HARDENED ALLOY COMPOSITE MATERIALS AND METHOD OF MANUFACTURE

Description

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under a Cooperative Research and Development Agreement with the United States Army Research Laboratory. The government has certain rights in the invention.

BACKGROUND

The present disclosure is directed generally to the manufacture of metal matrix composite (MMC) materials and, particularly, reinforced martensite hardenable alloys including bearing steel.

Bearings require high hardness to withstand continued stress. Bearing steel is hardened through martensitic transformation that occurs during quenching heat treatment. Conventional bearing steel has a hardness suitable for most applications but is very dense, which can negatively contribute to the weight of machines and can reduce efficiency.

There is a need for bearing steel that can be manufactured with tailored hardness and porosity to meet application demands.

SUMMARY

A hardened alloy composite material comprising an alloy matrix and reinforcing particles dispersed in the alloy matrix, the reinforcing particles having a shell and core structure, wherein the shell and the core have different chemical compositions, and wherein the reinforcing particles are formed by a MAX phase compound and/or a MAB phase compound.

In another aspect, a method of manufacturing a hardened alloy composite material includes combining an alloy powder and a reinforcing powder to form a powder mixture, consolidating the powder mixture by cold pressing to form a consolidated powder mixture, densifying the consolidated powder mixture by sintering to form a densified material, and hardening the densified material by application of a quenching heat treatment to form the hardened alloy composite material. The reinforcing powder comprises a MAX phase compound and/or a MAB phase compound. A microstructure of the hardened alloy composite comprises reinforcing particles interspersed in an alloy matrix, the reinforcing particles having a shell and core structure, wherein the shell and the core have different chemical compositions.

In yet another aspect, a hardened alloy composite material includes an alloy matrix, first reinforcing particles dispersed in the alloy matrix, the first reinforcing particles formed by a MAX phase compound and/or a MAB phase compound and having a shell and core structure, wherein the shell and the core have different chemical compositions; and second reinforcing particles in the alloy matrix, the second reinforcing particles formed by an oxide of the A element of the at least one MAX or MAB phase compound.

The present summary is provided only by way of example, and not limitation. Other aspects of the present disclosure will be appreciated in view of the entirety of the present disclosure, including the entire text, claims and accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with the color drawing will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 is a flow chart of a method for forming a metal matrix composite material.

FIGS. 2A and 2B are collections of surface images obtained by optical microscopy of a steel-MAX composite substrate formed according to the method of FIG. 1.

FIGS. 3A-3C are surface images of the steel-MAX composite substrate following quenching heat treatment. Surface images are obtained by scanning electron microscopy (SEM) with backscattered electron (BSE) imaging and secondary electron imaging (SE).

FIGS. 4A and 4B are enlarged SEM images of a portion 4A, 4B of FIG. 3A.

FIG. 5 is a surface image of the steel-MAX composite substrate following quenching heat treatment and obtained with SEM and including noted areas of analysis by energy dispersive spectroscopy (EDS)

FIG. 6 is graph comparing Vickers hardness measured for a porous 52100 steel substrate, a porous steel-MAX composite substrate, and a fully dense 52100 steel substrate.

FIG. 7 is a graph comparing an average coefficient of friction for a porous 52100 steel substrate and the porous steel-MAX composite substrate before and after quenching heat treatment.

FIG. 8 is a graph comparing variation of wear rate as a function of composition in dry, ethanol, and F-24 medium.

FIG. 9 is a collection of surface images of a dense steel-MAX composite substrate following quenching heat treatment and obtained by EDS (executed in color).

FIGS. 10 and 11 are polished and etched surface images of the dense steel-MAX composite substrate obtained by optical microscopy before and after heat treatment, respectively.

FIGS. 12 and 13 are polished and etched surface images of the dense steel-MAX composite obtained by SEM before and after heat treatment, respectively.

While the above-identified figures set forth embodiments of the present invention, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features, steps and/or components not specifically shown in the drawings.

