EP1499753A2 - Nanophase precipitation strengthened al alloys processed through the amorphous state - Google Patents

Abstract

Aluminum alloys having improved strength characteristics at elevated temperatures (300°C) are manufactured by combining selected transition metals (Ni, Co, Ti, Fe, Y, Sc) and selected rare earth materials (Er, Tm, Yb, Lu) in amounts of about 2 to 12% and 2 to 15% atomic percent respectively in an amorphous, glassy state and subsequently devitrifying the amorphous material to form a crystalline mix of fcc and L12 phase material. Devitrification from the amorphous state may be effected by various means including thermal and thermo mechanical processes.

Description

NANOPHASE PRECIPITATION STRENGTHENED Al ALLOYS PROCESSED THROUGH THE AMORPHOUS STATE

BACKGROUND OF THE INVENTION Activities relating to the development of the subject matter of this invention were funded at least in part by United States Government, U.S. Army Aviation & Missile Command Contract No. DAAH01-02-C-R125, and thus may be subject to license rights and other rights in the United States.

In a principal aspect, the present invention relates to aluminum-based alloys processed through an amorphous state or stage, preferably by means of a rapid solidification process from molten alloy, and then devitrified to a primarily crystalline structure by thermal or thermomechanical processing. In order to achieve a desired microstructure during processing, the aluminum alloy comprises a combination of rare earth and transition metal components with the aluminum. The final crystalline microstructure has stable high temperature strength (at or above about 300°C) characterized by nanoscale intermetallic precipitates, preferably Ll₂ phase precipitates.

Currently available commercial aluminum alloys, either manufactured with ingot or powder processing, are not capable of simultaneously achieving high strength and high temperature stability near 300°C; such characteristics being particularly important in applications such as the fan components in turbine engines where short-term tensile strengths of the order of 500MPa (40-60 ksi) are desired. Precipitation hardening introduced by aging is a known method to strengthen aluminum alloys. Thus, conventional high strength aluminum alloys in commercial applications utilize GP (Guinier-Preston) zones and subsequent precipitation when seeking to achieve high material strength. With this type of strengthening, precipitates are usually formed during aging at or below 250°C in order to produce appropriate strengthening precipitate dispersions.

Examples of aluminum alloys processed with relatively high aging temperatures in commercial practice include 2618 (200°C for 20 hours), 4032 (170-175°C for 8-12 hours), and 2218 (240°C for 6 hours) [Metals Handbook — Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Nolume 2, 10^th Edition, ASM International]. At the noted aging temperatures, these alloys have improved microstructure stability relative to other commercial aluminum alloys. These aluminum alloys, when age-hardened, usually possess a room temperature yield strength of about 600Mpa (85 ksi). However, at temperatures approaching 300°C, the precipitation strengthening efficiency in these alloys quickly and significantly degrades as a result of precipitate coarsening and/or dissolution. Due to the unstable strengthening precipitate size distribution at such high temperatures, the yield strength of currently available aluminum alloys at 300°C is often only 10% of the yield strength at room temperature, and thus renders such alloys unsuitable for high temperature applications above 150°C.

In order to achieve a combination of high strength and usable high temperature properties in aluminum alloys, researchers have investigated a variety of intermetallic precipitation dispersions. Since selected intermetallic precipitates may determine the strength and ductility of the final aluminum alloy, it is desirable to select cubic precipitate phases with high crystallographic symmetry to promote ductility. The aluminum-based Ll₂ phase (fee ordered) is one of the best-known precipitates to achieve a good combination of high strength and high toughness.

However, since crystalline aluminum has very limited solubility for most intermetallics, including Ll₂ forming components, it is difficult to produce fine intermetallic dispersions through crystalline solid-state heat treatments that utilize a step to dissolve the strengthening phase particles prior to reprecipitation during aging. However, rapid solidification processing (RSP) from the liquid state has been attempted or utilized. With an RSP process, it is possible to either (1) directly produce fine crystalline structure, or (2) produce partially or completely amorphous aluminum alloys. Nonetheless, crystalline RSP aluminum alloys have not been able to meet the high temperature strength requirements due to the difficulty of producing small, stable particle sizes at adequate volume fraction. Alternatively, the focus on amorphous aluminum alloys has been primarily on the glass formability. These glassy alloys in general have low ductility, and, when devitrified, they typically form crystalline microstructures that are far from optimal for fracture toughness and other requirements.

For example, RSP crystalline aluminum alloys, not designed to form glass with RSP, utilize a metastable and coherent Ll₂ dispersion (about 1%) which converts into other phase structures at high temperature. [Parameswaran, Weertman and Fine, Scripta Metall., vol.23, pp.147, 1989] U.S. Pat. Nos. 4,950,452, 5,074,935, 5,334,266, 5,458,700, 5,431,751, 5,053,085 and its derived divisional patents (U.S. Pat. Nos. 5,240,517, 5,320,688, and 5,368,658) utilize RSP to achieve either amorphous, a mixture of amorphous and crystalline, or crystalline microstructures, without the use of an Ll strengthening dispersion. The aluminum alloys of U.S. Pat. No. 4,889,582 achieve the high temperature strength with RSP process and age hardening to produce Al₃Fe (which has non-cubic DO₃, not Ll₂ structure) strengthening precipitates, using different alloying components. U.S. Pat. No. 4,464,199 describes high strength aluminum alloys with 4 to 15% iron processed with RSP to achieve high temperature stability. Use of iron promotes Al₃Fe precipitation. The aluminum alloys of U.S. Pat. No. 4,787,943 are composed of transition metals and rare earth metal components. However, with titanium and iron as the preferred transition metals, and gadolinium and cerium as rare earth elements, the alloy does not comprise a stable

Ll₂ strengthening dispersion.

U.S. Pat. No. 6,248,453B1 describes the use of an Ll strengthening dispersion for high strength aluminum alloys utilizing certain alloying components, but does not utilize a transition metal such as nickel and cobalt and an intermediate amoφhous state as a means to refine the

Ll₂ microstructure.

