US20150159240A1

US20150159240A1 - Melt fluxing method for improved toughness and glass-forming ability of metallic glasses and glass-forming alloys

Info

Publication number: US20150159240A1
Application number: US14/565,205
Authority: US
Inventors: Jong Hyun Na; Chase Crewdson; Danielle Duggins; Glenn Garrett; Maximilien Launey; Marios D. Demetriou; William L. Johnson
Original assignee: Apple Inc; Glassimetal Technology Inc
Current assignee: Apple Inc; Glassimetal Technology Inc
Priority date: 2013-12-09
Filing date: 2014-12-09
Publication date: 2015-06-11

Abstract

A method of fluxing the melt of metallic glass forming alloys is provided. Alloys fluxed according to the disclosed methods demonstrate a critical rod diameter that does not vary by more than 60% when varying the melt overheating. Moreover, metallic glasses produced from alloys fluxed according to the disclosed methods demonstrate notch toughness that does not vary by more than 30% when varying the melt overheating. Furthermore, a method by which used feedstock is purified such that its toughness and glass forming ability is restored for reuse is also disclosed. Recycled feedstock purified according to the disclosed method demonstrates critical rod diameter that is at least 70% of the critical rod diameter of the as-formed alloy. Also, metallic glasses produced from recycled feedstock demonstrate notch toughness of at least 70% of the notch toughness of a metallic glass produced from the as-formed alloy.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional 61/913,732, entitled “Melt Fluxing Method for Improved Toughness and Glass-Forming Ability of Metallic Glasses and Glass-Forming Alloys,” filed on Dec. 9, 2013, and U.S. Provisional 62/048,614, entitled “Melt Fluxing Method for Improved Toughness and Glass-Forming Ability of Metallic Glasses and Glass-Forming Alloys,” filed on Sep. 10, 2014, which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The disclosure is directed to a method of fluxing the melt of metallic glass forming alloys prior to quenching as means for improving the toughness and glass-forming ability, and also as means to recycle the alloy.

BACKGROUND

U.S. Pat. No. 5,797,443 is directed to a method of overheating the melt of a Zr-based metallic glass forming alloy above a threshold temperature, which is higher than the alloy liquidus temperature, in order to effect an increase in the degree to which the alloy can be undercooled to below the melting temperature by quenching. The authors in the U.S. Pat. No. 5,797,433 patent conjectured that by overheating the melt, certain oxide inclusions were dissolved into the melt and therefore could not serve as sites for heterogeneous nucleation of crystalline phases. The implication of a larger degree of undercooling is that the glass forming ability of the alloy is enhanced. However, the U.S. Pat. No. 5,797,433 patent did not demonstrate, suggest, or imply that overheating the melt above some threshold temperature would have any influence on the mechanical properties of the amorphous metal, such as the fracture toughness.
Patent application Ser. No. 14/161,434, entitled “Melt Overheating Method for Improved Toughness and Glass-Forming Ability of Metallic Glasses”, filed on Jan. 22, 2013, is directed to a method of overheating the melt of metallic glass forming alloys to improve the glass-forming ability of the alloy along with the toughness of the metallic glasses and metallic glass articles produced from such alloys. This provisional patent application discloses dramatic changes in the toughness of metallic glasses by increasing the overheating temperature during melt processing of the alloys. For example, the toughness of the metallic glasses changes from about 30 MPa m^1/2, when the melt is overheated to about 1200° C., to about 85 MPa m^1/2, when the melt is overheated to 1250° C.
There still remains a need to avoid overheating of alloys to high temperatures to achieve high toughness and high glass forming ability. Furthermore, a method by which used feedstock may be purified such that its toughness and glass forming ability is restored for reuse, that is, a method to “recycle” the feedstock, would be highly desirable from a technological perspective.

BRIEF SUMMARY

The disclosure is directed to methods of fluxing an alloy capable of producing a metallic glass and methods of producing metallic glasses from the fluxed alloy. In one embodiment, a method of producing a metallic glass, comprises:

- melting an alloy in contact with a fluxing agent to a fluxing temperature above the liquidus temperature of the alloy, T_liquidus;
- allowing the alloy melt to interact with the fluxing agent melt while in contact at the fluxing temperature to form a fluxed alloy;
- cooling the fluxed alloy to a temperature below the solidus of the alloy; and
- wherein the metallic glass produced from the fluxed alloy has a notch toughness that does not vary by more than 30% when varying the melt overheating temperature.

