon control rectifier (SCR) is used to "switch on" the discharge. The RLC circuit has a total resistance R, which consists of the output resistance of the SCR plus the inherent resistance of the capacitor bank, inductor, and electrical leads used to connect the components.

[0070] The metallic glass sample is then confined and disposed in a shaping configuration with the forming tool (24). For example, in the embodiment shown in FIG. 4, the BMG sample is confined by a ceramic plunger (26), which slip fits into the barrel (18). The plunger is loaded by an applied force, which is generated, for example, by a pneumatic, hydraulic, or magnetic drive system and urges the sample into the forming tool (24). During shaping, the capacitor is charged to a voltage V with stored energy (E= (1/2)CV²). The discharge is initiated by switching on the controller (22). A time varying current determined by the transient response of the RLC circuit then flows in the circuit. The dynamic magnetic field induced by the varying current in the coil induces an electric field and associated "eddy currents" in the sample, which rapidly heats an area of the sample determined by the penetration depth of the coupling to a processing temperature T determined by the total energy stored in the capacitor and the fraction of this energy that is inductively coupled to the sample.

[0071] Following heating, the sample may be dynamically molded. In the embodiment shown in FIG. 4, the softened metallic glass is injected under pressure applied by the plunger (26) through the gate (28) and into a mold tool cavity (24). It should be understood that though an injection molding apparatus is shown in FIG. 4, the BMG sample heated in accordance with the method of the invention may be shaped in accordance with any preferred shaping method, including, for example, injection molding, dynamic forging, stamp forging, blow molding, etc., to form an article on a time scale of less than one second. Further details of some exemplary forming techniques may be found in U.S. Patent Nos. 7,017,645; 7,708,844 and 7,883,592, and in U.S. Patent Publication Nos. 2009/0236017 and 2011/0079940, the disclosures of each of which are incorporated herein by reference.

[0072] The above discussion has focused on broad embodiments of the invention, below are provided specific preferred operational parameters that may be used with those broad embodiments. It should be understood that other operational parameters may be incorporated with the inductive heating method without departing from the scope of the current invention.

• Preferably, the metallic glass sample is uniformly heated at a rate of at least 100 K/s, and more preferably at rates of at least 1000 K/s.

• The metallic glass alloy material comprising the sample preferably has a resistivity,p, in the range 0.8-3.0 χ10^"6 Ω-ιη.

• The method preferably operates using a pulsed current waveform of typical pulse duration between 0.1 ms and Is, and typical pulse frequency components in the frequency range of 30 Hz to 3 kHz where, as discussed above, the frequency range is chosen so that the electromagnetic penetration depth, Λ, of the metallic glass for the given frequency range, is suitable to provide heating to the area of the BMG sample desired. For example, if the entire sample is to be heated uniformly then the penetration depth must be much large that the typical cross sectional dimension of the metallic glass sample (e.g. A>R, where R is the radius of a metallic glass cylinder). This condition ensures that the inductively coupled energy will be homogeneously deposited within the sample.

• In preferred embodiment, the length of the coil is chose to be at least twice the length of the sample, or more preferably at least 4 times the length of the sample. This condition ensures that the time varying magnetic field is uniform over the volume of the sample.

• The current pulse can be produced by the discharge of a capacitor within an RLC circuit, for example, a capacitor having a discharge time constant of from 10 μ≤ to 10 milliseconds may be used, or by using any suitable current pulse generator. For example, the pulse may alternatively be produced by a low frequency (< 3 kHz) Radio Frequency generator, which is switched on for a time period t, and then switched off. The time period t should be less than or of the order of 1 sec. A longer pulse duration will lead to heating rates less than the desired 100 K/s. [0073] Although the above has focused on methods of heating and shaping using inductive coupling, and shaping apparatus using such a technique, it will be understood that this technique may be specifically used to form parts for electronic devices. Exemplary electronic devices that can be formed using the present inventions are any devices that have power components or supplies, including portable, mobile, hand-held, or miniature consumer electronic devices. Illustrative electronic devices ca n include, but are not limited to, music players, video players, still image players, game players, other media players, music recorders, video recorders, cameras, other media recorders, radios, medical equipment, calculators, cellular phones, other wireless communication devices, personal digital assistances, programmable remote controls, pagers, laptop computers, printers, or combinations thereof. Miniature electronic devices may have a form factor that is smaller than that of hand-held devices. Illustrative miniature electronic devices ca n include, but are not limited to, watches, rings, necklaces, belts, accessories for belts, headsets, accessories for shoes, virtual reality devices, other wearable electronics, accessories for sporting equipment, accessories for fitness equipment, key chains, or combinations thereof. Some exemplary consumer electronics embodiments can be found at U.S. Pat. App. Nos. 12/700518, 11/302907, and 11/235873; and USPNs 4130862, 5528205, 7166795 and 7583500, the disclosures of each of which are incorporated herein by reference.