Finally, the high strength and high temperature aluminum alloys referenced have not found widespread application due to the lack of robust processing to produce those alloys and the deficiency of other alloy properties, such as toughness and ductility.

SUMMARY OF THE INVENTION

The present invention is directed to a new class of aluminum alloys characterized by formation from an intermediate amoφhous state to a final face centered cubic (fee) aluminum matrix strengthened by a precipitate dispersion in order to achieve high temperature strength with usable ductility. The disclosed aluminum alloys thus are comprised of a fine crystalline structure of primarily aluminum fee matrix strengthened by a precipitate phase with high cubic symmetry, preferably the Ll phase. An appropriate melt of aluminum with crystalline formers is first processed to achieve an intermediate amoφhous state to preserve the crystalline formers. The preferred method to achieve a primarily (above 70% in volume) amoφhous state is a rapid solidification process (RSP) from the molten alloy by process techniques such as powder atomization, melt spinning, and spray casting. The RSP process preferably should have a cooling rate of at least about 10³°C/sec, preferably at least 10⁴°C/sec. Other methods to achieve amoφhous microstructure through a solid-state process, such as mechanical milling, may also be used. The intermediate amoφhous alloys are then devitrified to a final primarily (above 70% in volume) fcc/Ll crystalline microstructure with at least about 30% fee phase in volume and at least about 10% Ll₂ phase in volume. Thermal treatment, thermomechanical treatment, or a combination of both, are the preferred methods to produce the final crystalline microstructure.

The alloys of the invention demonstrate high strength capability after both short and long exposure to a high temperature at or above 300°C by maintaining a fine and stable microstructure. For example, a short-term strength requirement of 40 - 60 ksi yield strength is expected. For the long-term strength, as characterized by creep strength, it has been demonstrated that optimal creep resistance at 300°C for aluminum alloys is obtained from a dispersion of 25 nm diameter Ll₂ particles [E. A. Marquis, Microstructural Evolution and Strengthening Mechanisms in Al-Sc and Al-Mg-Sc Alloys, Ph.D. thesis, Northwestern University, 2002.]. Also, it has been demonstrated [Ohtera, Inoue and Masumoto, in First International Conference on Processing Materials for Properties, Ed. Henein and Oki, The Minerals, Metals & Materials Society, p.713, 1993] that aluminum-based alloys devitrified by extrusion from amoφhous atomized powder can achieve aluminum fee grain size of 80 to 170 nm, with intermetallic phase dispersions as fine as 10 to 80 nm in diameter. Such grain or particle sizes may therefore encompass the desired optimal particle diameter for creep strengthening by an Ll₂ phase intermetallic dispersion. Accordingly, a process and an aluminum alloy product combining a sufficient amount of stable Ll₂ refined or converted by devitrification from a glass phase material by appropriate thermal or thermomechanical processing has been discovered. The alloy meets or exceeds high temperature strength requirements with optimized creep resistance as a result of intermetallic, small particle phase dispersions.

The aluminum alloys of this invention are produced by combining aluminum, selected metals, particularly transition metals, rare earth elements, and other optional elements into a melt which is converted to an amoφhous or glassy state and then subsequently to a crystalline state by a devitrification process. In the broadest sense about 2 to 15 atomic percent in the aggregate of at least one rare earth selected from the group consisting of erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) is provided for the melt. Further about 2 to 7 atomic percent in the aggregate of at least one transition metal selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe) and copper (Cu) is provided to enhance or facilitate the process step of forming an amoφhous or glassy material from the melt. Other materials included, optionally, in the melt comprise additional transition metals, lithium (Li) and magnesium (Mg). For example, of less than about 5% in the aggregate by atomic fraction scandium (Sc), yttrium (Y), titanium (Ti), zirconium (Zr), vanadium (V), chromium (Cr), manganese (Mn) and lithium (Li) incoφorated into the melt or alloy as a modifier. Less than about 7% in the aggregate by atomic fraction magnesium (Mg), zinc (Zn), silver (Ag) and niobium (Nb) may be incoφorated into the melt or alloy to accommodate fee misfit in the crystalline state. The selection of alloying components is based on the function or criteria of (1) good

Ll₂ formers, (2) adequate glass formability with RSP — this leads to the selection of components with strong short range ordering effects and slow long-range diffusing kinetics in molten aluminum, and (3) good fcc/Ll₂ coherency with low misfit which lead to a fine microstructure and good coarsening resistance at high temperature. Lanthanide rare earth Ll₂ formers such as erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) are slow diffusers and hence enhance glass formability while at the same time allowing Ll₂ formation during devitrification. Scandium (Sc) is a strong Ll₂ former which can be used in conjunction with rare earth components to promote Ll₂ formation and reduce the misfit between fee and Ll₂ phases. Transition metals such as nickel (Ni), cobalt (Co), iron (Fe), and copper (Cu) can enhance short range ordering in the molten aluminum, leading to the increase of glass transition temperature and promoting glass formability. Magnesium (Mg), as a component with relatively high solubility in crystalline aluminum, can be used in fee aluminum to further reduce the fcc/Ll misfit.

With efficient precipitation strengthening by the Ll₂ dispersion, the alloys of the subject invention achieve a hardness of 490 VHN at significantly higher aging temperature compared to conventional aluminum alloys indicating a usable strength at or beyond 300°C.

Thus, it is an object of the invention to provide a new class of high-strength Ll₂ phase strengthened aluminum alloys processed into the amoφhous state, preferably with a rapid solidification process, and then subsequently devitrified into a crystalline structure with a thermomechanical or thermal processes.

A further object of the invention is to provide aluminum alloys with usable strength at or above 300°C by selecting (1) slow diffusing Ll₂ formers to prevent fast coarsening kinetics at high temperature, (2) appropriate matrix fee and Ll₂ components to reduce the lattice misfit between them to further slow down the coarsening kinetics and to promote the initial finer Ll₂ moφhology.

