GB2554898A

GB2554898A - A Nickel-based alloy

Info

Publication number: GB2554898A
Application number: GB1617326.2A
Authority: GB
Inventors: Reed Roger; Crudden David
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2016-10-12
Filing date: 2016-10-12
Publication date: 2018-04-18
Anticipated expiration: 2036-10-12
Also published as: JP2019534946A; US20200048742A1; JP7155115B2; US11859267B2; CN110225985B; WO2018069666A1; US20240084424A1; GB2554898B; GB201617326D0; EP3526355A1; CN110225985A

Abstract

A nickel-based superalloy consisting of (by weight), of: between 4.4% and 6.9% aluminium, between 0.6% and 23.4% cobalt, between 9.1% and 11.9% chromium, between 5 0.1% and 4.0% molybdenum, between 0.6% and 3.7% niobium, between 0.0 and 1.0% tantalum, between 0.0% and 2.0% titanium, between 0.0% and 10.9% tungsten, between 0.02 wt.% and 0.35 wt.% carbon, between 0.001 and 0.2 wt.% boron, between 0.001 wt.% and 0.5 wt.%. zirconium, between 0.0 and 0.5% silicon, between 0.0 and 0.1% yttrium, between 0.0 and 0.1% lanthanum, between 0.0 and 0.1% cerium, between 0.0 and 0.003% sulphur, between 10 0.0 and 0.25% manganese, between 0.0 and 0. 5% copper, between 0.0 and 0.5% hafnium, between 0.0 and 0.5% vanadium, between 0.0 and 10.0% iron, the balance being nickel and incidental impurities.

Description

(71) Applicant(s):

Oxford University Innovation Limited Buxton Court, 3 West Way, Oxford, Oxfordshire, OX2 0JB, United Kingdom

1617326.2 ₍₅₁₎ |_{NT CL}C22C 19/05 (2006.01)

12.10.2016 (56) Documents Cited:

GB 1013347 A CN 104404308 A

RU 002148100 C1 US 3293030 A (58) Field of Search:

INT CL C22C, C30B

Other: Online: EPODOC, WPI, ALLOYS (72) Inventor(s):

Roger Reed David Crudden (74) Agent and/or Address for Service:

J A Kemp

South Square, Gray's Inn, LONDON, WC1R 5JJ, United Kingdom (54) Title ofthe Invention: A Nickel-based alloy

Abstract Title: A nickel-based super alloy comprising Al, Co, Cr, Mo, Nb, C, B and Zr (57) A nickel-based superalloy consisting of (by weight), of: between 4.4% and 6.9% aluminium, between 0.6% and 23.4% cobalt, between 9.1% and 11.9% chromium, between 5 0.1% and 4.0% molybdenum, between 0.6% and 3.7% niobium, between 0.0 and 1.0% tantalum, between 0.0% and 2.0% titanium, between 0.0% and 10.9% tungsten, between 0.02 wt.% and 0.35 wt.% carbon, between 0.001 and 0.2 wt.% boron, between 0.001 wt.% and 0.5 wt.%. zirconium, between 0.0 and 0.5% silicon, between 0.0 and 0.1% yttrium, between 0.0 and 0.1% lanthanum, between 0.0 and 0.1% cerium, between 0.0 and 0.003% sulphur, between 10 0.0 and 0.25% manganese, between 0.0 and 0. 5% copper, between 0.0 and 0.5% hafnium, between 0.0 and 0.5% vanadium, between 0.0 and 10.0% iron, the balance being nickel and incidental impurities.

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A Nickel-Based Alloy

The present invention relates to a nickel-based superalloy composition for use as a turbine wheel within an exhaust gas turbocharger device. Previously, there has been a tendency to migrate nickel-based superalloys proven on aeroengines to such applications. However, this has proven largely inappropriate probably because the necessary design intent - determined from factors such as exhaust gas temperature and production cost - is then not respected.

Examples of typical compositions of nickel-based superalloys which are used for turbine wheels within a turbocharger devices are listed in Table 1. The alloys IN713C and IN713LC are typically employed in applications where the maximal operation temperature does not exceed 900-950°C; beyond this temperature the tensile strength and creep resistance of these alloys is insufficient. For temperatures beyond 950°C it is necessary to use the MarM246 and Mar-M247, these alloys can be applied in temperatures up to 1050°C as they have better high temperature strength and creep resistance. However, the Mar-M246 and Mar-M247 alloys cost significantly more than IN713C and IN713LC, also the corrosion resistance of these alloy is substantially worse. The present invention provides an alloy designed to have an alloy with tensile strength and creep equivalent to the Mar-M246 and Mar-M247 alloy grades. These mechanical properties are achieved in combination with a reduction in alloy cost and an improvement in oxidation/corrosion resistance. The balance of properties for the new alloy make it suitable for many high temperature turbomachinery applications. In particular for use as a turbine wheel within a exhaust gas turbocharger device where increased exhaust gas temperatures require a high degree of mechanical strength as well as resistance to aggressive creep and corrosion damage.

Table 1: Nominal composition in wt. % of conventionally cast nickel-based superalloys used for automotive turbochargers..

Alloy (wt.%)	Al	Co	Cr	Mo	Nb	Ta	Ti	W	c	B	Zr	Hf
IN713C	6.0	0.0	13.5	4.5	2.0	0.0	0.8	0.0	0.10	0.01	0.06	0.00
IN713LC	5.8	0.0	12.0	4.3	2.0	0.0	0.7	0.0	0.06	0.01	0.06	0.00
Mar-M246	5.5	10.0	9.0	2.5	0.0	1.5	1.5	10.0	0.15	0.01	0.05	0.00
Mar-M247	5.5	10.0	8.2	0.6	0.0	3.0	1.0	10.0	0.16	0.015	0.05	1.50

These materials are used to produce the turbine wheel within an exhaust gas turbocharger device because of their outstanding resistance to mechanical and chemical degradation. They contain as many as ten different alloying elements, necessary to confer the desired combination of properties.

It is an aim of the invention to provide a nickel-based alloy used to produce the turbine wheel within an exhaust gas turbocharger device which has mechanical properties equivalent to the strongest alloys used for these applications combined with a reduction in cost and an improvement in oxidation/corrosion resistance.

The present invention provides a nickel-based alloy composition comprising or consisting, in weight percent, of: between 4.4% and 6.9% aluminium, between 0.3% and 23.4% cobalt, between 9.1% and 11.9% chromium, between 0.1% and 4.0% molybdenum, between 0.6% and 3.7% niobium, between 0.0 and 1.0% tantalum, between 0.0% and 2.0% titanium, between 0.0% and 10.9% tungsten, between 0.02 wt.% and 0.35 wt.% carbon, between 0.001 and 0.2 wt.% boron, between 0.001 wt.% and 0.5 wt.%. zirconium, between 0.0 and 0.5% silicon, between 0.0 and 0.1% yttrium, between 0.0 and 0.1% lanthanum, between 0.0 and 0.1% cerium, between 0.0 and 0.003% sulphur, between 0.0 and 0.25% manganese, between 0.0 and 0.5% copper, between 0.0 and 0.5% hafnium, between 0.0 and 0.5% vanadium, the balance being nickel and incidental impurities.

In an embodiment, the following equation is satisfied in which Wni>, Wt₃, Wti and Wai are the weight percent of niobium, tantalum, titanium and aluminium in the alloy respectively 19 < (WNb+Wra + Wn) + 3.2Vk_A1 < 24.5, preferably 20 < (W_Nb + lT,-_a + W_Ti) + 3.2IV_A1 < 24.5. This achieves a desired volume fraction ofy’ and thereby resistance to creep deformation and creep rupture life.

In an embodiment, the following equation is satisfied in which Ww and Wmo are the weight percent of tungsten and molybdenum in the alloy respectively 9.4 < Vk_w + 2.9Vk_Mo, preferably 11.6 < Vk_w + 2.9Vk_Mo. This ensures that the γ phase is strong.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 10.1% or more chromium. This provides even better oxidation/corrosion resistance.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 11.0% or less chromium. This minimises the risk of TCP phase formation.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 0.3% or more molybdenum, preferably 0.5% or more molybdenum, more preferably 1.0% or more molybdenum. This allows a stronger gamma matrix to be achieved as well as allowing a higher level of chromium thereby to achieve good oxidation/corrosion resistance without increasing the chance of TCP phase formation.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 3.0% or less molybdenum. This achieves a good balance between solid solution strengthening and oxidation/corrosion resistance.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 1.8% or less titanium, preferably 1.6% or less titanium. This limiting of the amount of titanium results in the best combination of strength and oxidation resistance.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 22.6% or less cobalt. This produces an alloy with a good balance of cost, solid solution strengthening of the matrix and creep resistance.

