Reactive phase sintering of oxide dispersion strengthened nanocrystalline alloys

Abstract

Provided are a sintered nanocrystalline alloy and a method of producing thereof. The sintered nanocrystalline alloy may include about 1-20 atomic % (at.%) magnesium; and balance aluminum, wherein the sintered nanocrystalline alloy comprises grains having a grain size less than about 100 nm and comprises MgO nanodispersoids. The method may include mixing about 1-20 at.% magnesium; a process control agent (PCA); and balance aluminum to form a mixture, milling the mixture using a ball mill to form a powder; sintering the powder to form the sintered nanocrystalline alloy.

Description

Attorney Docket No. 162152.51476 REACTIVE PHASE SINTERING OF OXIDE DISPERSION STRENGTHENED NANOCRYSTALLINE ALLOYS GOVERNMENT SUPPORT

[0001] The invention was made with government support under DE-SC0021060 awarded by the Department of Energy. The government has certain rights in the invention. TECHNICAL FIELD

[0002] The present disclosure is generally directed towards powder metallurgy and nanotechnology, focusing on the development of oxide dispersion-strengthened nanocrystalline alloys. It introduces a novel reactive phase sintering process designed to fabricate bulk alloys with enhanced mechanical properties and thermal stability, utilizing principles of grain boundary engineering.

[0003] The present disclosure is particularly relevant for the production of lightweight, high-strength materials, as useful, for example, in the aerospace, defense, and automotive industries. The core technological advancement involves the synthesis and processing of nanocrystalline aluminum-magnesium alloys, integrated with nanoscale oxide dispersoids, to achieve superior strength-to-weight ratios and resistance to thermal coarsening. BACKGROUND

[0004] For the past three decades, researchers have actively sought ways to enhance the strength of engineering materials through grain size reduction, focusing particularly on achieving nanocrystalline structures characterized by grains smaller than approximately 100 nanometers. This approach is rooted in the Hall-Petch relationship, which posits that reducingAttorney Docket No. 162152.51476 the grain size can significantly increase the material's strength. However, despite the potential for substantial strength improvements, nanocrystalline materials have not been widely adopted in industry due to thermomechanical processing challenges and intrinsic instabilities where thermodynamics favor grain growth to minimize the energetic penalty associated with a high density of interfaces.

[0005] Traditional nanocrystalline metals, including aluminum alloys, exhibit relatively low yield strengths and are prone to grain growth, which can diminish their mechanical properties. Moreover, the extreme strength-to-weight ratios desired for advanced structural materials in sectors such as aerospace and defense benefit from materials that can maintain their properties under both thermal and mechanical stress.

[0006] The development of oxide dispersion-strengthened (ODS) alloys offers a potential solution to these challenges, combining the benefits of nanocrystalline structures with enhanced thermal stability. These alloys typically incorporate finely dispersed oxide particles within the metal matrix, which act to impede grain growth and thus maintain the desired microstructural features even at elevated temperatures.

[0007] However, producing bulk nanocrystalline ODS alloys that are both thermally stable and mechanically robust has remained a significant challenge. SUMMARY

[0008] The reactive phase sintering process of the binary aluminum-magnesium binary alloy system, as detailed in the present disclosure, represents a breakthrough in this area, enabling the production of bulk ODS alloys with controlled nanocrystalline structures. This innovative approach not only addresses the key limitations of existing nanocrystalline materials, but also opens new avenues for the application of these materials in high- performance applications. 2 162759226.1Attorney Docket No. 162152.51476

[0009] The reactive phase sintering process creates unique nanostructures, leading to enhanced strength through the combined effects of Hall-Petch strengthening from stabilized magnesium nanograins and matrix dispersion strengthening from nano-oxides formed during the decomposition of the transient hydride phase. Regarding high strength-to-weight ratio alloys, these aluminum alloys find use, for example, in the transportation, aerospace, and defense sectors, with the capability to replace high-strength aluminum alloys in applications where strength (including at elevated temperatures) and wear resistance are essential material properties.

[0010] This processing technology is also applicable to other lightweight material systems, such as titanium, where sintering and oxide dispersion strengthening mechanisms can benefit alloys with magnesium additions. Additionally, there is a significant market opportunity for ferritic alloys with nano-oxides, offering enhanced strengthening and radiation tolerance. The inclusion of magnesium oxide alongside nanograins in these materials provides a high density of tailored defect sinks, reducing irradiation damage while significantly increasing strength, following the same mechanisms observed in the aluminum- magnesium system.

