US20210268585A1

US20210268585A1 - Dendrite-Reinforced Titanium-Based Metal Matrix Composites

Info

Publication number: US20210268585A1
Application number: US17/202,092
Authority: US
Inventors: Douglas C. Hofmann
Original assignee: California Institute of Technology CalTech
Current assignee: California Institute of Technology CalTech
Priority date: 2017-05-26
Filing date: 2021-03-15
Publication date: 2021-09-02
Also published as: US11014162B2; US20180339342A1; KR20200011470A; EP3630392A4; WO2018218247A1; EP3630392A1

Abstract

Ti-based metal matrix composites, methods of their additive manufacture, and parts manufactured therefrom and thereby are provided. Method include layer-by-layer additive manufacturing for fabricating Ti-based metal matrix composite parts thicker than 0.5 mm, in layers with thickness between 10-1000 micrometers. The parts formed may have one or more of the following properties: a tensile strength greater than 1 GPa, a fracture toughness greater than 40 MPa m1/2, a yield strength divided by the density greater than 200 MPa cm3/g, and a total strain to failure in a tension test greater than 5%.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 15/991,854, entitled “Dendrite-Reinforced Titanium-Based Metal Matrix Composites,” filed May 29, 2018, which claims priority to U.S. Provisional Application No. 62/511,668, entitled “Dendrite-Reinforced Titanium-Based Metal Matrix Composites for Additive Manufacturing”, filed May 26, 2017, the disclosures of which are incorporated herein by reference in their entireties.

STATEMENT OF FEDERAL FUNDING

The invention described herein was made in the performance of work under a NASA contract NNN12AA01C, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.

FIELD OF THE INVENTION

The current disclosure is directed to Ti-based metal matrix composites, methods of their additive manufacture, and parts manufactured therefrom and thereby.

BACKGROUND OF THE INVENTION

Bulk metallic glasses (BMGs), also known as amorphous or glassy metals, are alloys which can be quenched into a vitreous state at a relatively large casting thickness (generally over 1 mm). In turn, BMG matrix composites (BMGMCs) are two-phase materials, comprising a high strength BMG matrix and a dispersed crystalline phase grown in-situ during the alloy's cooling from melt due to chemical segregation. In contrast, ultra-fine grained metal matrix composites (UFGMCs) are rapidly cooled alloys that exhibit a nanocrystalline microstructure. UFGMCs can be thought of as insufficiently fast cooled metallic glasses that have been cooled fast enough to have a small grain size.
Metal additive manufacturing, also commonly known as metal 3D printing, is an emerging manufacturing technology, which is being rapidly integrated into commercial applications, such as fabrication of nozzles in aircraft and rocket engines. Typically, a 3D printing process comprises sequential deposition of a multitude of thin layers of a material to assemble a desired bulk part or structure. The most common forms of metal additive manufacturing are direct metal laser sintering, directed energy deposition, thermal spray additive manufacturing, laser foil welding, and ultrasonic additive manufacturing. Recently, it has been shown that bulk metallic glass parts can also be fabricated by additive manufacturing.

SUMMARY OF THE INVENTION

The current disclosure is directed to embodiments of Ti-based metal matrix composites, methods of their additive manufacture, and parts manufactured therefrom and thereby.
Many embodiments of the disclosure are directed to methods of fabricating a part thicker than 0.5 mm via layer-by-layer additive manufacturing including:

- providing an alloy having at least 85 atomic % of at least Ti and at least one component selected from the group of Zr, Hf, Ta, Nb, V, and Mo, and one or more additional components, X, selected from the group of Co, Fe, Ni, Cu, Al, B, Ag, Pd, Au, Pd, C, Si, and Sn, wherein the atomic % of Ti is greater than any other single component; disposing molten layers of the alloy atop one another additively;
- cooling each layer prior to disposition of the next at a rate such that upon solidification the alloy segregates phases into a metal matrix composite consisting of isolated crystalline dendrites in a continuous eutectic matrix material; and
- repeating the disposing and cooling to form a metal matrix composite part.

