METHOD FOR CARBIDE DISPERSION STRENGTHENED HIGH PERFORMANCE METALLIC MATERIALS

Abstract

A method of preparing a mixture of a metal or metal alloy and (Nb.sub.xTi.sub.1-x)C (where 0<x≤1) in which (Nb.sub.xTi.sub.1-x)C in particulate form (either with or without metal powder) is formed into a preform and then if necessary added to the metal. The resulting (Nb.sub.xTi.sub.1-x)C/metal mixture can then be heated to a temperature below the melting point of the (Nb.sub.xTi.sub.1-x)C and optionally dispersed in liquid metal and/or casted and cooled to produce a solid product with improved physical properties.

Claims

1. A method of preparing a mixture of a metal or metal alloy and (Nb.sub.xTi.sub.1-x)C where 0<x≤1, including the steps of: (Ai) providing (Nb.sub.xTi.sub.1-x)C in the form of particles, (Aii) mixing the (Nb.sub.xTi.sub.1-x)C with particles of metal or metal alloy, (Aiii) forming said particles into a preform by compression or by placing said particles into a mould, (Aiv) optionally adding said preform to metal or metal alloy, and (Av) optionally heating said preform to a temperature below the melting point of (Nb.sub.xTi.sub.1-x)C; or (Bi) providing (Nb.sub.xTi.sub.1-x)C in the form of particles, (Bii) forming said particles into a preform by compression or by placing said particles into a mould, and (Biii) adding said preform to metal or metal alloy, and (B iv) optionally heating said mixture to a temperature below the melting point of (Nb.sub.xTi.sub.1-x)C in order to melt the metal or metal alloy.

2. A method as claimed in claim 1, wherein said particles are compressed in order to form the preform and including the step of changing the compression applied to the particles in order to result in a preform which includes voids and wherein the void fraction is from 1% to 75% of the preform.

3. A method as claimed in claim 1, wherein said particles have an average size from 10 nm to 10 μm.

4. A method as claimed in claim 1, wherein x is from 0.01 to 1.

5. A method as claimed in claim 1, wherein a preform having a desired shape is formed by forming the preform into said shape or by including an additional step of removing a part of the preform in order to result in a preform having said shape, or a combination thereof.

6. A method as claimed in claim 5, wherein the removing step is carried out by drilling or machining the preform.

7. A method as claimed in claim 1, wherein the ratio of (Nb.sub.xTi.sub.1-x)C to metal or metal alloy is controlled to result in an amount of (Nb.sub.xTi.sub.1-x)C from 1 to 100 wt % of the final product.

8. A method as claimed in claim 1 wherein the metal or metal alloy in step (Aiv) or (Biii) is in the form of particles or is in liquid form at a temperature below the melting point of (Nb.sub.xTi.sub.1-x)C.

9. A method as claimed in claim 8, wherein the step of adding the preform to liquid metal or liquid metal alloy is carried out in the presence of an inert gas or a reduced partial pressure of oxygen in order to avoid oxidation.

10. A method as claimed in claim 8 wherein the preform has a void fraction of greater than 1% and wherein the liquid metal or metal alloy is infiltrated into said voids.

11. A method as claimed in claim 1, wherein the step of heating said preform to a temperature below the melting point of said (Nb.sub.xTi.sub.1-x)C is carried out in the presence of an inert gas or a reduced partial pressure of oxygen in order to prevent oxidation.

12. A method as claimed in claim 1 additionally including the step of solidifying the resulting mixture of a metal or metal alloy and (Nb.sub.xTi.sub.1-x)C by cooling said mixture.

13. A method as claimed in claim 1 wherein the metal of said metal or metal alloy is magnesium, aluminium, cobalt, nickel, silver, iron or steel.

14. A method as claimed in claim 1, wherein in steps (Ai) or (Bi) the (Nb.sub.xTi.sub.1-x)C particles are mixed with a substance which has a lower melting point than (Nb.sub.xTi.sub.1-x)C.

15. A method as claimed in claim 14, wherein said substance is a polyvinyl alcohol.

16. A method as claimed in claim 1 additionally including the step of adding the resulting mixture of a metal or metal alloy and (Nb.sub.xTi.sub.1-x)C to a metal or metal alloy in liquid form and dispersing said mixture in the liquid metal or metal alloy.