Another object of the invention is to utilize the cubic crystal structure of Ll₂ phase to promote the interfacial coherency with an aluminum fee matrix and the inherent ductile behavior of the Ll₂ crystalline phase itself, due to high number of slip systems in cubic Ll₂ structure, to maintain reasonable overall alloy ductility.

A further object of the invention is to combine alloying components so that the Ll₂ formation is enhanced while the likelihood of forming other undesirable intermetallics is reduced, in order to further promote alloy ductility of an aluminum alloy.

Another object of the invention is to provide adequate glass formability during the rapid solidification process such as powder atomization or melt spinning to form an amoφhous aluminum alloy structure, preserving the Ll₂ formers in the solution before a subsequent devitrification process. A further object of the invention is to utilize the intermediate, highly supersaturated amoφhous state, slow Ll₂ coarsening kinetics, and low Ll₂/fcc misfit to produce a fine microstructure in a strong aluminum alloy.

A further object of the invention is to provide aluminum alloys having high strength at high temperatures with appropriate toughness when processed with powder metallurgy techniques through an amoφhous state.

Another object of the invention is to combine transition metals and rare earth components to provide sufficient glass formability during a rapid solidification process and a nanoscale Ll₂ dispersion strengthened aluminum alloy fee crystalline structure after devitrification. These and other objects, advantages and features will be set forth in the detailed description which follows. BRIEF DESCRIPTION OF THE DRAWING

In the detailed description which follows, reference will be made to the drawing comprised of the following figures:

Figure 1 is a chart setting forth criteria for the design of aluminum alloys of the invention;

Figure 2 is a diagram illustrating the viscosity of liquid aluminum nickel alloys calculated at 1700°C in comparison with experimental data wherein the comparison is based upon calculations made using different thermodynamic descriptions of the aluminum nickel liquid (substitutional solution model without associate or associate model with AlNi associate); Figure 3 is an assessed Al - Yb phase diagram in comparison with experimental data; Figure 4a is a graph of the enthalpy of formation of cobalt and a rare earth, i.e. Er and Yb predicted by quantum mechanical calculation;

Figure 4b is a graph of the mixing enthalpy of the materials set forth in Figure 4a;

Figure 4c is a graph of the molar volume of the materials set forth in Figure 4a; Figure 5 is an X-ray diffractogram of the alloy of Example 1A as melt-spun with positions of fee pure aluminum reflections indicating a fully amoφhous state;

Figure 6 is an isochronic DSC thermogram of the alloy of Example 1A indicating devitrification steps upon heating;

Figure 7 is an X-ray diffractogram of the alloy of Example 1 A after devitrification at 425°C for 22 hours, with positions of reflections of pure fee Al, Al₃Yb, and Al₃Er phases, indicating the desired phases: FCC+L1₂;

Figure 8 is a SIMS elemental map of devitrified alloy of Example 1A indicating Er and Yb partition together to Ll₂ phase, and NI and Co which do not partition to Ll₂;

Figure 9 is a scanning electron microscope (SEM) secondary electron image of devitrified alloy of Example 1A indicating submicron moφhology;

Figure 10 is a 3DAP microscopy reconstruction of Al₃(Yb,Er)Ll₂ phase of the alloy of Example 1 A after devitrification at 424°C for 19 hours;

Figure 11 is an X-ray diffractogram of the alloy of Example 2 A as melt-spun, with positions of fee pure Al and Ll₂ reflections, indicating fully amoφhous status; Figure 12 is an X-ray diffractogram of the alloy of Example 2A after devitrification at

425°C for 19 hours, with positions of reflections of pure fee Al, Al₃Yb, and Al₃Er phases, indicating the desired phases: FCC+L1₂;

Figure 13 is an X-ray diffractogram of the alloy of Example 2B as melt-spun, with positions of fee pure Al and Ll₂ reflections, indicating fully amoφhous status; Figure 14 is an X-ray diffractogram of the alloy of Example 2B after devitrification at

425°C for 19 hours, with positions of reflections of pure fee Al, Al Yb, and Al Er phases, indicating the desired phases: FCC+L1₂ wherein the lattice parameter of Ll₂ is reduced by Sc;

Figure 15 is an X-ray diffractogram of the alloy of Example 2C as melt-spun, with positions of fee pure Al and Ll₂ reflections, indicating almost fully amoφhous status; Figure 16 is an X-ray diffractogram of the alloy of Example 2C after devitrification at

425°C for 19 hours, with positions of reflections of pure fee Al, Al₃Yb, and Al₃Er phases, indicating the desired phases: FCC+L1₂ wherein the lattice parameter of Ll₂ is expanded by Mg; Figure 17 is an X-ray diffractogram of the alloy of Example 2D as melt-spun, with positions of fee pure Al and Ll₂ reflections, indicating fully amoφhous status;

Figure 18 is an X-ray diffractogram of the alloy of Example 2D after devitrification at 425°C for 19 hours, with positions of reflections of pure fee Al, Al Yb, and Al Er phases, indicating the desired phases: FCC+L1₂ and wherein the lattice parameter of fee is expanded by Mg and that of Ll₂ is reduced by Sc;

Figure 19 is an X-ray diffractogram of the alloy of Example IC as melt-spun, with positions of fee pure Al and Ll₂ reflections, indicating non-amoφhous status due to the absence of transition metal elements; Figure 20 is an X-ray diffractogram of the alloy of Example IC after devitrification at

425°C for 19 hours, with positions of reflections of pure fee Al, Al Yb, and Al₃Er phases, indicating elements ER, Yb, and Sc can be completely intersoluble to form Ll₂ and wherein the lattice parameter of Ll₂ is reduced by Sc; and

Figure 21 is a scanning electron microscope (SEM) secondary electron image of a devitrified alloy of Example IC showing coarse microstructure in comparison with the alloy of Example 1A (see Figure 9) and indicating that passing through the glassy state can reduce the particle size of precipitation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