[0011] The use of ferritic nanostructured alloys produced by this method, as advanced structural materials, extends to generation IV fission reactors (e.g., molten salt, high- temperature gas, and others) and structural blankets in future fusion reactor platforms.

[0012] In some embodiments, a sintered nanocrystalline alloy is described. The sintered nanocrystalline alloy may include about 1-20 atomic % (at.%) magnesium; and balance aluminum, wherein the sintered nanocrystalline alloy comprises MgO nanodispersoids. 3 162759226.1Attorney Docket No. 162152.51476

[0013] The sintered nanocrystalline alloy may have about 10-20 at.% magnesium. The sintered nanocrystalline alloy may have a hardness of more than about 250 Vickers hardness (HV) at 0.3kgf.

[0014] The sintered nanocrystalline alloy of in some embodiments may have about 15 at.% magnesium. The sintered nanocrystalline alloy may have a hardness of about 275 Vickers hardness (HV) at 0.3kgf.

[0015] The sintered nanocrystalline alloy may further include an Al4C3minor second phase.

[0016] The sintered nanocrystalline alloy may have been sintered at least about 400 °C.

[0017] The sintered nanocrystalline alloy may be further annealed.

[0018] The sintered nanocrystalline alloy may be a bulk sintered nanocrystalline alloy.

[0019] The sintered nanocrystalline alloy may comprise grains having a grain size less than about 100 nm. In some embodiments, the grains may have a grain size less than about 50 nm. In some embodiments, the MgO nanodispersoids may be dispersed alongside the grain boundaries.

[0020] In some embodiments, a method for producing a sintered nanocrystalline alloy is described. The method may include mixing about 1–20 at.% magnesium, a process control agent (PCA), and balance aluminum to form a mixture, milling the mixture using a ball mill to form a powder; and sintering the powder to form the sintered nanocrystalline alloy, wherein the sintered nanocrystalline alloy may include MgO nanodispersoids.

[0021] The PCA may be methanol or ethanol present in about 2.75 wt%.

[0022] The ball mill may be a planetary ball mill equipped with a tungsten carbide milling jar using tungsten carbide balls. 4 162759226.1Attorney Docket No. 162152.51476

[0023] Compacting the powder may precede the sintering to form a bulk sintered nanocrystalline alloy.

[0024] About 10-20 at.% magnesium may be mixed with the PCA and the balance aluminum in the mixing. About 15 at.% magnesium may be mixed with the PCA and the balance aluminum in the mixing.

[0025] The method may further include an Al4C3 minor second phase.

[0026] The sintering may be performed at least about 400 °C.

[0027] The method may further include annealing the sintered nanocrystalline alloy.

[0028] The sintered nanocrystalline alloy may be a bulk sintered nanocrystalline alloy.

[0029] The sintered nanocrystalline alloy may include grains having a grain size less than about 100 nm. The grains may have a grain size less than about 50 nm. The MgO nanodispersoids may be dispersed alongside the grain boundaries. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The skilled artisan will understand that the drawings primarily are for illustrative purposes and are not intended to limit the scope of the inventive subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the inventive subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features.

[0031] FIG.1 presents a series of micrographs and related analyses for an annealed Al-7 atomic % (at.%) Mg sample, including (a) a bright-field STEM image with a corresponding selected area diffraction (SAD) pattern identifying predominantly nanocrystalline Al phases; (b) a correlative STEM-EDS map showcasing the distribution of Al and Mg; (c) detailed EDS maps of a magnesium-rich nanoprecipitate, alongside a STEM- 5 162759226.1Attorney Docket No. 162152.51476 HAADF image and an NBED pattern indexed for the HCP phase of magnesium; and (d) a graph depicting the bimodal size distribution of the Mg nanoprecipitates.

[0032] FIG.2 depicts TEM micrographs and corresponding analyses for Al-7 at.% Mg powders, including (a) the material's state as-milled; (b) post-annealing at 200°C, demonstrating the retention of the nanocrystalline structure; (c) grain size distributions; and (d) XRD patterns for both as-milled and annealed states. Additionally, (e) presents the lattice parameter, grain size, and microstrain values derived from Rietveld refinements.

[0033] FIG.3 exhibits data relating to the adjustments in lattice parameters due to interfacial stress, excess volume, and grain boundary (GB) segregation for both as-milled and annealed Al-Mg powders. This includes (a) changes in lattice parameters across various average magnesium concentrations; (b) a comparative analysis with values from the literature; and (c) GB concentration mapped as a function of lattice concentration, as deduced from these analyses.

