Alloy and a method of preparing the same

Abstract

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

Description

FIELD OF INVENTION

The present invention relates alloys. In particular, the present invention relates to medium entropy alloys that possess anti-corrosion properties.

BACKGROUND OF THE INVENTION

Metal corrosion is a critical issue in a variety of industries, including oil production, mining processing and aerospace industry. In practice, corrosion not only results in degradation of material properties, leading to high costs in repairing or replacement of the corroded materials, but also leads to failure of materials in a gradual manner, hence constituting a public risk if it is not recognized.

Among the corrosion mechanisms, pitting corrosion is commonly found in different metals, which may be initiated from the damage of surface oxide layers due to the localized chemical attack of aggressive species, such as chlorides (Cl⁻) and hydrogen ions (H⁺). On the other hand, acidic solutions are commonly used for cleaning and processing, acid picking and oil production, with hydrochloric acid (HCl) being one of the most economical and efficient choices. Therefore, a variety of corrosion inhibitors have been developed to alleviate corrosion in metals susceptible to the chemical attack in low-pH solutions containing Cl⁻ ions. However, most of the reported organic corrosion inhibitors are toxic, and there is a growing ecological concern about their high hazardous environmental implications. Therefore, the development of corrosion resistant alloys in low pH solutions without corrosion inhibitors is of great significance.

Accordingly, it is desirable to propose a metal alloy that has corrosion resistance that has little or no negative environmental impact.

SUMMARY OF THE INVENTION

In the first aspect, the invention proposes a medium entropy alloy (MEA) consisting of four transition metal elements, wherein the plurality of transition metal elements are arranged in a dual-phase structure; the dual phase structure comprising a face centre cubic (fcc) phase and a sigma phase.

Typically, the four transition metal elements are: molybdenum, chromium, nickel and cobalt. Preferably, the alloy has a chemical formula of Mo_xCrNiCo, with x=0.4 to 1.0. Advantageously, an alloy of this formula has corrosion resistance without need of additional corrosion inhibitors in the alloy.

More preferably, the sigma phase is the minority phase volumetrically and the fcc phase is the majority phase volumetrically; the signal phase forming a network around parcels of fcc phase. A network of sigma phase around a majority of fcc phase provides the possibility of corrosion resistance by the sigma phase while allowing the alloy to retain most of the advantageous physical properties of the fcc phase, such as toughness, without too much the disadvantages of the sigma such as brittleness.

Yet more preferably, the alloy has a chemical formula of Mo_xCrNiCo, with x=0.6.

In a second aspect, the invention proposes a method of preparing the alloy of the chemical formula Mo_xCrNiCo, with x=0.4 to 1.0, comprising the steps of: preparing an alloy composition by arc melting raw materials comprising four or more transition metal elements; heating the molten alloy to a temperature to precipitate a dual phase consisting of a sigma phase and an fcc phase according to the temperature and Mo range as shown in the phase diagram of FIG. 8:

Preferably, the arc melting is performed for multiple times under a Ti-gettered argon atmosphere.

More preferably, the chemical formula is Mo_xCrNiCo, with x=0.6, and the step of heating the molten alloy to a temperature to precipitate a dual phase consisting of a sigma phase and an fcc phase comprises heating the molten alloy to between 750 degrees Celsius and 1350 degrees Celsius; and quick quenching the molten alloy from above 750 degrees Celsius.

As shown in FIG. 8, quick quenching from 750 degrees Celsius prevents formation of miu-phase. As the skilled man knows, the miu phase is physically soft and undesirable in many applications.

Advantageously, the invention provides the possibility of a series of inhibitor-free chemically complex alloys (CCAs) with excellent corrosion resistance in low-pH solutions containing Cl⁻ ions (e.g. HCl). Compared with traditional alloys, the anti-corrosion properties of the chemically complex alloys of the present invention do not exhibit deterioration with reducing pH (i.e. increasing H⁺ ion concentration), or in other words, the transpassive potential of the chemically complex alloys increases with decreasing pH. These promising properties render the chemically complex alloys particularly useful in replacement of traditional alloys so as to extend service time and to reduce the risk of failure of materials in extremely corrosive environments.

BRIEF DESCRIPTION OF THE FIGURES

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.

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

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

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

FIG. 1d is the phase diagram of the alloy of FIG. 1;

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

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

FIG. 4 shows the cyclic potentiodynamic polarization curves of Mo_0.8CrNiCo in 1 M Cl-containing solution with different pH values of 7 (a), 5 (b), 2 (c) or 0 (d).

FIGS. 5(a) and 5(b) shows the corrosion morphologies of Mo_0.8CrNiCo after cyclic potentiodynamic polarization tests in 1 M Cl⁻ containing solution with different pH values of 7 (FIG. 5(a)) and 0 (FIG. 5(b)).

FIG. 6 shows the comparison of the corrosion current density (i_corr) and transpassivation potential (E_T) between MEAs and other materials in Cl-containing solution with reducing pH at room temperature.

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

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

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

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_xCrNiCo(atomic %)  (1)

• • where (x ranges from ˜0.4 to ˜1.0)

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.

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.