17. (canceled)

18. A method as claimed in claim 16 additionally including the step of casting said dispersed mixture in order to create a master alloy.

19. A method as claimed in claim 18 additionally including the step of adding said master alloy to a metal or metal alloy in liquid form and dispersing said master alloy in the liquid metal or metal alloy.

20. (canceled)

21. A method as claimed in claim 16 additionally including the step of casting and cooling said dispersed mixture in order to create a solid product.

22. A method as claimed in claim 19 additionally including the step of casting and cooling said dispersed mixture in order to create a solid product.

Description

[0059] A number of preferred embodiments of the present invention will now be described with reference to and as illustrated in the accompanying drawings, in which:

[0060] FIGS. 1a and 1b show schematic views of (a) nano-particle segregation to grain boundaries in conventional melt processed metal matrix composites and (b) nano-particle dispersions within magnesium grain rather than at the grain boundary, offering much needed dispersion strengthening by these nanoparticles;

[0061] FIG. 2 is an image of a 32 mm diameter pressure-less infiltrated NbC pellet;

[0062] FIG. 3 shows a photo of the cross-section of solidified metal of 87.5 vol % Mg-12.5 vol % NbC colloidal solution which shows uniform particle distribution at macroscopic level and the spatial variation of average Vicker's hardness from top to bottom of solidified metal (along the direction of gravitational force). A narrow range of 80 to 93 HV3 hardness value presents a well dispersion of (Nb.sub.xTi.sub.1-x)C particles

[0063] FIG. 4 shows a polarised optical microstructure of a hardened billet formed from the master alloy of FIG. 3; the colour contrast represents individual grains;

[0064] FIG. 5 depicts tensile stress-strain curves of the Mg alone and Mg with 11% NbC master alloy;

[0065] FIG. 6 depicts tensile stress-strain curves of AZ91 alloy and AZ91 alloy with 3 vol % (Nb.sub.x,Ti.sub.1-x)C particle addition under as-cast and T6 condition;

[0066] FIG. 7 shows polarised optical microstructures of AZ31 alloy (a) without and (b) with 0.2% NbC particle addition revealing the grain refinement. The colour contrast represents the individual Mg grains;

[0067] FIG. 8 is a graph of stiffness (elastic modulus) as a function of percentage of added (Nb.sub.x,Ti.sub.1-x)C to magnesium alloys;

[0068] FIG. 9(a) is an image showing an (Nb.sub.xTi.sub.1-x)C for x=1 pellet infiltrated by liquid Ni-alloy in a pressure-less environment;

[0069] FIG. 9 (b) is an image at a higher magnification than FIG. 9(a) showing uniform distribution of (Nb.sub.xTi.sub.1-x)C for x=1 particles in a Ni-alloy matrix;

[0070] FIG. 9 (c) is an image showing uniform distribution of (Nb.sub.xTi.sub.1-x)C for x=0.9 particles in a Ni-alloy matrix.

[0071] FIG. 9 (d) is scanning electron microscope image and also the chemical elements mapping of Ti, Nb, C, Ni, Cr and Fe elements in the sample. The figure on the top left shows the distribution of (Nb.sub.xTi.sub.1-x)C for x=0.8 particles in a Ni-alloy matrix.

[0072] FIG. 10 (a) is a scanning electron microscope image showing uniform distribution of (Nb.sub.xTi.sub.1-x)C for x=1 particles in a steel matrix;

[0073] FIG. 10 (b) is a scanning electron microscope image showing uniform distribution of (Nb.sub.xTi.sub.1-x)C for x=0.9 particles in a steel matrix

[0074] FIG. 11(a) is a scanning electron microscope image showing uniform distribution of (Nb.sub.xTi.sub.1-x)C for x=1 particles in a Co matrix;

[0075] FIG. 11(b) is a scanning electron microscope image showing uniform distribution of (Nb.sub.xTi.sub.1-x)C for x=0.9 particles in a Co matrix

[0076] FIG. 12 is a scanning electron microscope image showing NbC particles in a Ag matrix; and

[0077] FIG. 13 is an image of a microstructure showing NbC particles in an Al—Mg alloy.