[0034] FIG.4 displays Fourier-transform infrared (FTIR) spectra for Al-20 at.% Mg across a full wavenumber range (400-4000 cm-1), highlighting (a) low wavenumber regions showing Mg-O and Mg-H bonding signatures and (b) intermediate wavenumbers detailing peaks attributable to Mg-O(Et)2 bonding.

[0035] FIG.5 presents in situ X-ray diffraction (XRD) patterns captured during the heating of Al-15 at.% Mg powders up to 400 °C.

[0036] FIG.6 details (a) the phase fraction and (b) the heat flow as a function of temperature from the in situ XRD analyses.

[0037] FIG.7 provides a graphical representation of the sintering process tracked by die displacement during differential scanning calorimetry (DSC) for Al-15 at.% Mg powders, plotting (a) a sintering curve and an inset of the sintered bulk sample, and (b) hardness values 6 162759226.1Attorney Docket No. 162152.51476 against magnesium concentration, comparing them to benchmark aerospace-grade aluminum alloys – A7068-T6 and A7075-T6.

[0038] FIG.8 features (a) an XRD pattern for a bulk Al-15 at.% Mg alloy post- sintering at 400 °C, juxtaposed against the pattern from the milled powder to highlight phase formation; (b) a bright-field TEM image; and (c) a magnified HAADF image with corresponding EDS maps.

[0039] FIG.9 illustrates the thermal stability analysis of the Al-15 at.% Mg alloy, presenting (a) grain size and microstrain data, along with (b) the compositional distribution of minor oxides and carbides post-sintering and after thermal aging. DETAILED DESCRIPTION

[0040] The following description is made for the purpose of illustrating the general principles of the present compositions and / or methods and is not meant to limit the inventive concepts claimed in this document. Further, particular features described in this document can be used in combination with other described features in each of the various possible combinations and permutations.

[0041] Unless otherwise specifically defined in this document, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and / or as defined in dictionaries, treatises, etc.

[0042] It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. All publications mentioned in this document are incorporated by reference. Nothing in this document is to be 7 162759226.1Attorney Docket No. 162152.51476 construed as an admission that the embodiments described in this document are not entitled to antedate such disclosure by virtue of prior disclosure. As used herein, the term “comprising” means “including, but not limited to”.

[0043] Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about”.

[0044] As used herein, the term “fully dense”, or grammatical equivalents thereof, may refer to a material with a relative density of at least 98%—e.g., at least about 98%, about 99%, about 99.5%, or higher. The nanocrystalline material may have any value of relative density, depending on the material. Relative density may refer to the ratio between the experimentally measured density of the nanocrystalline material and the theoretical density of the nanocrystalline material accounting for alloying elements, oxide phases, and other minor phases.

[0045] As used herein, the “grain size”, or grammatical equivalents thereof, may refer to the average dimension of a grain. The dimension may refer to the diameter, length, width, or height of a grain, depending on the geometry thereof. In some embodiments, “grain size”, or grammatical equivalents thereof, may refer to the mean, median, or mode of the size distribution when measured using optical or electron micrographs. In some embodiments, the grains may be spherical, cubic, conical, cylindrical, needle-like, or any other suitable geometry. When quantified from X-ray diffraction, “grain size”, or grammatical equivalents thereof, may refer to the average dimension of a grain as determined from peak broadening through a Rietveld refinement or suitable methods to for pattern fitting. 8 162759226.1Attorney Docket No. 162152.51476

[0046] As used herein, the term “rich”, or grammatical equivalents thereof, may refer to a content of the element in the phase of at least about 50 at. %—e.g., at least about 60 at. %, at least about 70 a.t %, at least about 80 at. %, at least about 90 at. %, at least about 99 at.%, or higher.

[0047] As used herein, the term “phase”, or grammatical equivalents thereof, may refer to a state of matter. For example, in some embodiments, a phase may refer to a phase shown on a phase diagram.

[0048] As used herein, the term “nanodispersoids”, or grammatical equivalents thereof, refer to nano-sized dispersoids, which are particles or phases distributed within a different matrix.

[0049] As used herein, the term “free from”, or grammatical equivalents thereof, means that the substance is not measurably present in the composition.

[0050] The term “substantially free from”, or grammatical equivalents thereof, as defined herein means less than about 1%, or less than about 0.8%, or less than about 0.5%, or less than about 0.3%, or about 0%, by weight or atomic weight.

