Patent Application
20060137778
1. Field of the Invention
This invention resides in the fields of metallic glasses and nanotechnology, and particularly in methods for strengthening metallic glasses by the incorporation of crystalline particles in the glassy matrix.
2. Description of the Prior Art
Metallic glasses are known to be superior to conventional metals by virtue of the improved mechanical properties of the glasses, including their higher tensile strength, fatigue strength, hardness, axial fatigue, and fracture toughness. These qualities, combined with a mid-range density, have resulted in metallic glasses being used for certain high-performance and high-impact applications. Examples of products that have been manufactured from metallic glasses are aeronautical and industrial turbo-engines, airframes, knives, golf-club heads, and even wristwatches.
Metallic glasses are alloys that have an amorphous microstructure. The amorphous state is achieved by cooling the alloy composition from a melt at a cooling rate that is fast enough to avoid crystallization. In the early investigations of metallic glasses, the alloys that were used required cooling rates of 104-106 degrees Celsius per second (also designated as “K/s”) to bypass crystallization. Because of this requirement, cooling could only be performed on bodies of the melt with very small dimensions, such as layers less than 100 microns in thickness or small droplets, and the amorphous material was produced only in the form of thin ribbons or fine powders.
Subsequent investigations have led to the development of several families of alloys that can be cooled to an amorphous form at much slower cooling rates, such as 103 K/s or less, and most recently cooling rates within the range of 0.1-100 K/s. Among the leading investigators in this development are A. Peker, W. L. Johnson, and A. Inoue, whose investigations are reported in the literature, notably in Peker, A., and W. L. Johnson, Appl. Phys. Lett. 63, 2342 (1993), and U.S. Pat. No. 5,288,344 (Peker, A., and Johnson, W. L., assigned to California Institute of Technology), issued Feb. 22, 1994, A. Inoue, notably in Inoue, A., et al., “Fabrication of Bulky Zr-Based Glassy Alloys by Suction Casting into Copper Mold,” Materials Transaction, Japan Institute of Metals (English Version), vol. 36, no. 9, pp. 1184-1187 (1995), and Inoue, A., et al., “Preparation and Thermal Stability of Bulk Amorphous Pd40Cu30Ni10P2O Cylinder of 72 mm in Diameter,” Materials Transaction, Japan Institute of Metals (English Version), vol. 38, no. 2, pp. 179-183 (1997). The contents of these and all other literature and patent citations in this specification are incorporated herein by reference.
Alloys that can form amorphous solids at these low cooling rates have led to the emergence of a class of metallic materials known as bulk metallic glasses (BMGs) since the lower critical cooling rate permits these glasses to be produced in dimensions of several centimeters. By virtue of this flexibility, BMGs are suitable for many structural and functional applications, including the larger-scale products among those listed above and components in general for the defense industries, manufacturing industries, and recreational products, as well as magnetic materials, medical instruments, and implants.
An even more recent development in BMGs is the discovery that the mechanical properties of the glasses, and the magnetic properties of those used as magnets, can be further enhanced by the dispersion of crystallites throughout the amorphous matrix of the glass. Crystallites with sizes both in the nano-scale and the micro-scale have been investigated. Methods of achieving these dispersions, particularly of nanocrystallites, and the benefits that the dispersions offer are described for example in Perepezko, J. H., et al. (Wisconsin Alumni Research Foundation), United States Patent No. U.S. Pat. No. 6,261,386 B1 (issued Jul. 17, 2001). The methods generally consist of controlled cooling techniques that result in partial crystallization (devitrification) of the amorphous material. The crystals are thus grown by nucleation, however, which is generally accompanied by grain growth, and as noted by Perepezko et al., the quality of the resulting dispersion, particularly the number of crystals formed and their size and distribution throughout the amorphous matrix, are difficult to control. The crystals reported by Perepezko et al. are in the nano-range, i.e., 100 nm or less in diameter, which are of particular interest where a high-density dispersion is sought. To achieve nanocrystalline dispersions, the Perepezko et al. disclosure proposes the seeding of the amorphous matrix with elements that are insoluble in the amorphous matrix. This however involves the introduction of foreign matter into the melt, and requires control of the seed size, while still causing grain growth. The formation of and benefits offered by micro-range crystals are reported by Hays, C. C., et al., Phys. Rev. Lett. 84: 2901-4 (2000). The crystals reported in this paper are dendrites that improve the toughness and plasticity of the glasses.