EXAMPLES

Example 1 Synthesis of Solid Solution Particle and Nanoparticle

[0078] The starting nominal composition of (Nb.sub.xTi.sub.1-x)C (for x=0.9, 0.85, 0.8, 0.5, 0.2, 0.1) is blended and compressed and heat-treated in Ar atmosphere at elevated temperature of ˜2000° C. with intermediate grinding to obtain solid solution phase.

Example 2 Pressure-Less Infiltration into Solid Solution Carbide Pellet with Liquid Magnesium

[0079] The solid solution particle (Nb.sub.xTi.sub.1-x)C (for x=1) with particle size range of 300 nm to 2 μm is compressed at 1 ton and 2 ton pressure to produce pellets with 16 mm diameter×5 mm thickness and 32 mm diameter×10 mm thickness, respectively. The green pellets are preheated at 200° C. for 2 hours and placed in liquid Mg at 700° C. Liquid Mg is observed to infiltrate completely into the interior of pellet without any external pressure within 30 min for the 16 mm diameter pellet and 60 min for the 32 mm diameter pellet. Then the infiltrated pellets are cooled in protective atmosphere. The infiltrated 32 mm diameter pellet is shown in FIG. 2. The Vicker's hardness (HV0.1) for infiltrated Mg/(Nb.sub.xTi.sub.1-x)C (for x=1) pellet have the average of 325 in comparison with reference magnesium matrix of 26.5. The estimate volume fraction of NbC for this infiltrated pellet lies in the range of 50% to 60%.

Example 3 Magnesium-(Nb.SUB.x.Ti.SUB.1-x.)C Master Alloy Preparation

[0080] (Nb.sub.x,Ti.sub.1-x)C (for x=1) solid solution green pellets of 32 mm diameter×10 mm thickness, with particle size range from 300 nm to 2 μm, is compressed under 1-2 ton uniaxial pressure, preheated at 200° C. for 2 hours and placed in various liquid magnesium alloys, such as commercial pure Magnesium, AZ31 alloy, Elekto21 and AZ91D alloy, for pressure-less infiltration for 1 hour. Then the melt containing the pellets is stirred gently at 500 rpm to break the pellet and disperse the (Nb.sub.x,Ti.sub.1-x)C particles to obtain well dispersed Mg—(Nb.sub.x,Ti.sub.1-x)C colloidal solution. With this process, colloidal solutions consisting of different levels of particles were fabricated. After 2 hours holding, these concentrated solutions were cooled under a protective atmosphere to engulf the nano particles by Mg matrix and obtained solid master alloys with different levels of (Nb.sub.x,Ti.sub.1-x)C particles (Table 1).

TABLE-US-00001 TABLE 1 List of Mg master alloys consisting of NbC particles Metal NbC level vol % Magnesium 3.0 Magnesium 11.0 Magnesium 12.5 AZ31 2.5 Elektro21 3.75 AZ91D 5.0

[0081] For some of the samples produced in Example 3, the hardness and elastic modulus have been measured and tabulated in Table 2. This demonstrates that it is possible to produce materials with both high modulus and high hardness.

TABLE-US-00002 TABLE 2 The mechanical properties of solid solution reinforced magnesium metal matrix composites. Vicker's Elastic hardness HV3 modulus GPa Mg (reference) 34.5 41.0 Mg + 3 vol % particles 45.4 56.2 Mg + 12.5 vol % particles 87.0 122.5

[0082] For a magnesium master alloy consisting of 12.5 vol % dispersed (Nb.sub.x,Ti.sub.1-x)C with x=1, the Vickers hardness is 87.0 HV3 and elastic modulus is 122.5 GPa. For the reference magnesium material, the Vicker's hardness and elastic modulus are measured to be 34.5 HV3 and 41.0 GPa, respectively.