In an embodiment the nickel-based alloy composition consists of, in weight percent, of 7.5% or more cobalt, preferably 9.2% or more cobalt. This results in an alloy with good creep resistance.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 0.2% or less hafnium. This is beneficial for tying up incidental impurities in the alloy and for providing strength.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 2.9% or more tungsten. Increasing the minimum amount of tungsten results in better creep resistance.

In an embodiment, the nickel-based alloy composition consists, in weight percent, of 0.5% or less tantalum, preferably of 0.1% or less tantalum. Keeping the tantalum level low is advantageous as tantalum is very expensive compared to other elements which can take its place.

In an embodiment, the nickel-based alloy composition consists, in weight percent, of 4.5% or more aluminium, preferably of 4.8% or more aluminium. Raising the level of aluminium achieves the desired γ’ volume fraction without needing to use vast quantities of tantalum and thereby helps maintain the cost of the alloy low.

In an embodiment, the sum of elements cobalt, tungsten and molybdenum in weight percent is 11.2% or greater, preferably 18.1% or greater, more preferably 19.8% or greater. Increasing the sum of the elements cobalt, tungsten and molybdenum results in greater creep resistance.

In an embodiment, the sum of elements cobalt, tungsten and molybdenum is 26.6% or less, preferably 20.1% or less, more preferably 17.1% or less and most preferably 12.6% or less. This allows the niobium concentration to be kept low and thereby achieves a lower cost alloy whilst maintaining mechanical properties.

In an embodiment, the nickel-based alloy composition consists of, in weight percent, of 0.1% or more iron. This is preferred as it allows the alloy to be manufactured from recycled metals.

In an embodiment, the nickel-based alloy composition consists of in weight percent, of 8.0% or less iron, preferably 1.0% or less iron. This is preferred to reduce the propensity to form the unwanted Laves phase which can degrade the mechanical properties of the alloy.

In an embodiment, the sum of elements molybdenum and tungsten, in weight percent, is 10.6% or less, preferably 9.9% or less. This ensures the required level of microstructural stability given the chromium content.

In an embodiment, the sum of elements molybdenum and tungsten, in weight percent is 3.2% or more, preferably 3.6% or more, more preferably 4.0% or more. This allows the alloy to achieve a strong γ matrix phase and a suitable level of creep resistance.

In an embodiment, the nickel-based alloy composition consists, in weight percent, of 6.8% or less aluminium, preferably 6.7% or less aluminium. This allows a high strength to be achieved through a suitable fraction of γ’ being present.

In an embodiment, the following equation is satisfied in which WNb, WTa and Wti are the weight percent of niobium, tantalum and titanium in the alloy respectively WNb + Wt₃ + Wti > 2.6, preferably WNb + Wt₃ + Wti>3.1, more preferably WNb _ Wt₃ + Wti > 3.2, most preferably WNb _ W-r_a + Wti > 3.6. This allows a suitable amount of γ’ to be present combined with a high anti-phase boundary energy thereby achieving the desired strength.

In an embodiment, the ratio of the sum of the elements niobium, tantalum and titanium to aluminium by weight percent is greater than 0.45, preferably greater than 0.55, most preferably greater than 0.65. This achieves the desired combination of γ’ fraction and anti-phase boundary energy providing strength.

In an embodiment, the nickel-based alloy composition has between 55% and 70% volume fraction γ’, preferably between 58% and 70% volume fraction γ’. This provides the preferred balance between creep resistance, oxidation resistance and propensity to form TCP phases.

In an embodiment, a turbine wheel formed of the nickel-based alloy composition of any of the preceding claims.

In an embodiment, an exhaust gas turbocharger device comprises such a turbine wheel.

In an embodiment, a cast article is formed of the nickel-based alloy composition.

The term “consisting of’ is used herein to indicate that 100% of the composition is being referred to and the presence of additional components is excluded so that percentages add up to 100%. Unless otherwise stated, percents are expressed in weight percent.

The invention will be more fully described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows the partitioning coefficient for the main components in the alloy design space;

Figure 2 is a contour plot showing the effect of γ' forming elements aluminium and the sum of elements niobium, tantalum and titanium on volume fraction of γ' for alloys within the alloy design space, determined from phase equilibrium calculations conducted at 900°C;

Figure 3 is a contour plot showing the effect of elements aluminium and the sum of elements niobium, tantalum and titanium on yield strength (in terms of strength merit index), superimposed are limits for volume fraction of γ' between 55-70% taken from Figure 2;

Figure 4 is a contour plot showing the effect molybdenum and tungsten on solid solution strengthening (in terms of solid solution index), for alloys with a volume fraction of γ' between 55-70%;

Figure 5 is a contour plot showing the effect of niobium and the sum of elements cobalt, molybdenum and tungsten on raw material cost for alloys with a volume fraction of γ' between 55-70% when tantalum content is fixed at 0 wt.%;

Figure 6 is a contour plot showing the effect of niobium and the sum of elements cobalt, molybdenum and tungsten on raw material cost for alloys with a volume fraction of γ' between 55-70% when tantalum content is fixed at 1 wt.%;

Figure 7 is a contour plot showing the effect of niobium and the sum of elements cobalt, molybdenum and tungsten on raw material cost for alloys with a volume fraction of γ' between 55-70% when tantalum content is fixed at 2 wt.%;

Figure 8 is a contour plot showing the effect of tungsten and chromium on microstructural stability for alloys with a volume fraction of γ' between 55-70% when molybdenum content is fixed at 0 wt.%;

Figure 9 is a contour plot showing the effect of tungsten and chromium on 5 microstructural stability for alloys with a volume fraction of γ' between 55-70% when molybdenum content is fixed at 1 wt.%;

Figure 10 is a contour plot showing the effect of tungsten and chromium on microstructural stability for alloys with a volume fraction of γ' between 55-70% when molybdenum content is fixed at 2 wt.%;

Figure 11 is a contour plot showing the effect of tungsten and chromium on microstructural stability for alloys with a volume fraction of γ' between 55-70% when molybdenum content is fixed at 3 wt.%;

Figure 12 is a contour plot showing the effect of tungsten and chromium on microstructural stability for alloys with a volume fraction of γ' between 55-70% when molybdenum content is fixed at 4 wt.%;

Figure 13 is a contour plot showing the effect of tungsten and chromium on microstructural stability for alloys with a volume fraction of γ' between 55-70% when molybdenum content is fixed at 5 wt.%;

Figure 14 is a contour plot showing the effect of tungsten and chromium on 20 microstructural stability for alloys with a volume fraction of γ' between 55-70% when molybdenum content is fixed at 6 wt.%;

Figure 15 is a contour plot showing the effect of cobalt and the sum of molybdenum and tungsten on creep resistance (in terms of creep merit index), for alloys with a volume fraction of γ' between 55-70%;

Traditionally, nickel-based superalloys have been designed through empiricism. Thus their chemical compositions have been isolated using time consuming and expensive experimental development, involving small-scale processing of limited quantities of material and subsequent characterisation of their behaviour. The alloy composition adopted is then the one found to display the best, or most desirable, combination of properties. The large number of possible alloying elements indicates that these alloys are not entirely optimised and that improved alloys are likely to exist.

In superalloys, generally additions of chromium (Cr) and aluminium (Al) are added to 5 impart resistance to oxidation/corrosion, cobalt (Co) is added to improve resistance to sulphidisation. For creep resistance, molybdenum (Mo), tungsten (W), cobalt are introduced, because these retard the thermally-activated processes - such as, dislocation climb - which determine the rate of creep deformation. To promote static and cyclic strength, aluminium (Al), tantalum (Ta), niobium (Nb) and titanium (Ti) are introduced as these promote the formation of the precipitate hardening phase gamma-prime (γ'). This precipitate phase is coherent with the face-centered cubic (FCC) matrix phase which is referred to as gamma (γ).