EXEMPLARY EMBODIMENTS

[0074] The person skilled in the art will recognize that additional embodiments according to the invention are contemplated as being within the scope of the foregoing generic disclosure, and no disclaimer is in any way intended by the foregoing, non-limiting examples.

EXAMPLE 1: Coupling Efficiency Study

[0075] I n this exemplary embodiment, an analysis of the energy coupling efficiency of the discharge to a cylindrical sample located within the heating coil is provided. The engineering formula for inductance of a coil of length L, diameter d (both in inches), and total number of turns n results in a description for inductance, L, in μΗ in accordance with, L = (d²n²)/(18d+40L) (Eq. 12)

[0076] Accordingly, a critically damped RLC circuit will have a transient response current function, l(t) as shown in FIG. 5a. I n turn, FIG. 5b shows the derivative dl(t)/dt of the transient current. The electric field generated by Faraday induction within the sample coil will be proportional to dl/dt. The typical coupling efficiency and energy deposited in the sample are estimated for a typical case of interest in the following example.

[0077] I n this example, a capacitor of 0.264 F is used along with an inductive heating coil having length of 6", diameter of 0.75, and with n=40 turns (3.8 mm/turn), this gives an L of 3.55 μΗ. For the same coil, an RLC circuit will have a natural time constant of

The resistance of this "copper" coil is R_COii= p_CuL/A= (2xl0^~¾-m)(2.4m)/(2.85xl0^~4 m²) =1.68 ιτιΩ. Adding this to the circuit resistance of ~6 ιηΩ, gives a total circuit resistance of ~8 ΠΊΩ. NOW

ms, and also μΗ, and (LC)^0,5 =resonance period 2π/ω=(0.00125 s). Accordingly, the circuit damping factor is now

and a slightly over-damped circuit is obtained. By charging the capacitor to ~100V, ~1300 J is stored, and released in about 2-3ms with a typical current of l~ 10⁴ A. This will give a magnetic intensity in the coil of H = 40xl0⁴ A/ [0.15m] = 2.7xl0⁶ A/m, or equivalent to B = μ₀Η = 3.4 T. Now it is possible to estimate the power coupled to a sample (using):

power = πτΙΙ_Η²[πμρί]^1/2 CF (Eq. 13)

where C is a coupling factor related to sample geometry, F a "transmission" factor related to the penetration depth, and p is the resistivity of amorphous sample = 2xl0^"6 Qm and f the typical frequency or rate of discharge of the circuit.

[0078] Using a thin-walled cylindrical prism sample of outer diameter d=4mm, and length L=2cm, and where the wall thickness (s) is much smaller than the diameter (s«d), as shown in FIG. 6, it is possible to calculate a power = 163 kW xCxF, and a power transfer of ~160 J/ms. The total heat capacity of sample is ~1 J/K, so that heating rate is about dT/dt=160 K/ms or dT/dt = 1.6xl0⁵ K/s. Even with C=0.5 (we will have F~l since penetration is excellent), excellent coupling and heating rates are obtained. Accordingly, this example demonstrates that the energy discharged from the capacitor can be uniformly coupled to a sample shell of length 2cm and diameter 4mm in a time scale of ~lms.