In some embodiments, the fluxed alloy is cooled to a temperature below the glass-transition temperature at a cooling rate sufficiently rapid to prevent crystallization of the alloy to form a metallic glass.
In some embodiments, the notch toughness can remain substantially constant over a range of overheating temperatures.
In another embodiment, the metallic glass produced from the fluxed alloy has a notch toughness that does not vary by more than 20% when varying the melt overheating.
In another embodiment, the critical rod diameter of the fluxed alloy does not vary by more than 60% when varying the melt overheating.
In one embodiment, the alloy melt is allowed to interact with the fluxing agent for at least 60 seconds.
In another embodiment, the alloy melt is allowed to interact with the fluxing agent for at least 15 minutes.
In another embodiment, the alloy melt is allowed to interact with the fluxing agent for at least 2 hours.
In another embodiment, the fluxing temperature is at least 100° C. above the liquidus temperature, T_liquidus.
In another embodiment, the method further includes heating the fluxed alloy to a temperature above the liquidus temperature to form a fluxed alloy melt. In such embodiments, the fluxed alloy melt is cooled to a temperature below the glass-transition temperature at a cooling rate sufficiently rapid to prevent crystallization of the alloy to form a metallic glass. In other such embodiments, the fluxed alloy melt can optionally be shaped to form a metallic glass article.
In yet another embodiment, the method further includes shaping or forming the melt into an article and simultaneously or subsequently quenching the melt at a high enough rate to form a shaped metallic glass article.
In yet another embodiment, a metallic glass article made according to the present method having a lateral dimension of at least 0.5 mm is capable of undergoing macroscopic plastic deformation without fracturing catastrophically under a bending load.
In yet another embodiment, the alloy melt is heated by a process comprising heating of the melt inductively, resistively (in a furnace), by a plasma arc, or by joule heating, wherein the melt is held in a crucible made of fused or crystalline silica, a ceramic such as alumina or zirconia, graphite, or a water-cooled hearth made of copper or silver.
In yet another embodiment, the alloy melt is quenched by a process comprising quenching the crucible containing the melt made of any of the aforementioned materials in a bath of room temperature water, iced water, or oil, or quenching the melt by driving it under pressure or pouring it into a metal mold made of copper, brass, or steel.
In yet another embodiment, the base metal of the alloy is a late-transition metal.
In yet another embodiment, the base metal of the alloy is Pd, Pt, Au, Ni, Fe, Co, Cu, or combinations thereof.
In yet another embodiment, the alloy contains at least one metalloid, semi-metal, or non-metal.
In yet another embodiment, the alloy contains at least one of P, Si, Ge, B, or C, or combinations thereof.
In yet another embodiment, the alloy composition has the following formula:
X_100-a-bY_aZ_b
where:
X is Ni, Fe, Co, Pd, Pt, Au, Cu or combinations thereof;
Y is Cr, Mo, Mn, Nb, Ta, Ni, Cu, Co, Fe, Pd, Pt, Ag or combinations thereof;
Z is P, B, Si, Ge, C or combinations thereof;
a is between 2 and 45 at %; and
b is between 15 and 25 at %.
In yet another embodiment, the fluxing agent comprises boron and oxygen.
In yet another embodiment, the fluxing agent is boron oxide.
In yet another embodiment, the fluxing agent is boric acid.
In other embodiments, the disclosure is directed to a method to recycle a scrap alloy capable of producing a metallic glass. The method of recycling comprises:

- heating the scrap alloy in contact with a fluxing agent to a fluxing temperature above the liquidus temperature, T_liquidus;
- allowing the scrap alloy melt to interact with the fluxing agent melt while in contact at the fluxing temperature to form a fluxed scrap alloy melt;
- cooling the fluxed scrap alloy melt to a temperature below the solidus temperature of the alloy to obtain a recycled alloy having a critical rod diameter of at least 70% of the critical rod diameter of the alloy in its as-formed state, and
- wherein the metallic glass produced from the recycled alloy has a notch toughness of at least 70% of the notch toughness of the metallic glass produced from alloy in its as-formed state.

In another embodiment, at least a fraction of the alloy has undergone at least two casting cycles.
In another embodiment, at least a fraction of the alloy has undergone at least five shaping cycles.
In another embodiment, at least a fraction of the alloy has undergone at least six shaping cycles.
In another embodiment, at least a fraction of the alloy has undergone at least ten shaping cycles.
In another embodiment, the recycled alloy has a critical rod diameter of at least 80% of the critical rod diameter of the as-formed alloy.
In another embodiment, the recycled alloy has a critical rod diameter of at least 90% of the critical rod diameter of the as-formed alloy.
In another embodiment, the metallic glass produced from the recycled alloy has a notch toughness of at least 80% of the notch toughness of the metallic glass produced from the as-formed alloy.
In another embodiment, the metallic glass produced from the recycled alloy has a notch toughness of at least 90% of the notch toughness of the metallic glass produced from the as-formed alloy.
In another embodiment, the oxygen concentration in the recycled alloy or a metallic glass produced from the recycled alloy does not exceed the oxygen concentration in the as-formed alloy by more than 40%.
In another embodiment, the oxygen concentration in the recycled alloy or a metallic glass produced from the recycled alloy does not exceed the oxygen concentration in the as-formed alloy by more than 20%.
In another embodiment, the oxygen concentration in the recycled alloy or a metallic glass produced from the recycled alloy does not exceed the oxygen concentration in the as-formed alloy by more than 10%.
In other embodiments, the disclosure is also direct to a recycled alloy having a critical rod diameter of at least 70% of critical rod diameter of the alloy in its as-formed state. In some such embodiments, the recycled alloy is produced by a process comprising heating a scrap alloy in contact with a fluxing agent to a fluxing temperature above the liquidus temperature, allowing the scrap alloy melt to interact with the fluxing agent melt while in contact at the fluxing temperature, and cooling the fluxed scrap alloy to a temperature below the solidus temperature of the alloy to obtain the recycled alloy having critical rod diameter of at least 70% of the alloy in its as-formed state.
In other embodiments, the disclosure is also directed to a metallic glass article comprising the recycled alloy. The metallic glass article produced from the recycled alloy having a notch toughness of at least 70% of the notch toughness of a metallic glass produced from the alloy in its as-formed state.
In other embodiments, the disclosure is also directed to a feedstock processed by any of the aforementioned methods.
In other embodiments, the disclosure is also directed to a metallic glass produced by a feedstock processed by any of the aforementioned methods.
In other embodiments, the disclosure is also directed to a metallic glass article produced by a feedstock processed by any of the aforementioned methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to the following figures and data graphs, which are presented as various embodiments of the disclosure and should not be construed as a complete recitation of the scope of the disclosure.

FIG. 1 provides a plot of the notch toughness of metallic glass Ni₆₉Cr_8.5Nb₃P_16.5B₃, in accordance with embodiments of the disclosure, as a function of melt overheating for fluxed and unfluxed samples.

FIG. 2 provides a plot of the critical rod diameter of alloy Ni₆₉Cr_8.5Nb₃P_16.5B₃, in accordance with embodiments of the disclosure, as a function of melt overheating for fluxed and unfluxed samples.