A modelling-based approach used for the isolation of new grades of nickel-based superalloys is described here, termed the “Alloys-By-Design” (ABD) method. This approach utilises a framework of computational materials models to estimate design relevant properties across a very broad compositional space. In principle, this alloy design tool allows the so called inverse problem to be solved; identifying optimum alloy compositions that best satisfy a specified set of design constraints.

The first step in the design process is the definition of an elemental list along with the associated upper and lower compositional limits. The compositional limits for each of the elemental additions considered in this invention - referred to as the “alloy design space” - are detailed in Table 2.

Table 2: Alloys design space in wt. % searched using the “Alloys-by-Design ” method.

Alloy (wt.%)	Al	Co	Cr	Mo	Nb	Ta	Ti	w
Min	4.0	0.0	8.0	0.0	0.0	0.0	0.0	0.0
Max	8.0	25.0	16.0	8.0	4.0	4.0	4.0	12.0

The balance is nickel. The levels of carbon, boron and zirconium where fixed at 0.06%, 25 0.015% and 0.06% respectively.

The second step relies upon thermodynamic calculations used to calculate the phase diagram and thermodynamic properties for a specific alloy composition. Often this is referred to as the CAFPHAD method (CAFculate PHAse Diagram). These calculations are conducted at the typical service temperature for the new alloy (900°C), providing information about the phase equilibrium (microstructure).

A third stage involves isolating alloy compositions which have the desired microstructural architecture. In the case of nickel based superalloys which require superior resistance to creep deformation, the creep rupture life generally improves as the volume fraction of the precipitate hardening phase γ' is increased, the most beneficial range for volume fraction of γ' lies between 60%-70%. At values above 70% volume fraction of γ' a drop in creep resistance is observed.

It is also necessary that the γ/γ' lattice misfit should conform to a small value, either 10 positive or negative, since coherency is otherwise lost; thus limits are placed on its magnitude. The lattice misfit δ is defined as the mismatch between γ and γ' phases, and is determined according to

2(a,-a) + α_γ (1) where α_γ and α_γ, are the lattice parameters of the γ and γ' phases.

Rejection of alloy on the basis of unsuitable microstructural architecture is also made from estimates of susceptibility to topologically close-packed (TCP) phases. The present calculations predict the formation of the deleterious TCP phases sigma (σ), P and mu (μ) using CALPHAD modelling.

Thus the model isolates all compositions in the design space which are calculated to result in a desired volume fraction of γ', which have a lattice misfit γ' of less than a predetermined magnitude and have a total volume fraction of TCP phases below a predetermined magnitude.

In the fourth stage, merit indices are estimated for the remaining isolated alloy compositions in the dataset. Examples of these include: creep-merit index (which describes an alloy’s creep resistance based solely on mean composition), strength-merit index (which describes an alloy’s precipitation yield strength based solely on mean composition), solidsolution merit index (which describes an alloy’s solid solution yield strength based solely on mean composition), density and cost.

In the fifth stage, the calculated merit indices are compared with limits for required behaviour, these design constraints are considered to be the boundary conditions to the problem. All compositions which do not fulfil the boundary conditions are excluded. At this stage, the trial dataset will be reduced in size quite markedly.

The final, sixth stage involves analysing the dataset of remaining compositions. This can be done in various ways. One can sort through the database for alloys which exhibit maximal values of the merit indices - the lightest, the most creep resistant, the most oxidation resistant, and the cheapest for example. Or alternatively, one can use the database to determine the relative trade-offs in performance which arise from different combination of properties.

The example five merit indicies are now described.

The first merit index is the creep-merit index. The overarching observation is that timedependent deformation (i.e. creep) of a nickel-based superalloy occurs by dislocation creep with the initial activity being restricted to the γ phase. Thus, because the fraction of the γ' phase is large, dislocation segments rapidly become pinned at the γ/γ' interfaces. The rate-controlling step is then the escape of trapped configurations of dislocations from γ/γ' interfaces, and it is the dependence of this on local chemistry - in this case composition of the γ phase - which gives rise to a significant influence of alloy composition on creep properties.

A physically-based microstructure model can be invoked for the rate of accumulation of creep strain έ when loading is uniaxial and along the (00l) crystallographic direction. The equation set is >l) = Pm0p Aff (¹ - 0p )(¹ / 0p - Ο ^Sinh <

σδ²ω

46K_cvkT (2)

P_m=C£ (001) (3) where P_m is the mobile dislocation density, φ_ρ is the volume fraction of the γ' phase, and ω is width of the matrix channels. The terms CF and T are the applied stress and 25 temperature, respectively. The terms b and k are the Burgers vector and Boltzmann constant, respectively. The term P_CP = 1 + 20_p ^1/3 / 3^/3π (1-0 ^1/3) is a constraint factor, which accounts for the close proximity of the cuboidal particles in these alloys. Equation 3 describes the dislocation multiplication process which needs an estimate of the multiplication parameter C and the initial dislocation density. The term Z)_ff is the effective diffusivity controlling the climb processes at the particle/matrix interfaces.

Note that in the above, the composition dependence arises from the two terms φ_ρ and 5 . Thus, provided that the microstructural architecture is assumed constant (microstructural architecture is mostly controlled by heat treatment) so that φ_ρ is fixed, any dependence upon chemical composition arises through 7)_ff. For the purposes of the alloy design modelling described here, it turns out to be unnecessary to implement a full integration of Equations 2 and 3 for each prototype alloy composition. Instead, a first order merit index /V/ is employed which needs to be maximised, which is given by ^Mcreep=Z^^/^ (⁴) i

where A is the atomic fraction of solute i in the γ phase and is the appropriate interdiffusion coefficient.

The second merit index is for strength merit index. For high nickel-based superalloys, 15 the vast majority of strength comes from the precipitate phase. Therefore, optimising alloy composition for maximal precipitate strengthening is a critical design consideration. From hardening theory a merit index for strength, M_strength, is proposed. The index considers the maximum possible precipitate strength - determined to be the point where the transition from weakly coupled to strongly coupled dislocation shearing occurs - which can be approximated using, /^strength ⁼ (5)

Where M is the Taylor factor, y_APB is the anti-phase boundary (APB) energy, φ_ρ is the volume fraction of the γ' phase and b is the Burgers vector.

From Equation 5 it is apparent that fault energies in the γ' phase - for example, the antiphase boundary APB energy - have a significant influence on the deformation behaviour of nickel-based superalloys. Increasing the APB energy has been found to improve mechanical properties including, tensile strength and resistance to creep deformation. The APB energy was studied for a number of Ni-Al-X systems using density functional theory. From this work the effect of ternary elements on the APB energy of the γ' phase was calculated, linear superposition of the effect for each ternary addition was assumed when considering complex multicomponent systems, resulting in the following equation, γ_ΑΡΒ = 195 — 1.7x_Cr — i.7x_Mo + 4.6%^ + 2ΊΑχ_Τα + 21.4x_Nb -I- 15χ_π (6) where, xq·, xmo, xw, χτα, XNb and xn represent the concentrations, in atomic percent, of chromium, molybdenum, tungsten, tantalum, niobium and titanium in the γ' phase, respectively. The composition of the γ' phase is determined from phase equilibrium calculations.

The third merit index is solid solution merit index. Solid solution hardening occurs in (FCC) matrix phase which is referred to as gamma (γ), in particular this hardening mechanism is important at high temperatures. A model which assumes superposition of individual solute atoms on the strengthening of the matrix phase is employed. The solid solution strengthening coefficients, /q, for the elements considered in the design space: aluminium, cobalt, chromium, molybdenum, niobium, tantalum, titanium and tungsten are 225, 39.4, 337, 1015, 1183, 1191, 775 and 977 MPa/at.%^1/2, respectively. The solid-solution index is calculated based upon the equilibrium composition of the matrix phase using the following equation,

M.

solid-solution (7) where, M_solid__solution is the solid solution merit index and x_t is the concentration of element z in 25 the γ matrix phase.

The fourth merit index is density. The density, p, was calculated using a simple rule of mixtures and a correctional factor, where, pi is the density for a given element and xt is the atomic fraction of the alloy element.

p = 1.05[Σί%ίΡί] (8)

The fifth merit index was cost. In order to estimate the cost of each alloy a simple rule of mixtures was applied, where the weight fraction of the alloy element, x,, was multiplied by the current (2016) raw material cost for the alloying element, c,.

