Alloy and a method of preparing the same

Abstract

A novel medium entropy alloy having the chemical formula Mo.sub.xCrNiCo (atomic %) where (x ranges from ˜0.4 to ˜1.0).

Claims

1. An alloy of Mo.sub.0.6CrNiCo characterized in having a dual phase consisting of a face centered cubic (fcc) phase and a sigma phase, wherein the sigma phase is the minority phase volumetrically and the fcc phase is the majority phase volumetrically; and the sigma phase forms a network around parcels of the fcc phase.

2. A method of preparing the alloy of claim 1, comprising: arc melting molybdenum, chromium, nickel, and cobalt into a molten alloy; heating the molten alloy to a temperature between 750 degrees Celsius and 1350 degrees Celsius to precipitate the dual phase; and quick quenching the molten alloy from above 750 degrees Celsius.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention, in which like integers refer to like parts. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

(2) FIG. 1a shows the XRD patterns of an embodiment of the invention;

(3) FIG. 1b shows an fcc crystal structure which is found in the embodiment of FIG. 1a;

(4) FIG. 1c shows a sigma phase crystal structure which is also found in the embodiment of FIG. 1a;

(5) FIG. 1d is the phase diagram of the alloy of FIG. 1;

(6) FIG. 2a, FIG. 2b, FIG. 2c and FIG. 2d shows the microstructural features of the Mo.sub.xCrNiCo alloys observed on the etched cross section, where x is 0.4 (a), 0.6 (b), 0.8 (c) or 1.0 (d).

(7) FIG. 3 shows the potentiodynamic polarization curves of MoCrNiCo alloys in 1 M HCl solution.

(8) FIG. 4 shows the cyclic potentiodynamic polarization curves of Mo.sub.0.8CrNiCo in 1 M Cl-containing solution with different pH values of 7 (a), 5 (b), 2 (c) or 0 (d).

(9) FIGS. 5(a) and 5(b) shows the corrosion morphologies of Mo.sub.0.8CrNiCo after cyclic potentiodynamic polarization tests in 1 M Cl.sup.− containing solution with different pH values of 7 (FIG. 5(a)) and 0 (FIG. 5(b)).

(10) FIG. 6 shows the comparison of the corrosion current density (i.sub.corr) and transpassivation potential (E.sub.T) between MEAs and other materials in Cl-containing solution with reducing pH at room temperature.

(11) FIG. 7 shows comparison of point defect density of MEA and reported other metal and alloys in 1M HCl solution.

(12) FIG. 8 shows a phase diagram of a method of preparing an alloy according to the disclosure.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(13) The present invention relates to a medium entropy, chemically complex, alloy having a dual-phase structure. In the preferred embodiment, the alloy consists of four transition metal elements molybdenum, chromium, nickel, and cobalt having the general chemical formula:
Mo.sub.xCrNiCo(atomic %)  (1) where (x ranges from ˜0.4 to ˜1.0)

(14) The meaning of medium-entropy alloys (MEAs) has to be understood in the light of the meaning of high-entropy alloys. High-entropy alloys (HEAs) is a relatively novel class of materials that are multicomponent alloys which are formed by mixing equal or relatively large proportions of usually five or more elements in near equi-molar proportions. The term “high-entropy alloys” was coined because the “entropy increase of mixing” is substantially higher when the component elements are mixed.

(15) Entropy increase of mixing is a thermodynamic description relating to the increase in total entropy created when several separate and thermodynamically stable metals or elements are mixed together without chemical reaction into a new thermodynamic state of internal equilibrium, typically characterised at room temperature. The change in thermodynamic state can be due to differences of intermolecular forces or specific molecular effects between different elements, even though they are chemically non-reacting.

(16) A further definition of HEA is that the gas constant R is higher than 1.5. It suffices here to point out that, as the skilled reader would know, the gas constant is often used when calculating corrosion rate of metals. An offshoot of the HEA concept is therefore medium-entropy alloys, which are alloys of usually four elements of which R is between 1 R and 1.5 R, and having non-equimolar composition. For example, a medium-entropy alloy may comprise one matrix element and several equiatomic alloying elements.