In other embodiments, the alloy comprises a combination of Zr, a beta-stabilizer, and X, where the atomic percentage of the Ti, Zr, and beta-stabilizer is between 85 to 98 atomic % of the alloy, and where X comprises from 2 to 15 atomic % of the alloy.
In still other embodiments, the beta-stabilizer is selected from the group of V, Nb, Ta and Mo.
In yet other embodiments, Ti comprises at least 50 atomic % of the alloy.
In still yet other embodiments, the alloy comprises one or both Zr and Hf, a beta-stabilizer, B, and X, where the combination of Ti, Zr and Hf, and the beta-stabilizer is between 85 to 98 atomic % of the alloy, where B comprises from between 0.5 to 5 atomic % of the alloy, and where X comprises less than 10 atomic % of the alloy.
In still yet other embodiments, the beta-stabilizing components are selected from the group of V, Nb, Ta and Mo, and wherein X is selected from the group of Zr, B, Si, Cu, Co, Fe, and Pd.
In still yet other embodiments, the thickness of each of the layer is from between 10-1000 micrometers.
In still yet other embodiments, the cooling rate is greater than 10²K/s.
In still yet other embodiments, the crystalline dendrites comprise at least 60% by volume of the solidified alloy.
In still yet other embodiments, the hardness of the matrix is at least 5% larger than the hardness of the dendrites.
In still yet other embodiments, the composite part has at least one property selected from the group of a tensile strength of greater than 1 GPa, a fracture toughness of greater than 40 MPa m^1/2, a density of less than 6.0 g/cm³, total strain to failure of greater than 5% in a tension test, and a yield strength divided by the density greater than 200 MPa cm³/g.
In still yet other embodiments, the alloy is formed by adding components to Ti.
In still yet other embodiments, the solidus temperature of the alloy is less than 1600 Celsius.
In still yet other embodiments, the crystalline dendrites range in size from 1 to 20 micrometers in diameter after solidification.
In still yet other embodiments, the crystalline dendrites are less than 10 micrometers in diameter after solidification.
In still yet other embodiments, the alloy is heated to a semi-solid temperature region between the alloy solidus and liquidus during disposition.
In still yet other embodiments, the part is used in a structural application.
In still yet other embodiments, the heating and cooling disposition parameters are altered between the disposition of at least two layers of the part such that the one of either the size or the density of the dendrites is altered within at least two layers of the part such that a gradient of properties is formed within the part.
In still yet other embodiments, the disposition process is selected from one of powder bed fusion, direct energy deposition, laser foil welding, fused filament fabrication, electron beam fabrication, thermal spraying, and liquid deposition.
In still yet other embodiments, the disposition process is selected from one of binder jetting, friction stir additive manufacturing, cold spraying, and ultrasonic additive manufacturing.
In still yet other embodiments, the alloy comprises Nb and from 2 to 15 atomic % B.
In still yet other embodiments, the concentration of B is 5 atomic %.
In still yet other embodiments, the alloy is selected from the group of Ti₇₄V₁₀Zr₁₀Si₆, Ti₆₄V₁₀Zr₂₀Si₆, Ti₇₁V₁₀Zr₁₀Si₆Al₃, Ti₇₄Nb₁₀Zr₁₀Si₆, Ti₇₄Ta₁₀Zr₁₀Si₆, Ti₇₅Cu₇Ni₆Sn₂V₁₀, Ti₇₅Cu₇Ni₆Sn₂Nb₁₀, Ti₇₅CU₇Ni₆Sn₂Ta₁₀, (Ti₇₂Zr₂₂Nb₆)₉₅Co₅, (Ti₇₂Zr₂₂Nb₆)₉₂Co₅Al₃, (Ti₇₂Zr₂₂Ta₆)₉₅Co₅, (Ti₇₂Zr₂₂Ta₆)₉₂Co₅Al₃, (Ti₇₂Zr₂₂V₆)₉₅Co₅, (Ti₇₂Zr₂₂V₆)₉₂Co₅Al₃, Ti₉₀Nb₅Cu₅, Ti₈₅Nb₁₀Cu₅, Ti₈₀Nb₅Cu₁₀, Ti₈₀Nb₁₀Cu₁₀, Ti₉₀Ta₅Cu₅, Ti₈₅Ta₁₀Cu₅, Ti₈₀Ta₅Cu₁₀, Ti₈₀Ta₁₀Cu₁₀, Ti₉₀V₅Cu₅, Ti₈₅V₁₀Cu₅, Ti₈₀V₅Cu₁₀, Ti₈₀V₁₀Cu₁₀, Ti₈₅V₁₀B₅, Ti₈₅Ta₁₀B₅and Ti₈₅Nb₁₀B₅, Ti₅₇Zr₁₈V₁₂Cu₁₀Al₃or Ti₆₂Zr₁₈V₁₂Cu₅Al₃.
In still yet other embodiments, the metal matrix component part is selected form the group of biomedical implants, structural aerospace components, sporting equipment, medical devices, and engine components.
In still yet other embodiments, the matrix material and the crystalline dendrites are combined ex situ to form the metal matrix composite.
In still yet other embodiments, the matrix material and crystalline dendrites are in the form of powders with a size distribution within 10% of each other.
Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed subject matter. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying data and figures, wherein:

FIGS. 1A and 1B provide micrograph images of Ti-MMCs, wherein FIG. 1A shows a Ti-BMGMC, and FIG. 1B shows a Ti-MMC with a crystalline eutectic.

FIGS. 2A to 2E provide data showing properties of Ti-BMGMCs in accordance with the prior art.

FIGS. 3A and 3B provide a heat flow data graph (FIG. 3A), and images of cast parts (FIG. 3B) of Zr and Ti-BMGMCs in accordance with the prior art.

FIGS. 4A and 4B provide x-ray data and micrograph images (FIG. 4A and inset), and stress data (FIG. 4B) for Ti-UFGMCs in accordance with the prior art.

FIGS. 5A and 5B provide schematics of Ti-MCCs in accordance with embodiments.

FIG. 6 provides a table showing fracture toughness and yield strength for materials in accordance with the prior art.

FIGS. 7A to 7C provide micrograph images of MCCs with varying dendrite concentrations in accordance with embodiments.

FIG. 8 provides a periodic table showing compositional parameters for Ti-based MCCs in accordance with embodiments.

FIG. 9 provides images (top) of and x-ray crystal data for ingots formed using Ti—Nb₁₀₀(left plot) and Ti—Nb₉₅B₅(right plot) in accordance with embodiments.

FIGS. 10A and 10B provide micrograph images showing dendrite size-scales for alloys having the same dendrite concentrations in accordance with embodiments.

FIGS. 11A to 11C provide images (FIG. 11A) of ingots and a rod, and x-ray crystal data for the ingots (FIG. 11B) and rods (11C) in accordance with embodiments.

FIG. 12 provides a flowchart of a process for forming parts from Ti-based MMCs in accordance with embodiments.

FIG. 13 provides a schematic of an additive manufacturing process in accordance with embodiments.

DETAILED DISCLOSURE

The embodiments of the invention described herein are not intended to be exhaustive or to limit the invention to precise forms disclosed. Rather, the embodiments selected for description have been chosen to enable one skilled in the art to practice the invention.
Turning to the figures and data, embodiments of alloys for forming Ti-based metal matrix composites, methods of their additive manufacture, and parts manufactured therefrom and thereby are provided. Various embodiments are directed to layer-by-layer additive manufacturing methods for fabricating Ti-based metal matrix composite parts thicker than 0.5 mm, in layers with thickness between 10-1000 micrometers. In various such embodiments the cooling rate in each deposited layer is greater than 10³K/s. In some such embodiments the part may have one or more of the following properties: a tensile strength greater than 0.8 GPa, a fracture toughness greater than 40 MPa m^1/2, a yield strength divided by the density greater than 200 MPa cm³/g, and a total strain to failure in a tension test greater than 5% in either the as-printed conditions or a heat treated condition after printing.
In many such embodiments, upon solidification, the alloy segregates phases into a metal matrix composite consisting of isolated crystalline dendrites in a continuous matrix material. In some embodiments, the continuous matrix comprises a nanocrystalline metal alloy. In various such embodiments the most abundant element in the alloy is titanium, and comprises greater than 50% of the alloy by atomic percentage. In various other embodiments the atomic percentage of one or more of titanium, zirconium, hafnium, tantalum, niobium, vanadium and/or molybdenum present in the alloy sums to greater than 85 atomic percentage. In many embodiments the elements may be strategically added to titanium to produce a continuous matrix with dendrites upon solidification. In some such embodiments, the alloy exhibits at least 60% by volume crystalline dendrites in a continuous matrix. In some other such embodiments the hardness of the matrix is at least 5% larger than hardness of the dendrites. In various such embodiments the density of the alloy may be between 4 and 6 g/cm³.