Cost = Σί^Αί (9)

The estimates assume that processing costs are identical for all alloys, i.e. that the product yield is not affected by composition.

The ABD method described above was used to isolate the inventive alloy composition. The design intent for this alloy was to optimise the composition of a conventionally cast nickel10 based superalloy composition for tensile strength and creep resistance equivalent to the MarM246 and Mar-M247 alloy grades. These mechanical properties are achieved in combination with a reduction in alloy cost and an improvement in oxidation/corrosion resistance in comparison to the M246 and Mar-M247 alloy grades.

The material properties - determined using the ABD method - for the typical compositions of conventionally cast nickel-based alloys, listed in Table 1, are listed in Table

3. The design of the new alloy was considered in relation to the predicted properties listed for these alloys.

The rationale for the design of the new alloy is now described.

Table 3: Calculated phase fractions and merit indices made with the “Alloys-by-Design ” software. Results for nickel-based superalloys listed in Table 1.

Alloy	Phase Fractions	Creep Merit Index (m'²s x 10‘¹⁵)	Density (g/cm³)	Cost ($/lb)	γ/γ' Misfit (%)	HTW (°C)	Strength Merit Index (Mpa)	Solid Solution Merit Index (Mpa)
f	σ+μ+Ρ
IN713C	0.54	0.00	2.89	7.97	9.9	-0.40	133	1133	89
IN713LC	0.52	0.00	2.57	8.03	10.0	-0.25	145	1110	84
Mar-M246	0.59	0.03	7.34	8.51	11.9	-0.35	150	1246	90
Mar-M247	0.58	0.00	7.52	8.60	13.1	-0.16	141	1279	85

Optimisation of the alloy’s microstructure - primarily comprised of an austenitic face centre cubic (FCC) gamma phase (γ) and the ordered Lh precipitate phase (γ') - was required to maximise creep resistance. In the case of nickel-based superalloys which require superior resistance to creep deformation, the creep rupture life generally improves as the volume fraction of the precipitate hardening phase γ' is increased. At values above 70% volume fraction of γ' a drop in creep resistance is observed. A volume fraction of γ' of 55% or greater was desired to produce an alloy with creep rupture life greater than that of IN713C and IN713LC, preferably a volume fraction of γ' is greater than or equal to 58% so that creep resistance equivalent to or better than Mar-M246 and Mar-M247 is achieved.

The partitioning coefficient for each element included in the alloy design space was determined from phase equilibrium calculations conducted at 900°C, Figure 1. A partitioning coefficient of unity describes an element with equal preference to partition to the γ or γ' phase. A partitioning coefficient less than unity describes an element which has a preference for the γ' phase, the closer the value to zero the stronger the preference. The greater the value above unity the more an element prefers to reside within the γ phase. The partitioning coefficients for aluminium, tantalum, titanium and niobium show that these are strong γ' forming elements. The elements chromium, molybdenum, cobalt, and tungsten partition preferably to the γ phase. For the elements considered within the alloy design space aluminium, tantalum, titanium and niobium partition most strongly to the γ' phase. Hence, aluminium, tantalum, titanium and niobium levels were controlled to produce the desired γ' volume fraction.

Figure 2 shows the effect which elements added to form the γ' phase - predominantly aluminium, tantalum, titanium and niobium - have on the fraction of γ' phase in the alloy at the operation temperature, 900°C in this instance. The sum of the elements niobium, tantalum and titanium (Nb+Ta+Ti) has been considered as these elements are typically added to substitute for aluminium atoms in the γ' phase, such that the γ' phase is of composition Ni3(Al,Ti,Ta,Nb). The elements niobium, tantalum and titanium increase the anti-phase boundary (APB) energy of the γ' phase (Equation 6) having the technical effect of increasing the overall strengthening provided by the precipitate phase (Equation 5). Increasing the APB energy is beneficial for both tensile strength and creep resistance. For the design of this alloy a volume fraction of γ' between 55-70% was desired. Hence up to 7.6 weight percent (wt.%) of aluminium can be added to produce this volume fraction of γ' phase.

The change in γ' volume fraction was related to the change in aluminium and the sum of the elements niobium, tantalum and titanium content according to the formula

Z(y') = WnMa + Wri) + 3.21E_A1 where, /(γ') is a numerical value which ranges between 19.0 and 24.5 for an alloy with the desired γ' fraction, between 0.55 and 0.70 in this case, and JTni>, JTfa, JTfi and JTai are the weight percent of sum of the elements niobium, tantalum, titanium, and aluminium in the alloy respectively. More preferably /(γ') is a numerical value which is greater than 20.0 as this produces and alloy with the a preferred γ' fraction between 0.58 and 0.70.

Optimisation of aluminium, tantalum, titanium and niobium additions was also required to increase the yield stress of the alloy, predicted by the strength merit index (Equation 5). For turbocharger applications, where the turbine disc is rotating at a high speed and high temperature, a high yield stress is critical to be ensure resistance to disc burst. The current alloy compositions used have a strength merit index of ~1120 MPa for alloys IN713C and IN713LC and -1260 MPa for alloys Mar-M246 and Mar-M247. A minimum strength merit index of 1200MPa was desired such that the alloy will have a strength greater than IN713C and IN713LC. Preferably the target was to design an alloy with a strength merit index of 1250 MPa so that yield stress was equivalent to Mar-M246 and Mar-M247, most preferably the strength merit index should be greater than 1300 MPa so that the yield stress is greater than all currently used alloys.

Figure 3 shows the influence of aluminium and sum of the elements niobium, tantalum and titanium on the strength merit index. Dotted lines - taken from Figure 2 - are also superimposed, these identified the bounding limits for the required volume fraction of γ' (5570%). Modelling calculation showed that for alloys with a volume fraction of γ' between 5570% the sum of the elements niobium, tantalum and titanium must be greater than 2.6 wt.%, and the ratio of sum of the elements niobium, tantalum and titanium to aluminium by weight percent is greater than 0.45 (Nb+Ta+Ti/Al > 0.45), producing an alloy with a strength merit index of at least 1200 MPa. More preferably the sum of the elements niobium, tantalum and titanium must be greater than 3.1 wt.% and the ratio of sum of the elements niobium, tantalum and titanium to aluminium by weight percent is greater than 0.55 (Nb+Ta+Ti/Al > 0.55). Most preferably the sum of the elements niobium, tantalum and titanium must be greater than 3.6 wt.%, and the ratio of sum of the elements niobium, tantalum and titanium to aluminium by weight percent is greater than 0.65 (Nb+Ta+Ti/Al > 0.65), producing an alloy with a strength merit index of 1300 MPa or better.

The minimum ratio for the sum of elements tantalum, titanium and niobium to aluminium of 0.45 by weight percent results in aluminium additions being limited to a maximum of 6.9 wt.% so that the desired γ' volume fraction and strength merit index can be achieved (Figure 3). More preferably aluminium content should be limited to 6.8 wt.% so that a strength merit index of at least 1250 MPa is achieved, even more preferably the aluminium content should be limited to 6.7 wt.% so that a strength merit index of at least 1300 MPa is achieved.

In order to produce an alloy which a good combination of strength and oxidation resistance it is necessary to limit the additions of titanium to less than 2.0 wt.%. This limits the formation of titanium oxides which are not a protective oxide scales and are deleterious to the oxidation performance of the alloy. More preferably it is necessary to limit the additions of titanium to less than 1.8 wt.%. The best combination of strength and oxidation resistance is attained when additions of titanium are limited to 1.6 wt.% or less.