FIG. 3 provides a plot of the critical rod diameter of the as-formed and recycled feedstock of composition Ni_71.4Cr_5.52Nb_3.38P_16.67B_3.03at each casting/fluxing cycle, in accordance with embodiments of the disclosure.

FIG. 4 provides x-ray diffractograms of an 8-mm rod produced from the as-formed feedstock and of an 8-mm rod produced from the recycled feedstock that has undergone 6 casting/fluxing cycles.

FIG. 5 provides a plot of the notch toughness of metallic glasses produced from as-formed and recycled feedstock of composition Ni_71.4Cr_5.52Nb_3.38P_16.67B_3.03at each casting/fluxing cycle, in accordance with embodiments of the disclosure.

FIG. 6 provides differential calorimetry scans of a metallic glass produced from the as-formed feedstock and of a metallic glass produced from a recycled feedstock that has undergone 6 casting/fluxing cycles. The glass-transition, crystallization, solidus, and liquidus temperatures T_g, T_x, T_sand T_lfor the two metallic glasses are designated by arrows.

DETAILED DESCRIPTION

The disclosure is directed to a method of fluxing the melt of a metallic glass forming alloy prior to quenching as means for improving the toughness of the metallic glass produced from the alloy and the glass-forming ability of the alloy, and also as means to recycle the alloy. In some aspects of the disclosure, if an alloy is fluxed according to the disclosed methods, the notch toughness of the metallic glass produced from the fluxed alloy does not change substantially when varying the melt overheating temperature.
The notch toughness of a metallic glass can vary with changing processing parameters. If an alloy is not fluxed according to the disclosed methods, the notch toughness of the metallic glass produced from the alloy can vary dramatically if the alloy melt overheating temperature is varied prior to producing the metallic glass. In one non-limiting example, when the melt overheating temperature prior to producing the metallic glass is below 1250° C., the notch toughness of the metallic glass is about 30 MPa m^1/2. Meanwhile, when the melt overheating temperature prior to producing the metallic glass is above 1250° C., the notch toughness of the metallic glass increases with the increased melt overheating temperature (e.g. notch toughness reaches approximately 80 MPa m^1/2when the melt overheating temperature is 1350° C.). If an alloy is fluxed in accordance with the disclosed methods, the notch toughness of the metallic glass does not increase substantially when increasing the melt overheating temperature. In some embodiments of the disclosure, when an alloy is fluxed in accordance with the disclosed methods, the notch toughness of the metallic glass does not change by more than 30% when varying the melt overheating temperature prior to producing the metallic glass. In one non-limiting example, when an alloy is fluxed in accordance with the disclosed methods, the notch toughness of the metallic glass is between 70 and 90 MPa m^1/2when the melt overheating temperature is between 950° C. to 1350° C. In other embodiments, the notch toughness of the metallic glass can remain substantially constant over a range of melt overheating temperatures.
In other aspects, the disclosed methods are directed to recycling scrap alloy, that is, alloy that has been used in an operation requiring that the alloy be heated to a temperature above its glass-transition temperature in addition to the heating process in which it was formed from the elemental constituents. In some embodiments, such operations include shaping operations used to shape the heated alloy prior to cooling it to form a metallic glass article. In typical shaping operations, only a fraction of the feedstock used ends up as the article product, while the remainder of the feedstock ends up as scrap. There can be a considerable amount of scrap alloy. However, since the scrap alloy has undergone at least one heating process to a temperature above its glass-transition temperature in addition to the heating process in which it was formed from the elemental constituents, additional oxygen can be entrained in it as compared to the oxygen content in the as-formed state. In various aspects, additional oxygen is known to have an adverse effect on the glass forming ability of the alloy and mechanical properties (specifically toughness) of the metallic glass. In some embodiments of the disclosed methods, excess oxygen from high temperature processing is removed from glass-forming alloy scrap and allows for the reuse of the purified scrap to produce metallic glass products with substantially similar properties when compared to products made from as-formed alloy.