Properties of Ti-Based Metal Alloys

Ti-based metal alloys possess many advantageous properties and are widely used in high-performance applications, often outperforming very high strength materials, such as steel. Specifically, Ti alloys typically exhibit high tensile strengths, combined with low density, excellent corrosion resistance, superior hardness, high flexibility (especially relative to steel), and, in addition, Ti alloys have paramagnetic properties, which can be advantageous for many uses. Furthermore, titanium metal is a dimorphic allotrope with two possible crystal structure lattices: hexagonal closed packed (HCP, also known as the alpha (α) form) and body centered cubic (BCC, also known as the beta (β) form). As such, the additions of different elements to pure Ti can be used to control the crystal structure of the resulting alloy, with dramatic effects on the resulting mechanical properties. For example, β-Ti alloys typically comprise Ti alloyed with so-called β-stabilizing elements, such as Nb, Ta, V, Mo, Zr, or Sn, wherein the presence of these additional elements, even in small amounts, promotes BCC lattice formation. As compared to pure, non-alloyed Ti, or α-Ti alloys, β-Ti alloys exhibit higher density (due to the higher packing density of the BCC lattices), lower strength, softer, larger ductility, higher fracture toughness, and a greater fatigue endurance limit, as well as a lower specific strength. In contrast, α-Ti alloys, which contain α-stabilizing and HCP-phase promoting elements, such as Al, Cu, Co, Ni, or Si, demonstrate high strength, but lower toughness and ductility.
The most common alloy of Ti is Ti-6Al-4V, which is an α/β-Ti-alloy comprising both α and β stabilizing elements. This alloy is known for possessing a combination of excellent properties, as well as for its tolerance towards heat treatment. Specifically, in its annealed state, Ti-6Al-4V has a low density of 4.43 g/cm³, yield strength of 800 MPa, 10% total strain to failure in tension, and a specific strength of 264. However, despite exhibiting an optimal combination of properties, the Ti-6Al-4V alloy also possesses some detrimental characteristics that limit its use. For example, the alloy has poor wear resistance, a very low fracture toughness of 43 MPa m^1/2(especially as compared to steel, which has a fracture toughness of >100 MPa m^1/2), and a strength that is only ˜50% of that found for the best available steel. Moreover, the alloy becomes brittle when formed into parts with thicker cross-sections, is difficult to conventionally machine, and cannot be easily die cast or injection molded (due to the alloy's high melting temperature of >1600° C., which exceeds the melting temperature of most molds). In addition, while widely used in biomedical applications, Ti-6Al-4V is not completely non-toxic due to the presence of aluminum, which dissolves in the blood. Therefore, the development of Ti-based alloys that improve on all of the shortcomings of the ubiquitous Ti-6Al-4V is highly desired.
Creating new alloys that improve on the properties of Ti-6Al-4V is challenging due to the inherent limits of dislocation-based plasticity. In other words, any crystalline alloy in the Ti-6Al-4V family will likely have similar problematic physical properties. Typically, the strength and hardness of a crystalline alloy is inversely proportional to its ductility and fracture toughness. For example, increasing the strength of a crystalline Ti-alloy, which can be done easily, makes the alloy more prone to brittle fracture. Therefore, although various strategies for tuning the properties of Ti alloys have been reported (e.g. balancing of α and β stabilizing elements), none of these strategies have produced a Ti-alloy capable of exceeding the combined characteristics of the Ti-6Al-4V alloy (e.g., with a yield strength of greater than 1.5 GPa, a total strain to failure in tension of at least 5%, a fracture toughness larger than 43 MPa m^1/2, and, simultaneously, a specific strength greater than 250. In fact, since the specific strength is inversely proportional to the density (i.e. the specific strength is equal to the yield strength divided by the density), it is especially difficult to obtain this desired combination of properties, due to the low density of Ti-6Al-4V (4.43 g/cm³).
One alternative approach to improving the properties of Ti-based alloys is to form them as dendrite-reinforced metal matrix composite (MMC) materials. Such MMCs are also known as in-situ MMCs, because the alloy reinforcing dendrites are formed directly during the alloy solidification from the melt, rather than added separately, due to chemical desegregation. In the MMC approach, an alloy is formulated with a composition that makes it phase-separate upon solidification into a multi-phase composite, which is comprised of relatively soft, low shear modulus (G) Ti-based dendrites dispersed in a continuous matrix of a high-modulus and high-strength material. Consequently, the presence of a sufficient amount of soft Ti-based dendrites suppresses crack growth and brittle failure for the alloy under tension, enhancing its strength and toughness. Although the Ti-based dendrites that form will normally be inherently softer than the matrix, they can be softened further with the addition of a beta stabilizing element(s) into the alloy.
Two classes of MMCs have traditionally been considered for Ti-based alloys: (1) bulk metallic glass matrix-based MMCs (BMGMCs) and (2) ultra-fine grained metal matrix-based MMCs (UFGMCs). BMGMCs are formulated such that their matrix material forms a metallic glass (also known as an amorphous metal) upon cooling (as shown in FIG. 1A), while the UFGMCs are formulated such that they form a fine-grained or nanograined crystalline metal matrix (as shown in FIG. 1B). With heat treating or decreased cooling rate, the UFGMCs can also exhibit a crystalline eutectic with a larger grain size, if desired. Both classes comprise a Ti alloy reinforced with a beta-stabilizing element (to soften the Ti dendrites) and additional elements, which phase segregate upon solidification due to their low solubility in the (beta Ti) alloy. Consequently, the mechanical properties of the MMCs combine high strength (due to the strength of the continuous matrix) with high ductility and toughness (due to the presence of crack-stabilizing soft dendrites). Notably, the MMCs' advantageous combination of properties can, at times, exceed the rule of mixtures, which, typically, dictates a balance between strength and ductility.