The maximum tantalum and niobium content will be explained below with reference to Figures 5-7. This results in a tantalum range of up to 1.0 wt.%, a preferred range of up to 0.5 wt.%, or a more preferred range of up to 0.1 wt.%. The niobium range is limited to between 0.6 and 3.7 wt.% this results in a desirable combination of cost, strength and creep resistance (dealt with below). From Figure 2 it is seen that when the maximum concentrations of titanium (2 wt.%), tantalum (1.0 wt.%) and niobium (3.7 wt.%) are added, such that the sum of the elements tantalum, titanium and niobium is equal to 6.7 wt.% then a to produce the desired volume fraction of γ' a minimum of 4.4 wt.% aluminium is required. Therefore, an aluminium concentration of between 4.4 and 6.9 wt.% is required to achieve the desired γ' volume fraction. More preferably when the titanium is limited to 1.8% and 1.6% the preferred minimum aluminium content is 4.5 % to produce the desired volume fraction of γ'. Even more preferably when the tantalum content is less the 0.1% the preferred minimum aluminium content is 4.8 wt.% to produce the desired volume fraction of γ'.

As previously described the yield stress and creep resistance of the alloy is increased by controlling the γ' volume fraction and strength merit index. Further improvements in strength can be achieved by adding elements which partition to the face-centered cubic (FCC) matrix phase which is referred to as gamma (γ). The influence of elements on the strength of the γ phase is calculated using the solid solution merit index (SSI). The γ phase of the current invention is primarily composed of the elements, molybdenum, cobalt, chromium and tungsten. Chromium does not strongly effect solid solution strengthening of the γ phase and is added primarily increase the oxidation and corrosion resistance of the alloy. Cobalt does not strongly effect solid solution strengthening of the γ phase but has a beneficial effect upon the creep merit index, described in Figure 15. The elements molybdenum and tungsten were found to most strongly effect the solid solution index.

The effect of molybdenum and tungsten on the solid solution index is described in Figure 4. A minimum target for the solid solution index was 85 MPa, more preferably the minimum target was 90 MPa. The change in solid solution index was related to the change in tungsten and molybdenum content according to the formula /(SSI) = W_w + 2.9W_Mo where, /(SSI) is a numerical value, and JTw and JTmo are the weight percent of tungsten and molybdenum in the alloy respectively. The numerical value for /(SSI) should be greater than or equal to 9.4 in order to produce a value for SSI of at least 85 MPa, equivalent to alloys MarM246 and IN713FC. Preferably the numerical value for/(SSI) is greater than or equal to 11.6 to produce an alloy with a value for SSI of at least 90MPa, equivalent to alloys IN713C and Mar-M246.

The current (2016) raw material cost for the element tantalum is substantial higher than other elements in the invention and has the most significant effect on alloy cost. The element niobium is also expensive, but substantially lower cost than tantalum. Niobium has the same technical effect as tantalum, as determined by calculations for the strength merit index and γ' volume fraction; thus, preference for niobium over tantalum produces an improved balance of strength and cost. The elements cobalt, tungsten and molybdenum are of approximately similar cost, however, they are still more costly than nickel and thus increase alloy cost. The elements aluminium, titanium and chromium do not have the effect of increasing alloy cost.

Figures 5-7 presents the effect of elements tantalum, niobium and the sum of the elements cobalt, tungsten and molybdenum on alloy cost. The target for the invention was to have a cost of 11 $/lb, which is substantially lower than the Mar-M247 alloy and lower than Mar-M246 alloy. Preferably a cost which is less than 10.5 $/lb is desired, more preferably a cost of 10 $/lb is desired as this is equivalent to IN713C and IN713FC. Tantalum has the strongest influence on alloy cost, Figures 6-7. When tantalum is at 2 wt.% the required cost target cannot be satisfied (Figure 7), therefore tantalum is required to be less than 2 wt.%.

If the minimum required sum of the elements cobalt, tungsten and molybdenum is greater than or equal to 11.2 wt.% (Co+Mo+W >11.2 wt.%) an acceptable creep resistance is achieved; explained below with reference to Figures 15. Further improvements in creep are attained when the sum of the elements cobalt, tungsten and molybdenum is increased beyond 11.2 wt.%, most preferably the sum of the elements cobalt, tungsten and molybdenum is greater than 19.8 wt.% as this produces and alloy with a creep merit index equal to or better than MarM246 and Mar-M247. When the level of the tantalum in the alloy is at 1 wt.%, (Figure 6) the maximum concentration of the sum of the elements cobalt, tungsten and molybdenum is limited to 17.1 wt.% to meet the alloy’s cost target. When the percentage of tantalum is reduced to less than 1.0% more preferably less than 0.1 wt.% an improved balance of creep resistance and alloy cost is attained, Figure 5.

If the tantalum concentration in the alloy is limited to 0.1 wt.% the resulting minimum niobium concentration must be 0.6 wt.%, if (Nb+Ta+Ti > 2.6 wt.%) is to be satisfied. To achieve the cost target of less than or equal to 11 $/lb at a niobium concentration of 0.6 wt.% the sum of the elements cobalt, tungsten and molybdenum must be less than or equal to 26.6 wt.%. More preferably the sum of the cobalt, tungsten and molybdenum must be less than or equal to 20. lwt.% to produce an alloy with a cost lower than 10.5 $/lb, even more preferably less than or equal to 12.6 wt.% to produce an alloy with a cost lower than 10.0 $/lb.

Iron behaves in a similar way to nickel and can be added as a low cost alternative to nickel. Moreover tolerance to iron additions improves the ability of the alloy to be manufactured from recycled materials. Therefore, it is preferred that iron is present in an amount of at least 0.1 wt.%. However, additions of iron up to 10.0 wt.% can be made in order to substantially reduce the cost. Preferably the additions of iron are less than 8.0 wt.% in order to reduce the propensity to form the unwanted Laves phase which degrades the mechanical properties of the alloy. Most preferably iron additions are limited to 1 wt.% as this produces an alloy which has good ability to be recycled with no loss in material performance.

The addition of molybdenum, tungsten and chromium were found to increase the propensity to form unwanted TCP phases (Figure 8-14); primarily σ, P and μ phases. Additions of molybdenum and tungsten are necessary for both solid solution strengthening (Figure 4) as well as creep resistance (Figure 15). In combination with improved solid solution strengthening and creep resistance a high level of resistance to corrosion/oxidation is required. Improvements in oxidation and corrosion resistance come from additions of chromium. Thus a complex trade-off between mechanical performance, oxidation/corrosion resistance and microstructural stability must be managed. The alloy of this invention requires a chromium content of greater than 9.1 wt.% ensuring that oxidation/corrosion is better than Mar-M246 and Mar-M247. More preferably the chromium content is greater than 10.1 wt.% as this provides even better oxidation/corrosion resistance. It is desired that the new alloy contains less than 1% volume fraction of TCP phases at equilibrium at 900°C, ensuring that the alloy is microstructurally stable.

Figures 8-14 shows the effect of tungsten and chromium additions on the overall fraction of TCP phases (σ+μ+Ρ) for alloys containing different levels of molybdenum at equilibrium at 900°C. On each figure the minimum required tungsten content required to satisfy the constraint for solid solution strengthening, /(SSI), is delineated. It is seen that increasing the concentration of molybdenum limits the maximum concentration of chromium and tungsten if the alloy is to meet the requirements for limited TCP formation.

For alloys containing greater than 6 wt.% of molybdenum (Figure 14) an alloy with the minimum desired chromium level is difficult to achieve. Additions of molybdenum of 5 wt.% makes it difficult to achieve the required XSSI). When the molybdenum level is at 4 wt.% (Figure 12) the required /(SSI) can be achieved and a chromium content of up to 11 wt.% can be achieved, providing good balance of oxidation/corrosion resistance, solid solution strengthening and creep resistance. Therefore, molybdenum additions are limited to 4 wt.%.

Figures 8-13 allow the following observations to be made, based on a maximum of 0.01 phase fraction of TCP phase. If the alloy does not contain molybdenum (Figure 8) the chromium content is limited to 10.9 wt.%, it is also difficult to achieve the preferred value of /(SSI) at a chromium content of 9.1 wt.% or greater. When the alloy contains 1 wt.% of molybdenum (Figure 9) the maximum chromium content is limited to 11.9 wt.% for a value of /(SSI)=9.4, similar maximal chromium content is achieved for molybdenum contents of 2 wt.% and 3 wt.% (Figure 10-11), and the maximal chromium content reduces to 11.0 wt.% at 4 wt.% molybdenum (Figure 12). For the best solid solution strengthening (i.e./(SSI)>11.6) the maximal chromium content increases with increasing molybdenum content up to 3 wt.% molybdenum (Figure 9-12). Therefore a minimum molybdenum content is 0.1wt.%, preferably 0.3wt.% or 0.5 wt.%. At a molybdenum content of 1.0 wt.% the maximal chromium content when/(SSI)>11.6 is 10.1 wt.% this is preferred minimum chromium content. Therefore it is preferred that the molybdenum content ranges between 1.0 wt.% and 3.0 wt.%, as the best balance of solid solution strengthening and oxidation/corrosion resistance is attained.