EXAMPLE 2: Blow Molding Using Inductive Heating

[0079] As previously discussed, the inductive heating method of the current invention is particularly well-suited for use with thin-walled samples, such as hollow tubes. Such samples are often used as pre-forms or parisons in blow molding techniques. Accordingly, in one embodiment of the invention, the inductive heating tool of the instant invention would be used with a blow molding forming tool. In such an embodiment, as shown schematically in FIG. 4, a thin-walled cylindrically prismatic shaped parison (20) or pre-formed blank of BMG material is disposed in a shaping configuration with a mold inside an inductive coil (14). The parison is then heated by the inductive coil and once the temperature of the BMG parison reaches the requisite shaping temperature a pressure gradient is applied between the two surfaces of the parison to force the BMG into the mold or other forming tool (24). In this step the pressure gradient can be achieved either by applying pressure from the outer side of the parison, or by reducing the pressure in the mold cavity, or a combination of the two. Finally, releasing the pressure gradient terminates the forming process at which time the final article may be cooled.

DOCTRIN E OF EQUIVALENTS

[0080] Those skilled in the art will appreciate that the foregoing examples and descriptions of various preferred embodiments of the present invention are merely illustrative of the invention as a whole, and that variations in the steps and various components of the present invention may be made within the spirit and scope of the invention. Accordingly, the present invention is not limited to the specific embodiments described herein but, rather, is defined by the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method of rapidly heating and shaping an amorphous metal using inductive coupling comprising:

providing a closed-loop, thin-walled sample of amorphous material, said sample having a substantially uniform wall thickness and a prismatic shell shape;

discharging a transient current pulse of electrical energy and inductively coupling said discharge energy into said sample using a time varying magnetic field produced and directed along the axis of the sample, wherein the time variation of the magnetic field is configured such that the electromagnetic penetration depth of the magnetic filed is large in comparison to thickness of the sample wall such that the magnetic field is substantially homogeneous across said sample interior and within said sample wall to produce a flowing current within said sample that dissipates at a uniform power density within the entire sample, and which produces a substantially uniform heating of the entire sample via Ohmic dissipation to a processing temperature between the glass transition temperature and the equilibrium melting point of the amorphous material;

applying a deformational force simultaneous with or subsequent to said discharging to shape the heated sample into an amorphous article; and

cooling said article to a temperature below the glass transition temperature of the amorphous material sufficiently rapidly to avoid the crystallization of the amorphous sample.

2. The method of claim 1, wherein the sample is a cylindrical tube.

3. The method of claim 1, wherein the amorphous material has an electrical resistivity, p(T) that varies slowly with temperature such that -0.0003 < S < +0.0003 where S=(l/p)dp/dT and S is the relative change in resistivity per unit of temperature change.

4. The method of claim 1, wherein the temperature of the sample is increased at a rate of at least 500 K/sec.

5. The method of claim 1, wherein the amorphous material has a relative change of resistivity per unit of temperature change (S) of no greater than about 1 x 10^"4 °C ¹ and a resistivity at room temperature (p) between about 80-300 μΩ-cm.

6. The method of claim 1, wherein the quantum of electrical energy is delivered by a pulsed current waveform of typical pulse duration between 0.1 ms and Is.

7. The method of claim 1, wherein the processing temperature is such that the viscosity of the heated amorphous metal sample is from about 1 to 10⁴ Pas-sec.

8. The method of claim 1, wherein the sample is substantially defect free.

9. The method of claim 1, wherein the amorphous material is an alloy based on an elemental metal selected from the group consisting of Zr, Pd, Pt, Au, Fe, Co, Ti, Al, Mg, Ni and Cu.

10. The method of claim 1, wherein the step of discharging said quantum of electrical energy occurs through an inductive coil within which the sample is disposed, and which generates a varying magnetic field that induces eddy currents within said sample, and wherein the magnetic skin depth of the varying magnetic field generated is large compared to the wall thickness of the sample.

11. The method of claim 10, wherein the inductive coil has a shape symmetric to the sample.

12. The method of claim 11, wherein the sample is a tube and the coil is cylindrical.

13. The method of claim 10, wherein the ratio of the thickness of the sample wall to the outer (and inner) radius of the sample is selected such that the relative temperature difference generated across the thickness of said sample is less than 10%

14. The method of claim 1, wherein the sample is a closed-loop, thin-walled shape selected from the group consisting of a hollow circular cylinder, a hollow cylinder of elliptical cross- section, a square prism, or other symmetric shape.