General Summary

In general, the subject matter of the invention comprises a product (alloy) as well as a product by process and a process all directed to an aluminum alloy in crystalline form having higher or greater strength particularly at elevated temperatures, i.e. greater than at least 250°C and preferably 300°C. The alloy of aluminum is made by compounding a mixture of aluminum with selected rare earths (RE) and transition metals (TM) in an amoφhous or glassy state followed by devitrification of the material from the amoφhous state to a mixed crystalline state comprised of face centered cubic (fee) and Ll₂. (an ordered fee) state wherein the ratios of the crystalline states is within certain preferred ranges. Preferably the resultant alloy has at least about 30% by volume fee phase and at least about 10% by volume Ll₂ phase material with limited residual amoφhous or other phase material such as quasi-crystalline or ordered amoφhous material. The choice of starting materials may be varied as may the compounding processes, the glass phase formation processes and the devitrification processes. Importantly, when in the amoφhous state, there may be some crystalline material contained therein, but preferably no more than about 30% by volume in the fee phase. Thus forming the mixture in the amorphous or glassy intermediate state and subsequent devitrification constitute very important aspects of the invention.

The alloy materials, in addition to aluminum (Al), include one or more lanthanide rare earths (RE) selected or taken from the group of erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), and one or more transition metals (TM) taken or selected from the group of copper (Cu), nickel (Ni), cobalt (Co), titanium (Ti), iron (Fe), yttrium (Y), and scandium (Sc). Further additives in minor amounts (less than about five percent (5%) atomic percent) may be included such as magnesium (Mg). The range of various constituents is also important to maintain processability and preserve transition characteristics. Thus RE materials are utilized in the range of about 2 to 15 atomic percent (%), and the TM and metals are utilized in the aggregate range of about 2 to 12 atomic percent (%). Transition metals (TM) in the range of about 2 to 7 atomic percent are utilized.

The processes for mixing or forming the starting materials in the amoφhous state are not necessarily limiting. Thus, it is contemplated that solid state processing, liquid or melt processing as well as gas phase processing may be utilized, though liquid phase processing is preferred based upon examples as reported hereinafter. The completeness of the amoφhous state is preferably at least about 70% by volume and preferably greater. The amoφhous state is a solid noncrystalline state or phase prior to devitrification. Development Technique

Devitrification is accomplished via heat treating (thermal) processing, thermo- mechanical processing as well as other processing techniques.

Precipitation strengthened aluminum (Al) alloys are difficult to develop for high strength due to limited solubility of alloying elements in fee Al. The low solubility of strengthening dispersion elements prevents the solid-state solution treatment of the alloy to bring strengthening components into solution. It is hence not necessarily possible to allow subsequent aging to produce a desirable strengthening microstructure. Al alloys with high fractions of precipitate that cannot be completely solution treated therefore have very coarse particle sizes that tend to limit strength, corrosion resistance and toughness. In contrast, the Al alloys exhibit composition and microstructure characteristics that enable the Al alloys of invention to exhibit high strength, good ductility, and high temperature stability at or above 300°C.

By selecting an appropriate composition, processing techniques achieve a fully amoφhous state after rapid cooling of the Al alloy composition. Furthermore, with careful selection of composition this glass can then be thermal or thermo-mechanically processed such that the glass devitrifies into a fully crystalline fee matrix with intermetallic precipitates having a very fine size scale. By passing through the glass state, the equilibrium solidification that would produce coarse precipitates is avoided. Furthermore, the selection of the precipitate phase as Ll₂ allows the precipitate to be coherent or semi-coherent with the fee matrix improving the ductility of the alloy and the resistance of the precipitate to coarsen during subsequent thermal treatment.

Figure 1 is a system flow-block diagram that illustrates the processing/structure/properties relationships for alloys of the invention. The two-stage nature of the chart emphasizes the generally equal importance of processability and final performance in the design of alloys of the invention. The processability is governed by the glass forming ability; strength, ductility, and thermal stability are the required end properties.

Certain transition metals (TM) (such as Fe, Co, Ni, Cu) promote short-range ordering in liquid Al, which leads to low partial molar volume, low thermal expansion, and high viscosity that are beneficial to glass forming ability. Rare-earth (RE) elements (such as Ce, Gd, Yb, Er) with large atomic size exhibit low diffusivity in Al and thus retard crystal nucleation.

Therefore, Al-TM-RE comprise a class of glass forming system for Al alloys.

Viscosity of liquid metals also has a strong effect on the glass forming ability. Liquid phase metals having high viscosity usually becomes glass through fast cooling. A binary model to describe liquid's viscosity is set forth in Equation (1):

The variables x, and G_m represent the mole fraction and molar Gibbs energy of the liquid, respectively. The viscosity of pure components, η° , is available from literature, see Table 1.

From the above equation it is seen that the composition dependence of the viscosity is fully determined by the thermodynamic properties of the liquid. Table 1. Viscosity and related data for pure liquid metals [L. Battezzati and A.L. Greer, "The Viscosity of Liquid Metals and Alloys," Ada metall 37, 1791 (1989).]

T_m: melting temperature V_m: molar volume at T_m η(T_m): viscosity at T_π

C_A: constant E: activation energy for viscous flow B: B= E/RT_m η₀: pre-exponential viscosity Equation (1) has been validated for several binary systems where both experimental viscosity data and thermodynamic descriptions are available. An example is the Al-Ni system where an intermediate compound AlNi melts congruently at about 1650°C. Figure 2 shows the calculated viscosity in Al-Ni liquid at 1700°C as a function of Ni content, in comparison with experimental data. It is seen that the calculated viscosity shows a similar trend as the experimental data, but the latter indicates a stronger increase of the viscosity around 50 at.% Ni. The strong increase in viscosity is believed to be caused by molecular associates of Al-Ni in the liquid. In order to illustrate the effect of the AlNi associates on the viscosity, the Al-Ni system was thermodynamically reassessed with an associate model for the liquid free energy. The viscosity of Al-Ni liquid calculated using the AlNi associate model is also plotted in Figure 2 for comparison. The calculation shows a significant increase in the viscosity due to the formation of liquid AlNi associates.