Based upon the previous description it is found that the chromium content of the alloy is limited between 9.1 wt.% and 11.9 wt.%, more preferably between 10.1 wt.% and 11.9 wt.%. Most preferably it is limited between 10.1 wt.% and 11.0 wt.% as this produces the best balance of microstructural stability, solid solution strengthening and oxidation corrosion resistance.

The maximum allowable tungsten content is 10.9 wt.%, based upon the minimum level of chromium (9.1 wt.%) required (Figure 8). At the preferred upper limit of molybdenum of 3 wt.% it is desirable to include a minimum of 2.9 wt.% tungsten. This even allows for the preferred value for solid solution strengthening (f(SSI)> 11.6 ) to be achieved. In any case, an alloy with 2.9% or more tungsten will have improved solid solution strengthening and so this minimum level of tungsten is advantageous. Therefore a tungsten content between 2.9 wt.% and 10.6 wt.% is desired. However, the alloy may contain no tungsten, particularly at high levels of molybdenum, where a /(SSI) of 11.6 can be achieved with molybdenum at 4.0% alone. It is desirable to maintain the sum of the elements molybdenum and tungsten below 10.6 wt.% to provide the required level of microstructural stability for the minimum chromium content of 9.1 wt.% (Figures 8-14). More preferably the sum of molybdenum and tungsten should remain below 9.9 wt.% such that the preferred chromium content of greater than 10.1 wt.% can be achieved (Figure 8).

For the alloys which satisfied the previously described requirements it was necessary to optimise the levels of refractory elements for maximum creep resistance. The creep resistance was determined by using the creep merit index model. The influence which the sum of the elements molybdenum and tungsten and the additions of cobalt have on creep resistance is presented in Figure 15. It is desirable to maximise the creep merit index as this is associated with an improved creep resistance. It is seen that increasing the levels of the sum of molybdenum and tungsten and the additions of cobalt will improve creep resistance.

A creep merit index of 5.0xl0¹⁵ m²s or greater was desired to produce an alloy with creep resistance substantially better than that of IN713C and IN713FC (see Table 3). More preferably a creep merit index of 7.0xl0¹⁵ m²s is desired to produce alloys with creep performance which is equivalent to Mar-M246 and Mar-M247. Even more preferably a creep merit index of 7.5xl0¹⁵ m²s is desired to produce alloys with a creep resistance better than Mar-M246 and Mar-M247

The minimum concentration of the sum of the elements cobalt, molybdenum and tungsten in order to produce an alloy with a creep merit index of 5.0xl0¹⁵ m²s or greater is greater than 11.2 wt.% (Figure 15). The sum of the elements molybdenum and tungsten is desirably limited to 10.6 wt.%. Thus the alloy contains at least 0.6 wt.% cobalt. In order to achieve the alloy cost target of less than 11 $/lb the sum of the elements cobalt, tungsten and molybdenum is desirably less than or equal to 26.6 wt.%(Figure 5). The minimum sum of the elements molybdenum and tungsten is limited to 3.2 wt.%, more preferably 4.0 wt.%. Thus the maximum cobalt concentration is limited to 23.4 wt.%, more preferably 22.6 wt.% as this produces an alloy with a better balance of cost, solid solution strengthening and creep resistance.

For the best creep resistance the sum of cobalt, molybdenum and tungsten should be greater than 18.1 wt.% in order to produce an alloy with a creep merit index of 7.0xl0¹⁵ m²s equivalent to Mar-M246 and Mar-M247. Most preferably the sum of cobalt, molybdenum and tungsten must be greater than 19.8 wt.% to produce an alloy with a creep merit index of 7.5x10 ¹⁵ m'²s, better than Mar-M246 and Mar-M247. Thus, it is preferred that cobalt is greater than 7.5 wt.% and even more preferably 9.2 wt.% in which case the maximum content of molybdenum and tungsten is limited to 10.6 wt.%.

Additions of carbon, boron and zirconium are required in order to provide strength to grain boundaries. This is particularly beneficial for the creep and fatigue properties of the alloy. The carbon concentrations should range between 0.02 wt.% and 0.35 wt.%. The boron concentration should range between 0.001 and 0.2 wt.%. The zirconium concentrations should range between 0.001 wt.% and 0.5 wt.%.

It is beneficial that when the alloy is produced, it is substantially free from incidental impurities. These impurities may include the elements sulphur (S), manganese (Mn) and copper (Cu). The element sulphur preferably remains below 0.003 wt.% (30 PPM in terms of mass). Manganese is an incidental impurity which is preferably limited to 0.25 wt.%. Copper (Cu) is an incidental impurity which is preferably limited to 0.5 wt.%. The presence of Sulphur above 0.003 wt.% can lead to embrittlement of the alloy and sulphur also segregates to alloy/oxide interfaces formed during oxidation. This segregation may lead to increased spallation of protective oxide scales. If the concentrations of these incidental impurities exceed the specified levels, issues surrounding product yield and deterioration of the material properties of the alloy is expected.

Additions of hafnium (Hf) of up to 0.5wt.%, or more preferably up to 0.2wt.% are beneficial for tying up incidental impurities in the alloy and also for providing strength. Hafnium is a strong carbide former it can provide additional grain boundary strengthening.

Additions of the so called ‘reactive-elements’, Yttrium(Y), Lanthanum (La) and 10 Cerium (Ce) may be beneficial up to levels of 0.1 wt.% to improve the adhesion of protective oxide layers, such as AI2O3. These reactive elements can ‘mop-up’ tramp elements, for example sulphur, which segregates to the alloy oxide interface weakening the bond between oxide and substrate leading to oxide spallation. Additions of Silicon (Si) up to 0.5 wt.% may be beneficial, it has been shown that additions of silicon to nickel based superalloys at levels up to 0.5 wt.% are beneficial for oxidation properties. In particular silicon segregates to the alloy/oxide interface and improves cohesion of the oxide to the substrate. This reduces spallation of the oxide, hence, improving oxidation resistance.

Based upon the description of the invention presented in this section the broad range for the invention is listed in Table 4.

Table 4: Compositional range in wt. % for the newly design alloy.

Alloy (wt.%)	Al	Co	Cr	Mo	Nb	Ta	Ti	w
Min	4.4	0.6	9.1	0.1	0.6	0.0	0.0	0.0
Max	6.9	23.4	11.9	4.0	3.7	1.0	2.0	10.9

Table 5 describes example compositions for from the present invention. The calculated properties for these new alloys are compared with the currently used alloys in Table 6. The rationale for the design of these alloys is now described.

The alloys of Examples 1-5 are designed to provide the lowest overall cost, with each alloy having a cost Equivalent to that of IN713C and IN713LC. The alloys have much higher value for strength merit index than Mar-M246 and Mar-M247, as well as a higher volume fraction of γ', this provides good high temperature mechanical behaviour. The alloys have been designed for low cost at the expense of creep resistance. The chromium levels are much higher than Mar-M246 and Mar-M247 providing much better oxidation/corrosion resistance.

The alloys of Examples 6-10 are designed to provide a balance of cost and creep resistance. The creep resistance is substantially improved in comparison to alloy Examples 15 5 at the expense of increased cost and a lowering in maximum chromium levels which decreased oxidation/corrosion behaviour. However, the alloys are still substantially lower cost than Mar-M246 and Mar-M247. Moreover the chromium levels are higher than Mar-M246 and Mar-M247. As with Examples 1-5 the alloys have much higher value for strength merit index than Mar-M246 and Mar-M247, as well as a higher volume fraction of γ', this provides good high temperature mechanical behaviour.

Table 5: Nominal compositions in wt. % of the newly designed conventionally cast nickel-based superalloys compared with the alloys listed in Table 1.