DEFINITIONS

“Melt overheating temperature” means, for the purpose of this disclosure, the temperature to which the alloy melt is heated above the liquidus temperature, T_liquidus, prior to producing the metallic glass.
“Critical rod diameter” means, for the purpose of this disclosure, the maximum rod diameter that can be formed with the metallic glass phase by rapidly water quenching a quartz tube containing the alloy melt, whereas the quartz tube has wall thickness of 0.5 mm.
“Notch toughness” means, for the purpose of this disclosure, the stress intensity factor at crack initiation measured on a 3 mm diameter rod containing a notch with length between 1 and 2 mm and root radius between 0.1 and 0.15 mm, and is the measure of the material's ability to resist fracture in the presence of a notch. The notch toughness is also a measure of the work required to propagate a crack originating from a notch. A high Kq ensures that the material will be tough in the presence of defects.
“Shaping cycle” means, for the purpose of this disclosure, the process where the alloy, either in the crystalline or amorphous state, is heated above the glass transition temperature, and in some embodiments above the solidus temperature, then undergoes a shaping operation wherein the alloy is shaped into a desired geometry, and subsequently the shaped alloy cools. In some embodiments, the shaping operation may be casting, wherein the alloy is shaped by pouring it or injecting it into a mold. In other embodiments, the shaping operation may be forging or stamping, wherein the alloy is shaped by being pressed against dies. In yet other embodiments, the shaping operation may be blow molding, wherein the alloy is shaped by the application of gas pressure.
“As-formed alloy” or “as-formed feedstock” or “an alloy in its as-formed state” means, for the purpose of this disclosure, a material that has not undergone any heating process to a temperature above its glass-transition temperature other than the heating process in which it was formed from the elemental constituents.
“Scrap alloy” means, for the purpose of this disclosure, a material that has undergone at least one heating process to a temperature above its glass-transition temperature in addition to the heating process in which it was formed from its elemental constituents. In some embodiments, the oxygen concentration in a scrap alloy is higher than the alloy in its as-formed state.
The method for evaluating the critical rod diameter involves heating an alloy to a temperature above its glass-transition temperature, and more specifically above its melting temperature. It is noted here that when evaluating the critical rod diameter of an “as-formed” alloy, the evaluated critical rod diameter corresponds to the as-formed alloy regardless of the fact that the as-formed alloy was heated to a temperature above its glass-transition temperature for this evaluation.
Effect of Fluxing as Compared to the Effect of Overheating
The need for overheating the melt of metallic glass forming alloys in order to improve the glass forming ability of the alloys and the toughness of the metallic glasses produced from the alloys is associated with several technological drawbacks. First, handling metallic melts overheated to high temperatures is associated with adverse chemical effects such as higher oxidation of the melt and stronger reaction of the melt with the crucible leading to impurities in the alloys and metallic glasses produced from the melts. Furthermore, attempting to produce metallic glass articles by casting or pouring melts that have been overheated to high temperatures into metal tools may degrade the tool more quickly thereby shortening its tool life. Herein, a method is disclosed by which the glass forming ability of the alloys and the toughness of the metallic glasses produced from the alloys are improved without the need for overheating to high temperatures.
In patent application Ser. No. 14/161,434, the alloy melt was not fluxed prior to quenching to form a metallic glass. The present disclosure provides a method of producing metallic glass by fluxing the melt while avoiding overheating it to very high temperatures in order to achieve high toughness and high glass forming ability.
In accordance with the figures and disclosure, a method of fluxing the melt of a metallic glass forming alloy is provided. In many embodiments, this method involves heating the alloy in contact with a fluxing agent to a fluxing temperature above the liquidus temperature, T_liquidus, allowing the alloy melt to interact with the fluxing agent melt while in contact at the fluxing temperature for a certain fluxing time, and cooling the two melts to room temperature. In one embodiment, the fluxing time is at least 60 s. In another embodiment, the fluxing time is at least 15 minutes. In yet another embodiment the fluxing time is at least 2 hours.
Alloys fluxed according to embodiments of the disclosure may produce metallic glasses that may demonstrate notch toughnesses that do not vary substantially when varying the melt overheating. In some embodiments, alloys fluxed according to embodiments of the present disclosure may produce metallic glasses that may demonstrate notch toughnesses that do not vary by more than 30% when varying the melt overheating. In other embodiments, alloys fluxed according to embodiments of the present disclosure may produce metallic glasses that may demonstrate notch toughnesses that do not vary by more than 20% when varying the melt overheating. In such embodiments, the toughness is substantially independent of melt overheating. Furthermore, alloys fluxed according to the embodiments of the disclosure may demonstrate glass-forming abilities that do not vary substantially when varying the melt overheating. Specifically, alloys fluxed according to embodiments of the present disclosure may demonstrate glass forming abilities that do not vary by more than 60% when varying the melt overheating.
Embodiments of the disclosure are also extended to shaping or forming the melt into an article and simultaneously or subsequently quenching the melt at a rate high enough to prevent crystallization and form a shaped metallic glass article. Owing to the improvement in metallic glass toughness effected by the method, metallic glass articles made according to embodiments of the disclosure having a lateral dimension that are sufficiently thin, may be capable of undergoing macroscopic plastic deformation without fracturing catastrophically under a bending load. Specifically, metallic glass articles made according to embodiments of the disclosure having a lateral dimension of less than 0.5 mm may be capable of undergoing macroscopic plastic deformation without fracturing catastrophically under a bending load. In some embodiments, metallic glass articles made according to embodiments of the disclosure having a lateral dimension of less than 1 mm may be capable of undergoing macroscopic plastic deformation without fracturing catastrophically under a bending load.
In embodiments the alloy may be heated by any suitable process, including without limitation, heating inductively, resistively (in a furnace), by a plasma arc, or by joule heating. Melt crucibles may comprise materials that include, without limitation, fused or crystalline silica, a ceramic such as alumina or zirconia, graphite, or a water-cooled hearth made of copper or silver, or other suitable materials that are stable at the melt temperatures considered herein. The melt may be quenched by any suitable method, including without limitation, quenching the crucible containing the melt in a bath of room temperature water, iced water, or oil. Alternatively, the melt may be quenched by driving it under pressure or pouring it into a metal mold made of copper, brass, or steel.
The disclosed method is suitable for any metallic glass-forming alloy that can be fluxed with a specific fluxing agent. In many embodiments, the alloy has a base metal that is a late-transition metal. For example, the base metal of the alloy may be Pd, Pt, Au, Ni, Fe, Co, or Cu. In other embodiments, the alloy may contain at least one metalloid, semimetal, or nonmetal. For example, the alloy may contain one of P, Si, B, C, or combinations thereof. In yet other embodiments, the base metal of alloy may be a metal from the Iron Triad comprising Ni, Fe, Co or combinations thereof, it may contain other transition metals such as Cr, Mo, Mn, Nb, Ta or combinations thereof at combined atomic concentration ranging between 2 and 20%, and elements such P, B, Si, Ge, C or combinations thereof at combined atomic concentration ranging between 15 and 25%.
Embodiments of the disclosure are directed to any suitable fluxing agent. In one example, the fluxing agent may be based on boron and oxide. For example, the fluxing agent may be boron oxide.
To demonstrate the effects of the disclosed melt fluxing method on glass forming ability and toughness, metallic glass forming alloy Ni₆₉Cr_8.5Nb₃P_16.5B₃, disclosed in a recent application (U.S. patent application Ser. No. 14/067,521, entitled “Bulk Nickel-Based Chromium and Phosphorous Bearing Metallic Glasses with High Toughness”, filed on Oct. 30, 2012, which is incorporated herein by reference), was studied.