These strategies of using MMCs to improve the properties of Ti-6Al-4V have been previously demonstrated, including, most notably, by Hofmann et al. and illustrated in FIGS. 2A through 2D. (See, e.g., Hofmann, et al., PNAS, 105(51) 20136-20140 (2008), the disclosure of which is incorporated herein by reference.) Specifically, by modifying the alloys' composition and density, while balancing the volume fractions of metallic glass matrix and dendrites, Hofmann et al. developed a family of Ti—Zr—V—Cu—Be BMGMCs (the DV-DVAI family) with different dendrite volume fractions, wherein DV4, DVAI1 and DVAI2 all have >60% BCC dendrites (FIG. 2A). The resulting alloys were not only amenable to cooling into billets larger than 1 cm thick (FIG. 2B), but also possessed a host of enhanced mechanical properties, as compared to traditional crystalline alloys such as Ti-6Al-4V (FIGS. 2C and 2D). Specifically, the alloys demonstrated yield strength of up to 1,600 MPa, total strain to failure in tension of 12.5%, maximum specific strength of 315, density between 4.97-5.15 g/cm³, and fracture toughness greater than 60 MPa m^1/2. Notably, all of these alloys had a density of less than 6 g/cm³, which is the upper limit for crystalline Ti-based alloys used commonly. In addition, these prior art Ti-based BMGMCs had higher fatigue limits than Ti-6Al-4V (FIG. 2E), as well as a higher strength, specific strength, and fracture toughness. Moreover, the alloys had a solidus temperature that was approximately half that of conventional Ti-6-4 alloys (˜650° C.), which make them amenable to processing via die casting or injection molding. In addition, these enhanced alloys exhibited exceptional glass forming ability (GFA) and could be made with an amorphous matrix in thicknesses approaching one inch.
However, while Ti-based BMGMCs and UFGMCs both have some advantages (e.g., impart high strength to the composite), their use is not widespread due to problems associated with alloy formulation and manufacturing. For example, as shown in the tables and associated graphs, many Ti-based BMGMCs, such as those disclosed by Hofmann, contain beryllium in amounts of between 5-20 atomic % (0.9-3.4 weight %), which is highly undesirable due to its toxicity (FIG. 2C). However, Be is necessary for phase separation of the dendrites, due to its low solubility in beta titanium, and for good glass forming ability of the alloys, as well as for many other desirable properties. In addition, removing Be dramatically increases the melting temperature of the alloy, making it difficult to cast. For example, FIG. 3A illustrates that substituting Be in alloy DV1 (FIG. 2C) for Al to form Ti₅₂Zr₁₈V₁₂Cu₁₅Al₃, while maintaining approximately the same high Ti—Zr—V presence (in total of 80 and 82 atomic % correspondingly) and the same dendrite volume fraction, increases the solidus temperature by 210 degrees, making the new Al-containing alloy barely castable and no longer amorphous. Furthermore, the casting of dendrite-containing alloys is, in general, very problematic because the alloy viscosity and melting temperature are too high for the production of good quality cast parts, often leading to mold damage from overheating (which is necessary to lower the viscosity). For example, FIG. 3B demonstrates the challenges associated with casting Ti-BMGMC alloys that have a dendrite volume fraction of more than 50%, including the increase in cast part defects or incomplete casts. Accordingly, the manufacturing of complex parts with desirable properties from BMGMC alloys with high dendrite concentrations remains unattainable with conventional die-casting or injection molding. An additional detracting feature of Ti-BMGMCs is their very low services temperatures. Since the alloys contain a glassy matrix, they can only be used in service up to a temperature near their glass transition, at which point the matrix would begin to flow. This happens at approximately 300 C with Ti-BMGMCs. In contrast, crystalline Ti alloys, like Ti-6-4 have services temperatures in excess of 500 C.
Various attempts have been made to achieve the excellent mechanical properties of BMGMCs in non-Be Ti-based UFGMCs. One prominent example of such attempts was reported by Shultz et. al., in Nature Materials 2002. Specifically, Shultz et. al., describe the development of a Ti—Ta/Nb—Sn—Cu—Ni UFGMC, wherein the composition of Ti-based BMG Ti₈₀Cu₂₃Ni₂₀Sn₇(a poor glass former) was modified to include 0 stabilizing elements and have an increased Ti content, to create alloys Ti₆₀Cu₁₄Ni₁₂Sn₄Ta₁₀and Ti₆₀Cu₁₄Ni₁₂Sn₄Nb₁₀. It was found that when these new compositions were cooled at a sufficiently fast rate, the achieved microstructure of the high-strength nanocrystalline matrix (see FIG. 4a ) was reminiscent of the glassy microstructure of the Be-containing Ti-based BMGMCs. As such, these alloys exhibited high strength and good plasticity in compression (see FIG. 4b ). However, their ductility in tension and bending is poor, due to the high brittleness of the UFG matrix.
Other attempts to form Ti-based BMGMCs involved increasing the Ti content in a Ti-based BMG to more than 65 atomic %. For example, Whang et. al. studied the high Ti-content Ti₈₄Zr₁₀Si₆composition of the Ti—Zr—Si BMG family. (See, e.g., Whang et al., J. of Materials Science Letters, 4, 883-887 (1985), the disclosure of which is incorporated herein by reference). It was found that, upon solidification, rapidly quenched foils of this alloy possessed a 2-phase microstructure, comprising a crystalline phase dispersed in a nano-structured matrix. Notably, the 3-element alloy of Whang did not contain any R stabilizers for either softening the precipitating crystal phase or for improving its GFA. In another example, Lin et. al. developed an alloy within the same Ti—Zr—Si family—Ti₆₅Zr₁₀Ta₁₀Si₁₅, which formed a metallic glass upon rapid solidification. (See, e.g., Lin et al., Intermetallics, 2014, the disclosure of which is incorporated herein by reference). Here, although the addition of R stabilizing elements, such as Ta and Nb, did improve the alloy's GFA, the concurrent significant increase in the Si content counteracted some of the advantageous properties expected from BMGs. In particular, the focus on forming the materials into BMGs capable of casting means that the alloys show a brittleness and lack of strength not comparable with other Ti-based alloys.
Accordingly, Ti-based alloys that combine high strength and high toughness, yet do not contain Be and are easy to manufacture, remain elusive, since maximizing the strength and toughness properties via available methods, such as high-volume fraction dendrite MMCs, typically reduces the alloys' manufacturability via conventional casting or machining methods. Specifically, all attempts at improving the mechanical properties of Ti-based alloys reported to date, including via the BMGMC and UFGMC approaches, have been severely restricted by the requirement that, in order to be practical, the resulting alloys must remain amenable to casting into net-shapes or to machining, which, in turn, has dictated the acceptable (relatively low) alloy viscosities and melting temperatures.

