US20090114317A1

US20090114317A1 - Metallic mirrors formed from amorphous alloys

Info

Publication number: US20090114317A1
Application number: US11/577,052
Authority: US
Inventors: Steve Collier; Atakan Peker
Original assignee: Liquidmetal Technologies Inc
Current assignee: Liquidmetal Technologies Inc
Priority date: 2004-10-19
Filing date: 2005-10-19
Publication date: 2009-05-07
Also published as: WO2006060081A3; WO2006060081A2

Abstract

Metallic mirrors made of bulk-solidifying amorphous alloys, the bulk-solidifying amorphous alloys providing ruggedness, lightweight structure, excellent resistance to chemical and environmental effects, and low-cost manufacturing, and methods of making such metallic mirrors from such bulk-solidifying amorphous alloys are provided.

Description

FIELD OF THE INVENTION

The present invention is directed to metallic mirrors made of bulk solidifying amorphous alloys and mirror systems comprising components made of bulk solidifying amorphous alloys.

BACKGROUND OF THE INVENTION

Mirrors are optical devices designed to reflect and/or collect light for certain purposes. For the purposes of this disclosure, light is defined as an electromagnetic wave, which includes, but is not limited to, the frequencies of visible light. The most critical aspect of the mirror is the reflecting surface, which must be extremely smooth. Generally, the surface roughness of the reflecting surface is at the order of the wavelength of the reflected light, and preferably less than the wavelength of the reflected light. For high performance mirrors, surface roughness values of less than about 3 nm “rms” are desired and for certain mirror applications, surface roughness values of less than about 0.5 nm “rms” are preferred. The reflecting surface can be flat or curved, such as parabolic concave shapes.
Such demanding surface smoothness can be achieved cost effectively only with a limited number of materials. Silica-based glass material is the leading mirror material due to the relative ease of achieving highly smooth surfaces. Although silica-based glasses are broadly used in mirror applications, they have severe shortcomings due to their brittleness and extreme fragility. Moreover, silica based glasses need a reflective coating, generally a deposited metallic layer, which increases the processing costs and causes further complexities.
Metals and metallic alloys provide a remedy to the main shortcomings of silica base materials, namely the brittleness and fragility. However, the desired smoothness of the reflective surface cannot be readily achieved in metals. The polycrystalline grain nature of the microstructure, the multi-phases (especially in high strength and hardness alloy formulations), and impurities that can degrade the reflectivity of the material are the main obstacles for the use of conventional metals and alloys as reflective surfaces in high performance mirror systems. For example, the cost of achieving surface smoothness in metallic mirrors better than 3 about nm becomes very costly if at all possible. Furthermore, the directional characteristics of crystalline structure can also become an obstacle for achieving high surface smoothness as well as dimensional, environmental and thermal stability of metallic mirrors.
The mirror systems also comprise components other than the reflecting surface. Generally, a backing structure is utilized to support and provide durability to the reflecting surface, especially when it is made of silica base glasses. The supporting structure provides stiffness, stability (environmental and thermal) when such attributes can not be achieved with the reflecting surface itself. In certain cases, such as for the mobile and navigational mirror systems, the weight of the mirror system needs to be minimized and materials and structures with high-mass efficiency are needed.
Accordingly, a need exists for novel materials to be used in mirrors and mirror systems and also to develop novel mirror systems, which can provide remedy to the deficiencies of incumbent materials and structures.