Toughness may vary considerably by changing certain processing parameters, like for example, by varying the overheating temperature of the melt, by fluxing the melt versus not fluxing it, by changing the fluxing agent, by varying the fluxing temperature or fluxing time, among others. FIG. 1 provides a plot of the notch toughness of metallic glass Ni₆₉Cr_8.5Nb₃P_16.5B₃as a function of melt overheating for fluxed and unfluxed samples. As presented in FIG. 1, if the alloy is not fluxed, the notch toughness of the metallic glass varies dramatically with varying melt overheating. Specifically, when the melt overheating temperature is below 1250° C., the notch toughness of the metallic glass is roughly at a value of about 30 MPa m^1/2. While, when the melt overheating is above 1250° C., the notch toughness of the metallic glass increases with the increased melt overheating; reaching values as high as approximately 80 MPa m^1/2at a melt overheating temperature of 1350° C. By contrast, if the alloy is fluxed according to the disclosed method, the notch toughness of the metallic glass does not vary substantially with varying melt overheating temperature. Specifically, the notch toughness is between 70 and 90 MPa m^1/2when the melt overheating temperature ranges from 950 to 1350° C. This notch toughness range is essentially within the reported measurement error, thereby suggesting that notch toughness remains substantially constant with increasing melt overheating. FIG. 1 therefore demonstrates that the notch toughness of the metallic glasses formed from the fluxed alloys according to the disclosed fluxing method is substantially independent of the melt overheating temperature. This is a surprising result. For example, the notch toughness values corresponding to the fluxed alloy overheated to temperatures as low as 950° C. are about as high as the notch toughness values corresponding to the unfluxed alloy overheated to temperatures as high as 1350° C. The significance of this unexpected finding (that the high notch toughness of the metallic glass is achieved without having to overheat the melt) allows the melt to be processed without the need for high overheating temperatures. This is advantageous because high overheating temperatures can promote oxidation of the melt and degradation of the tooling.
FIG. 2 provides a plot of the critical rod diameter of metallic glass Ni₆₉Cr_8.5Nb₃P_16.5B₃as a function of melt overheating for fluxed and unfluxed samples. As presented in FIG. 2, if the alloy is not fluxed, the critical rod diameter varies dramatically when varying the melt overheating. Specifically, when the melt overheating is below 1050° C., the critical rod diameter is below 2 mm, when the melt overheating is between 1050 and 1250° C., the critical rod diameter increases from 2 mm to 10 mm, while when the melt overheating is above 1250° C., the critical rod diameter remains constant at 10 mm. By contrast, if the alloy is fluxed according to the method, the critical rod diameter varies substantially less with varying melt overheating. Specifically, when the melt overheating is below 1000° C., the critical rod diameter is constant at 6 mm, when the melt overheating ranges from 1000 to 1200° C., the critical rod diameter increases from 6 mm to 12 mm, while when the melt overheating is above 1200° C., the critical rod diameter remains constant at 12 mm. FIG. 2 therefore demonstrates that the critical rod diameter is higher for the fluxed alloy as compared to the unfluxed alloy for any overheating temperature. FIG. 2 also shows that the critical rod diameter of the fluxed alloy is constant at a considerably high value as the overheating decreases towards T_liquiduswhereas the critical rod diameter of the unfluxed alloy decreases rapidly to very low values as the overheating temperature decreases towards T_liquidus.
Use of Fluxing to Recycle Metallic Glass Forming Alloys
The ability to recycle scrap alloy, that is, alloy that has already been used in a shaping operation (i.e. it has been heated and processed at a high temperature) is of technological importance. In typical shaping operations, only a fraction of the feedstock used ends up as the article product, while the remainder of the feedstock ends up as scrap. For example, in a typical die casting process the article product is typically 50% or less of the original feedstock volume, while the remainder of the feedstock outside the article cavity ends up filling the gate, runners, overflows, etc. As such, there is a considerable amount of scrap alloy. But since the scrap has already been processed at a high temperature (in the molten state in the case of casting), it is expected that additional oxygen is entrained in it as compared to the oxygen content in the as-formed state. In metallic glass formers, oxygen is known to have an adverse effect on the glass forming ability of the alloy and mechanical properties (specifically toughness) of the metallic glass. Discovering a method by which excess oxygen from high temperature processing is removed from glass-forming alloy scrap would greatly improve the cycle time, reduce cost, and overall economics of the shaping process, as it would enable the reuse of the scrap to produce article products with substantially similar properties as products made from as-formed alloy.
Shaping operations that take place in the molten state, such as casting, are typically not performed in pure air, but in some kind of partially inert atmosphere where the oxygen concentration is significantly lower than that in pure atmospheric air, in order to limit the oxygen exposure of the high temperature melt. Partially inert atmospheres are typically achieved by first applying partial vacuum conditions in the environment where shaping will take place, and then backfilling with an inert gas such as argon, helium, nitrogen, etc. Partial vacuum conditions can range from 10⁻⁶to 1 mbar, but typically range from 10⁻⁵to 0.1 mbar, and more typically between 10⁻⁴and 0.01 mbar.
This process of purifying the scrap alloy of oxygen and reusing it is typically referred to as “recycling”. The scrap alloy to be recycled is only a fraction of the original feedstock, and after purification is typically mixed with more alloy to make up the volume of the new feedstock, the fraction of a given recycled feedstock in a new feedstock would diminish over time. Specifically, the fraction of a recycled feedstock that can be traced in a new feedstock over multiple cycles decreases as xn, where x is the volume fraction of the scrap in the total feedstock volume and n is the number of shaping and purification cycles. For example, if in a given shaping process the article product volume is 60% of the feedstock volume then x=0.4. The volume fraction of a given scrap feedstock in the new feedstock after the scrap feedstock has undergone 5 shaping/purification cycles, i.e. n=5, is 0.4⁵, or 1%. If the alloy properties have not been substantially degraded when the scrap volume fraction is only 1% of the total feedstock volume, then the scrap can be considered to be “solely recyclable”, as the alloy would be substantially untraceable if substantial degradation occurs at later cycles.
In some embodiments of the disclosure, if the alloy properties have not been substantially degraded when the scrap volume fraction is less than 10% of the total feedstock volume, then the scrap can be considered to be “solely recyclable.” In other embodiments of the disclosure, if the alloy properties have not been substantially degraded when the scrap volume fraction is less than 5% of the total feedstock volume, then the scrap can be considered to be “solely recyclable.” In yet other embodiments of the disclosure, if the alloy properties have not been substantially degraded when the scrap volume fraction is less than 2% of the total feedstock volume, then the scrap can be considered to be “solely recyclable.”
In some embodiments of the disclosure, the glass forming ability of the recycled alloy is considered to be substantially degraded if the critical rod diameter is less than 70% of the critical rod diameter of the as-formed alloy. In other embodiments, the glass forming ability of the recycled alloy is considered to be substantially degraded if the critical rod diameter is less than 80% of the critical rod diameter of the as-formed alloy. In yet other embodiments, the glass forming ability of the recycled alloy is considered to be substantially degraded if the critical rod diameter is less than 90% of the critical rod diameter of the as-formed alloy.
In some embodiments of the disclosure, the toughness of the recycled alloy is considered to be substantially degraded if the notch toughness of a metallic glass produced from the recycled alloy is less than 70% of the notch toughness of a metallic glass produced from the as-formed alloy. In other embodiments, the toughness of the recycled alloy is considered to be substantially degraded if the notch toughness of a metallic glass produced from the recycled alloy is less than 80% of the notch toughness of a metallic glass produced from the as-formed alloy. In yet other embodiments, the toughness of the recycled alloy is considered to be substantially degraded if the notch toughness of a metallic glass produced from the recycled alloy is less than 90% of the notch toughness of a metallic glass produced from the as-formed alloy.
To demonstrate the ability of the fluxing method disclosed herein to recycle scrap feedstock, metallic glass forming alloy Ni_71.4Cr_5.52Nb_3.38P_16.67B_3.03was studied, disclosed in U.S. patent application Ser. No. 14/067,521, entitled “Bulk Nickel-Based Chromium and Phosphorous Bearing Metallic Glasses with High Toughness,” filed on Oct. 30, 2013, which is incorporated herein by reference. As a shaping operation, the counter-gravity casting process is used, wherein molten liquid contained in fused silica is injected upwards into a mold using gas pressure. The mold had a rod-shaped cavity where a 3-mm diameter rod can be cast.
Six cycles of shaping by casting and fluxing were performed using the same feedstock and the critical rod diameter of the recycled alloy and notch toughness of a metallic glass made from the recycled alloy are evaluated every cycle, and compared to those corresponding to the as-formed feedstock.
The critical rod diameter of the recycled feedstock at each cycle is presented in Table 1 and plotted in FIG. 3. As seen in Table 1 and FIG. 1, the critical rod diameter after each casting/fluxing cycle remains unchanged at 8 mm, which is equal to the critical rod diameter of the as-formed feedstock. The x-ray diffractograms of an 8-mm rod produced from the as-formed feedstock and of an 8-mm rod produced from the recycled feedstock that has undergone 6 casting/fluxing cycles are presented in FIG. 4, verifying the fully amorphous structure of the rods.
The notch toughness of the recycled feedstock at each cycle is presented in Table 1 and plotted in FIG. 5. As seen in Table 1 and FIG. 5, the notch toughness after each casting/fluxing cycle remains substantially unchanged, as it remains roughly within the error margin of the measurement.