[0051] In some embodiments of the present disclosure, advancements in powder metallurgical processing are described, which, combined with targeted alloying using a grain boundary segregating species, may enable the retention of a nanocrystalline microstructure with grain sizes of e.g., less than about 100 nm and nanoscale oxide dispersoids through high- temperature processing.

[0052] In some embodiments, “nanocrystalline” materials may typically consist of grains that are in the size range of a few nanometers, specifically under approximately 1000 nm. This may include sizes such as 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, 200 nm or less, 150 nm or less, 100 nm or less, 50 nm or less, 30 nm or less, 20 nm or less, 10 nm or less, 5 nm or less, 2 nm or less, 9 162759226.1Attorney Docket No. 162152.51476 or even smaller. In one embodiment, the nanocrystalline material may be a polycrystalline material. In another embodiment the nanocrystalline material may be a single crystalline material.

[0053] In some embodiments, the nanocrystalline material may be in the form of particulates. The shape of the particulates may be spherical, cubical, conical, cylindrical, needle-like, irregular, or any other suitable geometry.

[0054] The following three criteria are beneficially met for the selection of alloying species: (i) the alloying elements should exhibit an energetic preference for segregating to the grain boundaries, thereby reducing their interfacial energy and the corresponding driving force for grain growth; (ii) the alloying elements should possess a greater affinity for forming oxides compared to the solvent; and (iii) metal hydrides formed through the reaction of the process control agent with the alloying species should have decomposition temperatures below the maximum sintering temperature.

[0055] The Applicant has surprisingly found that the Al-Mg system meets the three criteria: (i) in aluminum, magnesium exhibits an energetic preference to segregate to the grain boundaries, based on the enthalpy of mixing and segregation estimates along with computational thermodynamics modeling; (ii) magnesium exhibits a greater oxidation potential than aluminum, indicating its preferential oxidation; and (iii) hydrides formed with magnesium decompose at temperatures around 400 °C.

[0056] In some embodiments, alloying of aluminum with magnesium can be accomplished through high-energy ball milling, using a process control agent (PCA) known in the art that are organic compounds, such as but not limited to alcohols (e.g., ethanol, methanol, or hexane) or fatty acids (e.g., C6 to C24 fatty acid such as stearic acid) that may contain substantial hydrogen, in concentrations of e.g., up to about 10 wt%. Process control variables, including mill rotational speed, milling time, and duty cycle, can be systematically 10 162759226.1Attorney Docket No. 162152.51476 tuned to achieve a predominantly single-phase solid solution with a minor solute-rich second phase in the milled powders, while minimizing cold welding.

[0057] For the Al-Mg system, in some embodiments, feedstock materials may comprise commercially pure aluminum (99.9% purity, -325 mesh) and magnesium (99.9% purity, -325 mesh), combined in atomic ratios ranging from about 1 to 20 at.% with about 2.75 wt% ethanol serving as the process control agent (PCA). Milling may be performed using a ball mill. In some embodiments, the ball mill can be a planetary ball mill equipped with a tungsten carbide milling jar, which is loaded under an inert atmosphere. Tungsten carbide balls, approximately 5 mm in diameter, may be used as the milling media. The milling process can be executed at a constant speed of about 550 RPM for a duration of approximately 16 hours, with a ball-to-powder weight ratio (BPR) of about 5:1 to ensure optimal particle size reduction and uniformity. In some embodiments, the RPM may be In some embodiments, the BPR may be about 2:1 to 5:1.

[0058] In some embodiments, the production of bulk alloys from the milled powders can be achieved through sintering methods such as Field-Assisted Sintering (FAS), also known as Direct Current Sintering (DCS) or Spark Plasma Sintering (SPS). In certain cases, approximately 3 g of the alloyed powder may be compacted within a graphite die with an inner diameter of approximately 2.5 mm to shape the final specimen. Sintering parameters can be meticulously set to include, for example, an applied pressure of approximately 60 MPa, a maximum pulse current of approximately 1700 A, and a voltage of approximately 4.5 V. The sintering cycle may involve a heating phase where the samples are heated to a temperature of approximately 400 °C within a span of about 4 minutes, followed by a dwell time of approximately 5 minutes at the same temperature to enhance particle bonding and densification, while minimizing grain growth. The sintering temperature may be at least about 400 °C, at least about 500 °C, at least about 600 °C, at least about 700 °C, at least about 11 162759226.1Attorney Docket No. 162152.51476 800 °C, at least about 900 °C, at least about 1000 °C, , at least about 1100 °C, or at least about 1200 °C. Selection of the sintering temperature is dependent on the alloy system where higher melting temperature materials will require increased sintering temperatures.

