EP0006951A1 - Improvements in or relating to nickel-, cobalt-, and iron based alloys. - Google Patents

Abstract

The oxidation resistance and the corrosion resistance of an alloy based on nickel, cobalt or iron can be improved by including in the composition of the alloy a metal of the platinum group, such as osmium, iridium, platinum, ruthenium, rhodium or palladium, and one or more elements complementary to platinum, such as titanium, scandium, yttrium, lanthanum, hafnium, tantalum, zirconium, niobium and lanthanides in balanced proportions. The composition of the resulting alloy consists of at least 5 percent by weight of chromium, 0 to 3 percent by weight of carbon, an x component, a z component, and a balance of one or more nickel components , cobalt and iron with elements and possible impurities, or the component x is formed by one or more of the following compositions: i) at least 2 percent by weight in total of one or more of the following elements: aluminum, titanium, tantalum and niobium; ii) at least 5 percent by weight in total of one or both of the following: tungsten, molybdenum, and iii) at least 60 percent by weight of nickel, the component z comprises mp percent by weight of one or several metals of the platinum group mixed with mc percent by weight of one or more metals complementary to platinum with 0.1 <= mp + mc <= 5 and 0.3 <= mp / mc <= 20. The quantity mp of platinum group metal is preferably between 50% and 95% by weight of the total (mp + mc), and more particularly the quantities of platinum group metal and of complementary metal of platinum are chosen in stochiometric proportions relative to the intermetallic compounds which can form between them. The improved alloys are particularly suitable for use with the components of a gas turbine.

Description

Title: Nickel-, Cobalt- and Iron-based Alloys, This invention relates to nickel-, cobalt- and iron-based alloys comprising those suitable for use at high temperatures under oxidising conditions or corrosive conditions, and more particularly, but not exclusively, is concerned with directionslly solidified nickel-based alloys for use in these conditions.

Alloys capable of resisting oxidation, corrosion and high mechanical stresses at elevated temperatures are increasingly required, particularly in the gas turbine field. In these applications slight increases in permissible blade temperatures have a considerable and very favourable effect upon engine output and efficiency. It is an unfortunate characteristic of gas turbine alloy development, however, that changes in alloy composition which lead to improved high temperature strength tend also to reduce the oxidation resistance of the alloys. Many of the strongest gas turbine alloys presently known have a relatively low resistance to oxidation and corrosion,so that they must be protected against high temperature attack, ie corrosion and oxidation, by coatings which either remain on the surface of the alloy components or are caused to diffuse into the body of the components on which they are deposited. Is is a disadvantage of the various coating processes employed that they are costly, and that they tend also to have a deleterious effect upon the high temperature mechanical properties of the components to which they are applied. This invention seeks to provide high temperature nickel, cobalt and iron-based alloys having oxidation and corrosion resistance made good by controlled alloying additions which do not have any substantial adverse effect on the high temperature mechanical strength of the alleys in which they are incorporated and which, at least in some cases, lead to enhanced oxide scale adhesion.

According to a first aspect of the present invention there is provided on alloy consisting of at least 5 wt % of chromium, from 0 to 3 wt % of carbon, a component X, a component Z, and a balance of one or more of nickel, cobalt and iron together with incidental elements and impurities if any, wherein component X is one or more of;

(i) at least 2 wt % in total of one or more of aluminium, titanium, tantalum and niobium, (ii) at least 5 wt % in total of one or both of tungsten and molybdenum, and

(iii) at least 60 wt % of nickel; and component Z comprises m_pwt % of one or more platinum group metals (as herein defined) together with m_c wt % of one or more platinum-complementing metals (as herein defined) with

0.1 ≤ m_p + m_c ≤5 and

0.3 ≤ m_p / m_c≤ 20

According to a second aspect of the present invention there is provided a method of modifying the oxidation resistance and corrosion resistance of a nickel based, cobalt based or iron based alloy, which comprises including in the alloy composition an amount m_p wt % of a platinum group metal (as herein defined) together with an amount m_c wt % of one or more platinum complementing elements (as herein defined), and wherein

0.1≤m_p + m_c ≤ 5 and

0.3≤m_p / m_c ≤ 20 with percentages being relative to the alloy composition which is the product of the method. In this specification, the expression "platinum group metal" should be taken to mean one of osmium, iridium, platinum, ruthenium, rhodium and palladium, and the expression "platinum-complementing element" should be taken to mean one of the following:- titanium, scandium, yttrium, lanthanum, hafnium, tantalum, zirconium, niobium, and any of the lanthanide elements (Ce to Lu).