DETAILED DESCRIPTION

The exemplary embodiments disclosed herein are directed particularly to the manufacture of reinforced bearing steel, which can be used in many applications, including but not limited to aerospace, automotive, and industrial applications, and can be used for a variety of components, including, for example, power transmission bearings and gears, wear components, bushings, and high pressure fuel pumps and injectors, among others. The embodiments disclosed herein are exemplary in nature and are intended to provide an explanation of the present invention. The present invention is not limited to the embodiments disclosed. It will be understood by one of ordinary skill in the art that various modifications and variations can be made to the invention without departing from the scope and spirit of the invention.

Metal matrix composites (MMC) are composite materials having a metal or alloy matrix strengthened by metal, ceramic, or intermetallic reinforcing compounds. The present disclosure is particularly directed to MMC materials comprising a martensite hardenable alloy (particularly, alloy steel) reinforced by one or more MAX phase compounds, one or more MAB phase compounds, or a combination thereof. As disclosed herein, reinforcing compounds can be used to tailor hardness and porosity of a material, which are critical requirements for many applications. While the present disclosure is directly relevant to martensite hardenable alloy steel, the methods disclosed herein can be applied to other martensite hardenable alloys, including but not limited to titanium alloys such as nitinol (NiTi).

MAX phase compounds are layered ternary transition metal carbides or nitrides. MAX phase compounds have the general chemical formula: M_n+1AX_n, wherein M is at least one early transition metal selected from groups IIIB, IVB, VB, and VIB, A is at least one element selected from groups IIIA, IVA, VA, VIA, and VIIA, X is one or both of carbon and nitrogen, and n is an integer between 1 and 3. MAX phase compounds can include, for example, Cr₂AlC, V₂AlC, Ti₃AlC₂, Ti₄AlN₃, Ta₂AlC, Ta₄AlC₃, and Ti₃SiC₂. As disclosed herein, Cr₂AlC has been shown to be particularly beneficial in the manufacture of bearing steel.

MAB phase compounds are layered ternary or quaternary transition metal borides. MAB phase compounds can include, for example, Zr₂SeB and Hf₂SeB. Some examples of Al-containing MAB phases are MAlB (M=Ti, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc), M2AlB2 (M=Sc, Ti, Zr, Hf, V, Cr, Mo, W, Mn, Tc, Fe, Rh, Ni), M3Al2B2 (M=Sc, T, Zr, Hf, Cr, Mn, Tc, Fe, Ru, Ni), M3AlB4 (M=Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe), and M4AlB6 (M=Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo).

MAX phase compounds and MAB phase compounds can be used to enhance tribological properties (friction and wear properties) of martensite hardenable alloys. MAX phase compounds and MAB phase compounds are additionally highly damage resistant, creep resistant, readily machinable, conductive, and oxidation resistant. As disclosed herein, the addition of reinforcing particles formed in the incorporation of MAX phase compounds and/or MAB phase compounds to a martensite hardenable alloy matrix can result in increased hardness and greater wear resistance through matrix-particle interaction.

Martensite hardenable alloys are hardened through a quenching heat treatment process, which promotes martensite transformation. As disclosed herein, porosity can reduce the effect martensite transformation has on hardening of a material. In fact, porous substrates formed of some martensite hardenable alloys may not be hardened through quenching heat treatment despite occurrence of martensite transformation. As disclosed herein, the addition of reinforcing particles resultant of additions of small volume fractions of MAX phase compounds and/or MAB phase compounds to a martensite hardenable alloy matrix can result in increased hardness over martensite formation alone. Importantly, the inventors have determined that the reinforcing particles can have a synergistic effect on martensite transformation during quenching heat treatment, providing hardening of a porous alloy steel that is unattainable with martensite formation alone. The synergistic effect can allow for significant weight reductions in manufactured components as porous MAX and/or MAB reinforced martensite hardenable alloys can be produced with a hardness substantially equal to or even greater than the hardness of their non-reinforced denser counterparts.

The addition of MAX and/or MAB phase compounds can additionally improve the material properties of martensite hardenable alloys that have been fully densified, for example, through a hot isostatic pressing (HIP) process. As discussed further herein, the addition of small volume fractions of MAX phase compounds has been shown to increase the hardness of fully dense bearing steel through dispersion hardening.