SUMMARY OF THE INVENTION

The present invention is directed to a metallic mirror made of a bulk solidifying amorphous alloy.
In another embodiment of the invention, the metallic mirror has a flat reflecting surface
In yet another embodiment of the invention, the metallic mirror comprises a curved reflecting surface.
In still yet another embodiment of the invention, the metallic mirror comprises a reflecting surface and a back-structure for supporting the reflecting surface.
In still yet another embodiment of the invention, the metallic mirror comprises a reflecting surface and a back-structure as a single integral structure.
In still yet another embodiment of the invention, the metallic mirror comprises a reflecting surface and a back-structure joined together.
In still yet another embodiment of the invention, the reflecting surface of the metallic mirror comprises a deposited dielectric coating layer.
In still yet another embodiment of the invention, the reflecting surface of the metallic mirror comprises a deposited coating layer comprised of one or more of noble metals.
In still yet another embodiment of the invention, the amorphous alloy is described by the following molecular formula: (Zr, Ti)a(Ni, Cu, Fe)b(Be, Al, Si, B)c, wherein “a” is in the range of from 30 to 75, “b” is in the range of from 5 to 60, and “c” is in the range of from 0 to 50 in atomic percentages
In still yet another embodiment of the invention, the amorphous alloy is described by the following molecular formula: (Zr, Ti)a(Ni, Cu)b(Be)c, wherein “a” is in the range of from 40 to 75, “b” is in the range of from 5 to 50, and “c” is in the range of from 5 to 50 in atomic percentages.
In still yet another embodiment of the invention, the amorphous alloy can sustain strains up to 1.5% or more without any permanent deformation or breakage.
In still yet another embodiment of the invention, the bulk solidifying amorphous alloy has a high fracture toughness of at least 20 ksi-in^0.5.
In still yet another embodiment of the invention, the bulk solidifying amorphous alloy has a ΔT of 60° C. or greater.
In still yet another embodiment of the invention, the bulk solidifying amorphous has a hardness of 7.5 Gpa and higher.
In another alternative embodiment, the invention is also directed to methods of manufacturing metallic mirrors from bulk-solidifying amorphous alloys.