This viscosity model has further been extended to multi-component systems, which provides guidance of glass forming ability for the design of alloys of the invention.

A model to predict glass transition temperature, T_g, has been developed for the Al-TM- RE system. The model is based on the parabolic relationship between T_g and room temperature glass shear modulus, G, and it assumes that G is linear in atomic fraction, X, of each element according to equation (2): = Al, Mg, Ni, Co, Er, Yb, Sc, Y, etc.

The coefficient for each element, K„ has been calibrated to experimental DSC data of melt-spun amoφhous ribbons. These coefficients quantitatively represent the relative effectiveness of the aforementioned molecular associates in promoting glass forming ability by varying T_g. This model also provides guidance of glass forming ability for the design of alloys of the invention. The prediction error of equation (2) compared to available experimental data is about 9 K with a standard deviation of 10 K, which is within the experimental precision.

The desired performance for applications (e.g. aircraft structural components, high temperature turbine components, etc.) determines a set of alloy properties required, including strength, ductility and thermal stability resistance up to 300°C. The desired properties can be achieved through a fine, stable, multicomponent Ll₂ phase dispersion in the Al matrix after devitrification of the glassy Al, as long as a sufficient volume fraction of Ll₂ and low misfit with the Al matrix and thus low coarsening rate can be obtained.

The elements Er, Lu, Tm and Yb are known as the only stable rare earth (RE) Ll₂ formers. The melting point and cost of these rare earth elements are presented in Table 2. Some properties of corresponding trialuminides are also given in Table 2, including lattice parameter, density, and Ll₂ decomposition or congruent melting temperature. Among the four RE elements, according to Table 2, Yb has the smallest lattice parameter and relatively lower cost. Er has the lowest cost. Both Yb and Er have relatively lower melting point. Usually, the lower the melting temperature, the higher the glass forming ability. As a consequence, Alloys of the invention utilize Yb and Er as preferred Ll₂ formers rather than Tm and Lu.

Table 2. Stable Ll₂ formers and their properties.

For 1-5 kg cast metal ingots from 99.9%-grade oxides

Sc is the only transition metal (TM) element that can form a stable Ll₂ with Al. In comparison with RE Ll₂ formers, Sc can form Ll₂ with a smaller lattice parameter of 4.101 A, thus leading to smaller misfit between LI 2 and the Al matrix. However, Sc is by far the most expensive of the elements considered, as seen in Table 2. The alloy of the invention therefore seeks to limit the use of Sc as much as possible. Efforts have been made to search for other transition metals to substitute for Sc. A preliminary requirement for such substitution is solubility. Experiments have shown that Ti has a substantial solubility in Al₃Sc, up to 12.5 at.%, i.e. Alo. ₅Sco..25Tio.i2₅. In addition, Ti has the lowest diffusion coefficient in solid Al among the transition metals. For instance, the diffusivity in Al at 300°C is 9.0x10^"20 m²/s for Sc, 6.3x10^"24 m²/s for Zr, and 2.7 10^"25 m²/s for Ti. Adding Ti to Al₃Sc will thus reduce the coarsening rate of Ll₂ precipitates. Moreover, addition of Ti decreases the lattice parameter of Al₃(Sc,Ti) and hence minimizes the lattice misfit with Al. The lattice parameters for seven hypothetical Ll₂ compounds with transition metals are calculated at zero degree Kelvin (0 K) using the first-principle quantum mechanical approach. Results are shown in Table 3. All seven hypothetic Ll₂ compounds show significant smaller lattice parameters than those stable Ll₂ phases. Thus, alloys of the invention incoφorate Yb, Er and Sc as base Ll₂ formers but are not limited to these elements. TMs such as Ti, V, Zr, etc, which will result in low misfit and thus lower interfacial energy to yield a low driving force for particle coarsening are considered useful.

Table 3 Lattice Parameters of hypothetical Ll TM Compounds through First Principle

Calculations at 0 K.

It should be pointed out that the compound Al₃Ti is metastable as the Ll₂ phase but stable as D0₂₂. It can be transformed to the Ll₂ structure if Al is substituted partially by Cr, Mn, Fe, Co or Ni.

For a multicomponent Ll₂ phase, the lattice parameter a can be calculated based on a linear model similar to Vegard's law. As an example, we have the following equation for LI ₂ with the formula Al₃(Er_xYb_ySc_z) is utilized: α[Al₃ (Er_x Yb Sc_z ] = x * a(Al_iEr) + y * a(Al₃Yb) + z * a(Al,Sc) (3) Assuming Ll₂ has a linear thermal expansion, its lattice parameter at elevated temperature can be calculated as follows (T in °C): a(L\₂, T) = a(Ll₂,RT) x [1 + (T - 25) x α(El₂ )] (4) where α(Ll₂) represents the thermal expansion estimated according to the following reciprocal relation: a(M) _ T_m(Al₃M)

(5) a(Al,M) T_m(M) where M represents the corresponding pure metal in A1₃M and T_m is the melting point (°K). Table 4 summarizes melting point, thermal expansion of relevant elements in alloys of the invention and the estimated thermal expansion as well as lattice parameter at 25 °C and 300°C for stable Ll₂ or meta-stable Ll₂.

Table 4. Estimation of Thermal Expansion Coefficients and Lattice Parameters.

In the practice of the invention, decrease of the misfit by expanding the lattice parameter of the Al matrix is a useful technique. There are few elements that have significant solubility in Al. Mg is the only one that can substantially increase the lattice spacing of Al. According to literature information, in dilute AlMg alloys, every one atom percent of Mg will expand the Al lattice by 0.0045 A. Further, every one atom percent of Mg will increase the linear coefficient of thermal expansion of Al by 1.179 l0^"7/°C.