TABLE 1

Critical rod diameter of as-formed and recycled feedstock
and notch toughness of metallic glass produced from
as-formed and recycled feedstock of composition
Ni_71.4Cr_5.52Nb_3.38P_16.67B_3.03at each casting/fluxing cycle.

Casting/fluxing	Critical rod diameter	Notch Toughness
cycles	d_cr(mm)	K_Q(MPa m^1/2)

0	8	87.9 ± 4.3
1	8	89.8 ± 4.8
2	8	85.4 ± 5.2
3	8	85.1 ± 3.6
4	8	92.2 ± 5.2
5	8	87.5 ± 3.3
6	8	89.4 ± 2.2

The differential calorimetry scans of a metallic glass produced from the as-formed feedstock and of a metallic glass produced from a recycled feedstock that has undergone 6 casting/fluxing cycles are presented in FIG. 6. The glass-transition temperature T_g, crystallization temperature T_x, solidus temperature T_s, and liquidus temperature T_l, for the two metallic glasses are designated by arrows, from left to right, in FIG. 6 and listed in Table 2. As seen in FIG. 6 and Table 2, casting and fluxing the alloy up to 6 times has not substantially altered these transition temperatures.

TABLE 2

Glass-transition, crystallization, solidus, and liquidus
temperatures corresponding to metallic glasses of
composition Ni_71.4Cr_5.52Nb_3.38P_16.67B_3.03produced
from the as-formed feedstock and from the recycled
feedstock that has undergone 6 casting/fluxing cycles.

Casting/fluxing
cycles	T_g(° C.)	T_x(° C.)	T_s(° C.)	T_l(° C.)

0	395.6	435.8	830.5	880.4
6	392.2	430.0	831.9	876.3

Chemical analysis was performed to investigate the concentration of C, N, O, and H impurities in a metallic glass produced from the as-formed feedstock and a metallic glass produced from a recycled feedstock that has undergone 6 casting/fluxing cycles. The concentrations of impurities in the two metallic glasses are presented in Table 3. As seen in Table 3, the concentrations of all impurities are lower in the metallic glass produced from a recycled feedstock that has undergone 6 casting/fluxing cycles. Even the concentration of 0 impurity is lower, in spite of the fact that the recycled metallic glass has undergone multiple heating cycles to the high temperature liquid state where the alloy is highly prone to oxidation. The lower concentration of oxygen as well as other impurities in the metallic glass produced from a recycled feedstock that has undergone 6 casting/fluxing cycles can be attributed to the ability of the fluxing process to purify the melt. This purification ability explains why no degradation in the glass forming ability and the toughness is observed on the recycled feedstock and metallic glass produced from the recycled feedstock.