[0083] The spatial variation of average Vicker's hardness value for 12.5 vol % (Nb.sub.x,Ti.sub.1-x)C containing master alloy from top to bottom (i.e, along the direction of gravitational force) varied within a narrow range of 87±8 HV3, as shown in FIG. 3. The uniform hardness across the sample suggests lack of particle sedimentation, which further indicates stability of colloidal solution with well dispersed particles in Mg. Similarly, the average Vicker's hardness of 11 vol % (Nb.sub.x,Ti.sub.1-x)C containing master alloy is 74±8 HV3. For a magnesium master alloy containing 11 vol % dispersed (Nb.sub.x,Ti.sub.1-x)C with x=1, exhibits a tensile yield strength of 137.8 MPa and ultimate strength of 165.4 MPa, whereas the reference magnesium material in the same condition shows yield strength of 16.4 MPa and ultimate strength of 70.5 MPa. The tensile stress-strain curve is presented in FIG. 5.

[0084] The engulfed particles by Mg grain are also observed to distribute uniformly as shown in FIG. 4. These engulfed particles interact with dislocations and could contribute to significant strength through Orowan strengthening mechanism. The particle engulfment feature seen here is unique to the (Nb.sub.xTi.sub.1-x)C/Mg system compared to conventional magnesium based metal matrix composites produced by conventional casting methods, in which engulfment is much harder to achieve.

Example 4: Method for Preparation of Diluted Mg/(Nb.SUB.x.Ti.SUB.1-x.)C Colloidal Solution

[0085] The master alloy prepared in Example 3 with compositions of (100-y)Mg+y(Nb.sub.xTi.sub.1-x)C, for x=1 and y=5, 11, 12.5 vol % are preheated to 200° C. and added to liquid Mg alloy (AZ91D) for obtaining, 1, 2 and 3 vol % of (Nb.sub.xTi.sub.1-x)C. The melt is protected under SF6+N2 gas flow to avoid oxidation. The melt is gently stirred with a metal rod followed by impeller mixing at 100-200 rpm to ensure mixing without creating turbulence and oxide inclusions. The stability of the colloidal solution with 3 vol % particles has been investigated for 15 mins and 30 mins of holding time. The micro-hardness (HV0.1) across solidified billets is measured at 70±5. The low variation demonstrates the absence of particle sedimentation.

[0086] The solutions prepared in this method are fed to various die casting processes such as gravity die casting, twin roll casting and high pressure die casting processes to obtain the final product, in which (Nb.sub.xTi.sub.1-x)C particles are remained engulfed by the Mg matrix during solidification.

Example 5: Solid Solution Nanoparticle Strengthened AZ91 Alloy

[0087] By following the method described in Example 4, 3 vol % solid solution nanoparticles are introduced into liquid AZ91 magnesium alloy (9 wt % Al, 0.8 wt % Zn and 0.2 wt % Mn) to form particle dispersion strengthened AZ91 alloy by diluting the magnesium master alloy containing 12.5 vol % solid solution nanoparticle. The introduced solid solution nanoparticle is of a particle size range of 300 nm to 2 μm and was uniformly dispersed in the AZ91 magnesium alloy matrix. In the as-cast condition, nanoparticle strengthened AZ91 alloy resulted in a tensile yield strength of 125 MPa and ultimate strength of 179 MPa, whereas AZ91 alloy without particle addition reached 102.2 MPa of yield strength and 150.9 MPa of ultimate strength. With T6 heat treatment (i.e. 413° C. for 16 hours and 168° C. for 16 hours) solid solution nanoparticle strengthened AZ91 alloy had a tensile yield strength of 161.5 MPa and ultimate strength of 240.6 MPa, whereas reference AZ91 alloy reached 129.7 MPa of yield strength and 232.1 MPa of ultimate strength. The stress-strain curves of solid solution strengthened AZ91 alloy and reference AZ91 alloy are presented in FIG. 6.

Example 6: Grain Refinement of AZ31 Magnesium Alloy

[0088] The NbC particle can also enhance the heterogeneous nucleation of magnesium grain in the solidification process. The AZ31 magnesium alloy has been tested for grain refinement with NbC particle size of 2 μm. For this a master alloy prepared in Example 3 has been added to liquid AZ31 alloy holding at 40° C. super heat and gently stirred manually, after 10 min holding the mixture was cast into a steel mould. The grain size of solid solution particle refined AZ31 is of 198±14 μm and for reference AZ31 it is 464±97 μm. The microstructure is presented in FIG. 7.