It has now been discovered that a composite metallic material consisting of crystallites dispersed throughout a metallic glass matrix can be achieved with a high degree of control by annealing a metallic glass while passing an electric current, preferably a DC current, through the glass. The sizes of the crystallites and their density (i.e., volume fraction) can be controlled by adjustment of the current density. This discovery permits the formation of a composite amorphous-nanocrystalline or amorphous-microcrystalline material with a high volume fraction of crystallites with little risk of undesired crystal growth that typically accompanies devitrification in thermal annealing processes and without the introduction of foreign materials. Although applicable to amorphous systems in general, this invention is of particular interest to bulk metallic glasses.
It has further been discovered that the achievement of crystallite dispersions by this method occurs both in glasses that do not undergo phase changes during the annealing process prior to crystallization and those that do undergo phase changes. The phase changes referred to herein are redistributions of the alloy components causing changes in alloy composition while still maintaining the amorphous structure. These phase changes may be desirable or undesirable, depending on which phase is preferred and on the character and properties sought in the ultimate product.
This invention also resides in the resulting composites themselves, which are novel in view of the high degree of control that can be imposed on their manufacture. These and other features, objects, advantages, and preferred embodiments will be best understood from the description that follows.
The terms “a” and “an” as used in the claims appended hereto denote “one or more,” and the term “comprising” denotes “including but not limited to.”
Alloy compositions to which this invention can be applied include the wide range of compositions that can form a glass upon cooling from a melt and that once formed will remain a glass upon further cooling to ambient temperature. For some alloys, as noted above, the formation of a glass by cooling from a melt requires a very fast cooling rate while for others, slower cooling rates, such as those that permit the formation of bulk metallic glasses, can be used. Among the various classes of alloys that can form a glass in this manner are: (i) aluminum-based glasses that include transition metals and rare earth elements, (ii) iron-based, nickel-based, and cobalt-based glasses that may include transition metals and rare earth elements and in many cases, boron, (iii) zirconium-based glasses, (iv) titanium-based glasses, (v) palladium-based glasses, (vi) platinum-based glasses, and other transition-metal-based glasses that include copper, nickel, boron, or phosphorus as alloying elements, often in various combinations. Some of the more recently developed classes of glass-forming alloy systems and publications in which they are described, are the following, in which “Ln” denotes lanthanide metal and “TM” denotes transition metal:
Still other classes of glass-forming alloys exist, as known to those skilled in the art, and are contemplated for inclusion within the practice of the present invention. Both magnetic and non-magnetic alloys are included. The terms “[metal]-based” in which “[metal]” refers to a specified metallic element and “primary.constituent” are used herein to denote the metal that constitutes the highest weight percent component of the alloy. The term “secondary constituent” denotes a component present in the alloy at a lower weight percent than the primary constituent.
One class of preferred alloy compositions is that in which the primary constituent is either zirconium or titanium or both in combination. Certain alloys of this class contain beryllium as a secondary constituent. Among these are alloys that contain zirconium and titanium in combination with beryllium, copper and nickel. Alloys of this class are available from Amorphous Technologies International (Laguna Niguel, Calif. USA) and Liquidmetal Technologies (Tampa, Fla., USA) under the name VITRELOY®. Examples of zirconium-based VITRELOY alloys that contain beryllium are Vit1 and Vit1A, examples of those that do not contain beryllium are Vit 105 (the metals of which are Zr—Ti—Ni—Co—Al) and Vit 106 (the metals of which are Zr—Nb—Ni—Co—Al), and an example of one that is copper-titanium-based is Vit 101 (the metals which are Ti—Cu—Zr—Ni). Another class of preferred alloy compositions is one in which the primary constituent is an upper transition metal that is either palladium, iron, cobalt, manganese, ruthenium, or silver. The secondary constituent(s) in this class of alloy compositions are preferably copper, nickel, or phosphorus, or two or all of these elements. A preferred subgroup of this class is the Pd—Cu—Ni—P group included in the list above, and a particularly preferred subgroup are those having the formula Pd60-xCu30Ni10Px where x=10-20. A third class of preferred alloy compositions is that in which the alloys include a transition metal, phosphorus, and boron. Further preferred alloy compositions are as listed above.