Alloy (wt.%)	Al	Co	Cr	Mo	Nb	Ta	Ti	W	c	B	Zr	Hf
IN713C	6.0	0.0	13.5	4.5	2.0	0.0	0.8	0.0	0.10	0.010	0.06	0.00
IN713LC	5.8	0.0	12.0	4.3	2.0	0.0	0.7	0.0	0.06	0.010	0.06	0.00
Mar-M246	5.5	10.0	9.0	2.5	0.0	1.5	1.5	10.0	0.15	0.010	0.05	0.00
Mar-M247	5.5	10.0	8.2	0.6	0.0	3.0	1.0	10.0	0.16	0.015	0.05	1.50

Example 1	6.2	7.0	10.0	2.0	1.0	2.0	4.0	0.10	0.015	0.06
Example 2	6.2	6.0	10.5	2.0	1.0	2.0	4.5	0.10	0.015	0.06
Example 3	6.0	5.5	11.5	1.5	1.0	2.0	5.5	0.10	0.015	0.06
Example 4	6.2	6.0	11.0	2.0	1.0	2.0	4.5	0.10	0.015	0.06
Example 5	6.0	6.0	11.9	2.0	1.3	2.0	4.0	0.10	0.015	0.06

Example 6	6.2	12.0	11.1	2.5	2.2	1.2	4.5	0.10	0.015	0.06
Example 7	6.0	11.0	10.1	2.0	1.0	2.0	6.0	0.10	0.015	0.06
Example 8	6.0	11.0	10.1	2.0	1.3	1.8	6.0	0.10	0.015	0.06
Example 9	6.0	12.0	10.5	3.0	1.8	1.6	4.0	0.10	0.015	0.06
Example 10	5.8	12.0	11.5	2.5	1.3	2.0	4.5	0.10	0.015	0.06

Example 11	5.4	13.0	10.1	2.0	2.5	1.5	6.5	0.10	0.015	0.06
Example 12	5.6	13.0	10.1	2.5	3.0	1.2	5.5	0.10	0.015	0.06
Example 13	5.8	14.0	10.1	2.5	1.5	1.8	5.0	0.10	0.015	0.06
Example 14	5.8	13.0	10.1	2.0	2.0	1.5	6.0	0.10	0.015	0.06
Example 15	5.6	13.0	10.1	2.5	1.5	1.8	6.0	0.10	0.015	0.06

The alloys of Examples 11-15 are designed to provide the highest levels creep 15 resistance, substantially better than Mar-M246 and Mar-M247. The creep resistance is substantially improved in comparison to alloy Examples 1-10 at the expense of increased cost and a lowering in maximum chromium levels which decreases oxidation/corrosion behaviour. However, the alloys are still substantially lower cost than Mar-M246 and Mar-M247 and the chromium levels are higher than Mar-M246 and Mar-M247. As with Examples 1-10 the alloys have much higher value for strength merit index than Mar-M246 and Mar-M247, as well as a higher volume fraction of γ', this provides good high temperature mechanical behaviour.

Table 6: Calculated phase fractions and merit indices made with the “Alloys-by-Design ” software. Results for nickel-based superalloys usedfor producing a turbine wheel within an exhaust gas turbocharger device Table 1 and the nominal composition of the new alloys 10 listed in Table 5.

Alloy	Phase Fractions	Creep Merit Index (m²s x 10¹⁵)	Density (g/cm³)	Cost ($/lb)	γ/γ- Misfit (%)	HTW (°C)	Strength Merit Index (Mpa)	Solid Soluiton Merit Index (Mpa)
γ’	σ+μ+Ρ
IN713C	0.54	0.00	2.89	7.97	9.9	-0.40%	133	1133	89
IN713LC	0.52	0.00	2.57	8.03	10.0	-0.25%	145	1110	84
Mar-M246	0.59	0.03	7.34	8.51	11.9	-0.35%	150	1246	90
Mar-M247	0.58	0.00	7.52	8.60	13.1	-0.16%	141	1279	85

Example 1	0.65	5.01	8.04	10.1	-0.24%	103	1300	84
Example 2	0.66	5.23	8.05	10.1	-0.30%	103	1304	85
Example 3	0.63	5.43	8.10	10.1	-0.29%	114	1302	84
Example 4	0.65	5.28	8.05	10.1	-0.33%	105	1301	85
Example 5	0.62	5.18	8.04	10.1	-0.30%	110	1308	85

Example 6	0.65	7.11	8.07	10.6	-0.39%	96	1331	89
Example 7	0.64	7.06	8.15	10.5	-0.35%	110	1313	88
Example 8	0.63	7.02	8.17	10.6	-0.33%	109	1300	87
Example 9	0.62	7.02	8.07	10.6	-0.41%	106	1312	91
Example 10	0.59	7.02	8.10	10.6	-0.37%	116	1305	89

Example 11	0.58	7.76	8.28	11.0	-0.25%	112	1360	90
Example 12	0.59	7.51	8.22	10.9	-0.31%	112	1329	90
Example 13	0.59	7.57	8.17	10.9	-0.32%	113	1303	88
Example 14	0.61	7.60	8.22	11.0	-0.29%	103	1334	88
Example 15	0.58	7.62	8.24	10.9	-0.34%	119	1306	91

Claims

Claims

1. A nickel-based alloy composition consisting, in weight percent, of: between 4.4% and 6.9% aluminium, between 0.6% and 23.4% cobalt, between 9.1% and 11.9% chromium, between 0.1% and 4.0% molybdenum, between 0.6% and 3.7% niobium, between 0.0 and 1.0%

5 tantalum, between 0.0% and 2.0% titanium, between 0.0% and 10.9% tungsten, between 0.02 wt.% and 0.35 wt.% carbon, between 0.001 and 0.2 wt.% boron, between 0.001 wt.% and 0.5 wt.%. zirconium, between 0.0 and 0.5% silicon, between 0.0 and 0.1% yttrium, between 0.0 and 0.1% lanthanum, between 0.0 and 0.1% cerium, between 0.0 and 0.003% sulphur, between 0.0 and 0.25% manganese, between 0.0 and 0. 5% copper, between 0.0 and 0.5% hafnium,

10 between 0.0 and 0.5% vanadium, between 0.0 and 10.0% iron, the balance being nickel and incidental impurities.
2. The nickel-based alloy composition according to claim 1, wherein the following equation is satisfied in which WNb, Wt₃, Wti and Wai are the weight percent of niobium,

15 tantalum, titanium and aluminium in the alloy respectively

19 < (W_Nb+lT_ra + W_Ti) + 3.2Vk_A1 < 24.5 preferably 20 < (W_Nb+lT_ra + W_Ti) + 3.2IV_A1 < 24.5
3. The nickel-based alloy composition according to claim 1 or 2, wherein the following 20 equation is satisfied in which Ww and Wmo are the weight percent of tungsten and molybdenum in the alloy respectively

9.4 < W_w + 2.9Vk_Mopreferably 11.6 < Vk_w + 2.9Vk_Mo
4. The nickel-based alloy composition of any of claims 1-3 consisting of, in weight percent, of 10.1% or more chromium.
5. The nickel-based alloy composition of any of claims 1-4 consisting of, in weight percent, of 11.0% or less chromium.
6. The nickel-based alloy composition of any of claims 1-5 consisting of, in weight 5 percent, of 0.3% or more molybdenum, preferably 0.5% or more molybdenum, more preferably

1.0% or more molybdenum.
7. The nickel-based alloy composition of any of claims 1-1-6 consisting of, in weight percent, of 3.0% or less molybdenum.
8. The nickel-based alloy composition of any of claims 1-7 consisting of, in weight percent, of 1.8% or less titanium, preferably 1.6% or less titanium.
9. The nickel-based alloy composition of any of claims 1-8 consisting of, in weight 15 percent, of 22.6% or less cobalt.
10. The nickel-based alloy composition of any of claims 1-9 consisting of, in weight percent, of 7.5% or more cobalt, preferably 9.2% or more cobalt.

20 11. The nickel-based alloy composition of any of claims 1-10 consisting of, in weight percent, of 0.2% or less hafnium.

12. The nickel-based alloy composition of any of claims 1-11 consisting of, in weight percent, of 2.9% or more tungsten.

13. The nickel-based alloy composition of any of claims 1-12 consisting, in weight percent, of 0.5% or less tantalum, preferably of 0.1% or less tantalum.