Thermodynamic modeling and assessments have been performed for Al-TM-RE systems with relevant elements incoφorated in alloys of the invention. The assessed Al-Yb binary phase diagram is shown in Figure 3. The Ll₂ Al₃Yb is the strengthening phase that is used in alloys of the invention. According to Figure 3, Al₃Yb is not solution treatable. The only way to avoid coarse precipitation from Al liquid is using fast cooling. It is possible to get fine crystalline precipitates directly by rapid solidification processing (RSP) from the melt. However, much finer precipitates can be generated through devitrification of amoφhous state due to the larger driving force. In addition, the precipitation process can be controlled through devitrification. Quantum mechanical calculations have been performed in Al₃Er-Al₃Co and Al₃Yb-

Al₃Co systems. The predicted enthalpy of formation, mixing enthalpy and molar volume in the corresponding systems are plotted in Figure 4. According to Figure 4, there is a strong repulsive Co-RE interaction indicating negligible Co solubility in Ll₂ AI₃RE. This sets a challenge in producing good glass forming alloys through conventional technology in the same composition intended to precipitate a significant amount of Ll₂ precipitation through devitrification. Experimental Results

The present invention alloys, through computational design of multi-component Al- TM-RE systems incoφorate, desired processing properties - glass forming ability and the desired microstructure - a fine dispersion of Ll₂ after devitrification in the Al matrix. Alloys of the invention are summarized in Table 5 for their composition, status in melt-spun and devitrification conditions, as well as the lattice parameter of the Al fee and the LI ₂ phases and the misfit between Ll₂ and Al matrix.

Table 5 Summary of Composition and Microstructure of Al Alloys

According to Table 5, alloys with TM elements (e.g. Ni and Co) are fully glass or almost fully glass in the as melt-spun conditions. These alloys include alloy 1A, IB, 2 A, 2B, 2C and 2D. Alloys without Ni and Co primarily show FCC Al and Ll₂ crystalline phases although they are partially amoφhous in the as melt-spun conditions. These alloys include alloy IC, ID, IE, IF, IG, IH and II. The results confirm the prediction that TM elements such as Ni and Co are critical elements to promote the glass forming ability in the alloys of the invention. Through devitrification, all alloys precipitate mainly Ll₂ in the Al matrix. It should be emphasized that the particle size of alloys passing through a fully or almost fully glassy state is much finer than that of alloys without passing through the glassy state or only passing through a partially glassy state with Ll₂ already present in the as-spun condition. Following is a detailed demonstration of characteristics of alloys in Table 5. Example 1

Alloy 1A in Table 5 was fabricated into both powders using gas atomization and ribbons using melt spinning technology (wheel speed 55 m/s). In the as melt-spun condition, Alloy 1A was initially characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The diffractometer trace (Cu-Kα radiation ) of as-spun Alloy 1A in Figure 5 shows a broad peak characteristic of glass, and the ribbons show very good ductility, also characteristic of the glassy state. A DSC trace run at 40°C/min. shown in Figure 6 indicates three stages of glass devitrification at temperatures near 200°C, 315°C and 370°C. DSC thermograms indicated that for heating at a constant rate of 10-40°C/min, the dominant crystallization reactions occur below about 400°C for all alloys under investigation. Based on this information, a temperature of 425°C was selected for the devitrification anneal. Specifically, alloy samples were encapsulated in quartz tubes under a protective atmosphere of approximately 500 mbar Argon with 99.999% purity. The encapsulated specimens were kept at 425°C for periods of 19 or 22 hrs, respectively, to ensure completion of the crystallization reaction and grain growth to a size that facilitates experimental characterization.

Figure 7 shows an X-ray diffractogram of Alloy 1A after devitrification at 425°C for 22 hours. At this point transformations I-III are fully completed and the ribbons are fully devitrified in the sense that no significant amount of amoφhous material can be detected by XRD and TEM. The peak positions of (111), (200), and (220) reflections calculated for FCC pure Al (lattice parameter 4.0496 A) are indicated in red. Within the accuracy of this experiment (that is 0.01° at 2Θ = 38.505° (Al (111)), the measured peak positions are identical to pure Al, indicating the equilibrium solubility of Ni, Co, Yb, and Er in Al is very small. Also marked in red are the positions of the 'forbidden' reflections (110), (210), (21 1), and (221) for FCC Al, which are superlattice reflections of the Ll₂ structure. The peak positions for Ll₂ Al₃Yb (lattice parameter 4.202 A marked in green, and that for Ll₂ Al₃Er (lattice parameter 4.215 A) are marked in brown. It can be seen that the measured peak positions are perfectly matched by Ll₂ structure reflections. Therefore, Alloy 1A mainly consists of FCC Al and Ll₂ phases as devitrified microstructure. It is also noticed that there is a minor third phase x, which crystalline structure is to be identified. The FCC AI+LI₂ microstructure of Alloy 1A was investigated by the secondary ion mass spectrometry (SIMS) technology. Figure 8 indicates that Yb and Er partition together into LI ₂ precipitates, while Ni and Co have limited partitions.

The moφhology of FCC+L1₂ microstructure of Alloy 1A is shown in Figure 9. This SEM secondary electron image indicates that the majority of Ll₂ precipitates are less than 1 micron after devitrification at 425°C, 22 hours. Based upon previously discussed experience in devitrified Al alloys, this indicated the potential to obtain nanoscale LI₂ precipitates with thermomechanical processing.

The three dimension atom probe microscopy (3DAPM) analysis confirms that Ll₂ exists in Alloy 1A with formula Alo.₇₂(Yb,Er)o 28- Yb and Er have about the same site fraction. Ni and Co are negligible in LI ₂ phase. See Figure 10.

Alloy 1A was also studied through microhardness testing in ribbon conditions after thermal cycles required for powder consolidation and extrusion. Results are presented in Table 6. Through devitrification at a temperature near 300°C, Alloy 1A has demonstrated a hardness of 490 VHN, which is equivalent to the room temperature tensile strength exceeding 1000 MPa according to Inoue's work in similar alloy systems. (Akihisa Inoue, "Amoφhous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems", Progress in Materials Science 43 (1998) 365-520). This result indicates high efficient strengthening of Ll₂.