The starting material can be an alloy that is entirely amorphous in microstructure, or one that contains both amorphous and crystalline regions, preferably with the amorphous region constituting at least 50% by volume of the microstructure.
While the invention is applicable to any alloy composition that is capable of forming a metallic glass, preferred alloy compositions are those that have a critical cooling rate of less than about 1,000 K/s, preferably less than about 500 K/s, and most preferably within the range of about 0.1 K/s to about 100 K/s. The “critical cooling rate” is the slowest cooling rate at which the composition will form an amorphous solid upon cooling from a melt. In terms of the glass transition temperature (Tg), preferred alloy compositions are those that have a glass transition temperature within the range of about 250° C. to about 600° C., and most preferably within the range of about 300° C. to about 500° C., as determined at a heating rate of 10 degrees Celsius per minute. In terms of a still further parameter, the difference between the crystallization temperature and the glass transition temperature, preferred alloy compositions are those in which this difference is at least about 30 degrees Celsius (at a heating rate of 10° C./min), and most preferred are those in which the difference is at least about 50 degrees Celsius.
As noted above, alloy compositions for use in the practice of this invention can be purchased from commercial suppliers. Alternatively, the alloy compositions can be prepared by methods known in the art and disclosed in the literature. A method in current use is vacuum die casting, as described in the following United States patents naming Colvin, G., as inventor: U.S. Pat. No. 5,287,910, issued Feb. 22, 1994, U.S. Pat. No. 6,021,840, issued Feb. 8, 2000, and U.S. Pat. No. 6,070,643, issued Jun. 6, 2000.
The passage of an electric current through the glass during the annealing procedure in the practice of this invention can be achieved by conventional means. The current density, the duration of the application of the current, and the temperature maintained during the current can vary, although variations in these parameters will result in variations in the volume fraction of the crystallites formed and, in many cases, the sizes of the crystallites, which can range from several nanometers to tens of micrometers. As noted above, in cases where the sizes of the crystallites are affected by the current density and its duration, any increase in size can be readily controlled, and is considerably more controllable than in the thermal devitrification processes of the prior art. Thus, the current density and duration will be varied in accordance with the volume fraction and in some cases size of the crystallites that are desired for the product composite. In most cases, particularly when crystallites in the nano-size range are sought, best results will be obtained with current densities of at least about 100 A/cm2, preferably at least about 300 A/cm2, more preferably from about 300 A/cm2 to about 5,000 A/cm2, and most preferably from about 500 A/cm2 to about 2,500 A/cm2. Best results in most cases will also be achieved by applying the current for a duration of at least about 30 minutes, preferably at least about one hour, more preferably from about one hour to about eight hours, and most preferably from about two hours to about six hours. The annealing temperature at which the current is applied is preferably below the laboratory glass transition temperature (measured at a heating rate of 10 K/min), and preferably within about 25 degrees Celsius, most preferably within about 10 degrees Celsius, of the laboratory glass transition temperature. The applied current can be an alternating current (AC), a direct current (DC), or a pulsed direct current. For maximum control of the crystallite size, a DC current is preferred.
As noted above, the operating conditions can be controlled to achieve crystallites within the micro-range or the nano-range, or both. Preferably, the conditions are controlled to produce crystallites that are about 50 microns or less in diameter, preferably 1 to 30 microns in diameter, when micro-range crystallites are either acceptable or desired. When nano-range crystallites are sought, the preferred size range of the crystallites will be about 100 nanometers or less, and most preferably about 2 nanometers to about 100 nanometers.
The following examples are offered for purposes of illustration and are not intended to limit the scope of the invention.