[0059] In some embodiments, the nanocrystalline material may be a bulk nanocrystalline alloy. A bulk nanocrystalline alloy may be a material that is not in the form of a thin film or a powder. For instance, a bulk nanocrystalline alloy may be defined as a material whose smallest dimension is at least approximately 1 micron—this may include dimensions of at least about 10 microns, at least about 25 microns, at least about 50 microns, at least about 75 microns, at least about 100 microns, at least about 250 microns, at least about 500 microns, at least about 1 mm, at least about 5 mm, at least about 10 mm, or larger.

[0060] Referring to FIG.1, the bright-field scanning transmission electron microscopy (STEM) image and corresponding energy-dispersive spectroscopy (EDS) maps reveal the formation of Mg-rich nanoprecipitates in the alloyed powders. The magnified high- angle annular dark-field (HAADF) image and EDS mapping, coupled with nanobeam electron diffraction (NBED), indicate that these precipitates are of the hexagonal close- packed (HCP) phase of magnesium. The sizes of the precipitates are generally smaller than about 20 nm, but there is a bimodal distribution with a small fraction of larger precipitates, with sizes ranging from about 40 to 100 nm.

[0061] The addition of magnesium results in a stable nanocrystalline grain structure in the powders up to about 200°C, as evidenced by the bright-field micrographs and cumulative size distributions presented in FIG.2: (a) the material in its as-milled state; and (b) post-annealing at 200°C. This is further supported by (c) grain size distributions; (d) representative XRD patterns for the as-milled and annealed Al-7 at.% Mg samples; and (e) the lattice parameter, grain size, and microstrain values derived from Rietveld refinements. 12 162759226.1Attorney Docket No. 162152.51476

[0062] From the X-ray diffraction (XRD) analysis, the powders are shown to be predominantly a face-centered cubic (FCC) solid solution of aluminum containing magnesium, with a small fraction of hexagonal close-packed (HCP) magnesium, consistent with the STEM-EDS analysis. The annealed powders retain the same phase signature, indicating that magnesium remains within the solid solution, with a small fraction phase- separated as HCP magnesium. Lattice parameter, grain size, and microstrain can be determined from a Rietveld analysis, indicating a direct scaling of lattice parameter with magnesium concentration, while the grain size decreases up to about 10 at.% Mg. Microstrain initially increases but then decreases with increasing magnesium concentration, with a uniform reduction upon heat treatment indicative of relaxation due to the elimination of two- dimensional processing defects (e.g., dislocations, stacking faults) during annealing. The degree of magnesium segregation to the grain boundaries can be estimated by considering contributions from interfacial stress, excess free volume, HCP precipitation, and grain boundary segregation to the scaling of the lattice parameter. Each effect is plotted in FIG.3a as a function of magnesium concentration, with the grain boundary segregation component extracted in FIG.3b to map the degree of segregation in FIG.3c. Segregation of magnesium is maximized in the range of about 5–7.5% magnesium, thus confirming the role of segregation in stabilizing the nanocrystalline grain structure.

[0063] Employing magnesium as an alloying element for thermal stabilization can have secondary effects when milled in the presence of ethanol as the process control agent (PCA). As summarized in FIG.4, Fourier-transform infrared (FTIR) spectroscopy measurements of the Al-20 at.% Mg powders, over a wavenumber range of 400–4000 cm⁻¹ in absorbance mode, reveal a complex magnesium bonding environment.

[0064] First, absorption peaks evident for Mg-H bonding at approximately 456.90 and 3091.6 cm⁻¹ in FIG.4a are associated with the vibration modes of normal and parahydrogen, 13 162759226.1Attorney Docket No. 162152.51476 respectively, indicating the presence of magnesium hydride. Second, subtle absorption peaks around 427.91 and 492.65 cm⁻¹ may be attributed to Mg-O bonding, with the higher wavenumber of the former indicative of the stretching of the Mg-O bond. A closer examination over the wavenumber range of 1300–3000 cm⁻¹ reveals absorption peaks corresponding to the stretching and bending of C-O and C-H bonds, as summarized in Table 1.Table 1

[0065] The combination of these observations with the presence of Mg-O bonding suggests that magnesium is interacting with ethanol during milling to form magnesium ethoxide (Mg-(OC₂H₅)₂, henceforth referred to as Mg-(OEt)₂). This is further supported by the stretching of the Mg-O bonds; due to the more electropositive nature of magnesium compared to carbon, its bond strength with oxygen will be greater than that of O-C in (OEt)₂, resulting in a shift of the Mg-O stretching frequency to a higher wavenumber. Finally, the 14 162759226.1Attorney Docket No. 162152.51476 broad doublet band at 2300 cm⁻¹ likely corresponds to atmospheric CO₂ and therefore may not derive from the milled powders. Additionally, the absence of broad band absorption peaks over the range of 3100–3600 cm⁻¹ suggests that moisture is not absorbed in the powders.