Embodiments of Ti-Based MMCs

The current application is directed to embodiments of methods and alloy systems for developing Ti-based alloy metal matrix composite compositions with superior mechanical properties, especially high toughness and strength. As discussed above, to date, the development of Ti-based MMCs (whether BMGMC or UFGMC) has primarily been restricted by the requirement that they are amenable to casting, and, therefore, efforts have focused on the alloys with <50% dendrites by volume and with <85% sum total of Group 4-6 metals (e.g. Ti, Zr, Nb, Ta, Hf, Mo, V). However, as also discussed above, the alloys obtained under such restrictions tend to be brittle, as their microstructures do not arrest the development of fatal cracks. Accordingly, alloys of embodiments are directed to MMC materials comprising a soft crystalline metal dendrite phase dispersed throughout a continuous high strength crystalline eutectic matrix phase (e.g., at least 5% harder than the dendrite phase) in sufficient concentration capable of resisting fracture.
The deficiency, in prior art attempts to arrest fracturing relates to the nature of crack propagation and the interaction of the propagating crack with the dendritic phase. As shown in the schematic provided in FIGS. 5A and 5B, a Ti-based MMC alloy generally comprises at least two phases: a soft crystalline metal Ti-dendrite phase (shown by the circles and voids) dispersed throughout a continuous high strength matrix phase. It has been shown that a certain minimum dendrite amount/volume fraction is required in the solidified alloy in order to achieve the desired crack suppression effect and, as a result, to minimize brittleness and acquire satisfactory ductility. For example, it has been shown that for Be-containing BMGMCs the dendrite volume fraction must be 60% or above in order for the alloy's properties to be “casting invariant.” In other words, rapidly quenched BMGMC alloys will have roughly the same ductility as large ingots of the material. (See, e.g., the Table provided in FIG. 2C.)
However, the ability to use dendritic concentrations as low as 60% by volume in BMGMCs is the result of the unique properties of metallic glasses and high toughness of their matrix (see, e.g., FIG. 6), which better resists fracturing and allows for the use of lower volume fractions of dendrites. By contrast, ultra-fine or nano-grained (e.g., eutectic) MMCs, such as those provided in embodiments, are far more brittle than that of BMGMCs, and, therefore, require a larger volume fraction of dendrites. This is also true if the grain size of the matrix is enlarged, through heat treating steps, which would form brittle phases with larger grains. Exemplary micrographs of crack propagation for different concentrations of dendrites in exemplary materials are provided in FIGS. 7A to 7C. As shown, at dendritic concentrations below 60% by volume (FIG. 7A), cracks propagate without difficulty through the brittle matrix. By contrast, at dendritic concentrations at and above 60% by volume (FIGS. 7B and 7C) crack propagation is impeded and stopped by the density of the dendritic phase. Accordingly, in various embodiments the Ti-based MMCs comprise a dendritic reinforced matrix wherein the dendritic phase upon solidification from melt comprises in many embodiments at least 60% by volume of the MMC, in various embodiments at least 70% by volume, in other various embodiments at least 80% by volume, and in still other various embodiments up to 98% by volume. It is important to note that Ti-MMC formed in-situ will still have a continuous matrix with as little as 2% matrix and 98% dendrites due to the physics of dendrite formation. This innovation allows the volume fraction of the dendrites to remain exceedingly large while still retaining a high strength, which is carried by the continuous matrix.
Obtaining Ti-based MMCs with high dendritic volumes according to embodiments, in turn, requires high combined concentrations of Ti and one or more ß-stabilizers in the composition. In general terms this means maximizing the concentration of elements on the composition from left side of the periodic table (e.g., the early transition metals), at the expense of component elements from the right side of the periodic table (e.g., late transition metals). Accordingly, in many embodiments, the Ti-based MMCs formulated comprise a combined total of 85% of Ti and one or more ß-stabilizers selected from the Group 4-6 metals, as shown schematically in FIG. 8. In many embodiments the combined compositional amount of Ti and R-stabilizers (including, in some embodiments, Zr) is between 85-98 atomic %. In many embodiments, the alloys with 85-98 combined atomic % of Ti and ß-stabilizers afford a sufficient amount of dendrites to produce a Ti-alloy part with satisfactory ductility, strength, and toughness according to the methods of the application. In many embodiments, the relevant Group 4-6 metals are chosen from a list that includes, but is not limited to: titanium, zirconium, hafnium, tantalum, niobium, vanadium, molybdenum, and any combination thereof. In some embodiments, beta stabilizers include Zr and/or other Group 4 through 6 metals selected from the list above. In many such embodiments, ß-stabilizers include at least Zr.
In many embodiments, the remaining 2-15 atomic % of the alloy composition comprise elements that, together, have a very low solubility in the BCC phase of Ti, such that they form a second phase upon solidification from the melt. Typically, such element combinations can be determined from the relevant phase diagrams, but, in many embodiments, the acceptable elements are metalloids or late transition metals selected from the group comprising: B, Si, Co, Cu, Fe, Ni, Sn, Pd, Al. As an example, adding as little as 5 atomic % of low-solubility B turns a monolithic (Ti—Nb)₁₀₀alloy into a 2-phase, dendrite-infused continuous matrix composite (as demonstrate by the crystalline X ray plots obtained for such alloys provided in FIG. 9). Accordingly, in many embodiments, a small amount (e.g., 2-15 atomic %) of one or more elements capable of affording phase segregation in Ti-alloys is included in the compositions of the instant application to reduce the alloy's overall melting temperature and ensure phase separation and dendrite production.
Note, although the above discussion has excluded Be, in principle, Be is also a suitable insoluble second phase former, and, together with other non-metals B and Si, represents the most potent group of second phase formers with a very low solubility in BCC Ti, as demonstrated by observing the solubility of these elements with Ti in binary phase diagrams. However, in many embodiment Be is excluded from all compositions of the instant application due to its toxicity and the problems it poses during manufacturing, especially with powder.
In addition to the volume concentration of dendrites in the Ti-based MMCs, the size-scale of dendritic microstructures may also be controlled to improve the properties of the materials according to embodiments. Specifically, FIGS. 10A and 10B provide micrograph images of the dendrites in two Ti-MMCs having the same concentration of dendrites (e.g., 66% by volume). However, in the Ti-based MMC shown in FIG. 10A the dendrites have a size-scale of around ˜20 μm, while in FIG. 10B the dendrites have a size-scale of 2 μm. These smaller size-scale dendrites according to embodiments can have a significant impact on the toughness of the materials and their susceptibility to fracture. In accordance with embodiments, smaller dendrites allow for closer packing and, ultimately for a higher volume fraction of dendrites in the alloy. In turn, smaller, tighter packed dendrites are more effective at arresting crack propagation that might plague the otherwise brittle matrices of MMCs, and, therefore, greatly enhance the alloy's strength and toughness. Accordingly, in many embodiments, the Ti-based MMC comprises an ultra-fine grained metallic matrix composite. In various embodiment, the dendrite size of such embodiments is less than 20 microns in diameter. In various other embodiments, the dendrite size is smaller than 10 microns in diameter. In still various other embodiments, the dendrite size is smaller than 5 microns in diameter.
One method of obtaining Ti-based MMC incorporating smaller, more closes packed dendrites is to rapidly cool the material from the melt. Specifically, FIGS. 11A to 11C, illustrate how varying cooling rates can lead to different microstructures. To this end, FIG. 11A show images of an ingot and a 3 mm rod both formed using a the same Ti-based MMC. However, in order to form the rod, the cooling rate on solidification from the melt is increased as a result of the smaller cross-section. As shown in FIGS. 11B and 11C, this change in cooling rate has a significant impact on the microstructure of formed pieces. Specifically, x-ray scans for the two pieces show that the slower cooled piece shows much more intense crystalline peaks indicating larger dendritic phases and larger grained eutectic (FIG. 11B), whereas the x-ray scans for the faster cooled rod shows less intense crystalline peaks indicating a more disordered structure and thus smaller dendritic phases and finer grained eutectic (FIG. 11C). This difference can also be observed in the physical properties of the parts formed using different cooling rates. For example, the ingot is soft and ductile, whereas the rod behaves as a high strength MMC. Accordingly, in many embodiments, cooling rates of at a sufficiently fast rate to afford a material comprising a matrix infused with ultra-high volume fraction of small dendrites. In some such embodiments, the cooling rate upon formation is least 1000 K/s.
In view of the discussion above, Ti-based MMCs in accordance with many embodiments include at least the following compositional features:

- They are formulated to produce dendrite reinforced Ti-based metal matrix composites from alloys having high concentrations (e.g., 85% or more of two or more) of Group 4-6 metals. In some such embodiments, the concentration of Ti in the composition is at least 50%. In various such embodiments, the composition further comprises form from 2 to 15% of a component having poor BCC solubility, including, for example, B, Si, Co, Cu, Fe, Ni, Sn, Pd, Al.
- The MMC's incorporate dendrites in high concentrations (e.g., at least 60% by volume, and in some embodiments up to 98% by volume).
- In various embodiments, the dendrites are small, wherein the diameter of a single dendrite is less than 20 micrometer.

Although specific compositions and combinations of materials are described above, it will be understood that other combinations of materials and concentrations may be used to form Ti-based MMCs without departing from embodiments of the invention.

Embodiments of Additive Manufacturing Methods for Fabricating Ti-Based MMC Parts

As discussed above, Ti-based MMCs in accordance with embodiments incorporate high concentrations of dendrites, however, as found by prior art studies an increased dendrite volume fraction also makes the alloys more viscous and significantly raises their melting temperature, making them more difficult to cast. Recently, metal additive manufacturing (AM) technology (also known as 3D printing) has made rapid advances in the fabrication of net-shaped parts from novel metal alloys. Typically, metal AM has been used to fabricate components from conventional crystalline metal alloys, such as titanium, steel, casting aluminum alloys, and Inconel. However, the rapid cooling rates inherent to many AM technologies have also allowed for the fabrication of metastable alloys, such as bulk metallic glasses (BMGs) and nanocrystalline metals. Specifically, cooling rates in excess of 10³K s⁻¹for each deposited layer have allowed thick, net-shaped components to be printed from alloys that have microstructures (or lack thereof) that cannot be cast or otherwise manufactured in bulk form.
As discussed above, high cooling rates are advantageous in the formation of Ti-based MMCs in accordance with embodiments. Accordingly, many embodiments are also directed to methods of forming objects that take advantage of the very fast rates of cooling afforded by 3D printing to form discrete thin layers to assemble bulk parts from the Ti-based MMC compositions according to embodiments. In many such embodiments, additive manufacturing may be used to fabricate parts of enhanced strength and toughness, wherein each deposited layer of the alloy composition formulated according to the methods of the application is cooled at the sufficiently fast rate to afford a material comprising a matrix infused with ultra-high volume fraction of small dendrites.
FIG. 12 provides a flowchart, and FIG. 13 a schematic of additive manufacturing (AM) methods in accordance with many embodiments. As shown and described, in many embodiments, parts may be manufactured from the Ti-based MMC compositions formulated according to the methods of the application in a layer by layer additive manufacturing process, wherein each layer has a sufficient volume fraction of soft Ti-based dendrites to obtain a three-dimensional part having desired properties. In some such embodiments the individual layers of the Ti-based MMC comprise at least 60% by volume of a soft dendrite phase in the high strength matrix. In various other embodiments, each layer is cooled rapidly enough to create a UFG matrix. In some such embodiments the UFG matrix incorporates dendrites with a size-scale of less than 20 μm. Although various deposition and manufacturing parameters may be used, in many embodiments the thickness of each layer being deposited and cooled during the AM process according to the methods of the application is from 10 to 100 micrometers to ensure a 100 to 1,000 K/s or higher cooling rate per layer.
In many embodiments, the feedstock for 3D printing is pre-alloyed, so as to, upon solidification of the deposited layer, form the appropriate size Ti-dendrites dispersed within the composite's matrix in a sufficient volume fraction. Forming the alloys in accordance with embodiments may be done in situ in the melt, or ex situ, where a hard Ti alloys is combined with the soft Ti components to form a composite. In some such embodiments the hard and soft Ti materials may be provided in the form of powders with a size distribution within 10% of each other. In other such embodiments, the different Ti feedstocks may have different melting temperatures such that during disposition of the metallic glass forming alloy melts and the crystalline phase at most partially melts. In many embodiments, wherein full or partial melting of the material occurs during deposition, Ti-based MMCs of the application are cooled rapidly from above the solidus temperature. In such embodiments, the substrate being built becomes a heat-sink, allowing for cooling rates exceeding 100 to 1,000 K/s to be achieved in each layer. In many embodiments, such high cooling rates are sufficient to form the matrix of the Ti-based MMC into either a metallic glass or an ultra-fine grained alloy, but are also slow enough to allow dendrite precipitation. In many embodiments, additive manufacturing methods of the application afford sufficient cooling rates to fabricate parts with thicknesses exceeding the feedstock alloys' critical casting dimensions. In many embodiments the additive manufacturing technique of the method is selected from a group that includes (but is not limited to): powder bed fusion, direct energy deposition, laser foil welding, thermal spray additive manufacturing, binder jetting, liquid deposition, wire arc additive manufacturing, fused filament fabrication, cold spraying, any combination thereof.