In an exemplary embodiment, AISI 52100 bearing steel or equivalent bearing steel (i.e., 100Cr6, GCr15, and SUJ2 bearing steel) is reinforced with a MAX phase compound or a MAB phase compound in a powder metallurgy process. AISI 52100 bearing steel is a high carbon, chromium containing through hardening alloy steel. AISI 52100 bearing steel contains iron with, by wt. %, 0.93% to 1.05% carbon, 1.35% to 1.60% chromium, 0.15% to 0.35% silicon, 0.25% to 0.45% manganese, less than or equal to 0.10% molybdenum, less than or equal to 0.25% nickel, less than or equal to 0.35% copper, less than or equal to 0.025% phosphorus, less than or equal to 0.015% of each of sulfur and oxygen, and less than or equal to 0.05% aluminum. Using the disclosed quenching heat treatment, fully dense AISI 52100 bearing steel was determined to have a hardness of approximately 7,700 MPa (increase of approximately 5,300 MPa) as measured through Vickers hardness testing, while AISI 52100 bearing steel having a relative density of 86% (approximately 14% porosity) was found to have no increase in hardness following quenching heat treatment. The incorporation of a small volume fraction (5 vol. %) of MAX and/or MAB phase compounds in the porous AISI bearing steel can increase the hardness of the porous AISI 52100 bearing steel to a value substantially matching the hardness of fully dense AISI 52100 bearing steel following quenching heat treatment. In addition, the incorporation of MAX and/or MAB phase compounds is shown to improve tribological properties including the coefficient of friction and wear.

As disclosed further herein, the selection of MAX and MAB phase compounds, volume fraction, and particle size can be made to tailor porosity and hardness of the martensite hardenable alloy as well as tribological properties. Furthermore, improved hardening of martensite hardenable alloys can be achieved without hot isostatic pressing, which can simplify manufacturing and reduce manufacturing costs.

FIG. 1 is a flow chart of powder metallurgy method 10 for forming an MMC, and particularly, a reinforced martensite hardenable alloy composite material. In step 12, a martensite hardenable alloy powder and a powder of one or more MAX phase compounds, MAB phase compounds, or combination thereof, are combined. The martensite hardenable alloy powder can be, for example, an alloy steel. A MAX phase compound can be, for example, Al-containing MAX phases, such as: Ti₂AlC, V₂AlC, Cr₂AlC, Nb₂AlC, Ta₂AlC, Ti₂AlNb Zr₂AlC, Hf₂AlC, Ti₃AlC₂ Ta₃AlC₂, Zr₃AlC₂, Hf₃AlC₂, Ti₅Al2C₃, Ti4AlN3, Ta6AlC5, Ta4AlC3, Nb4AlC3, V4AlC3; and Si-containing MAX phases, such as: Ti3SiC2, Ti4SiC3, Ti5Si2C3, Ti7Si2C5.

A MAB phase compound can be, for example, Al-containing MAB phases, such as MAlB (M=Ti, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc), M2AlB2 (M=Sc, Ti, Zr, Hf, V, Cr, Mo, W, Mn, Tc, Fe, Rh, Ni), M3Al2B2 (M=Sc, T, Zr, Hf, Cr, Mn, Tc, Fe, Ru, Ni), M3AlB4 (M=Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fc), and M4AlB6 (M=Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo). MAX and MAB phase compounds and combinations thereof can be selected based on desired hardness, porosity, and/or tribological properties. MAB phase compounds, for example, may provide increased hardness values as compared to MAX phase compounds.

Martensite hardenable alloy powders are readily available in various particle sizes. MAX and MAB phase compounds are readily available as pre-reacted powders of various sizes. Alternatively, MAX and MAB phase compounds can be manufactured according to methods known in the art. Powder particles can be further ground to a desired size (step 14).

Powders of the martensite hardenable alloy and MAX and/or MAB phase compound(s) can be combined in varying ratios to provide a desired hardenability, porosity, tribological property, or other material property suitable for a particular application. Preferably, the combined powders can contain at least 5 percent by volume of the MAX and/or MAB phase compound(s). In some embodiments, the combined powders can less than 5 vol. % MAX and/or MAB phase compound(s), however, it is expected that hardness increases with increased MAX and/or MAB concentrations. The volume fraction of MAX and/or MAB phase compounds can be selected based on the material composition and desired porosity and hardness.