[0066] Stability and phase evolution in the milled Al-15 at.% Mg powders can be traced using in situ synchrotron XRD measurements, with the resulting patterns plotted as a function of temperature in FIG.5. In addition to the primary peaks of the FCC Al solid solution, consistent with the benchtop XRD patterns shown in FIG.2 for the as-milled and annealed powders, several additional peaks can be identified over different temperature ranges up to about 400°C. At intermediate temperatures, peaks corresponding to the tetragonal MgH₂ phase increase in intensity before disappearing above about 400°C, coinciding with the appearance of aluminum carbide (Al₄C₃). Cubic MgO peaks also emerge at intermediate temperatures and are retained throughout the entire in situ heating experiment, even upon the melting of the Al-Mg solid solution above about 550°C. The Mg(OEt)₂ phase may not be visible in the XRD patterns due to several factors, the most prominent being the small fraction of this compound (approximately 0.1 wt.%) present in the powders, thus at the detection limit even for synchrotron XRD. A second contributing factor could be the weak / diffuse scattering intrinsic to the Mg(OEt)₂ crystal structure, which hinders the formation of clear diffraction peaks. Lastly, Mg(OEt)₂ may form through a surface reaction during milling, enhancing the disorder of this phase with a consequent reduction in Bragg peak intensity. Similar disordered / non-crystalline structures have been observed in the thermal decomposition behavior of magnesium alkoxide powders and in i-propoxide- modified magnesium alkoxide pore architectures for heterogeneous Ziegler–Natta catalysts.

[0067] Phase evolution as a function of temperature can be determined using Rietveld refinements on the in-situ XRD patterns, as shown in FIG.6a. From this representation, the 15 162759226.1Attorney Docket No. 162152.51476 formation temperature of MgO is now clearly evident, just below approximately 200°C, as is the onset of Al₄C₃ upon the decomposition of MgH₂ at around 400°C. However, the mechanism for MgO formation may not be captured in the XRD data due to its inability to resolve the Mg(OEt)₂ phase. DSC can be employed to determine the reaction pathway, with results depicted in FIG.6b. Upon heating, an endothermic process becomes apparent up to about 100°C, consistent with stored hydrogen desorption from the ethoxide framework, which partially reacts with magnesium to form MgH₂, as evidenced by the small but non- negligible increase in the MgH₂ phase fraction in FIG.6a above about 100°C. An increase in temperature produces a significant exothermic reaction that initiates at about 150°C, with a subsequent maximum in the heat release rate at 198.9°C, collectively aligning with Mg(OEt)₂ oxidation and decomposition. The second exothermic peak, occurring above 200°C, may be correlated with the formation of MgO during the decomposition of Mg(OEt)₂, continuing through about 300°C where the sharp reduction in exothermic heat flow combined with the plateau in the MgO phase fraction signals the completion of this transformation. Finally, above about 300°C, hydrogen begins desorbing from the MgH₂ phase, as evidenced by the onset of a third exothermic peak in the DSC data, with decomposition at about 360°C aligning with the XRD phase analysis and consistent with prior MgH₂ desorption studies. The breakdown of the hydride phase evolves hydrogen locally, creating a reducing atmosphere, contributing to the plateau in the MgO fraction above 350°C, aligning with its decomposition. Following the decomposition of MgH₂, the formation of Al₄C₃ occurs above 400°C. This suggests that the sintered alloy comprises an Al₄C₃ minor phase. In some embodiments, the sintered alloy may be free from or substantially free from MgH₂.