Although most metal additive manufacturing techniques are high temperature methods that melt (or at least partially melt) the material during deposition, cold metal 3D printing methods also exit. Accordingly, in some embodiments, the alloys formulated according to the methods of the application may also be used with the low temperature additive manufacturing techniques chosen from a list that includes, but is not limited to: binder jetting, ultrasonic welding, friction stir additive manufacturing, or cold spraying. In such embodiments, the feedstock material is manufactured in such a way as to achieve the high cooling rate required to form the UFG matrix. In many such embodiments, the feedstock alloy is provided in the form of one of: metal injection molding powder, spray atomized powder, ribbon, or wire.
In many embodiments, additive manufacturing techniques are used to create complex net shapes from the Ti-based MMC alloys of the instant application that cannot be otherwise conventionally cast or machined. In many embodiments, additive manufacturing is used with alloys formulated according to the methods of the application to afford UFGMCs or BMGMCs upon layer solidification from the melt. In many embodiments, the properties of the resulting parts exceed those of conventional crystalline titanium alloys. In many embodiments, the Ti-alloy part fabricated according to the methods of the application exhibits an ultimate tensile strength of greater than 1 GPa, a total strain to failure of at least 5% or more, and a fracture toughness of at least 40 MPa m^1/2. In many embodiments, Ta and Nb elements in the alloy composition, if any, are replaced with low-density V to achieve a density of less than 5.1 g/cm³.
Accordingly, the systems and methods of the application are compatible with most additive manufacturing techniques and are very versatile in their ability to build a net-shaped part with a host of high-performance properties. In addition, in many embodiments, the UFGMC alloys of the application (which have more heat resistant phases than BMGMC alloys) are heat treated post production in order to reduce their porosity and/or improve other properties. Typically, heat treatment of UFFGMCs causes grain growth within the alloy's matrix, which, in turn, negatively affects their mechanical properties. However, in many embodiments, the significant presence (i.e. high volume fraction) and tight packing of the dendrites in the UFGMC alloys prepared according to the methods of the application counteract the detrimental effects of the heat treatment.

EXEMPLARY EMBODIMENTS

The following examples are set forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use the present invention, and are not intended to limit the scope of what the inventors regard as their invention nor are they intended to represent that the experiments below are all or the only experiments performed. Efforts have been made to ensure accuracy with respect to numbers used (e.g. amounts, temperature, etc.) but some experimental errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, molecular weight is weight average molecular weight, temperature is in degrees Celsius, and pressure is at or near atmospheric.

Example 1. Representative Alloy Compositions for Additive Manufacturing of Parts

In many embodiments, exemplary compositions, such as, Ti₇₄Ta₁₀Zr₁₀Si₆and Ti₇₅Cu₇Ni₆Sn₂Ta₁₀represent alloys suitable for the additive manufacturing methods of the application. Specifically, both of these alloys comprise a sufficient amount of Ti and ß-stabilizers (94 and 85 atomic %, correspondingly), which, in turn, ensures the sufficient volume fraction of dendrites 60%) in their microstructure upon solidification from the melt. Such high dendrite content prevents these alloys from easy casting, however, they can be used in 3D printing methods of the application to afford tough parts of any complexity.

Example 2. Representative Alloy Compositions for Additive Manufacturing of Low Density Parts

In many embodiments, exemplary compositions, such as, Ti₇₄V₁₀Zr₁₀Si₆and Ti₇₅Cu₇Ni₆Sn₂V₁₀represent low density alloys suitable for the additive manufacturing methods of the application. In these compositions, Ta element of the compositions from Example 1 above is replaced with V element, lowering the resulting alloys' densities to 4.9 and 5.1 g/cm³respectively. Notably, additions of Al and/or B can lower the alloy density even further. Accordingly, in many embodiments, the low density, high strength alloys composed according to the methods of the application are used with additive manufacturing methods of the application to produce parts with excellent specific strengths.

Example 3. Importance of High Cooling Rates for Formation of Ti-based MMCs

In many embodiment, exemplary compositions, such as, Ti₉₀Nb₅Cu₅alloy may be used through a rapid cooling manufacturing process, such as, AM to form parts having smaller, tighter packed dendrite microstructure. In accordance with such embodiments, high strength MMCs parts may be produced from the alloys known to afford soft and ductile cast parts, when used according to the methods of the application.

Doctrine of Equivalents

This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.

Claims

1. A method of manufacturing a part comprising a metal composite material comprising:

providing an alloy comprising Ti at least one β-stabilizer, wherein a β-stabilizer is an element selected from the group consisting of Zr, Hf, Ta, Nb, V, Sn, and Mo; and X, wherein X represents one or more additional elements selected from the group consisting of Co, Fe, Ni, Cu, Al, B, Ag, Pd, Au, Pd, C, and Si wherein the atomic % of Ti in the alloy is greater than the amount of any other element in the alloy, wherein the combined amounts of Ti and the at least one β-stabilizer comprise at least 85 atomic % of the alloy, and wherein the alloy does not contain Be;

depositing a molten portion of the alloy; and

cooling the molten portion at a cooling rate such that upon solidification the alloy segregates phases to produce a metal matrix composite comprising a plurality of isolated crystalline dendrites characterized by a dendrite size and a dendrite density dispersed within a continuous crystalline eutectic material matrix.