"Incidental elements and impurities" can comprise elements such as silicon, manganese and boron or, to a lesser extent vanadium, which elements are usually found in commercial iron-based alloys, and will also generally comprise small amounts of oxygen, nitrogen, hydrogen, phosphorus and sulphur. Nickel-, cobalt- and iron-based gas turbine alloys depend for their high temperature strength on carefully controlled micro-structures which generally contain, among several other phases, carbides based on Ti(Mo)C, Ti(Eb)C or other transition element compounds. Otherwise, the micro-structures may contain less stable components, such as Cr₃C₂. (It has been proposed to provide Cr₃C₂ in a directionally solidified alloy in the form of slender reinforcing fibres). If these strengthening carbides are to retain their integrity and reinforcing ability at high temperatures, the matrix of the alloy must have a low affinity either for carbon or for the metal from which the carbide is formed. Certain metals known for their solution strengthening capabilities have a high affinity for one or the other of the components of these strengthening carbides. Their addition has been shown to render the reinforcing carbides less stable. Thus zirconium, for example, which strengthens solid solutions very effectively in other alloy systems, cannot, in general, be added safely to superalloys because of its very high affinity for carbon, which tends to decompose any titanium or niobium carbides in its vicinity. It is well known that reactive metals such as Y and La, when present in suitable concentrations, can improve the high temperature oxidation and corrosion resistance of nickel-, cobalt- and iron-based alloys. However, these elements have, like zirconium, a high affinity for carbon, When they are present above a critical concentration level they have a tendency to attack the reinforcing constituents of the alloy in which they are incorporated. For example, consider a Ni-Ni-Al-Cr-Cp directionally solidified eutectic alloy which depends for its high temperature strength upon fine longitudinal fibres of Cr₃C₂. The Applicants have found that rare earth metals such as yttrium, when present in excess in the alloy, tend to decompose these reinforcing fibres, thus limiting the high temperature mechanical properties of this material, although its oxidation resistance is improved. Relatively small additions of one of the six platinum group metals (Os, Ir, Pt, Ru, Ki, Pd) are known by the present Applicants to enhance the oxidation and corrosion/of specific nickel-, iron- and cobalt-based alloys, particularly when the alloy to which additions are made is one of those which form a protective layer of aluminium oxide. Substantial additions of platinum group metals can rarely be made to such materials, however, because these metals have a tendency to decompose any carbides upon which the superalloy depends for mechanical reinforcement. This decomposition is caused, not because of the affinity of the platinum metals for carbon, which is very small, but because of their exceedingly high affinity for the metals which form stable carbides. Itis known, for example, that platinum and iridium are capable of decomposing lanthanum carbide at temperatures as low as 1000ºC. When platinum additions are made, therefore, to the directionally solidified

Ni-Ki₃Al-Cr₃C₂ eutectic composite mentioned above, the aligned Cr₃C₂ reinforcing fibres are partly decomposed and carbon is released in the form of graphite flakes. This leads to a deterioration in mechanical properties.

Considerations such as those outlined above appear therefore, at first glance, to preclude the use of the strongly carbide-forming elements and of the carbide-decomposing elements as beneficial additions to existing high temperature nickel-, cobalt- and iron-based alloys.

