Forming of ferromagnetic metallic glass by rapid capacitor discharge

Abstract

An apparatus and method of uniformly heating, rheologically softening, and thermoplastically forming magnetic metallic glasses rapidly into a net shape using a rapid capacitor discharge forming (RCDF) tool are provided. The RCDF method utilizes the discharge of electrical energy stored in a capacitor to uniformly and rapidly heat a sample or charge of metallic glass alloy 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. Once the sample is uniformly heated such that the entire sample block has a sufficiently low process viscosity it may be shaped into high quality amorphous bulk articles via any number of techniques including, for example, injection molding, dynamic forging, stamp forging, sheet forming, and blow molding in a time frame of less than 1 second.

1 .- 17 . (canceled) 
     
     
         18 . A rapid capacitor discharge apparatus comprising:
 a source of electrical energy comprising a capacitive discharge circuit configured to generate a current pulse through a sample of metallic glass formed from a metallic glass forming alloy;   at least two electrodes configured to interconnect said source of electrical energy to said sample of metallic glass, said electrodes configured to attach to said sample such that substantially intimate connections are formed between said electrodes and said sample;   a shaping tool disposed in forming relation to said sample;   wherein said source of electrical energy is configured to discharge a quantum of electrical energy sufficient to heat the sample to a processing temperature between the glass transition temperature of a ferromagnetic metallic glass and the equilibrium melting point of the alloy, and wherein the electromagnetic skin depth of the electric field generated in said sample by the quantum of electrical energy is increased to be greater than the radius, width, thickness, and length of the sample, but wherein the rise time of the current pulse does not exceed the time associated with crystallizing the metallic glass at the processing temperature; and   wherein said shaping tool is configured to apply a deformational force sufficient to form said heated sample to a net shape article.   
     
     
         19 . The method of  claim 18 , wherein the shaping tool is selected from the group consisting of an injection mold, a dynamic forge, a stamp forge and a blow mold. 
     
     
         20 . The apparatus of  claim 18 , wherein the shaping tool is at least partially formed from at least one of the electrodes. 
     
     
         21 . The apparatus of  claim 18 , wherein the shaping tool further comprises a temperature-controlled heating element for heating said tool to a temperature preferably above the Curie temperature and below the glass transition temperature of the amorphous metal. 
     
     
         22 . The apparatus of  claim 18 , wherein the metallic glass has a resistivity that does not increase with temperature. 
     
     
         23 . The apparatus of  claim 18 , wherein the source of electrical energy is configured to increase the temperature of the sample at a rate of at least 500 K/sec. 
     
     
         24 . The apparatus of  claim 18 , wherein the metallic glass has a relative change of resistivity per unit of temperature change (S) of no greater than about 1×10 4 ° C. −1  and a resistivity at room temperature (ρ 0 ) between about 80 and 300 μΩ-cm. 
     
     
         25 . The apparatus of  claim 18 , wherein the quantum of electrical energy is at least about 100 J and the rise time for the current pulse is between about 1 ms and 100 ms. 
     
     
         26 . The apparatus of  claim 18 , wherein the processing temperature is about half-way between the glass transition temperature of the metallic glass and the equilibrium melting point of the alloy. 
     
     
         27 . The apparatus of  claim 18 , wherein the processing temperature is such that the viscosity of the heated amorphous metal is from about 1 to 10 4  Pas-sec. 
     
     
         28 . (canceled) 
     
     
         29 . The apparatus of  claim 18 , wherein the electrode is selected from the group consisting of Cu, Ag, or Ni, or an alloy containing at least 95 at % of one of Cu, Ag or Ni. 
     
     
         30 . The apparatus of  claim 18 , wherein the rise time of the current pulse is increased by increasing the capacitance of the circuit to achieve greater skin depth. 
     
     
         31 . The apparatus of  claim 18 , wherein the time constant of the capacitive discharge circuit is increased to achieve greater skin depth. 
     
     
         32 . The apparatus of  claim 31 , wherein the time constant of capacitive discharge circuit is increased by increasing the capacitance of the circuit to achieve greater skin depth. 
     
     
         33 . The apparatus of  claim 18 , wherein the source is configured to supply a pre-heating discharge configured to pre-heat the sample to a pre-heating temperature above the Curie temperature prior to discharging the quantum of electrical energy. 
     
     
         34 . The apparatus of  claim 33 , wherein the pre-heating temperature is above the Curie temperature and below the glass transition temperature. 
     
     
         35 . The apparatus of  claim 18 , wherein a sample of ferromagnetic metallic glass is disposed between the electrodes. 
     
     
         36 . The apparatus of  claim 18 , wherein the capacitive discharge circuit comprises one capacitor or more capacitors. 
     
     
         37 . The apparatus of  claim 31 , wherein the capacitive discharge circuit comprises an inductor in series with the sample to increase the time constant of the circuit to achieve greater skin depth. 
     
     
         38 . The apparatus of  claim 30 , wherein the capacitive discharge circuit comprises an inductor in series with the sample to increase the rise time to achieve greater skin depth.