(17) The embodiment of the chemical formula (1) above may be deemed a medium-entropy alloy. However, there are more and more research into materials classed as HEA and medium-entropy alloy, and their definition may still be evolving. Hence, the term medium entropy is used herein loosely merely for skilled reader to appreciate the advance provided by the embodiments of this invention.

(18) The alloy of chemical formula (1) is prepared by arc melting using Mo, Cr, Ni, Co metals of high purity (99.95%) as raw materials. Typically, to ensure the chemical homogeneity, ingots of the alloy are fully re-melted at least five times in a Titanium-gettered argon atmosphere, before the melt is subsequently cast into a water-cooled Cu mould (50×10×5 mm.sup.3).

(19) The CoCrNi ternary alloy forms a single phase fcc crystal structure. However, according to the semi-empirical rules, addition of Mo into CoCrNi promotes the precipitation of sigma phase. FIG. 1 shows the XRD patterns of samples of the embodiments (hereafter denoted as Mo-x), where the peaks identifying sigma-phase are indicated.

(20) As the skilled man knows, a sigma phase crystal is a metallic compound having a tetragonal crystalline structure, and having a typical precipitation temperature between 600 degrees Celsius and 1000 degrees Celsius. Sigma phase can increase the hardness and decrease the toughness, as well as the elongation, of the alloy. With much surprise, sigma phase in an alloy of the described chemical formula improves resistance to corrosion.

(21) FIG. 1b shows a fcc crystal structure and FIG. 1c shows a sigma crystal structure (images taken from Wikipedia).

(22) FIG. 1d is the phase diagram of the Mo.sub.xCrNiCo alloy where x=0.4 to 1.0. A dual fcc and sigma phase can be formed over a wide range of Mo, from 0.1 to 1.0. FIG. 1d shows how the temperature in which sigma phase may be precipitated steadily increases until Mo 0.6, after which further amounts of Mo in the alloy does not result in any increase in temperature for forming a dual phase having sigma precipitate, and the eutectic form of the alloy will begin to manifest instead.

(23) FIG. 2a to FIG. 2d show the as-cast microstructure of the CoCrNiMox alloys having different Mo content. The lighter regions are the fcc phase while the secondary, sigma phase appeared among the fcc phase. FIG. 2a shows the darker sigma phase scattered among the lighter fcc phase when Mo is 0.4. FIG. 2b shows the darker sigma phase in greater area fraction among the lighter fcc phase when Mo is 0.6. FIG. 2c shows that at around 0.8 Mo content, an eutectic lamellar microstructure manifests which was made up of a mixture of fcc with an average size of 10 to 20 um, and an eutectic structure with an average lamellar spacing of 800 nm. FIG. 2d shows that at 1.0 Mo, a fully eutectic lamellar microstructure manifests, the sigma phase becoming interconnected, displaying block shapes with an average size of 10 to 20 um.

(24) Table 1 shows the volume fraction of the phases in different regions based on the SEM images. The fcc matrix phase in Mo-0.4 and Mo-0.6 exhibits a relatively uniform elemental distribution of Co, Cr and Ni with a low Mo content. By comparison, the eutectic regions are enriched with Mo in Mo-0.8 and Mo-1.0.

(25) TABLE-US-00001 TABLE 1 Chemical Composition (%) Sample Microstructure Phase Cr Co Ni Mo Mo.sub.0.4CrNiCo fcc matrix with fcc 30.9 ± 1 29.8 ± 1 29.4 ± 1  9.9 ± 1 discontinuous Sigma Sigma 28.2 ± 1 45.4 ± 1 26.4 ± 1 0 Mo.sub.0.6CrNiCo fcc Matrix with fcc 29.7 ± 2 29.2 ± 2 28.9 ± 2 12.2 ± 2 network of sigma Sigma 30.1 ± 2 27.6 ± 2 26.7 ± 2 15.6 ± 2 Mo.sub.0.8CrNiCo Fully eutectic Eutectic fcc 25.0 ± 1 28.2 ± 1 31.6 ± 1 15.2 ± 1 Eutectic Sigma 28.1 ± 1 24.1 ± 1 18.9 ± 1 27.9 ± 1 Mo.sub.1.0CrNiCo Mixture of Sigma 27.2 ± 2 24.5 ± 2 18.6 ± 2 29.7 ± 2 eutectic and Sigma Eutectic fcc 23.8 ± 1 28.0 ± 1 32.7 ± 1 15.5 ± 1 Eutectic Sigma 27.1 ± 1 23.7 ± 1 20.2 ± 1 29.0 ± 1