The following experiments illustrate the use of a DC current to form crystallites in two metallic glasses. One of the glasses had the empirical formula Zr42.6Ti12.4Cu11.25Ni10Be23.75, has a critical cooling rate of 1 K/s, a glass transition temperature of 628 K (355° C.) at a heating rate of 10 K/min, and a AT (the difference between the crystallization temperature and the glass transition temperature) of about 97 K, and is a glass known to undergo decomposition (phase change) before crystallization. The other glass had the empirical formula Pd40Cu30Ni10P20 (referred to hereinafter as “PCNP”), has a critical cooling rate of less than 1 K/s, a glass transition temperature of 582 K (309° C.) at a heating rate of 10 K/min, and a AT of about 88 K, and is believed to crystallize by classical nucleation and growth, i.e. without decomposition. The Zr-based glass was a product obtained from Howmet Research Corporation (Whitehall, Mich., USA) and identified by the product name Vit1A. The Pd-based glass was prepared by induction melting of Pd, Cu73.4P26.6, Ni2P and P in vacuum of 10−3 mbar inside a silica tube 5 mm in diameter, using B2O3 oxide flux, followed by quenching with water. The materials used were Pd metal pieces (99.95% purity), Ni2P powder (99.5%), copper phosphorus shots (Cu:P; 85:15 weight percent) and additional phosphorus lumps (99.999+%). The Vit1A glass sample was sectioned into rectangular sheets measuring of 3.25×3.80×0.5 mm. The PCNP glass was sectioned into circular disks 0.5 mm in thickness and 5 mm in diameter.
The sectioned samples were exposed to a DC current in an apparatus consisting of two copper electrodes surrounded by a cylindrical furnace made from tantalum sheets cut in a serpentine design as described by Bertolino, N., et al., in Scripta Mater. 44, 737 (2001), and Phil. Mag. B 82, 969 (2002). Copper foil was placed between the samples and the electrodes to ensure good contact. DC current was then applied at current densities of 810 and 1620 A/cm2 for the Vit1A samples and 509 and 1018 A/cm2 for the PCNP samples while the samples were annealed. Further samples of each material were annealed under identical conditions except in the absence of a current. The annealing temperatures for all samples, regardless of the presence or absence of a current or the current density, were 623 K for the Vit1A samples and 577 K for the PCNP samples. Each of these temperatures is approximately 5 K below the laboratory Tg values of the respective glass. For those samples that were exposed to a current, the samples reached the steady-state temperature resulting from the Joule heating in approximately 5 minutes. Since these temperatures were lower than the desired annealing temperatures, the furnace was then activated to achieve the target annealing temperature. In all cases, the annealing temperatures were controlled to within approximately ±2 K of the target. Temperatures were monitored by a shielded and grounded Type K thermocouple in direct contact with the center of each sample. The thermocouple was calibrated by determination of the melting points of Zn and Sn.
After annealing, the samples were analyzed by small-angle neutron scattering (SANS) at the Paul Scherrer Institute (Villigen, Switzerland). Using a wavelength of λ=6 Å and sample-detector distances of 1.8 m and 8 m for the Vit1A samples, and 20 m for the PCNP samples, the Q-range covered was 0.1-3.5 nm−1 for the Vit1A samples and 0.03-3.5 nm−1 for the PCNP samples (Q=4π sin θ/λ, with θ=half the scattering angle).
The data representing the samples annealed for 210 minutes at 623 K in the absence of a DC current (the asterisks) indicate only a very small scattering contribution in the low Q region. This shows that the sample is essentially amorphous after this heat treatment and still homogeneous in composition. In contrast, the data representing the samples annealed at the same conditions but under the influence of a current (the squares and triangles), show a strong scattering contribution in SANS. This indicates that the imposition of a current has a strong influence on either phase separation, crystallization, or both in this metallic glass.