Example 7 Fabrication of Dispersion Strengthened AZ91 Containing 3 Vol % (Nb.SUB.0.85.Ti.SUB.0.15.)C

[0089] 3 vol % (Nb.sub.0.85Ti.sub.0.15)C solid solution particles are compressed into pellet (32 mm diameter) under 1 ton load and then introduced into liquid AZ91 magnesium alloy (9 wt % Al, 0.8 wt % Zn and 0.2 wt % Mn). Particles are dispersed in the liquid by gently stirring the liquid magnesium alloy. This colloidal solution is cast into a permanent mould. In the as-cast condition, tensile yield strength of 133.8 MPa and ultimate strength of 172.4 MPa are observed for particle strengthened AZ91 alloy, whereas for AZ91 alloy without particle addition 120.8 MPa of yield strength and 150.9 MPa of ultimate strength are observed.

Example 8: Stiffness as a Function of Amount of (Nb.SUB.x.Ti.SUB.1-x.)C Particles

[0090] FIG. 8 is a graph showing the elastic modulus of Mg/(Nb.sub.xTi.sub.1-x)C composite as a function of particle addition (blue line experimental data, red is theoretical). The dispersion strengthened Mg alloys are produced as in Example 3, 4 and 5.

[0091] The measured modulus is similar to the predicted values, using rule of mixture concept. In conventional metal matrix composites, the predicted values are very different from the measured ones due to the reinforcement particles' agglomeration and segregation at grain boundaries. If the particles are uniformly distributed within the matrix (as in this invention), then the value linearly increases with volume fraction of particles.

[0092] The results in the graph demonstrate that alloys can be designed with varied modulus (stiffness).

Example 9 Strengthening Approach for Nickel-Based Super Alloy by Introducing Compressed Pellet into Nickel-Based Alloy Melt

[0093] (Nb.sub.xTi.sub.1-x)C for x=1, 0.9 and 0.8 with an average particle size of about 1.2 micron is compressed at 0.5 ton or 1 ton pressure to produce pellets with 6 mm diameter×1.1 mm thickness. The estimated porosity in the green body is 55% and 49% for 0.5 ton and 1 ton loads respectively. The green body pellets are placed on Ni-based Inconel 718 alloy powder layer in Al.sub.2O.sub.3 crucibles.

[0094] In 5N purity Ar atmosphere (0.2 l/min flow rate), the temperature is raised to 1450° C. at 3 K/s so that the Ni alloy is in molten state. For each loading condition, the melt is kept for 60 s for one set of samples and 180 s for another set and then cooled to room temperature at 10 K/s. During this period the pellet sinks into liquid metal and the pressure-less infiltration is clearly observed and the (Nb.sub.xTi.sub.1-x)C pellet is completely wetted by the liquid Ni alloy.

[0095] FIG. 9(a) shows an electron microscopy image at lower magnification for x=1 and FIG. 9(b) shows an image at higher magnification. In FIG. 9(a), it can be seen that the liquid Ni infiltrated into the green pellet and filled the porosity in the green body pellet. In FIG. 9(b), uniform dispersion of NbC particles within Ni-alloy matrix can be seen. FIG. 9(c) shows microstructure of (Nb.sub.xTi.sub.1-x)C for x=0.9. FIG. 9(d) shows chemical elements mapping of alloy consisting of (Nb.sub.xTi.sub.1-x)C for x=0.8 particles. Coexistence of solid solution (Nb.sub.xTi.sub.1-x)C phase particles within the Inconel 718 matrix is clearly evident in this chemical mapping analysis.

Example 10 Strengthening Approach for Cast Iron, Tool Steel and Stainless Steel by Introducing Compressed Pellet into Alloy Melt

[0096] (Nb.sub.xTi.sub.1-x)C for x=1 and 0.9 with an average particle size of about 1.2 micron is compressed at 0.5 ton or 1 ton pressure to produce pellets with 6 mm diameter×1.1 mm thickness. The estimated porosity in the green body is 55% and 49% for 0.5 ton and 1 ton loads respectively. Green pellets are placed on 316 L alloy powder layer in Al.sub.2O.sub.3 crucibles.