DESCRIPTION OF THE INVENTION

The current invention is directed to metallic mirrors made of bulk-solidifying amorphous alloys, the bulk-solidifying amorphous alloys providing ruggedness, lightweight structure, excellent resistance to chemical and environmental effects, and low-cost manufacturing for highly smooth reflecting surfaces. Another object of the current invention is a method of making metallic mirrors from such bulk-solidifying amorphous alloys.
Bulk solidifying amorphous alloys are a recently discovered family of amorphous alloys, which can be cooled at substantially lower cooling rates, of about 500 K/sec or less, and substantially retain their amorphous atomic structure. As such, they can be produced in thicknesses of 1.0 mm or more, substantially thicker than conventional amorphous alloys, which are typically limited to thicknesses of 0.020 mm, and which require cooling rates of 10⁵K/sec or more. U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, the disclosures of which are incorporated herein by reference in their entirety, disclose such bulk solidifying amorphous alloys.
A family of bulk solidifying amorphous alloys can be described as (Zr, Ti)a(Ni, Cu, Fe)b(Be, Al, Si, B)c, where a is in the range of from 30 to 75, b is in the range of from 5 to 60, and c is in the range of from 0 to 50 in atomic percentages. Furthermore, these basic alloys can accommodate substantial amounts (up to 20% atomic, and more) of other transition metals, such as Nb, Cr, V, Co. A preferable alloy family is (Zr, Ti)a(Ni, Cu)b(Be)c, where a is in the range of from 40 to 75, b is in the range of from 5 to 50, and c is in the range of from 5 to 50 in atomic percentages. Still, a more preferable composition is (Zr, Ti)a(Ni, Cu)b(Be)c, where a is in the range of from 45 to 65, b is in the range of from 7.5 to 35, and c is in the range of from 10 to 37.5 in atomic percentages. Another preferable alloy family is (Zr)a(Nb, Ti)b(Ni, Cu)c(Al)d, where a is in the range of from 45 to 65, b is in the range of from 0 to 10, c is in the range of from 20 to 40 and d is in the range of from 7.5 to 15 in atomic percentages.
Another set of bulk-solidifying amorphous alloys are ferrous metals (Fe, Ni, Co) based compositions. Examples of such compositions are disclosed in U.S. Pat. No. 6,325,868 and in publications to (A. Inoue et. al., Appl. Phys. Lett., Volume 71, p 464 (1997)), (Shen et. al., Mater. Trans., JIM, Volume 42, p 2136 (2001)), and Japanese patent application 2000126277 (Publ. No. 2001303218 A), all of which are incorporated herein by reference. One exemplary composition of such alloys is Fe72Al5Ga2P11C6B4. Another exemplary composition of such alloys is Fe72Al7Zr10Mo5W2B15. Although, these alloy compositions are not processable to the degree of the Zr-base alloy systems, they can still be processed in thicknesses of 1.0 mm or more, sufficient enough to be utilized in the current invention.
Bulk-solidifying amorphous alloys have typically high strength and high hardness. For example, Zr and Ti-base amorphous alloys typically have yield strengths of 250 ksi or higher and hardness values of 450 Vickers or higher. The ferrous-base version of these alloys can have yield strengths up to 500 ksi or higher and hardness values of 1000 Vickers and higher. As such, these alloys display excellent strength-to-weight ratio especially in the case of Ti-base and Fe-base alloys. Furthermore, bulk-solidifying amorphous alloys have good corrosion resistance and environmental durability, especially the Zr and Ti based alloys. Amorphous alloys generally have, high elastic strain limit approaching up to 2.0%, much higher than any other metallic alloy.
In general, crystalline precipitates in bulk amorphous alloys are highly detrimental to the properties of amorphous alloys, especially to the toughness and strength of these alloys, and as such it is generally preferred to minimize the volume fraction of these precipitates. However, there are cases in which, ductile crystalline phases precipitate in-situ during the processing of bulk amorphous alloys, which are indeed beneficial to the properties of bulk amorphous alloys, especially to the toughness and ductility of the alloys. Such bulk amorphous alloys comprising such beneficial precipitates are also included in the current invention. One exemplary case is disclosed in (C. C. Hays et. al, Physical Review Letters, Vol. 84, p 2901, 2000), which is incorporated herein by reference.
As a result of the use of these bulk-solidifying amorphous alloys, the metallic mirrors of the present invention have characteristics that are much improved over conventional metallic mirrors made of ordinary metallic materials. The surprising and novel advantages of using bulk-solidifying amorphous alloys in producing metallic mirrors will be described in various embodiments below.
First, the unique amorphous atomic structure, of the bulk solidifying amorphous alloys provide a featureless microstructure, wherein high surface smoothness can be achieved substantially better than conventional metallic alloys. The general obstacles to high surface finish, such as poly-crystalline microstructure, are not applicable. The inventors discovered that the surfaces of exemplary bulk solidifying amorphous alloys can be polished to very high degrees of smoothness. Initial trials demonstrate that surface smoothness of 3 nm rms can be readily achieved and surface smoothness of less than 1 nm rms is within practicality. Moreover, such high surface smoothness can be achieved over large areas more than several inches square. Accordingly, the quality of the reflective surfaces of bulk solidifying amorphous alloys substantially become better than conventional metals and alloys.