In step 14, the combined powders are uniformly mixed by ball milling or other suitable mixing process. Powders can be ground in a ball milling process to produce smaller and more uniform particle sizes. Powder size can be, for example, equal to or less than approximately 40 μm (−325 mesh). The reaction rate increases and material properties can be enhanced at lower particles sizes. Mixing is conducted to provide uniform distribution of the reinforcing particles in the final product.

In step 16, the powder mixture is consolidated into one or more composite pellets or other suitable material shape. Consolidating can be performed by cold pressing. Cold pressing can be performed using any conventional mechanical process used to compact powder materials, including but not limited to cold isostatic pressing. Cold pressing pressure can be selected to achieve powder consolidation. For example, in some embodiments, cold pressing can be conducted at a pressure of 150 MPa. Consolidation can produce, for example, composite pellets having a porosity of 30-40%.

In step 18, the consolidated powder mixture is densified. Densification can be conducted by sintering in an unpressurized furnace. A pressureless sintering process can be conducted at a temperature not exceeding a melting temperature of the powder materials. Pressureless sintering can be conducted in an inert atmosphere (i.e., in the presence of argon gas). Pressureless sintering causes the powder particles to fuse together while retaining porosity within the material. Following sintering, the densified material is allowed to cool to room temperature to reset the microstructure.

In alternative embodiments, densification can be conducted by a hot pressing or hot isostatic pressure-derived sintering process to further decrease the composite porosity and thereby increase the composite density. The temperature and pressure can be selected based on the material composition and desired density.

During the sintering process, the reinforcing particles decompose forming a core-shell structure due to instability of the MAX and/or MAB crystal lattice at high temperature. During decomposition, A elements can be released from the MAX and/or MAB crystal lattice and can diffuse into the alloy matrix, leaving MX or MB core reinforcing particles. As discussed with respect to the example disclosed herein, a MAX phase compound of Cr₂AlC decomposes during sintering to form a chromium carbide core surrounded by an aluminum shell. The aluminum preferentially diffuses outward and into the alloy steel matrix forming a thin shell around the chromium carbide core. The escaped aluminum absorbs excess oxygen in the alloy matrix in the process of decomposition. Typically, aluminum or silicon is added to steel to absorb oxygen to make killed steel (steel in which oxygen has been absorbed). The MAX and/or MAB phase compounds of the disclosed hardenable alloy can be a reservoir for aluminum or silicon to absorb oxygen and make killed steel.

In powder metallurgical processing that includes HIP, A elements (e.g., aluminum or silicon) can diffuse further into the material matrix providing increased strength due to dispersion hardening. For example, as discussed further herein, aluminum can form a shell around a chromium carbide core and can also diffuse into the material matrix, where it can react with oxygen, forming reinforcing alumina particles in the matrix.

In step 20, the densified composite material undergoes a quenching heat treatment. Heat treatment can be conducted, for example, in a box furnace. Heat treatment can be conducted in air or, preferably, in an inert atmosphere. A heat treatment temperature and duration can vary depending on the material composition.

Following heating above the austenizing temperature, the composite material is immersed in a quenching fluid to quickly cool the material to room temperature. The quenching fluid can be, for example, deionized water or oil. The cooling rate can be, for example, 430° C./s to 2.4° C./s. Rapid cooling promotes martensite transformation.

Generally, martensite transformation increases the hardness of a martensite hardenable alloy, however, as discussed further herein, a synergistic effect of reinforcing particles formed by the MAX and/or MAB phase compounds and martensite transformation promotes increases in hardness in materials not hardened by martensite transformation alone.

EXAMPLE 1

Method 10 was used to form a MAX-reinforced bearing steel substrate (referred to further herein as steel-MAX composite) as described in detail herein. A variety of analysis and testing methods, as described with respect to FIGS. 2-7, was performed to assess the material composition and properties.

In the following example, AISI 52100 bearing steel (or equivalent) was reinforced with the MAX phase compound Cr₂AlC. For comparison, a non-reinforced 52100 steel substrate was formed according to the method disclosed below without the addition of Cr₂AC. Both the steel-MAX composite the non-reinforced (or pure) 52100 steel substrate were porous, each having a porosity of around 14%. The non-reinforced 52100 steel substrate is referred to herein as “porous 52100 steel” to distinguish from a convention dense 52100 steel, which was used for comparison in hardness testing.