Example 2

Alloy 2A in Table 5 was fabricated into ribbons using melt spinning technology (wheel speed 55 m/s). In the as melt-spun condition, Alloy 2A was characterized by X-ray diffraction. The diffractometer trace (Cu-Kα radiation ) of as-spun Alloy 2 A in Figure 11 shows a broad peak characteristic of glass, and the ribbons show reasonable good ductility, also characteristic of the glassy state.

Figure 12 shows an X-ray diffractogram of Alloy 2 A after devitrification at 425°C for 19 hours. At this point the ribbon is fully devitrified in the sense that no significant amount of amoφhous material can be detected by XRD. The peak positions of (111), (200), and (220) reflections calculated for FCC pure Al (lattice parameter 0.40496 A) are indicated in red. The measured peak positions are identical to pure Al, indicating the equilibrium solubility of Ni, Co, Yb, and Er in Al is very small. Also marked in red are the positions of the 'forbidden' reflections (110), (210), (211), and (221) for FCC Al, which are superlattice reflections of the LI₂ structure. The peak positions for Ll₂ Al₃Yb (lattice parameter 4.202 A) are marked in green, and that for Ll₂ Al Er (lattice parameter 4.215 A) are marked in brown. It can be seen that the measured peak positions are perfectly matched by Ll₂ structure reflections. Therefore, Alloy 2A mainly consists of FCC Al and Ll₂ phases as devitrified microstructure. It is noticed that the third phase x, in Example 2A, is less than in Example 1A, reflecting the desire to reduce the amount of x phase by reducing Ni and Co in Alloy 2A while retaining reasonable glass forming ability.

Example 3

Alloy 2B in Table 5 was fabricated into ribbons using melt spinning technology (wheel speed 55 m/s). In the as melt-spun condition, Alloy 2B was characterized by X-ray diffraction.

The diffractometer trace (Cu-Kα radiation) of as-spun Alloy 2B in Figure 13 shows a broad peak characteristic of glass, and the ribbons show reasonable good ductility, also characteristic of the glassy state.

Figure 14 shows an X-ray diffractogram of Alloy 2B after devitrification at 425°C for 19 hours. At this point the ribbon is fully devitrified in the sense that no significant amount of amoφhous material can be detected by X-ray diffraction (XRD). The peak positions of (111), (200), and (220) reflections calculated for FCC pure Al (lattice parameter 4.0496 A) are indicated in red. The measured peak positions are identical to pure Al, indicating the equilibrium solubility of Ni, Co, Yb, Er and Sc in Al is very small. Also marked in red are the positions of the 'forbidden' reflections (110), (210), (211), and (221) for FCC Al, which are superlattice reflections of the Ll₂ structure. It can be seen that the measured peak positions are matched by LI₂ structure reflections with lattice parameter 4.150 A. It demonstrates that Yb, Er and Sc can be completely intersoluble to form LI 2, and the lattice parameter has been significantly reduced by adding Sc, which is another means of reducing misfit to keep the Ll₂ precipitates coarsening resistant. The misfit between FCC Al and Ll₂ is 2.52% for Alloy 2B.

Example 4 Alloy 2C in Table 5 was fabricated into ribbons using melt spinning technology (wheel speed 55 m/s). In the as melt-spun condition, Alloy 2C was characterized by X-ray diffraction. The diffractometer trace (Cu-Kα radiation ) of as-spun Alloy 2C in Figure 15 shows a broad peak characteristic of glass, and the ribbons show reasonable good ductility, also characteristic of the glassy state. Figure 16 shows an X-ray diffractogram of Alloy 2C after devitrification at 425°C for

19 hours. At this point the ribbon is fully devitrified in the sense that no significant amount of amoφhous material can be detected by XRD. The peak positions of (11 1), (200), and (220) reflections calculated for FCC pure Al (lattice parameter 4.0496 A) are indicated in red. The measured peak positions indicate that the lattice space of Al has been dilated to 4.060 A, reflecting another feature of the invention: adding Mg to expand the lattice space of Al and thus decrease the misfit between Al and Ll₂. Also marked in red are the positions of the 'forbidden' reflections (110), (210), (211), and (221) for FCC Al, which are superlattice reflections of the Ll₂ structure. It can be seen that the measured peak positions are perfectly matched by Ll₂ structure reflections. Therefore, Alloy 2C mainly consists of FCC Al and LI₂ phases as devitrified microstructure.

Example 5

Alloy 2D in Table 5 was fabricated into ribbons using melt spinning technology (wheel speed 55 m/s). In the as melt-spun condition, Alloy 2D was characterized by X-ray diffraction. The diffractometer trace (Cu-Kα radiation ) of as-spun Alloy 2D in Figure 17 shows a broad peak characteristic of glass, and the ribbons show very good ductility, also characteristic of the glassy state.

Figure 18 shows an X-ray diffractogram of Alloy 2D after devitrification at 425°C for

19 hours. At this point the ribbon is fully devitrified in the sense that no significant amount of amoφhous material can be detected by XRD. The peak positions of (111), (200), and (220) reflections calculated for FCC pure Al (lattice parameter 4.0496 A) are indicated in red. The measured peak positions indicate that the lattice space of Al has been dilated to 4.057 A. Also marked in red are the positions of the 'forbidden' reflections (110), (210), (211), and (221) for FCC Al, which are superlattice reflections of the LI 2 structure. It can be seen that the measured peak positions are matched by Ll₂ structure reflections with lattice parameter 4.155 A. It demonstrates that Yb, Er and Sc can be completely intersoluble to form Ll₂, and the lattice parameter has been significantly reduced by adding Sc. Alloy 2D reflects the means of reducing misfit by adding Mg to dilate the lattice space of Al and adding Sc to decrease the lattice parameter of Ll₂. Thus Alloy 2D achieved the lowest misfit so far:2.42% without losing the glass forming ability. Alloy 2D mainly consists of FCC Al and Ll₂ phases as devitrified microstructure.