[0068] The significant exothermic reaction associated with the vaporization of Mg(OEt)₂ and the subsequent decomposition of MgH₂ may underlie the reaction sintering mechanism, facilitating the retention of the nanocrystalline grain structure in consolidated 16 162759226.1Attorney Docket No. 162152.51476 bulk form and the formation of the nano-oxide dispersion phase. Displayed in FIG.7a is the die displacement curve associated with the consolidation and sintering of the milled nanocrystalline Al-15 at.% Mg powders. The die displacement rate increases at approximately 55 seconds, corresponding to a temperature of about 100°C, where the significant exothermic release originates from Mg(OEt)₂ vaporization. A reduction in the displacement rate occurs shortly after about 2.5 minutes at a temperature of about 225°C, as the heat release from the vaporization process declines. However, the rate increases again around 3 minutes due to the formation of MgO and the subsequent decomposition of MgH₂, resulting in a fully dense sintered compact, as shown in the inset of FIG.7a. The Mg concentration varies with samples sintered at about 1–20 at.%, and the resulting hardness values are plotted in FIG.7b relative to two structural aerospace Al alloys – Alloy A7068 and A7075, both undergoing T6 heat treatment. Specifically, the sintered nanocrystalline alloy containing 10-20 at.% magnesium exhibits a hardness exceeding 250 Vickers (HV) at 0.3 kgf. In particular, the sintered nanocrystalline alloy with 15 at.% magnesium displays a hardness of approximately 275 Vickers (HV) at 0.3 kgf.

[0069] A structural analysis of the sintered Al-15 at.% Mg bulk alloy is shown in FIG.8, where the broad peaks in the XRD patterns of the sintered material align with those of the milled powder and are indicative of a nanocrystalline grain size. This size contains FCC Al (as a solid solution), cubic MgO, and rhombohedral Al₄C₃. The latter two phases may not be present in the milled powders, consistent with their formation during sintering, as demonstrated by in situ XRD and DSC experiments. Additionally, the sintered bulk alloy displays the same peaks. The bright-field TEM micrograph in FIG.8b confirms the nanocrystalline microstructure, with the magnified STEM-HAADF image and EDS maps revealing the presence or formation of MgO nanodispersoids. 17 162759226.1Attorney Docket No. 162152.51476

[0070] From the Rietveld refinements applied to the XRD datasets, the grain size, microstrain, and phase fractions for the Al-15 at.% Mg alloy can be determined. These are plotted in FIG.9 for both the as-sintered condition and following heat treatment at 400°C. The sintered grain size is decidedly nanocrystalline, at less than approximately 50 nm, with only minor increases observed during thermal aging for up to 28 hours. The microstrain, resulting from extended defects (e.g., dislocation content), may be reduced by the four-hour heat treatment, which promotes recovery and a reduction in the effective defect density. Annealing may also result in additional MgO formation; however, this tends to plateau after four hours, while the Al₄C₃ fraction continues to increase due to the presence of interstitial carbon retained from the process control agent (PCA).

[0071] Collectively, the Hall-Petch strengthening resulting from the nanocrystalline grain size, along with precipitation hardening from the oxides and carbides, may account for the exceptional strength exhibited by these alloys. Their excellent thermal stability may also be attributed to these microstructural features, combined with the probable segregation of free Mg to the grain boundaries, which reduces the driving force for thermal coarsening.

[0072] While the specific embodiments of the present disclosure have been illustrated and described, it will be obvious to those skilled in the art that the present disclosure may be variously modified and changed without departing from the technical spirit of the present disclosure defined in the appended claims.

[0073] It should be understood that variations, clarifications, or modifications are contemplated. Applications of the technology to other fields not mentioned are also contemplated.

[0074] Example methods and compositions are described. Since numerous modifications and changes will readily be apparent to those having ordinary skill in the art, it is not desired to limit the present disclosure to only the exact constructions as demonstrated 18 162759226.1Attorney Docket No. 162152.51476 in this disclosure. Accordingly, all suitable modifications and equivalents may be resorted to falling within the scope of the present disclosure.

[0075] Thus, for example, any sequence(s) and / or temporal order of steps of various processes or methods that are described herein are illustrative and should not be interpreted as being restrictive except as it would be generally understood from the context and description. Accordingly, it should be understood that although steps of various processes or methods or connections or sequence of operations may be shown and described as being in a sequence or temporal order, but they are not necessarily limited to being carried out in any particular sequence or order. For example, the steps in such processes or methods generally may be carried out in various different sequences and orders, while still falling within the scope of the present disclosure.

[0076] It should be understood that claims that include fewer limitations, broader claims, such as claims without requiring a certain feature or process step in the appended claim or in the specification, clarifications to the claim elements, different combinations, and alternative implementations based on the specification, or different uses, are also contemplated by the embodiments of the present disclosure.