In a first step, Cr₂AlC was combined with AISI 52100 bearing steel powder. The total Cr₂AlC content in the combined powder was equal to approximately 5 vol. %, as calculated by weight and density of the materials.

The combined AISI 52100 bearing steel powder and Cr₂AlC powder was mixed via ball milling for 5 minutes to provide a substantially uniform powder mixture and particle size of approximately 40 μm (−325 mesh).

The powder mixture was placed in a vessel and consolidated via cold pressing at a pressure of approximately 150 MPa to form a composite pellet or consolidated powder mixture. The composite pellet had a porosity of approximately 30-40%.

The composite pellet was densified by pressureless sintering in a furnace containing an argon atmosphere to produce a densified composite material. Pressureless sintering was conducted at a temperature of approximately 1400° C. for 30 minutes to fuse the powder particles of the composite pellet and decompose Cr₂AlC. The densified composite material retained approximately 14% porosity following pressureless sintering.

The densified composite material was allowed to cool in the furnace to room temperature following sintering to reset the microstructure of the composite material.

Following sintering, the densified composite material was heat treated in a box furnace heated to approximately 840° C. for 23 minutes in air. Following heating, the densified composite material was quenched in deionized water to rapidly cool the composite material to room temperature to promote martensite transformation and to form the steel-MAX composite.

The steel-MAX composite substrate was polished to a surface roughness of approximately equal to 0.2 μm, for analysis.

The steel-MAX composite substrate was evaluated according to known methods described herein to determine density, hardness, microstructure, elemental composition, and tribological properties. The steel-MAX composite substrate was compared the porous 52100 steel.

FIGS. 2A and 2B are collections of surface images obtained by optical microscopy of the steel-MAX composite substrate. FIG. 2A shows images for each of the steel-MAX composite substrate and the porous 52100 steel substrate before quenching heat treatment. FIG. 2A shows images of unetched samples and samples that were etched with nital. FIG. 2B shows images for each of the steel-MAX composite substrate and the porous 52100 steel substrate and after quenching heat treatment. FIG. 2B shows images of unetched samples and samples that were etched with nital. Etching was conducted to show martensite transformation. Alloy steel matrix 21, reinforcing particles 22, pores 24, and martensite 26 are shown.

Alloy steel matrix 21 is porous 52100 bearing steel having pores 24. Reinforcing particles 22 are formed by a core-shell structure of decomposed Cr₂AlC distributed throughout the alloy steel matrix. Martensite 26 forms upon quenching heat treatment and is observed throughout alloy steel matrix 21.

As shown in FIG. 2A, porosity is visible at the polished surface of each of the steel-MAX composite substrate and the porous 52100 steel substrate prior to quenching heat treatment. As shown in FIG. 2A, porous 52100 bearing steel and the steel-MAX composite substrate have similar microstructures before quenching heat treatment. As shown in FIG. 2B, the addition of Cr₂AlC leads to a martensite-rich microstructure after quenching heat treatment.

FIGS. 3A-3C are surface images obtained by scanning electron microscopy (SEM) of the steel-MAX composite substrate following quenching heat treatment. FIGS. 3A-3C are images of the same location on the steel-MAX composite substrate. FIG. 3A is obtained by secondary electron imaging (SE) imaging. FIG. 3B is obtained by backscattered electron (BSE) imaging in compositional mode. FIG. 3C is obtained by BSE imaging in topographical mode. SEM analysis shows that reinforcing particles 22 are well distributed throughout alloy steel matrix 21. Porosity remains visibly present as indicated by pores 24.

FIGS. 4A and 4B are enlarged SEM images of portion 4A, 4B of FIG. 3A. FIG. 4A is obtained by BSE imaging in compositional mode. FIG. 4B is obtained by BSE imaging in topographical mode. SEM analysis shows micron-sized pores 24 around reinforcing particles 22.

FIG. 5 is a surface image of the steel-MAX composite substrate following quenching heat treatment and obtained by SEM with concurrent energy dispersive spectroscopy (EDS). FIG. 5 illustrates the shell-core structure formed by quenching heat treatment of reinforcing particles 22. The image was divided into six regions for chemical analysis. The regions were identified from an innermost portion of reinforcing particle 22 outward as core 27, core shell 28, intermediate region 30, intermediate shell 32, outer region 34, outer shell 36, and matrix 21. The elemental composition of each region is provided in Table 1.