It has been found that, in accordance with the present invention, the above-mentioned carbide-forming and carbide-decomposing groups of metals can in certain circumstances be jointly added to superalloys in quantities up to a total of 5% by weight without any deleterious effect upon structure or mechanical properties, and with some improvement in their resistance to oxidation and corrosion at high temperatures. It is thought that this is possible because the platinum group (carbide-decomposing) metals have an affinity for the carbide-forming elements which is comparable to and in most instances higher than the affinity of these carbide-forming elements for carbon. The strengthening carbides can thus remain stable, and the platinum group metals can therefore be safely added without detriment to the high temperature properties of nickel, cobalt- or iron-based alloys, provided that they are suitably associated with one or more of the platinum complementing elements titanium, scandium, yttrium, lanthanum, hafnium, tantalum, zirconium, niobium and any of the lanthanide elements (Ce to Lu).

While the most beneficial effects are obtained when the composition of the component Z is stoichiometrically adjusted to provide for example the compounds listed below in Table 1, precise adjustment is not essential, and preferably component Z contains between 50 and about 93% by weight of the platinum group metals. In any case, there must be more than about 0.025 wt % of a platinum group metal present in component Z (corresponding to a lower limit of 0.3 in the quantity m_p/m_c , given a lower limit of 0.1 in the quantity(m_p + m_c). The present invention will now be illustrated by the followingExamples:-

EXAMPLE 1 Tb an alloy having a nominal composition (expressed in wt %) as set out below (incidental elements and impurities amounting to 1 wt %)

various additions were made to give alloys A, B, C and D as shown in Table 2.

Alloys C and D are according to the invention. The basic alloy and alloys A and B are for comparison. The observations set out below were made on the five alloy compositions given in Table 2. Microstructure.

Directional solidification of the basic alloy at a rate of 300mm/hour in a temperature gradient of about 13°Kmm^-1produced an ingot in which were present Cr₃C₂ fibres well aligned within a gamma nickel matrix which contained equi-axed particles of gamma prime (Ni₃Al). The alloys A and D contained yttrium in excess of that required for the formation of Pt₅Y and, in addition to the phases mentioned above, these two alloys also exhibited an elongated eutectic-like constituent which tended to run parallel to the aligned carbide fibres. ϊhis irregular constituent is thought to contain an yttrium-carbon compound. The alloy to which 1.96% by weight of platinum(alloy B) had been added contained a quite different irregular phase. This phase is thought to be pure graphite deposited due to the release of carbon on the formation of a highly stable platinum-chromium compound.

No additional phases or micro constituents were observed in the alloy to which the platinum and yttrium in the ratio needed to form the compound Pt₅Y (alloy C) had been added. (If the compound Pt₅Y retained its separate identity when added to the alloy it must, presumably, have been in the form of a dispersion too fine to resolve with the optical microscope). Ihe alloy displayed a regular aligned eutectic structure with thin fibres of Cr₃C₂ supported in a matrix consisting of nickel containing finely distributed particles of the compound Ni₃Al. Oxidation Resistance.

The basic alloy to which no addition had been made had a relatively poor resistance to oxidation when exposed to air at high temperatures either cyclically or under isothermal conditions The initially formed scale of Al₂O₃ spalled readily and oxidation continued with the formation of Cr₃O₃, nickel-chromium spinels, and with internal oxidation. ϊhe addition of yttrium alone (alloy A) improved the oxidation resistance significantly by stabilising the layer of Al₂O₃ which formed initially. Even so, the Al₂O₃ scale which formed was not completely tenacious, and cpalling occurred under tests carried out in a high velocity gas stream approximating in composition and speed to the hot gas passing over the first stage blading in a gas turbine, when the alumina scale was removed as rapidly as it was formed leaving behind the more tenacious skin of nickel-chromium oxide.

The alloy to which the εtoichiometrically adjusted Pt₅Y addition has been made (alloy C) developed on oxidation testing in air a protective skin of alumina which was resistant to spalling when cycled in temperature and also when handled at room temperature. After exposure to air at atmospheric pressure for 1000 hours at 1000°C no measurable oxide skin was observed and the carbide fibres retained their integrity to the specimen surface.