[0097] In 5N purity Ar atmosphere (0.2 l/min flow rate), the temperature is raised to 1500° C. at 3 K/s so that the 316 L alloy is in molten state. For each loading condition, the melt is kept for 60 s for one set of samples and 180 s for another set and then cooled to room temperature at 10 K/s. During this period the pellet sinks into liquid metal and the pressure less infiltration is clearly observed and the (Nb.sub.xTi.sub.1-x)C pellet is completely wetted by the liquid alloy.

[0098] FIG. 10 (a) shows an electron microscopy image at higher magnification in which uniform dispersion of (Nb.sub.xTi.sub.1-x)C for x=1 particles within the steel matrix are clearly visible.

[0099] FIG. 10 (b) shows an electron microscopy image at higher magnification in which uniform dispersion of (Nb.sub.xTi.sub.1-x)C for x=0.9 particles within the steel matrix are clearly visible.

Example 11 Strengthening Approach for Cobalt by Introducing Compressed Pellet into Melt

[0100] (Nb.sub.xTi.sub.1-x)C for x=1 and 0.9 with an average particle size of about 1.2 micron is compressed at 0.5 ton or 1 ton pressure to produce pellets with 6 mm diameter×1.1 mm thickness. The estimated porosity in the green body is 55% and 49% for 0.5 ton and 1 ton loads respectively. Green pellets are placed on Co powder layer in Al.sub.2O.sub.3 crucibles.

[0101] In 5N purity Ar atmosphere (0.2 l/min flow rate), the temperature is raised to 1600° C. at 3 K/s so that the Co powder is in molten state. For each loading condition, the melt is kept for 60 s for one set of samples and 180 s for another set and then cooled to room temperature at 10 K/s. During this period the pellet sinks into liquid metal and the pressure less infiltration is clearly observed and the (Nb.sub.xTi.sub.1-x) pellet is completely wetted by the liquid metal.

[0102] FIG. 11(a) shows an electron microscopy image in which uniform dispersion of (Nb.sub.xTi.sub.1-x) for x=1 particles within Co matrix.

[0103] FIG. 11(b) shows an electron microscopy image in which uniform dispersion of (Nb.sub.xTi.sub.1-x)C for x=0.9 particles within Co matrix.

TABLE-US-00003 TABLE 3 Vickers hardness (HV.sub.1) for Co, 316L and IN718 alloys hardened with NbC 45 vol % 51 vol % HV.sub.1 0 vol % NbC NbC Co 146.8 873.5 970.9 316L 126.7 554.8 612.1 In718 288.7 725.2 782.0

Example 12 Strengthening Approach for Silver by Introducing Compressed Pellet into Melt

[0104] NbC with an average particle size of about 1.2 micron is compressed at 0.25 ton to produce pellet with 6 mm diameter×1.1 mm thickness. The estimated porosity in the green body is 55%. The green body (pellet) is placed on Ag powder layer in Al.sub.2O.sub.3 crucibles.

[0105] In 5N purity Ar atmosphere (0.2 l/min flow rate), the temperature is raised to 1200° C. at 3 K/s so that the Ag powder is in molten state. The melt is kept for 60 s and then cooled to room temperature at 10 K/s. During this period the pellet sinks into liquid metal and the pressure less infiltration is clearly observed and the NbC pellet is completely wetted by the liquid metal.

[0106] FIG. 12 shows an electron microscopy image in which NbC particles within Ag matrix can be seen. The hardness (HV.sub.0.1) for pure Ag and Ag+36 vol % NbC are 40 and 158, respectively.

Example 13 Strengthening Approach for Aluminium Alloys by Introducing Mg—NbC Master Alloy

[0107] When NbC is added directly to liquid Al it is observed that the NbC is chemically active with liquid Al and a chemical reaction is observed to occur. To minimise the chemical reaction, the master alloy with 50 wt % Mg-50 wt % NbC composition, prepared by following Examples 2 & 3, has been placed in liquid Al melt.

[0108] The melt was held at 730° C. for 2.5 hours and cast into a steel mould. Microstructure of cast sample shows dispersion of NbC particles. The hardness (HV.sub.0.1) for pure Al is 22 and Al containing NbC are observed to range from 100 to 250 depending on local concentration of NbC content.

[0109] FIG. 13 shows an image of a microstructure showing NbC particles in Al—Mg alloy

[0110] All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

[0111] The disclosures in UK patent application number 2011863.4, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.