TABLE 1
Elemental composition of steel-MAX composite
substrate following quenching heat treatment

Wt. %	Fe	C	O	Cr	Al	Si

Core (27)	26	11	7	57	0	0
Core shell (28)	44	9	17	30	0	0
Intermediate region (30)	32	5	26	37	0	0
Intermediate shell (32)	57	4	26	4	9	0
Outer region (34)	70	4	23	3	0.5	0
Outer shell (36)	61	6	19	13	1	0.2
Matrix (20)	89	6	1	4	0.2	0.1

As indicated by elemental composition analysis and shown in FIG. 5, aluminum segregates from chromium to alloy with iron in matrix 21, forming intermediate shell 32 and chromium carbide core 27. EDS showed some oxidation of chromium and aluminum-iron. This may be avoided by conducting all steps of the powder metallurgical process in an inert atmosphere.

FIG. 6 is graph comparing Vickers hardness measured for the porous 52100 steel substrate (having a porosity of about 14% or relative density of about 86%), the porous steel-MAX composite substrate (having a porosity of about 14% relative density of about 86%), and a fully dense 52100 steel substrate (i.e. having no measurable porosity). The fully dense 52100 steel substrate was obtained from a commercial vendor. FIG. 6 compares the hardness of the materials before quenching heat treatment and after quenching heat treatment. The same quenching heat treatment was applied to each material.

The hardness, as measured by Vickers hardness testing, of the porous 52100 steel substrate was approximately equal to the hardness of the fully dense 52100 steel substrate before heat treatment, while the hardness of the porous steel-MAX composite substrate was slightly greater. Following quenching heat treatment, the hardness increased significantly for each of the fully dense 52100 steel substrate and the porous steel-MAX composite substrate, while the hardness of the porous 52100 steel substrate remained relatively unchanged.

As shown in FIG. 2B, martensite transformation was observed for both the porous 52100 steel substrate and the porous steel-MAX composite substrate following quenching heat treatment. However, as shown in FIG. 6, martensite transformation did not result in increased hardness for the porous 52100 steel. Increased hardness was only observed with the combined martensite transformation and matrix-particle interaction of the reinforcing particles in the steel-MAX composite substrate.

FIG. 7 is a graph comparing an average coefficient of friction for the porous 52100 steel substrate and the porous steel-MAX composite substrate before and after quenching heat treatment, as measured by tribometer with F-24 fuel. As shown in FIG. 7, the coefficient of friction following quenching heat treatment was reduced by nearly half with the addition of the Cr₂AlC to the porous 52100 steel. The porous steel-MAX composite substrate had an average coefficient of friction of 0.073 following quenching heat treatment, while the porous 52100 steel had an average coefficient of friction of 0.130. Additional friction testing was conducted by dry sliding and with ethanol, as shown in FIG. 8. The addition of Cr₂AlC particles was effective in decreasing the wear rate under dry and sliding in the ethanol medium although in F-24 medium the wear rate was slightly higher than base steel.

As disclosed herein, reinforcing particles formed by MAX and/or MAB phase compounds incorporated into martensite hardenable alloys can be used to tailor hardness and porosity of a material, which will lead to critical advancements in many applications. While the exemplary embodiment disclosed herein is a steel-MAX composite formed by incorporation of Cr₂AlC into 52100 bearing steel in a powder metallurgy process, the synergistic effect observed of the core-shell reinforcing particle structure on martensite transformation is applicable to other martensite hardenable alloys and MAX and/or MAB phase compounds. Powder size, volume fraction, and material composition (i.e., MAX and/or MAB phase compound selection) can be modified to tailor hardness, porosity, and tribological properties for a given application.

EXAMPLE 2

In another example, densification (step 18) was performed using HIP to produce a dense steel-MAX composite with a density of 98.6% with a standard deviation of 1.7%, as measured using the Archimedes method. The porosity is therefore 1.4%. This sample was prepared as described above in steps 12 and 14, but then was hot-isostatic pressed to a temperature of 1200° C. at a pressure of 25,400 psi with a 30-minute hold at these conditions. Reference to “dense” steel-MAX composites herein refers to steel-MAX composites that have been densified by HIP and have a porosity less than 3%.