(26) FIG. 4 shows the cyclic potentiodynamic polarization curves of Mo.sub.0.8CrNiCo in 1 M Cl.sup.− containing solution with different pH values of 7 (a), 5 (b), 2 (c) or 0 (d).

(27) FIGS. 5(a) and 5(b) shows the corrosion morphologies of Mo.sub.0.8CrNiCo after cyclic potentiodynamic polarization tests in 1 M Cl.sup.− containing solution with different pH values of 7 (FIG. 5(a)) and 0 (FIG. 5(b)).

(28) FIG. 6 shows the comparison of the corrosion current density (i.sub.corr) and transpassivation potential (E.sub.T) between medium-entropy alloys and other materials in Cl.sup.− containing solution with reducing pH at room temperature. It can be seen that the alloys of chemical formula (1) has lower corrosion current than Mg alloys, Al alloys pure metals and is as good as still even at the lower pH.

(29) Table 2 shows the electrochemical parameters of MoCrNiCo alloys in 1 M HCl solution obtained from the potentiodynamic polarization curves of FIG. 3.

(30) TABLE-US-00002 TABLE 2 E.sub.corr I.sub.corr E.sub.T Samples (mV.sub.SCE) (A .Math. cm.sup.−2) (mV.sub.SCE) Mo.sub.0.4CrNiCo −49 10.72 × 10.sup.−8 852 Mo.sub.0.6CrNiCo −76  4.12 × 10.sup.−8 889 Mo.sub.0.8CrNiCo −44  7.96 × 10.sup.−8 879 Mo.sub.1.0CrNiCo −57 13.20 × 10.sup.−8 866

(31) Table 3 which shows the electrochemical parameters of Mo.sub.0.8CrNiCo alloys obtained from the cyclic potentiodynamic polarization curves of FIG. 4.

(32) TABLE-US-00003 TABLE 3 E.sub.corr I.sub.corr E.sub.T Samples Solution (mV.sub.SCE) (A .Math. cm.sup.−2) (mV.sub.SCE) Mo.sub.0.8CrNiCo Pure NaCl −365 2.63 × 10.sup.−8 612 (pH = 7) Mo.sub.0.8CrNiCo NaCl + HCl −304 4.57 × 10.sup.−8 640 (pH = 5) Mo.sub.0.8CrNiCo NaCl + HCl −148 4.58 × 10.sup.−8 753 (pH = 2) Mo.sub.0.8CrNiCo Pure HCl −44 7.96 × 10.sup.−8 879 (pH = 0)

(33) Table 4 shows the summary of electrochemical parameters of reported other metal and alloys in Cl.sup.− containing solution with reducing pH.