Secondly, the combination of high strength and high strength-to-weight ratio of the bulk solidifying amorphous alloys significantly reduces the overall weight and bulkiness of the metallic mirrors of the current invention, thereby allowing for the reduction of the thickness of these metallic mirrors without jeopardizing the structural integrity and operation of mirror systems into which these metallic mirrors are integrated. The ability to fabricate metallic mirrors with thinner walls is also important in reducing the bulkiness of the mirror system and increasing the efficiency per -volume of the mirror system. This increased efficiency is particularly useful for the application of mirror systems in mobile devices and equipment, such as in navigational instruments and space vehicles.
Although other materials, such as silica base glasses, are considered in these reflecting surfaces, there are major fabrication and assembly deficiencies with those materials. For example, silica based glasses lack any flexibility and are therefore actually quite fragile. Other conventional metallic alloys, although not fragile, however, are prone to permanent deformation, denting and scratching due to low hardness values. The very large surface area and very small thicknesses of metallic mirrors makes such problems even more significant. However, bulk-solidifying amorphous alloys have reasonable fracture toughness, on the order of 20 ksi-sqrt(in), and high elastic strain limit, approaching 2%. Accordingly, high flexibility can be achieved without permanent deformation and denting of the metallic mirror and high hardness of bulk solidifying amorphous alloys provide better resistance against scratching of the reflecting surface. As such, metallic mirrors made of bulk-solidifying amorphous alloys can be readily handled during fabrication and assembly, reducing the cost and increasing the performance of the mirror system.
As discussed, bulk solidifying amorphous alloys have very high elastic strain limits, typically around 1.5% or higher. This is an important characteristic for the use and application of mirror system metallic mirrors. Specifically, high elastic strain limits are preferred for devices mounted in mobile devices, or in other applications subject to mechanical loading or vibration. A high elastic strain limit allows the metallic mirror to take even more intricate shape and to be thinner and lighter, high elastic strain limits also allow the metallic mirrors to sustain loading and flexing without permanent deformation or destruction of the device, especially during assembly.
In addition, metallic mirrors made of bulk solidifying amorphous alloy also have good corrosion resistance and high inertness. The high corrosion resistance and inertness of these materials are useful for preventing the metallic mirrors from being decayed by undesired chemical reactions between the metallic mirror and the environment of the mirror system. The inertness of bulk solidifying amorphous alloy is also very important to the life of the mirror system because it does not tend to decay the reflective nature of the reflecting surface.
Another aspect of the invention is the ability to make metallic mirrors with isotropic characteristics. Generally non-isotropy in metallic articles causes degraded performance for those portions of metallic articles that require precision fit, such as in the contact surfaces of the formed metallic mirrors due to variations in temperature, mechanical forces, and vibration experienced across the article. Moreover, the non-uniform response of ordinary metals in various directions would also require extensive design margins to compensate, and as such would result in heavy and bulky structures. Accordingly, the isotropic response of the metallic mirrors in accordance with the present invention is crucial, at least in certain designs, given the intricate and complex patterns and the associated large surface areas and very small thicknesses of the metallic mirrors, as well as the need to utilize high strength construction material. For example, castings of ordinary alloys are typically poor in mechanical strength and are distorted in the case of large surface area and very small thickness. Accordingly, using metallic alloys for casting such large surface areas with high tolerance in flatness (or precisely curved shapes) is not generally feasible. In addition, for ordinary metallic alloys, extensive rolling operations would be needed to produce the metallic mirror sheet in the desired flatness and with the desired high strength. However, in this case the rolled products of ordinary high-strength alloys generate strong orientation, and as such lack the desirable isotropic properties. Indeed, such rolling operations typically result in highly oriented and elongated crystalline grain structures in metallic alloys resulting in highly non-isotropic material. In contrast, bulk-solidifying amorphous alloys, due to their unique atomic structure lack any microstructure as observed in crystalline and grainy metal, and as a result articles formed from such alloys are inherently isotropic.
Another function of the metallic mirror is to provide structural rigidity and complex patterns of back structure to provide a stiff support. The high strength, high elastic strain limit and high surface finishes of the bulk amorphous alloys allow for the ready production of metallic mirrors with seals of relatively high integrity back structures. As discussed below, the near-to-net shape forming ability of the bulk solidifying alloys allows the, use design features, such as ribs and ridges, to improve the stiffness and structural integrity of the support structures and mirror systems.
Another object of the invention is providing a method to produce metallic mirrors in net-shape form from bulk solidifying amorphous alloys. By producing metallic mirrors in net-shape form manufacturing costs can be significantly reduced while still forming metallic mirrors with good flatness, intricate surface features comprising precision curves, and high surface finish on the reflecting areas.
One exemplary method of making such metallic mirrors comprises the following steps:

- 1) Providing a sheet feedstock of amorphous alloy being substantially amorphous, and having an elastic strain limit of about 1.5 % or greater and having a ΔT of 30° C. or greater;
- 2) Heating the feedstock to around the glass transition temperature;
- 3) Shaping the heated feedstock into the desired shape; and
- 4) Cooling the formed sheet to temperatures far below the glass transition temperature.

Herein, ΔT is given by the difference between the onset of crystallization temperature, Tx, and the onset of glass transition temperature, Tg, as determined from standard DSC (Differential Scanning Calorimetry) measurements at typical heating rates (e.g. 20° C./min).
Preferably ΔT of the provided amorphous alloy is greater than 60° C., and most preferably greater than 90° C. The provided sheet feedstock can have about the same thickness as the average thickness of the final metallic mirror. Moreover, the time and temperature of the heating and shaping operation is selected such that the elastic strain limit of the amorphous alloy is substantially preserved to be not less than 1.0%, and preferably not being less than 1.5%. In the context of the invention, temperatures around glass transition means the forming temperatures can be below glass transition, at or around glass transition, and above glass transition temperature, but always at temperatures below the crystallization temperature Tx. The cooling step is carried out at rates similar to the heating rates at the heating step, and preferably at rates greater than the heating rates at the heating step. The cooling step is also achieved preferably while the forming and shaping loads are still maintained.
Upon the finishing of the above-mentioned fabrication method, the shaped metallic mirror can be subjected further surface treatment operations as desired such as to remove any oxides on the surface. Chemical etching (with or without masks) can be utilized as well as light buffing and polishing operations to provide improvements in surface finish so that high quality reflectivity and surface matching with other components can be achieved.
Another exemplary method of making metallic mirrors in accordance with the present invention comprises the following steps:

- 1) Providing a homogeneous alloy feedstock of amorphous alloy (not necessarily amorphous);
- 2) Heating the feedstock to a casting temperature above the melting temperatures;
- 3) Introducing the molten alloy into shape-forming mold; and
- 4) Quenching the molten alloy to temperatures below glass transition.

Bulk amorphous alloys retain their fluidity from above the melting temperature down to the glass transition temperature due to the lack of a first order phase transition. This is in direct contrast to conventional metals and alloys. Since, bulk amorphous alloys retain their fluidity, they do not accumulate significant stress from their casting temperatures down to below the glass transition temperature and as such dimensional distortions from thermal stress gradients can be minimized. Accordingly, metallic mirrors with large surface area and small thickness can be produced cost-effectively.
Although specific embodiments are disclosed herein, it is expected that persons skilled in the art can and will design alternative amorphous alloy metallic mirrors and methods to produce the amorphous alloy metallic mirrors that are within the scope of the following claims either literally or under the Doctrine of Equivalents.

Claims

1. A metallic mirror comprising at least one reflective surface made of a bulk solidifying amorphous alloy.

2. The metallic mirror of claim 1, wherein the reflective surface is flat.

3. The metallic mirror of claim 1, wherein the reflective surface is curved.

4. The metallic mirror of claim 1, wherein the reflective surface further comprises a deposited dielectric coating layer.

5. The metallic mirror of claim 4, wherein reflective surface further comprises a deposited coating layer comprised of one or more of noble metals.

6. The metallic mirror of claim 1, wherein the amorphous alloy is described by the following molecular formula: (Zr, Ti)a(Ni, Cu, Fe)b(Be, Al, Si, B)c, wherein “a” is in the range of from 30 to 75, “b” is in the range of from 5 to 60, and “c” is in the range of from 0 to 50 in atomic percentages.

7. The metallic mirror of claim 1, wherein the amorphous alloy is described by the following molecular formula: (Zr, Ti)a(Ni, Cu)b(Be)c, wherein “a” is in the range of from 40 to 75, “b” is in the range of from 5 to 50, and “c” is in the range of from 5 to 50 in atomic percentages.