EXAMPLE 2

The densified composite material was heat treated as described with respect to Example 1 in step 20. The resulting dense steel-MAX composite substrate was polished to a surface roughness of approximately equal to 0.2 μm, for analysis. FIG. 9 is collection of surface images of the dense steel-MAX composite substrate following quenching heat treatment and obtained by EDS. FIG. 9 illustrates the shell-core structure formed by quenching heat treatment with diffusion of aluminum into the matrix and reinforcing particles of oxidized chrome carbide and alumina particles dispersed in the matrix. FIGS. 10 and 11 are etched surface images of the dense steel-MAX composite substrate obtained by optical microscopy before and after heat treatment, respectively. FIGS. 10 and 11 show the overall structure in optical micrographs of the dense microstructure with low porosity and the modified microstructure due to heat treatment, respectively. FIGS. 12 and 13 are polished and etched surface images of the dense steel-MAX composite obtained by SEM before and after heat treatment, respectively. FIG. 12 shows a lamellar microstructure dominated by pearlite 38 with chrome and aluminum in metallic, carbide, and oxide forms 40. FIG. 13 shows martensitic microstructure 42 with fine carbides 44 interspersed and aluminum oxides 46 in between grains closer to the chrome-aluminum core-shell structure 48 visible in the lower left of the micrograph.

Vickers hardness was measured for the dense steel-MAX composite substrate prior to and following heat treatment (step 20). Multiple dense steel-MAX composite samples were evaluated. Prior to heat treatment, an average measured Vickers hardness was approximately 4000 MPa. Following heat treatment, samples exhibited a Vickers hardness of greater than 8000 MPa and, in some instances, greater than 9000 MPa. The average Vickers hardness following heat treatment of the dense steel-MAX composite substrate was approximately 8500 MPa, which is greater than the Vickers hardness measured for non-reinforced fully dense AISI 52100 bearing steel (shown in FIG. 6). The measured Vickers hardness of the heat-treated dense steel-MAX composite substrate was corroborated by bulk hardness measurement of approximately 66 HRC and greater, which is equivalent to 8483 MPa and greater, as shown in Table 2. Table 2 shows samples HIP-1-02 HT and HIP-1-05 HT taken from two locations of the heat-treated steel-MAX composite and repeatedly evaluated.

TABLE 2
Rockwell C Hardness of heat treated and HIP-
processed steel-5% Cr2AlC samples.
HRC

Pt	HIP-1-02 HT	HIP-1-05 HT

1	66	66.9
2	66.5	66.6
3	66.5	67
4	66.6	77.6
5	66	67.6
Avg	66.32	69.14
St dev	0.263818119	4.242452121

As discussed herein, the synergistic effect of reinforcing particles formed by the MAX and/or MAB phase compounds and martensite transformation promotes increases in hardness in materials not hardened by martensite transformation alone. The methods provided herein can be used to produce both porous and fully dense steel-MAX/MAB composites having a hardness surpassing that of martensite hardenable alloy steel alone.

Any relative terms or terms of degree used herein, such as “substantially”, “essentially”, “generally”, “approximately” and the like, should be interpreted in accordance with and subject to any applicable definitions or limits expressly stated herein. In all instances, any relative terms or terms of degree used herein should be interpreted to broadly encompass any relevant disclosed embodiments as well as such ranges or variations as would be understood by a person of ordinary skill in the art in view of the entirety of the present disclosure, such as to encompass ordinary manufacturing tolerance variations, incidental alignment variations, transient alignment or shape variations induced by thermal, rotational or vibrational operational conditions, and the like. Moreover, any relative terms or terms of degree used herein should be interpreted to encompass a range that expressly includes the designated quality, characteristic, parameter or value, without variation, as if no qualifying relative term or term of degree were utilized in the given disclosure or recitation.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments of the present invention.

A hardened alloy composite material comprising an alloy matrix and reinforcing particles dispersed in the alloy matrix, the reinforcing particles having a shell and core structure, wherein the shell and the core have different chemical compositions, and wherein the reinforcing particles are formed by a MAX phase compound and/or a MAB phase compound.

The hardened alloy composite material of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, and/or additional components:

The hardened alloy composite material according to the preceding paragraphs can have a porosity equal to or greater than 14 percent.