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.

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.

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 mm³).

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.

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.

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

FIG. 1d is the phase diagram of the Mo_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.

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.

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.

	TABLE 1
	Chemical Composition (%)
Sample	Microstructure	Phase	Cr	Co	Ni	Mo
Mo0.4CrNiCo	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
Mo0.6CrNiCo	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
Mo0.8CrNiCo	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
Mo1.0CrNiCo	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

FIG. 4 shows the cyclic potentiodynamic polarization curves of Mo_0.8CrNiCo in 1 M Cl⁻ containing solution with different pH values of 7 (a), 5 (b), 2 (c) or 0 (d).

FIGS. 5(a) and 5(b) shows the corrosion morphologies of Mo_0.8CrNiCo after cyclic potentiodynamic polarization tests in 1 M Cl⁻ containing solution with different pH values of 7 (FIG. 5(a)) and 0 (FIG. 5(b)).

FIG. 6 shows the comparison of the corrosion current density (i_corr) and transpassivation potential (E_T) between medium-entropy alloys and other materials in Cl⁻ 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.

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

	TABLE 2
		Ecorr	Icorr	ET
	Samples	(mVSCE)	(A · cm−2)	(mVSCE)
	Mo0.4CrNiCo	−49	10.72 × 10−8	852
	Mo0.6CrNiCo	−76	4.12 × 10−8	889
	Mo0.8CrNiCo	−44	7.96 × 10−8	879
	Mo1.0CrNiCo	−57	13.20 × 10−8	866

Table 3 which shows the electrochemical parameters of Mo_0.8CrNiCo alloys obtained from the cyclic potentiodynamic polarization curves of FIG. 4.

TABLE 3
		Ecorr	Icorr	ET
Samples	Solution	(mVSCE)	(A · cm−2)	(mVSCE)
Mo0.8CrNiCo	Pure NaCl	−365	2.63 × 10−8	612
	(pH = 7)
Mo0.8CrNiCo	NaCl + HCl	−304	4.57 × 10−8	640
	(pH = 5)
Mo0.8CrNiCo	NaCl + HCl	−148	4.58 × 10−8	753
	(pH = 2)
Mo0.8CrNiCo	Pure HCl	−44	7.96 × 10−8	879
	(pH = 0)

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

	TABLE 4
		pH	Icorr	Eb
	Material	value	(A/cm2)	(mVSCE)
	Ti	2	1.1 × 10−6	—
		1.5	2.0 × 10−6	—
		1	2.9 × 10−6	—
		0.5	6.0 × 10−6	—
		0.25	9.0 × 10−6	—
	Ni	6	1.4 × 10−5	—
		2	3.4 × 10−5	—
	Al	6	1.6 × 10−5	—
		2	4.0 × 10−5	—
	CuCrZr	7	4.3 × 10−2	—
		5	1.4 × 10−1	—
		3	4.2 × 10−1	—
		1	2.0	—
	high strength	7	2.6 × 10−5	−409
	pipeline steel	4	1.9 × 10−5	−562
	254SMO	5	8.6 × 10−7	920
	stainless steel	2	7.3 × 10−6	719
		0.1	5.4 × 10−5	892
	AlSl 410	4.25	—	295
	stainless steel	2.25	—	65
	Cr23N1.2	6	7.9 × 10−7	278
	high nitrogen	2	8.5 × 10−5	276
	chromium steel	1	7.0 × 10−4	−181
	SAF 2205	8.5	—	252
	DSS	3	—	72
		1.5	—	−124
	AZ63	8	9.2 × 10−4	—
	magnesium alloy	3	1.1 × 10−3	—
		2	3.3 × 10−3	—
	AZ91D	7.25	6.6 × 10−4	—
	Magnesium alloy	2	1.3 × 10−1	—
	AA7075	7	3.4 × 10−5	−600
	aluminum alloy	3	5.1 × 10−5	−278
		0.85	2.0 × 10−3	−552
	7050-T7451	4	5.3 × 10−4	—
	aluminum alloy	2	1.3 × 10−3	—
		1	1.3 × 10−2	—
	Ni50Al50	6	1.2 × 10−6	—
		2	2.4 × 10−5	—

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

TABLE 5
                                 Point defect
Materials                        density/cm3
Mo0.6CrNiCo                      2.63 × 1020
Ti3SiC2                          3.80 × 1021
Ti                               4.85 × 1020
Nb                               7.42 × 1021
Al80Mo20                         7.20 × 1020
Al75Mo25                         1.70 × 1021
Fe68.8C7.0Si3.5—B5P9.6Cr2.1Mo2.0Al2.0 9.95 × 1020
LTON-treated 304 SS              3.89 × 1021
304 SS                           3.78 × 1022

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.

In particular, the alloy having a chemical formula of Mo_0.8CrNiCo was found to be the most corrosion resistant, illustrated poignantly in FIG. 3 where Mo_0.8CrNiCo has high pitting potential than the alloys with other Mo content.

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_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_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.

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.

Claims

1. An alloy of Mo0.6CrNiCo 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; andthe 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; andquick quenching the molten alloy from above 750 degrees Celsius.