2. The method of claim 1, wherein the alloy comprises Ti, Zr, at least one additional beta-stabilizer, and X; and wherein the atomic % of the sum of Ti, Zr, and the at least one additional beta-stabilizer comprises between 85 and 98 atomic % of the alloy, and X comprises from 2 and 15 atomic % of the alloy.

3. The method of claim 2, wherein the at least one additional beta-stabilizer is selected from the group consisting of V, Nb, Ta and Mo.

4. The method of claim 1, wherein Ti comprises at least 50 atomic % of the alloy.

5. The method of claim 1, wherein the alloy comprises Ti, one or both Zr and Hf, at least one additional beta-stabilizer, B, and at least one additional X; wherein the sum of Ti, the on or both Zr and Hf, and the at least one additional beta-stabilizer comprises between 85 and 98 atomic % of the alloy, B comprises between 0.5 and 5 atomic % of the alloy, and X comprises less than 10 atomic % of the alloy.

6. The method of claim 5, wherein the at least one additional beta-stabilizer is selected from the group consisting of V, Nb, Ta and Mo, and the at least one additional X is selected from the group consisting of Si, Cu, Co, Fe, and Pd.

7. (canceled)

8. The method of claim 1, wherein the cooling rate is greater than 10²K/s.

9. The method of claim 1, wherein the plurality of isolated crystalline dendrites comprises at least 60% by volume of the solidified alloy.

10. The method of claim 1, wherein the continuous crystalline eutectic material matrix is characterized by a matrix hardness and the plurality of isolated crystalline dendrites is characterized by a dendrite hardness, and wherein the matrix hardness is at least 5% larger than the dendrite hardness.

11. The method of claim 1, wherein the metal matrix composite part has at least one property selected from the group consisting of: a tensile strength of greater than 1 GPa, a fracture toughness of greater than 40 MPa m^1/2, a density of less than 6.0 g/cm³, a total strain to failure of greater than 5% in a tension test, and a yield strength divided by the density greater than 200 MPa cm³/g.

12. The method of claim 1, wherein the alloy is characterized by a solidus temperature of less than 1600 Celsius.

13. The method of claim 1, wherein the dendrite size ranges from 1 to 20 micrometers in diameter.

14. The method of claim 13, wherein the dendrite size is less than 10 micrometers in diameter.

15. The method of claim 1, wherein depositing further includes heating the alloy portion to a semi-solid temperature region between the alloy's solidus and liquidus.

16. The method of claim 1, wherein the metal matrix composite part is used in a structural application.

17. The method of claim 1, wherein the part is used in a structural alloy is selected from the group consisting of Ti₇₄V₁₀Zr₁₀Si₆, Ti₆₄V₁₀Zr₂₀Si₆, Ti₇₁V₁₀Zr₁₀Si₆Al₃, Ti₇₄Nb₁₀Zr₁₀Si₆, Ti₇₄Ta₁₀Zr₁₀Si₆, Ti₇₅CU₇Ni₆Sn₂V₁₀, Ti₇₅CU₇Ni₆Sn₂Nb₁₀, Ti₇₅CU₇Ni₆Sn₂Ta₁₀, (Ti₇₂Zr₂₂Nb₆)₉₅Co₅, (Ti₇₂Zr₂₂Nb₆)₉₂Co₅Al₃, (Ti₇₂Zr₂₂Ta₆)₉₅Co₅, (Ti₇₂Zr₂₂Ta₆)₉₂Co₅Al₃, (Ti₇₂Zr₂₂V₆)₉₅Co₅, (Ti₇₂Zr₂₂V₆)₉₂Co₅Al₃, Ti₉₀Nb₅Cu₅, Ti₈₅Nb₁₀Cu₅, Ti₈₀Nb₅Cu₁₀, Ti₈₀Nb₁₀Cu₁₀, Ti₉₀Ta₅Cu₅, Ti₈₅Ta₁₀Cu₅, Ti₈₀Ta₅Cu₁₀, Ti₈₀Ta₁₀Cu₁₀, Ti₉₀V₅Cu₅, Ti₈₅V₁₀Cu₅, Ti₈₀V₅Cu₁₀, Ti₈₀V₁₀Cu₁₀, Ti₈₅V₁₀B₅, Ti₈₅Ta₁₀B₅and Ti₈₅Nb₁₀B₅, Ti₅₇Zr₁₈V₁₂Cu₁₀Al₃or Ti₆₂Zr₁₈V₁₂Cu₅Al₃.

18. The method of claim 1, wherein the metal matrix composite part is a type of a part selected form the group consisting of: biomedical implant, structural aerospace component, sporting equipment, medical device, and engine component.

19. The method of claim 1, wherein the cooled portion forms a metal matrix composite part with a thickness of at least 0.5 mm.

20. A method of manufacturing a metal composite part comprising:

providing a metal composite material comprising:

an alloy comprising Ti; at least one β-stabilizer, wherein a β-stabilizer is an element selected from the group consisting of Zr, Hf, Ta, Nb, V, Sn, and Mo; and X, wherein X represents one or more additional elements selected from the group consisting of Co, Fe, Ni, Cu, Al, B, Ag, Pd, Au, Pd, C, and Si; wherein the atomic % of Ti in the alloy is greater than the amount of any other element in the alloy, wherein the combined amounts of Ti and the at least one β-stabilizer comprise at least 85 atomic % of the alloy; and wherein the alloy does not contain Be, wherein the alloy is segregated into phases comprising a plurality of isolated crystalline dendrites dispersed within a continuous crystalline eutectic material matrix; and

depositing and forming a portion of the metal composite material without heating to form a metal matrix composite part with a thickness of at least 0.5 mm.

21. The method of claim 20, where the alloy comprises Ti, Nb and from 2 to 15 atomic % B.

22. The method of claim 21, wherein the concentration of B is 5 atomic %.