(34) TABLE-US-00004 TABLE 4 pH I.sub.corr E.sub.b Material value (A/cm.sup.2) (mV.sub.SCE) Ti 2 1.1 × 10.sup.−6 — 1.5 2.0 × 10.sup.−6 — 1 2.9 × 10.sup.−6 — 0.5 6.0 × 10.sup.−6 — 0.25 9.0 × 10.sup.−6 — Ni 6 1.4 × 10.sup.−5 — 2 3.4 × 10.sup.−5 — Al 6 1.6 × 10.sup.−5 — 2 4.0 × 10.sup.−5 — CuCrZr 7 4.3 × 10.sup.−2 — 5 1.4 × 10.sup.−1 — 3 4.2 × 10.sup.−1 — 1 2.0 — high strength 7 2.6 × 10.sup.−5 −409 pipeline steel 4 1.9 × 10.sup.−5 −562 254SMO 5 8.6 × 10.sup.−7 920 stainless steel 2 7.3 × 10.sup.−6 719 0.1 5.4 × 10.sup.−5 892 AlSl 410 4.25 — 295 stainless steel 2.25 — 65 Cr23N1.2 6 7.9 × 10.sup.−7 278 high nitrogen 2 8.5 × 10.sup.−5 276 chromium steel 1 7.0 × 10.sup.−4 −181 SAF 2205 8.5 — 252 DSS 3 — 72 1.5 — −124 AZ63 8 9.2 × 10.sup.−4 — magnesium alloy 3 1.1 × 10.sup.−3 — 2 3.3 × 10.sup.−3 — AZ91D 7.25 6.6 × 10.sup.−4 — Magnesium alloy 2 1.3 × 10.sup.−1 — AA7075 7 3.4 × 10.sup.−5 −600 aluminum alloy 3 5.1 × 10.sup.−5 −278 0.85 2.0 × 10.sup.−3 −552 7050-T7451 4 5.3 × 10.sup.−4 — aluminum alloy 2 1.3 × 10.sup.−3 — 1 1.3 × 10.sup.−2 — Ni50Al50 6 1.2 × 10.sup.−6 — 2 2.4 × 10.sup.−5 —

(35) Table 5 shows the summary of point defect density of medium-entropy alloy and reported other metal and alloys in 1M HCl solution.

(36) TABLE-US-00005 TABLE 5 Point defect Materials density/cm.sup.3 Mo.sub.0.6CrNiCo 2.63 × 10.sup.20 Ti.sub.3SiC.sub.2 3.80 × 10.sup.21 Ti 4.85 × 10.sup.20 Nb 7.42 × 10.sup.21 Al.sub.80Mo.sub.20 7.20 × 10.sup.20 Al.sub.75Mo.sub.25 1.70 × 10.sup.21 Fe.sub.68.8C.sub.7.0Si.sub.3.5—B.sub.5P.sub.9.6Cr.sub.2.1Mo.sub.2.0Al.sub.2.0 9.95 × 10.sup.20 LTON-treated 304 SS 3.89 × 10.sup.21 304 SS 3.78 × 10.sup.22

(37) Accordingly, an alloy of chemical formula (1) provides a unique dual-phase structures and high entropy passive film effectively prevents severe pitting corrosion, leading to more general corrosion. It is found that all chemically complex alloys of chemical formula (1) exhibit a low corrosion rate, and good passivation as well as repassivation ability in HCl. The anti-corrosion ability was determined to be substantially unchanged with decreasing pH. Moreover, the transpassivation potential of the chemically complex alloys chemical formula (1) shows an increase with reducing pH. Such promising anti-corrosion ability may be attributed to the formation of protective passive oxide film and the dual-phase structure of the chemically complex alloys, which prevent severe pitting corrosion within the chemically complex alloys' structure.

(38) In particular, the alloy having a chemical formula of Mo.sub.0.8CrNiCo was found to be the most corrosion resistant, illustrated poignantly in FIG. 3 where Mo.sub.0.8CrNiCo has high pitting potential than the alloys with other Mo content.

(39) Accordingly, the configuration of the dual-phase alloy of FIG. 2b is the preferred embodiment. The sigma phase forms a network around larger pockets of fcc phase. This provides protection of the Mo.sub.0.8CrNiCo alloy from corrosion and, possibly, imparts superior hardness by the sigma phase network while allowing the alloy to retain the properties of the fcc phase. The dual phase Mo.sub.0.8CrNiCo alloy can be formed between 750 degrees Celsius to 1350 degrees Celsius, and quick quenched in water when the alloy is above 750 degree Celsius to avoid the formation of miu (p) phase.

(40) While there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design, construction or operation may be made without departing from the scope of the present invention as claimed.