For an analysis of the SANS data, SANS intensity was assumed to occur from spherical particles with log-normally distributed particle diameters. In this case, the scattering intensity is
where Δη is the scattering length density contrast, Fp is the particle form factor, Vp(R) is the volume of the (spherical) particle and N(R)dR is the incremental volume fraction in the size interval between R and R+dR. Assuming a log-normal distribution, N(R) can be parameterized by the amplitude A, width a, and position R0 of the log-normal distribution. The three parameters are obtained from fitting the scattering curves (the solid lines in
The plot shows that the results were similar to those obtained with the Vit1A samples: in the samples where no current was applied, annealing near the glass transition only leads to a small scattering contribution at low Q, whereas in the samples where annealing was accompanied by the application of a current, a stronger SANS intensity was observed. The log-log presentation of the sample annealed with a current of 1018 A/cm2, which is included in the Figure as an inset, shows that the SANS intensity mainly follows a Q−4 law. The exponent −4 indicates that most of the structure information was at very low Q, outside the resolution of the SANS experiment. Although the only information about the crystal size was that it exceeded 100 nm, the result shows that the crystallization behavior changes when an external current is applied.
This experiment shows that the application of an electric current can produce crystals of various sizes ranging from the nanometer range into the micrometer range. Depending on the alloy and its use, both nano-sized and micro-sized dispersions offer superior properties over microstructures that are either monolithic glasses or fully crystalline. A composite consisting of micrometer-sized crystals in a glassy matrix shows for example 5% plasticity under tensile conditions while the monolithic glass shows nearly no plasticity and fails under the formation of one dominant shear band.
To obtain further information about the crystallization behavior, differential scanning calorimetry (DSC) was performed on Vit1A samples that had been processed and annealed in the same way as those that were analyzed by SANS.
While the glass transition temperature, at Tg=625±3 K, is the same within experimental error for the different annealing conditions, the crystallization behavior is obviously different. The sample annealed without external current (the lower line) shows crystallization peaks centered at 731 K and 737 K, with the main crystallization peak at 737 K. In contrast, the samples annealed with a current (middle and upper lines) show a lower thermal stability. The crystallization peaks in these scans are centered at 715 K and 731 K, with the main crystallization peak at 731 K. Furthermore, the overall crystallization enthalpy of the sample annealed with a current of 1620 A/cm2 (the upper line) is 77.4 J/g, which is slightly smaller than that of the sample annealed with 810 A/cm2 (ΔHcryst=78.8 J/g) (middle line). Similar observations are made in the PCNP samples. Thus, while the glass transition does not change, modifications in the crystallization behavior are observed with variations in the current density.
In both the Vit1A samples and the PCNP samples, the SANS data for the samples annealed with current included no interference maxima, while the literature reports the occurrence of interference maxima in SANS data of samples annealed in the absence of current. The presence of interference maxima implies a phase separation process on the nanometer scale. In Vit1A, for example, Löffler, J. F., et al., J. Appl. Cryst. 33, 500 (2000), report interference maxima occurring in a temperature range between 603 and 643 K when the samples are annealed for 15 hours in the absence of a current. Only at temperatures above 643 K do these interference maxima vanish. Comparing this with the experimental data presented herein leads to the conclusion that annealing in the presence of the current can cause the glass to crystallize without a prior phase separation process on the nanometer scale. The current thus has an effect on crystallization that is similar to that resulting from an increase in temperature.
Comparing the DSC patterns in
As reported by Löffler, J. F., et al., Mater. Sci. Forum 343-346, 179 (2000), PCNP forms micrometer-sized crystals even in the deeply undercooled liquid regime and does not decompose on the nanometer scale. While SANS can only give qualitative results on the current-induced crystallization behavior, the strong increase of the SANS intensity at low Q shown in the data presented herein indicates that the current also influences the crystallization in Pd-based alloys.
To summarize, the data collectively show that electric current has a large influence on the volume fraction and crystal size in metallic glasses, and that high current densities facilitate crystallization. Since the microstructure is much easier to control with an electric current than with temperature in thermal devitrification processes of the prior art, crystallite dispersions can be formed in metallic glasses with a high degree of control over the size of the particles and the density and uniformity of the dispersion.
The foregoing descriptions are offered primarily for purposes of illustration. Further variations and substitutions in the materials and equipment used, the processing conditions, and other parameters of the system will be apparent to those of skill in the art and can be made without departing from the spirit and scope of the invention.
[00011 This invention was made with Government support by Grant No. DAAD19-01-1-0493, awarded by the Army. The Government has certain rights in this invention.