TABLE 3

Concentration of C, N, O, and H impurities in metallic
glasses of composition Ni_71.4Cr_5.52Nb_3.38P_16.67B_3.03produced
from the as-formed feedstock and a recycled feedstock
that has undergone 6 casting/fluxing cycles.

Casting/fluxing

Impurity (ppm wt. %)

cycles	C	N	O	H

0	110	6	41	4
6	28	<5	28	4

Therefore, in embodiments according to the disclosure, the oxygen concentration in the recycled alloy or a metallic glass produced from the recycled alloy does not exceed the oxygen concentration in the as-formed alloy by more than 40%. In another embodiment, the oxygen concentration in the recycled alloy or a metallic glass produced from the recycled alloy does not exceed the oxygen concentration in the as-formed alloy by more than 20%. In another embodiment, the oxygen concentration in the recycled alloy or a metallic glass produced from the recycled alloy does not exceed the oxygen concentration in the as-formed alloy by more than 10%. In another embodiment, the oxygen concentration in the recycled alloy or a metallic glass comprising the recycled alloy is less than the oxygen concentration in the as-formed alloy.
Method of Producing the Alloy Ingots
In some embodiments, a method for producing the alloy ingots in the disclosure involves inductive melting of the appropriate amounts of elemental constituents in a quartz tube sealed under partial atmosphere of argon. The alloy ingots in the disclosure were produced using this method. The particular purity levels of the constituent elements were as follows: Ni 99.995%, Cr 99.996%, Nb 99.95%, B 99.5%, and P 99.9999%.
Description of the Method of Fluxing the Alloy Ingots
The particular method for fluxing the alloy ingots in the present disclosure involves melting the alloy ingot in contact with boron oxide in a quartz tube under one atmosphere of argon, allowing the two melts to interact for 15 minutes at about 1200° C., and subsequently quenching in a bath of room temperature water. The boron oxide used had purity of 99.999%, and contained 200 ppm of H₂O or less.
Method of Shaping the Metallic Glasses Using Counter-Gravity Casting
In various embodiments, a metallic glass produced from the recycled alloy can be shaped using a counter-gravity casting process. In the counter-gravity casting process, molten liquid contained in fused silica is injected upwards (against gravity) into a mold using gas pressure. The metallic glasses used in the disclosure to investigate the use of fluxing to recycle glass-forming alloys were shaped using this method. Feedstock of about 100 g was used. An inert atmosphere was created in the melt chamber by first applying vacuum at 5×10⁻²mbar and subsequently following several purges with argon, an argon atmosphere was established having a pressure of −20 in-Hg. The feedstock is heated inductively first to 1350° C. and back to 1300° C. to create a homogeneous high temperature melt, and is subsequently urged upwards using an argon pressure of 10 psi through a fused silica tube of 4 mm inner diameter into an H13 tool steel mold having a rod-shaped cavity with a diameter of 3 mm and length of 100 mm. The melt is rapidly cooled in the mold to produce an amorphous rod 3 mm in diameter and 100 mm in length. After each casting cycle, the cast rod can be used for notch toughness and chemical analysis testing, while the scrap material undergoes a new fluxing/casting cycle.
Method of Producing the Metallic Glasses Using Quartz-Tube Water-Quenching
In various embodiments, a metallic glass rod can be produced from an alloy using quart-tub water quenching. The method involves melting the alloy ingot in a quartz tube having 0.5 mm thick walls and a certain inner diameter in a furnace under high purity argon. After processing at a specific temperature, the melt is rapidly quenched in a room-temperature water bath to produce a cylindrical rod having a certain diameter and a length that is at least twice the diameter. If the rod diameter is equal to or less the alloy critical rod diameter, the rod would be amorphous (i.e. a metallic glass).
Method of Assessing the Critical Rod Diameter
In various embodiments, the critical rod diameter of an alloy can be assessed by producing metallic glass rods of various diameters using the quartz-tube water-quenching method and determining the maximum diameter of the rod at which the metallic glass phase can be formed. The metallic glass phase is verified by performing x-ray diffraction analysis on a cross section of the rod. The critical rod diameter of the alloys used in the present disclosure to investigate the use of fluxing to recycle glass-forming alloys and the effect of fluxing as compared to the effect of overheating was assessed using this method.
Method of Measuring Notch Toughness
Measurement of notch toughness of the metallic glasses of the disclosure was performed on 3-mm diameter amorphous rods at room temperature according to the particular method as follows: the rods were notched using a wire saw with a root radius of between 0.10 and 0.15 mm to a depth of approximately half the rod diameter. The notched specimens were tested in a 3-point beam configuration with a span of 12.7 mm, and with the notched side carefully aligned and facing downward. The critical fracture load was measured by applying a monotonically increasing load at constant crosshead speed of 0.001 mm/s using a screw-driven testing frame. At least three tests were performed, and the variance between tests is included in the notch toughness plots. The stress intensity factor for the geometrical configuration employed here was evaluated using the analysis by Murakimi (Y. Murakami, Stress Intensity Factors Handbook, Vol. 2, Oxford: Pergamon Press, p. 666 (1987)). The notch toughness of metallic glass rods produced by the counter-gravity casting method and used in the disclosure to investigate the use of fluxing to recycle glass-forming alloys was assessed using this method. The notch toughness of metallic glass rods produced by the quartz-tube water-quenching method and used in the disclosure to investigate the effect of fluxing as compared to the effect of overheating was also assessed using this method.
Metallic glasses described herein, or produced by the methods or from alloys disclosed herein, can be valuable in the fabrication of electronic devices. An electronic device herein can refer to any electronic device known in the art. For example, it can be a telephone, such as a mobile phone, and a land-line phone, or any communication device, such as a smart phone, including, for example an iPhone®, and an electronic email sending/receiving device. It can be a part of a display, such as a digital display, a TV monitor, an electronic-book reader, a portable web-browser (e.g., iPad®), and a computer monitor. It can also be an entertainment device, including a portable DVD player, conventional DVD player, Blue-Ray disk player, video game console, music player, such as a portable music player (e.g., iPod®), etc. It can also be a part of a device that provides control, such as controlling the streaming of images, videos, sounds (e.g., Apple TV®), or it can be a remote control for an electronic device. It can be a part of a computer or its accessories, such as the hard drive tower housing or casing, laptop housing, laptop keyboard, laptop track pad, desktop keyboard, mouse, and speaker. The article can also be applied to a device such as a watch or a clock.
Having described several embodiments, it will be recognized by those skilled in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the disclosure. Those skilled in the art will appreciate that the presently disclosed embodiments teach by way of example and not by limitation. Therefore, the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present disclosure. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.