15. The method of claim 1 wherein the step of shaping uses a shaping tool selected from the group consisting of injection molding, dynamic forging, stamp forging and blow molding.

16. The method of claim 15, wherein the shaping tool is heated to a temperature preferably around the glass transition temperature of the amorphous material.

17. The method of claim 1, wherein the deformational force is applied such that the heated sample is deformed at a rate sufficiently slow to avoid flow-front breakup (splashing) or shear tearing of the sample shell.

18. The method of claim 1, wherein the heating and shaping of the sample are complete in a time of between about 100 μ≤ to 1 s.

19. The method of claim 1, wherein the pulse frequency is in the range of 30 Hz to 3 kHz.

20. The method of claim 19, wherein the skin depth of the inductive coupling is determined by the pulse frequency and is less than the thickness of the sample wall thickness.

21. A rapid capacitor discharge apparatus for shaping an amorphous material using inductive coupling comprising:

a closed-loop, thin-walled sample of an amorphous material, said sample having a substantially uniform cross-section;

a source of stored electrical energy;

an inductive coil interconnected to said source of electrical energy, the sample of amorphous metal being disposed within said coil;

a shaping tool disposed in forming relation to said sample;

wherein said source of electrical energy is capable of producing a transient current pulse of electrical energy and the coil is capable of inductively coupling said current pulse into said sample via a magnetic field having a penetration depth sufficient to ensure said magnetic field is homogenous across said sample thickness and over the sample surface such that a uniform eddy current is established within said sample, and wherein the walls of said sample are sufficiently thin in comparison to the outer dimension of the sample cross-section that the temperature gradient across said sample is sufficiently small such that said sample is rapidly and substantially uniformly heated via Ohmic dissipation to a processing temperature between the glass transition temperature and the equilibrium melting point of the amorphous material; and wherein said shaping tool is capable of applying a deformational force sufficient to form said heated sample to a net shape article.

22. The apparatus of claim 21, wherein the inductive coil has a shape symmetric to the sample.

23. The apparatus of claim 21, wherein shaping tool is selected from the group consisting of injection molding, dynamic forging, stamp forging and blow molding.

24. The apparatus of claim 21, wherein the shaping tool further comprises a temperature- controlled heating element for heating said tool to a temperature preferably around the glass transition temperature of the amorphous material.

25. The apparatus of claim 21, further comprising one of either a pneumatic or magnetic drive system in operative relation to the shaping tool for applying the deformational force to the sample.

26. The apparatus of claim 21, wherein the amorphous metal has a resistivity that does not increase with temperature.

27. The apparatus of claim 21, wherein the temperature of the sample is increased at a rate of at least 500 K/sec.

28. The apparatus of claim 21, wherein the amorphous material has a relative change of resistivity per unit of temperature change (S) of no greater than about 1 x 10^"4 °C ¹ and a resistivity at room temperature (p₀) between about 80 and 300 μΩ-cm.

29. The apparatus of claim 21, wherein the quantum of electrical energy is at least about 100 J and a time constant of between about 10 μ≤ and 10 ms.

30. The apparatus of claim 21, wherein the processing temperature is about half-way between the glass transition temperature of the amorphous material and the equilibrium melting point of the alloy.

31. The apparatus of claim 21, wherein the processing temperature is such that the viscosity of the heated amorphous material is from about 1 to 10⁴ Pas-sec.

32. The apparatus of claim 21, wherein the sample is substantially defect free.

33. The apparatus of claim 21, wherein the ratio of the thickness of the sample wall to the radial dimension of the sample is selected such that the temperature variation across the sample thickness and over its area is less than 10% of the total induced average sample temperature.

34. The apparatus of claim 21, wherein the amorphous material is an alloy based on an elemental metal selected from the group consisting of Zr, Pd, Pt, Au, Fe, Co, Al, Mg, Ti, Ni and Cu.

35. The apparatus of claim 21, wherein the apparatus is capable of forming the article from the room temperature sample in a time of from about 100 μ≤ to about 1 s.

36. The apparatus of claim 21, wherein the coil generates a varying magnetic field that induces an eddy current within said sample, and wherein the magnetic skin depth of the varying magnetic field generated is large compared to wall thickness of the sample.