Example 6

Alloy IE in Table 5 was fabricated into ribbons using melt spinning technology (wheel speed 55 m/s). In the as melt-spun condition, Alloy IE was characterized by X-ray diffraction. The diffractometer trace (Cu-Kα radiation ) of as-spun Alloy IE in Figure 19 shows crystalline reflection peaks, indicating non-amoφhous or partially non-amorphous state. It demonstrates that TMs are necessary elements to promote glass in Al-TM-RE system. In addition, the ribbons are very brittle, also characteristic of the non-glassy state.

Figure 20 shows an X-ray diffractogram of Alloy IE after devitrification at 425°C for 19 hours. This diffractogram shows that Alloy IE only has two phases: fee A1+L1₂. It also demonstrates that Yb, Er and Sc can be completely intersoluble to form Ll₂, and the lattice parameter has been significantly reduced by adding Sc. The misfit between FCC Al and Ll₂ is 3.03%.

The moφhology of fcc+Ll₂ microstructure of Alloy IE is shown in Figure 21. This SEM secondary electron image indicates that the particle size of Ll₂ precipitates is about 3 times larger than that of Alloy 1A, see Figure 9, although the misfit of Alloy 1A is a little bit larger:3.735%. It indicates that devitrification of glassy state can significantly reduce the particle size of precipitation in comparison with that by crystallization directly from fast cooling at the same rate.

In review, the range of composition processing parameters, microstructure characterization and physical properties is summarized in the following Table 7: Table 7

Variations of the described aluminum alloy as well as the process for manufacture thereof and the product created by the process are available to provide the expected functionality of high short-term and long-term strength at temperatures above about 300°C. Thus the invention is to be limited only by the following claims and equivalents thereof.

Claims

CLAIMSWhat is claimed is:

1. An aluminum alloy characterized by high strength including high strength in the temperature range greater than about 250°C comprising, in combination: an alloy mixture in primarily crystalline form having at least about 30% by volume fee phase and at least about 10% by volume Ll₂ ductile precipitate phase, said alloy consisting essentially of at least one transition metal selected from the group consisting of about 2 to 12 atomic percent Ni, Co, Ti, Fe, Y, Sc, Cu, Zn, V, Cr, Mn and Li, and at least one rare earth material selected from the group consisting of about 2 to 15 atomic percent Er, Tm, Yb, Lu, and the balance Al.

2. The alloy of claim 1 further including Mg in an amount up to about 5% atomic fraction.

3. The alloy of claim 1 having a tensile strength of at least about 275Mpa at 250°C.

4. A method for manufacture of an aluminum alloy characterized by high strength including high strength in a temperature range greater than about 250°C comprising the steps of:

(a) forming a melt mixture of aluminum, about 2 to about 12 atomic percent of at least one metal selected from the group consisting of Ni, Co, Ti, Fe, Y, Sc, Cu, Zn, V, Cr, Mn and Li, and at least one rare earth selected from the group consisting of about 2 to 15 atomic percent Er, Tm, Yb, and Lu; (b)_a converting said melt to an amorphous state wherein the mixture comprises at least about 70% by volume amoφhous material; and

(c) devitrifying said amoφhous material to at least in part a crystalline matrix of at least about 30% by volume fee and at least about 10% by volume LI ₂ phase.

5. The method of claim 4 wherein the step of converting the melt comprises cooling in the range of at least about 10 °C per second.

6. The method of claim 4 wherein the step of devitrification is selected from a group consisting of thermoprocessing, thermomechanical processing and combinations thereof. A high temperature and high strength aluminum-based alloy processed through a primarily greater than about 70% in volume amoφhous state and then devitrified into greater than about 70% in volume crystalline microstructure with fee matrix and strengthening by about a ductile LI ₂ precipitate phase greater than about 10% L , and fee greater than about 30% in volume.

7. The method of claim 4 wherein converting the melt comprises a step selected from the group consisting of gas powder atomization, water powder atomization, melt spinning, spray casting and combinations thereof.

8. The method of claim 4 wherein the step of devitrifying amoφhous material comprises a step selected from the group consisting of hot isostatic pressing, thermal aging, extrusion and combinations thereof.

9. An aluminum alloy characterized by high strength including high strength at a temperature greater than about 250°C made by a process comprising the steps of:

(a) formulating a melt comprised of Al; at least one transition metal (TM) selected from the group consisting of Ni, Co, Ti, Fe, Y, Sc, Cu, Zn, V, Cr, Mn, Li and

Mg; and at least one rare earth selected from the group consisting of Er, Tm, Yb, Lu;

(b) converting the melt to at least about 70% by volume amoφhous material; and

(c) devitrifying at least in part the amoφhous material to a mixture of fee and L 1₂ crystalline precipitate phase material.

10. The alloy product by the process of claim 9 wherein the transition metal is provided in an amount of about 2 to 10 atomic percent.

11. The alloy product by the process of claim 9 wherein the rare earth material is provided in an amount of about 2 to 10 atomic percent.

12. The alloy product by the process of claim 9 further including an additive of Mg to the melt.

13. The alloy product by the process of claim 9 wherein devitrifying the amoφhous material comprises forming at least about 30% by volume fee material.

14. The alloy product by the process of claim 9 wherein devitrifying the amoφhous material comprises forming at least 10% by volume LI ₂ material.

15. The alloy product by the process of claim 9 wherein converting the melt to amoφhous material comprises at least one step selected from the group consisting of gas powder atomization, water powder atomization and melt spinning.

16. The alloy product by the process of claim 9 wherein devitrification comprises at least one step selected from the group consisting of hot isostatic pressing, thermal aging, and extrusion.

17. The method of claim 4 wherein converting the melt comprises the step of rapid solidification processing.

18. The product by the process of claim 9 wherein converting the melt comprises rapid solidification processing.

19. The alloy of claim 1 or claim 9 wherein the precipitate phase has a grain size diameter in the range of less than about 80 nm.