8. The metallic mirror of claim 1, wherein the amorphous alloy can sustain strains up to 1.5% or more without any permanent deformation or breakage.

9. The metallic mirror of claim 1, wherein the amorphous alloy amorphous alloy has a ΔT of 60° C. or greater.

10. The metallic mirror of claim 1, wherein the amorphous alloy has a hardness of 7.5 Gpa and higher.

11. The metallic mirror of claim 1, wherein the reflective surface has a surface smoothness of less than about 3 nm rms.

12. The metallic mirror of claim 1, wherein the reflective surface has a surface smoothness of less than about 1 nm rms.

13. A metallic mirror system comprising:

a reflective surface; and

a support structure, wherein at least one of the components the mirror system is made of a bulk solidifying amorphous alloy.

14. The metallic mirror system of claim 13, wherein the reflective surface and the support structure are a single integral structure made of a bulk solidifying amorphous alloy.

15. The metallic mirror system of claim 13, wherein the reflective surface and the support structure comprise separate pieces, each made of a bulk solidifying amorphous alloy, that are joined together into a single integral structure.

16. The metallic mirror system of claim 13, wherein the amorphous alloy is described by the following molecular formula: (Zr, Ti)a(Ni, Cu, Fe)b(Be, Al, Si, B)c, wherein “a” is in the range of from 30 to 75, “b” is in the range of from 5 to 60, and “c” is in the range of from 0 to 50 in atomic percentages.

17. The metallic mirror system of claim 13, wherein the amorphous alloy is described by the following molecular formula: (Zr, Ti)a(Ni, Cu)b(Be)c, wherein “a” is in the range of from 40 to 75, “b” is in the range of from 5 to 50, and “c” is in the range of from 5 to 50 in atomic percentages.

18. The metallic mirror system of claim 13, wherein the amorphous alloy can sustain strains up to 1.5% or more without any permanent deformation or breakage.

19. The metallic mirror system of claim 13, wherein the amorphous alloy has a high fracture toughness of at least 20 ksi-in^0.5.

20. The metallic mirror system of claim 13, wherein the amorphous alloy amorphous alloy has a ΔT of 60° C. or greater.

21. The metallic mirror system of claim 13, wherein the reflective surface has a surface smoothness of less than about 1 nm rms.

22. A method of making metallic mirrors of bulk solidifying amorphous alloy, comprising the steps of:

providing a sheet feedstock of a bulk-solidifying amorphous alloy being in a substantially amorphous state, and having an elastic strain limit of about 1.5% or greater and having a ΔT of 30° C. or greater;

heating the feedstock to around the glass transition temperature of the bulk-solidifying amorphous alloy;

shaping the heated feedstock into a desired mirror shape; and

cooling the formed mirror to temperatures far below the glass transition temperature.

23. The method of claim 22, wherein the ΔT of the amorphous alloy is greater than 90° C.

24. The method of claim 22, wherein the elastic strain limit of the amorphous alloy is substantially preserved during processing to be not less than 1.5%.

25. The method of claim 22, further comprising a finishing process selected from the group consisting of oxide removal, chemical etching, buffing, and polishing.

26. A method of making metallic mirrors of bulk solidifying amorphous alloy, comprising the steps of:

providing a homogeneous alloy feedstock of a bulk solidifying amorphous alloy in either an amorphous or non-amorphous state;

heating the feedstock to a casting temperature above the melting temperature of the bulk solidifying amorphous alloy;

introducing the molten alloy into a shape-forming mold; and

quenching the molten alloy to a temperature below the glass transition temperature of the bulk solidifying amorphous alloy.

27. The method of claim 26, wherein the amorphous alloy has a ΔT of greater than 60° C.

28. The method of claim 26, further comprising a finishing process selected from the group consisting of oxide removal, chemical etching, buffing, and polishing.