[0077] It should be understood that combinations of described features or steps are contemplated even if they are not described directly together or not in the same context.

[0078] The terms or words that are used herein are directed to those of ordinary skill in the art in this field of technology and the meaning of those terms or words will be understood from terminology used in that field or can be reasonably interpreted based on the plain English meaning of the words in conjunction with knowledge in this field of technology. This includes an understanding of implicit features that for example may involve multiple possibilities, but to a person of ordinary skill in the art a reasonable or primary understanding or meaning is understood. 19 162759226.1Attorney Docket No. 162152.51476

[0079] Unless defined otherwise, all technical and scientific terms used herein have same meaning as commonly understood by the person of ordinary skill in the art to which this disclosure belongs.

[0080] Embodiments of the present disclosure can include methods and / or compositions.

[0081] It should be understood that the above description of the present disclosure and specific examples, while indicating preferred embodiments of the present disclosure, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present disclosure may be made without departing from the spirit thereof, and the present disclosure includes all such changes and modifications. 20 162759226.1

Claims

Attorney Docket No. 162152.51476 WHAT IS CLAIMED IS:

1. A sintered nanocrystalline alloy comprising: about 1-20 atomic % (at.%) magnesium; and balance aluminum, wherein the sintered nanocrystalline alloy comprises MgO nanodispersoids.

2. The sintered nanocrystalline alloy of claim 1, comprising about 10-20 at.% magnesium.

3. The sintered nanocrystalline alloy of claim 2, wherein the sintered nanocrystalline alloy has a hardness of more than about 250 Vickers hardness (HV) at 0.3kgf.

4. The sintered nanocrystalline alloy of claim 2, comprising about 15 at.% magnesium.

5. The sintered nanocrystalline alloy of claim 4, wherein the sintered nanocrystalline alloy has a hardness of about 275 Vickers hardness (HV) at 0.3kgf.

6. The sintered nanocrystalline alloy of claim 1, further comprising an Al4C3minor second phase.

7. The sintered nanocrystalline alloy of claim 1, wherein the sintered nanocrystalline alloy has been sintered at least about 400 °C.

8. The sintered nanocrystalline alloy of claim 1, wherein the sintered nanocrystalline alloy is further annealed.

9. The sintered nanocrystalline alloy of claim 1, wherein the sintered nanocrystalline alloy is a bulk sintered nanocrystalline alloy.

10. The sintered nanocrystalline alloy of claim 1, wherein the sintered nanocrystalline alloy comprises grains having a grain size less than about 100 nm.

11. The sintered nanocrystalline alloy of claim 10, wherein the grains have a grain size less than about 50 nm. 21 162759226.1Attorney Docket No. 162152.51476 12. The sintered nanocrystalline alloy of claim 10, wherein the MgO nanodispersoids are dispersed alongside the grain boundaries.

13. A method for producing a sintered nanocrystalline alloy comprising: mixing about 1–20 at.% magnesium, a process control agent (PCA), and balance aluminum to form a mixture; milling the mixture using a ball mill to form a powder; and sintering the powder to form the sintered nanocrystalline alloy; wherein the sintered nanocrystalline alloy comprises MgO nanodispersoids.

14. The method of claim 13, wherein the PCA is methanol or ethanol present in about 2.75 wt%.

15. The method of claim 13, wherein the ball mill is a planetary ball mill equipped with a tungsten carbide milling jar using tungsten carbide balls.

16. The method of claim 13, wherein compacting the powder precedes the sintering to form a bulk sintered nanocrystalline alloy.

17. The method of claim 13, wherein about 10-20 at.% magnesium is mixed with the PCA and the balance aluminum in the mixing.

18. The method of claim 17, wherein about 15 at.% magnesium is mixed with the PCA and the balance aluminum in the mixing.

19. The method of claim 13, further comprising an Al4C3minor second phase.

20. The method of claim 13, wherein the sintering is performed at least about 400 °C.

21. The method of claim 13, further comprising annealing the sintered nanocrystalline alloy.

22. The method of claim 13, wherein the sintered nanocrystalline alloy is a bulk sintered nanocrystalline alloy. 22 162759226.1Attorney Docket No. 162152.51476 23. The method of claim 13, wherein the sintered nanocrystalline alloy comprises grains having a grain size less than about 100 nm.

24. The method of claim 23, wherein the grains have a grain size less than about 50 nm.

25. The method of claim 23, wherein the MgO nanodispersoids are dispersed alongside the grain boundaries. 23 162759226.1

Images