Claims

1. A method of producing a metallic glass, comprising:

melting an alloy in contact with a fluxing agent by heating to a fluxing temperature above the liquidus temperature, T_liquidusof the alloy;

allowing the alloy melt to interact with the fluxing agent melt while in contact at the fluxing temperature to form a fluxed alloy;

cooling the fluxed alloy to a temperature below the solidus temperature of the fluxed alloy;

wherein the metallic glass formed of the fluxed alloy has a notch toughness that does not vary by more than 30% when varying the melt overheating temperature.

2. The method of claim 1, wherein the fluxed alloy is cooled to a temperature below the glass-transition temperature at a cooling rate sufficiently rapid to prevent crystallization of the alloy to form a metallic glass.

3. The method of claim 1 further comprising:

heating the fluxed alloy to a temperature above T_liquidusto form a fluxed alloy melt;

cooling the fluxed alloy melt to a temperature below the glass-transition temperature at a cooling rate sufficiently rapid to prevent crystallization of the alloy to form a metallic glass.

4. The method of claim 1, wherein the metallic glass formed of the fluxed alloy has a notch toughness that does not vary by more than 20% when varying the melt overheating temperature.

5. The method of claim 1, wherein the alloy melt interacts with the fluxing agent melt while in contact at the fluxing temperature for a fluxing time of at least 60 seconds.

6. The method of claim 1, wherein the fluxing temperature is at least 100° C. above the liquidus temperature, T_liquidus.

7. The method of claim 1, wherein the critical rod diameter of the fluxed alloy does not vary by more than 60% when varying the melt overheating temperature.

8. The method of claim 1, wherein the metal with the highest atomic fraction in the alloy is Pd, Pt, Au, Ni, Fe, Co, or Cu.

9. The method of claim 1, wherein the alloy comprises a metalloid, semi-metal, or non-metal, where a metalloid, semi-metal, or non-metal is P, Si, Ge, C, B, or combinations thereof.

10. The method of claim 1, wherein the fluxing agent comprises boron and oxygen.

11. A method of recycling a scrap alloy capable of producing a metallic glass, comprising:

heating the scrap alloy in contact with a fluxing agent to a fluxing temperature above the liquidus temperature, T_liquidus;

allowing the scrap alloy melt to interact with the fluxing agent melt while in contact at the fluxing temperature to form a fluxed scrap alloy melt;

cooling the fluxed scrap alloy melt to a temperature below the glass transition of the alloy to obtain a recycled alloy having a critical rod diameter of at least 70% of the critical rod diameter of the alloy in its as-formed state; and

wherein the metallic glass produced from the recycled alloy has a notch toughness of at least 70% of the notch toughness of the metallic glass produced from the alloy in its as-formed state.

12. The method of claim 11, wherein the scrap alloy has undergone at least two shaping cycles.

13. The method of claim 11, wherein the recycled alloy has a critical rod diameter of at least 80% of the critical rod diameter of the as-formed alloy.

14. The method of claim 11, wherein the metallic glass produced has a notch toughness of at least 80% of the notch toughness of the metallic glass produced from the alloy in its as-formed state.

15. The method of claim 11, wherein the recycled alloy has an oxygen concentration that does not exceed the oxygen concentration of the as-formed alloy by more than 40%.

16. The method of claim 11, wherein the metal with the highest atomic fraction in the alloy is Pd, Pt, Au, Ni, Fe, Co, or Cu.

17. The method of claim 11, wherein the alloy comprises a metalloid, semi-metal, or non-metal, where a metalloid, semi-metal, or non-metal is P, Si, Ge, C, B, or combinations thereof.

18. The method of claim 11, wherein the fluxing agent comprises boron and oxygen.

19. A recycled alloy having a critical rod diameter of at least 70% of the critical rod diameter of the alloy in its as-formed state, the recycled alloy produced by a process comprising:

heating a scrap alloy in contact with a fluxing agent to a fluxing temperature above the liquidus temperature, T_liquidus;

allowing the scrap alloy melt to interact with the fluxing agent melt while in contact at the fluxing temperature;

cooling the fluxed scrap alloy melt to a temperature below the glass transition temperature of the alloy to obtain the recycled alloy having the critical rod diameter of at least 70% of the critical rod diameter of the alloy in its as-formed state.

20. A metallic glass article formed comprising the recycled alloy of claim 19, wherein the metallic glass article produced from the recycled alloy has a notch toughness of at least 70% of the notch toughness of a metallic glass produced from the alloy in its as-formed state.