Hot salt corrosion resistance

Specimens (in the form of cylinders 6mm dia × 44 mm) of alloys A and C and of the basic alloy were tested in a gas burner rig in conditions of hot salt corrosion at a temperature of 850°C. A commercial superalloy designated IN 713 LC was also tested for purposes of comparison. Two specimens of each alloy were tested, The principal impurities in the fuel used in the burner were 0.15ppm sulphur and 50ppm sodium, the latter being introduced in the form of sodium carboxylite. In addition 50ppm of sodium chloride was injected into the air feed in the form of sea water, The specimens were removed and examined at 24 hr intervals and the tests were run for a total of 300 hrs. At the end of the tests all the alloys showed distint evidence of attack although all of the eutectic-based alloys retained a more regular cylindrical shape than the IN 713 LC specimens, The latter was subject to internal attack along grain boundaries giving both more rapid and irregular corrosion than with the former which degraded by regular surface attack, ϊhe depths of penetration of the corrosion products, determined from transverse sections of the specimens are given in Table 3 below, from which it will be seen that the most corrosion resistant alloys are the basic (eutectic) alloy and the alloy C according to the invention.

Creep Behaviour

Table 2 above shows the results of creep tests performed in air on the various alloys at 1000°C under a direct tensile stress of 100mPa. ϊhe alloy C retained the mechanical properties of the yttrium doped alloy (alloy A) and both were substantially stronger at high temperatures than those/either Pt or Y (alloys B and D respectively) above the level required to form stoichiometric Pt₅Y. EXAMPLE 2 The nominal compositions of the alloys studied are shown in Table 3. Alloy K is according to the invention and the remaining alloys are for comparison.

TABLE 3

Alloy Cr Al Pt Ht wt %

J 10 11 1 - )

K 10 11 0.9 0.3)

L 10 11 - - )

H 10 11 - 1 )

N 10 11 - 0.3) balance cobalt

Oxidation experiments were carried out in static air at 1 sphere pressure in a horizontal tube furnace, ϋheraogravimetric measurements were performed in a Sartorius automatic recording raicrobalance at 1100°C. Eeεults are as set out below and as shown in Table k. Alloy Microstructure

No microstructual differences between alloys J and L were observed. In alloy J, neither electron probe micro-analysis or optical examination were able to detect any intennetallics containing platinum. Alloy K (containing 0.3 Hf - 0.9 Pt) had a grain size smaller than alloy L and similar to that in Hf-containing alloys with no platinum (alloys M and N). Again, no platinum-containing intermetallics could be detected in alloy K and it would appear that the hafnium and platinum additions were both completely soluble in the alloy, at least at the concentrations used here. On oxidation of the samples of alloy K, the hafnium usually oxidised internally, but there was no apparent segregation of the platinum. Oxidation Kinetics Figures 1 and 2 of the accompanying drawings show the effect of Pt and Pt + Hf additions on the rate of weight gain of the basic Co-10Cr-11Al alloy L under isothermal oxidising conditions at 1100°C. Figure 1 shows that the addition of 1%Pt (alloy J) results in a slight decrease in the isothermal oxidation rate. Figure 2 is a plot of wt gain versus time each on a logarithmic scale. When measured over the period 10 to 100 h to avoid the initial transient stages of oxidation, the slope of the curve for alloy L has a value of 0.5, corresponding almost exactly to a parabolic rate law. The slope is reduced to 0.4 for the Co-10Cr-11Al-1Pt alloy J.

For the alloys containing 0.3 wt % Hf (alloys K and N) the situation was rather different. Neither conformed to a parabolic rate law, the Co-10Cr-11Al-0.3Hf alloy N had a slope of 0.28 whilst for the alloy containing 0.9 Pt (alloy K) the slope was 0.18. In addition, Figure 1 shows that the initial stages of oxidation of alloy K were terminated more rapidly than in alloy N, and that with both these Hf-containing alloys the transient stage was shorter than with the Hf-free alloys J and L. As indicated previously, it is difficult to define precisely the end of the transient stage, but typically it lasted for ½-1 h. Table 4 compares the weight gains of the four alloys after this period (1 h) and after 120 h exposure. Also included for comparison are the data for an alloy R known to the Applicants to have a particularly low overall weight gain under these conditions. This alloy R is Co-10Cr-11Al-0.3Hf internally oxiddzed for 300 h at 1200°C. TABLE 4