14. The nickel-based alloy composition of any of claims 1-13 consisting, in weight percent, of 4.5% or more aluminium, preferably of 4.8% or more aluminium.

5 15. The nickel-based alloy composition of any of claims 1-14, wherein the sum of elements cobalt, tungsten and molybdenum, in weight percent, is 11.2% or greater, preferably 18.1% or greater, more preferably 19.8% or greater.

16. The nickel-based alloy composition of any of claims 1-15, wherein the sum of elements 10 cobalt, tungsten and molybdenum, in weight percent, is 26.6% or less, preferably 20.1% or less, more preferably 17.1% or less and most preferably 12.6% or less.

17. The nickel-based alloy composition of any of claims 1-16 consisting of, in weight percent, of 0.1% or more iron.

18. The nickel-based alloy composition of any of claims 1-17 consisting of, in weight percent, of 8.0% or less iron, preferably 1.0% or less iron.

19. The nickel-based alloy composition of any of claims 1-18, wherein the sum of elements 20 molybdenum and tungsten, in weight percent, is 10.6% or less, preferably 9.9% or less.

20. The nickel-based alloy composition of any of claims 1-19, wherein the sum of elements molybdenum and tungsten, in weight percent, is 3.2% or more, preferably 3.6% or more, more preferably 4.0% or more.

21. The nickel-based alloy composition of any of claims 1-20, consisting of, in weight percent, of 6.8% or less aluminium, preferably 6.7% or less aluminium.

22. The nickel-based alloy composition of any of claims 1-21, wherein the following equation is satisfied in which Wni>, Wt₃ and Wn are the weight percent of niobium, tantalum and titanium in the alloy respectively

5 W_Nb + W_Ta + W_Ti > 2.6, preferably WNb + W-r_a + Wn >3.1 more preferably WNb _ WTa + Wti >3.2 most preferably WNb _ Wt₃ + Wti > 3.6.

23. The nickel-based alloy composition of any of claims 1-22, wherein the ratio of the sum 10 of the elements niobium, tantalum and titanium to aluminium by weight percent is greater than

0.45, preferably greater than 0.55, most preferably greater than 0.65.

24. The nickel-based alloy composition of any of claims 1-23, having between 55% and 70% volume fraction γ’, preferably between 58% and 70% volume fraction γ’.

25. A cast article formed of the nickel-based alloy composition of any of claims 1-24.

26. A turbine wheel formed of the nickel-based alloy composition of any of claims 1-24.

20 27. An exhaust gas turbocharger device comprising a turbine wheel according to claim 26.

06 03 18

Amendment to the claims have been filed as follows

Claims

1. A nickel-based alloy composition consisting, in weight percent, of: between 4.4% and 6.9% aluminium, between 0.6% and 23.4% cobalt, between 10.1% and 11.9% chromium, between 0.1% and 4.0% molybdenum, between 0.6% and 3.7% niobium, between 0.0 and 1.0%

5 tantalum, between 0.0% and 2.0% titanium, between 0.0% and 10.9% tungsten, between 0.02 wt.% and 0.35 wt.% carbon, between 0.001 and 0.2 wt.% boron, between 0.001 wt.% and 0.5 wt.%. zirconium, between 0.0 and 0.5% silicon, between 0.0 and 0.1% yttrium, between 0.0 and 0.1% lanthanum, between 0.0 and 0.1% cerium, between 0.0 and 0.003% sulphur, between 0.0 and 0.25% manganese, between 0.0 and 0. 5% copper, between 0.0 and 0.5% hafnium,

10 between 0.0 and 0.5% vanadium, between 0.0 and 10.0% iron, the balance being nickel and incidental impurities.

2. The nickel-based alloy composition according to claim 1, wherein the following equation is satisfied in which Wni>, Wt₃, Wti and Wai are the weight percent of niobium,

15 tantalum, titanium and aluminium in the alloy respectively

19 < (W_Nb+W_Ta + Wn) + 3.2VK_A1 < 24.5 preferably 20 < (W_Nb+W_Ta + W_Ti) + 3.214^ < 24.5

3. The nickel-based alloy composition according to claim 1 or 2, wherein the following

20 equation is satisfied in which Ww and Wmo are the weight percent of tungsten and molybdenum in the alloy respectively

9.4 < W_w + 2.9W_Mopreferably 11.6 < Vf_w + 2.9Vf_Mo

25 4. The nickel-based alloy composition of any of claims 1-3 consisting of, in weight percent, of 11.0% or less chromium.

5. The nickel-based alloy composition of any of claims 1-4 consisting of, in weight percent of 0.3% or more molybdenum.

6. The nickel-based alloy composition of any of claims 1-5 consisting of, in weight percent, of 0.5% or more molybdenum, preferably 1.0% or more molybdenum.

06 03 18

5 7. The nickel-based alloy composition of any of claims 1-1-6 consisting of, in weight percent, of 3.0% or less molybdenum.

8. The nickel-based alloy composition of any of claims 1-7 consisting of, in weight percent, of 1.8% or less titanium, preferably 1.6% or less titanium.

9. The nickel-based alloy composition of any of claims 1-8 consisting of, in weight percent, of 22.6% or less cobalt.

10. The nickel-based alloy composition of any of claims 1-9 consisting of, in weight 15 percent, of 7.5% or more cobalt, preferably 9.2% or more cobalt.
11. The nickel-based alloy composition of any of claims 1-10 consisting of, in weight percent, of 0.2% or less hafnium.

20
12. The nickel-based alloy composition of any of claims 1-11 consisting of, in weight percent, of 2.9% or more tungsten.
13. The nickel-based alloy composition of any of claims 1-12 consisting, in weight percent, of 0.5% or less tantalum, preferably of 0.1% or less tantalum.
14. The nickel-based alloy composition of any of claims 1-13 consisting, in weight percent, of 4.5% or more aluminium, preferably of 4.8% or more aluminium.

5
15. The nickel-based alloy composition of any of claims 1-14, wherein the sum of elements cobalt, tungsten and molybdenum, in weight percent, is 11.2% or greater, preferably 18.1% or greater, more preferably 19.8% or greater.
16. The nickel-based alloy composition of any of claims 1-15, wherein the sum of elements 10 cobalt, tungsten and molybdenum, in weight percent, is 26.6% or less, preferably 20.1% or less, more preferably 17.1% or less and most preferably 12.6% or less.
17. The nickel-based alloy composition of any of claims 1-16 consisting of, in weight percent, of 0.1% or more iron.
18. The nickel-based alloy composition of any of claims 1-17 consisting of, in weight percent, of 8.0% or less iron, preferably 1.0% or less iron.
19. The nickel-based alloy composition of any of claims 1-18, wherein the sum of elements 20 molybdenum and tungsten, in weight percent, is 10.6% or less, preferably 9.9% or less.
20. The nickel-based alloy composition of any of claims 1-19, wherein the sum of elements molybdenum and tungsten, in weight percent, is 3.2% or more, preferably 3.6% or more, more preferably 4.0% or more.
21. The nickel-based alloy composition of any of claims 1-20, consisting of, in weight percent, of 6.8% or less aluminium, preferably 6.7% or less aluminium.
22. The nickel-based alloy composition of any of claims 1-21, wherein the following equation is satisfied in which Wni>, Wt₃ and Wti are the weight percent of niobium, tantalum and titanium in the alloy respectively

5 W_Nb + W_Ta + Wti > 2.6, preferably WNb + Wia + Wti >3.1 more preferably WNb _ WTa + Wti >3.2 most preferably WNb _ Wt_;, + Wti > 3.6.
23. The nickel-based alloy composition of any of claims 1-22, wherein the ratio of the sum 10 of the elements niobium, tantalum and titanium to aluminium by weight percent is greater than

0.45, preferably greater than 0.55, most preferably greater than 0.65.
24. The nickel-based alloy composition of any of claims 1-23, having between 55% and 70% volume fraction γ’, preferably between 58% and 70% volume fraction γ’.
25. A cast article formed of the nickel-based alloy composition of any of claims 1-24.
26. A turbine wheel formed of the nickel-based alloy composition of any of claims 1-24.

20 27. An exhaust gas turbocharger device comprising a turbine wheel according to claim 26.

Intellectual

Property

Office

Matthew Lawson

23 March 2017

GB1617326.2

1-27