The hardened alloy composite material of any of the preceding paragraphs can have a hardness greater than 6,000 MPa as determined by the Vickers hardness test.

In an embodiment of the hardened alloy composite material of any of the preceding paragraphs, a constituent of the MAX phase and/or a MAB phase can alloy with iron in the alloy matrix to form a shell around remaining constituents of the MAX phase and/or a MAB phase.

In an embodiment of the hardened alloy composite material of any of the preceding paragraphs, the alloy matrix can be a bearing steel.

In an embodiment of the hardened alloy composite material of any of the preceding paragraphs, the bearing steel can be AISI 52100 steel.

In an embodiment of the hardened alloy composite material of any of the preceding paragraphs, the MAX phase compound can have the chemical formula: M_n+1AX_n, wherein M is at least one early transition metal selected from groups IIIB, IVB, VB, and VIB, A is at least one element selected from groups IIIA, IVA, VA, VIA, and VIIA, X is one or both of carbon and nitrogen, and n is an integer between 1 and 3, and wherein A forms the shell.

In an embodiment of the hardened alloy composite material of any of the preceding paragraphs, the MAX phase compound can be Cr₂AlC.

The hardened alloy composite material of any of the preceding paragraphs can be fully dense and can have a hardness of greater than 8,000 MPa as determined by the Vickers hardness test.

In an embodiment of the hardened alloy composite material of any of the preceding paragraphs, the MAX phase or MAB phase compound can form a deoxidizing agent that absorbs excess oxygen in the alloy matrix.

In another aspect, a method of manufacturing a hardened alloy composite material includes combining an alloy powder and a reinforcing powder to form a powder mixture, consolidating the powder mixture by cold pressing to form a consolidated powder mixture, densifying the consolidated powder mixture by sintering to form a densified material, and hardening the densified material by application of a quenching heat treatment to form the hardened alloy composite material. The reinforcing powder comprises a MAX phase compound and/or a MAB phase compound. A microstructure of the hardened alloy composite comprises reinforcing particles interspersed in an alloy matrix, the reinforcing particles having a shell and core structure, wherein the shell and the core have different chemical compositions.

The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, additional components, and/or steps:

In an embodiment of the preceding method, the alloy powder can be a bearing steel.

In an embodiment of the method of any of the preceding paragraphs, the alloy powder can be AISI 52100 steel.

In an embodiment of the preceding method, the powder mixture can include at least 5% by volume the MAX phase compound or the MAB phase compound.

In an embodiment of the preceding method, the MAX phase compound has the chemical formula: M_n+1AX_n, wherein M is at least one early transition metal selected from groups IIIB, IVB, VB, and VIB, A is at least one element selected from groups IIIA, IVA, VA, VIA, and VIIA, X is one or both of carbon and nitrogen, and n is an integer between 1 and 3, and wherein A forms the shell.

In an embodiment of the preceding method, the reinforcing powder can comprise Cr₂AlC.

In an embodiment of the preceding method, the core can comprise chromium carbide.

In an embodiment of the preceding method, the microstructure of the hardened alloy composite material can further comprise martensite.

In an embodiment of the preceding method, each of the alloy powder and the reinforcing powder can have a particle size equal to or less than approximately 40 micrometers.

In an embodiment of the preceding method, densifying the consolidated powder mixture can include sintering in an unpressurized furnace, the resulting densified material having a porosity equal to or greater than 14 percent.

In an embodiment of the preceding method, a hardness of the hardened alloy composite can be greater than 6,000 MPa as determined by the Vickers hardness test.

In an embodiment of the preceding method, densifying the consolidated powder mixture can include hot isostatic pressing.

In an embodiment of the preceding method a hardness of the hardened alloy composite can be greater than 8,000 MPa as determined by the Vickers hardness test.

In yet another aspect, a hardened alloy composite material includes an alloy matrix, first reinforcing particles dispersed in the alloy matrix, the first reinforcing particles formed by a MAX phase compound and/or a MAB phase compound and having a shell and core structure, wherein the shell and the core have different chemical compositions; and second reinforcing particles in the alloy matrix, the second reinforcing particles formed by an oxide of the A element of the at least one MAX or MAB phase compound.

While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.