Weight Gain Data at 1100°C ; Isothermal Exposure

Weight Gain

Alloy mg/cm²

1 h 120 h

L : Co-10Cr-11Al 0.15 0.9 J : Co-10Cr-11Al-1Pt 0.16 0.72

N : Co-10Cr-11Al-0.3Hf 0.2 0.53 K : Co-10Cr-11Al-0.3Hf-0.9Pt 0.1 0.2 R : Co-10Cr-11Al-0.3Hf 0.09 0.18 (internally oxidized 300 h at 1200°C).

Scale Morphology

The Al₂O₃ scale which formed on the alloy Co-10Cr-11Al-1Pt

(alloy J) after 265 h oxidation at 1200°C was not adherent and spalled from the alloy on cooling, The oxide was multi-layered in many locations, particularly at the corners of the sample, and the outer layer of oxide at the gas/scale interface was heavily wrinkled. Similar features were observed with the ternary Co-Cr-Al alloy (alloy L) oxidized tinder similar conditions. Surface examinations of the alloy Co-10Cr-11Al-0.3Hf-0.9pt(alloy K) after oxidation at 1200°C revealed features similar to those of the alloy Co-10Cr-11Al-0.3Hf (alloy N). ϊhe Al₂O₃ scale was tightly adherent to the substrate and spalled during cooling only from small discrete areas, ϊhe major difference between the two alloys was that, with the Pt-free alloy N, the substrate surface appeared to be more heavily convoluted than with the Pt-containing alloy K.

Claims

WHAT WE CLAIM IS:

1. An alloy consisting of at least 5 weight percent of chromium, from 0 to 3 weight percent of carbon, a component X, a component Z, and a balance of one or more of nickel, cobalt and iron together with incidental elements and impurities if any, wherein component

X is one or more of:

(i) at least 2 weight percent in total of one or more of aluminium, titanium, tantalum and niobium;

(ii) at least 5 weight percent in total of one or both of tungsten and molybdenum; and

(iii) at least 60 weight percent of nickel; and component Z comprises m weight percent of one or more platinum group metals (as herein defined) together with m_cweight percent of one or more platinum - complementing metals (as herein defined), where

0.1 ≤ m_p + m_c ≤ 5 and 0.3 ≤ m_p/m_c ≤ 20 all the weight percentages being relative to the total weight of the alloy.

2. An alloy according to claim 1, wherein component Z contains from 50 to 95% by weight of one or more platinum group metals.

3. An alloy according to claim 2, wherein component Z contains substantially stoichiometric quantities of a platinum group metal and a platinum - complementing metal corresponding to the composition of a compound of the metals.

4. An alloy according to any of claims 1 to 3, wherein component X comprises at least6θweight percent of nickel.

5. An alloy according to any of claims 1 to 3, wherein component X comprises at least 5 weight percent in total of one or both of tungsten and molybdenum.

6. An alloy according to any of claims 1 to 3, wherein component X comprises at least 2 weight percent in total of one or more of aluminium, titanium, tantalum and niobium.

7. A method of modifying the oxidation resistance and corrosion resistance of a nickel-based, cobalt-based or iron-based alloy, which comprises including in the alloy composition an amount m_p weight percent of a platinum group metal (as herein defined) together with an amount m_c weight percent of one or more platinum complementing elements (as herein defined), and wherein 0.1 ≤ m_p + m_c ≤ 5

0.3 ≤m_p/m_c ≤ 20 with percentages being relative to the alloy composition which is the product of the method.

8. A method according to claim 8, wherein m comprises from 50 to 95% of the total additive (m_p + m_c).

9. A method according to claim 9 wherein a platinum group metal and a platinum complementing metal are added in stoichiometric quantities corresponding to the composition of a compound of the metals.