Nickel-based alloy and turbine wheel incorportating same

Abstract

Turbocharger turbine wheels including nickel-based alloys are disclosed herein. In one exemplary embodiment, a turbocharger turbine wheel includes as, at least part of its constituency, nickel-based alloy composition consisting, on a weight basis, of: 11.0% to 13.5% chromium, 5.2% to 8.5% iron, 6.0% to 7.2% aluminium, 3.0% to 4.4% molybdenum, 1.15% to 2.00% titanium, 0.1% to 0.2% carbon, 0.03% to 0.07% zirconium, 0.01% to 0.02% boron, up to 2.0% niobium, up to 0.2% silicon, up to 0.03% copper, up to 0.005% phosphorous, up to 0.02% manganese, up to 0.003% sulphur, with the balance being nickel and incidental / inevitable / unavoidable impurities. The turbocharger turbine wheel may be configured for operating at 770 °C to 850 °C.

Description

[0001] The present disclosure is generally directed to metal alloys with improved high-temperature oxidation and fatigue resistance for use in turbocharger applications. More particularly, the present disclosure is directed to nickel-based alloys and turbine wheels including nickel-based alloys.BACKGROUND

[0002] Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine wheel to spin within the housing. The exhaust gas-driven turbine wheel is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft and housed in a compressor housing. Thus, rotary action of the turbine wheel also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the turbine housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.

[0003] The turbine wheel, usually a cast nickel-based alloy, used in turbochargers is a highspeed rotating component that is exposed to high temperature combustion gasses between temperatures of about 750°C to about 1050°C, depending upon the application. To withstand such a high temperature, the nickel-based alloy material should be oxidation resistant, creep resistant as well as thermo-mechanical fatigue resistant while rotating in the approximately 100,000 - 300,000 RPM range. Existing alloys for such applications include the Mar-M246, Mar-M247, IN713C, nickel-based alloys. The Mar-M246 and Mar-M247 alloys can be used at high operating temperatures of 950 °C - 1050°C but are expensive. The IN713C alloy is cheaper but cannot be used at such high temperatures. It is typically used where the operating temperature does not exceed 900 - 950°C.

[0004] GMR-235 was developed for producing cast turbine buckets and has a higher iron content and lower quantities of particularly expensive alloying elements such as niobium and tantalum than the other mentioned alloys, and is therefore cheaper again. GMR-235D is a modified version of the original GMR-235 alloy, designed to have improved rupture life over GMR-235 when vacuum melted. Neither of these alloys are, however, suitable to be used at temperatures above 770°C, and both have inferior properties (both mechanical properties and oxidation resistance) to IN713C at the top end of their operating temperature range. In particular, their high cycle fatigue resistance is too low for these alloys to be good candidates for use in turbine wheels for turbochargers.

[0005] Therefore, it will become apparent to those skilled in the art that there remains a present and continuing need for the provision of improved nickel-based alloys and turbine wheels that include such nickel-based alloys. Particularly, it would be desirable to provide an alloy suitable for use at temperatures up to 850°C based on IN713C but with an improved chemistry that reduces costs significantly while still retaining the desirable high-temperature oxidation and corrosion, creep and thermo-mechanical fatigue resistance, for example, of IN713C. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.BRIEF SUMMARY

[0006] Nickel-based alloys and turbocharger turbine wheels including nickel-based alloys are disclosed herein. In one exemplary embodiment, a nickel-based alloy consists of, on a weight basis of the overall alloy: 11.0% to 13.5% chromium, 5.2% to 8.5% iron, 6.0% to 7.2% aluminium, 3.0% to 4.4% molybdenum, 1.15% to 2.00% titanium, 0.1% to 0.2% carbon, 0.03% to 0.07% zirconium, 0.01% to 0.02% boron, 0% to 2.0% niobium, 0% to 0.2% silicon, 0% to 0.03% copper 0% to 0.02% manganese, 0% to 0.005% phosphorous 0% to 0.003% sulphur, with the balance being nickel and incidental / inevitable / unavoidable impurities.

[0007] With regard to the foregoing alloy embodiments: the amount of chromium may be limited to 11.0 wt% to 12.0 wt%; alternatively or additionally, the amount of iron may be limited to 7.5 wt% to 8.5 wt%; alternatively or additionally, the amount of aluminium may be limited to 6.0 wt% to 7.0 wt%; alternatively or additionally, the amount of molybdenum may be limited to 3.0 wt% to 4.0 wt%; alternatively or additionally, the amount of titanium may be limited to 1.35 wt% to 1.85 wt%; alternatively or additionally, the amount of niobium may be limited to 1.0 wt% to 2.0 wt%.

[0008] In another exemplary embodiment, a turbocharger turbine wheel includes or consists of, at least as a part of its overall composition, a nickel-based alloy, wherein the nickel-based alloy consists of, on a weight basis of the overall alloy: 11.0% to 13.5% chromium, 5.2% to 8.5% iron, 6.0% to 7.2% aluminium, 3.0% to 4.4% molybdenum, 1.15% to 2.00% titanium, 0.1% to 0.2% carbon, 0.03% to 0.07% zirconium, 0.01% to 0.02% boron, 0% to 2.0% niobium, 0% to 0.2% silicon, 0% to 0.03% copper 0% to 0.02% manganese, 0% to 0.005% phosphorous 0% to 0.003% sulphur, with the balance being nickel and incidental / inevitable / unavoidable impurities.

[0009] With regard to the foregoing turbine wheel embodiments: the amount of chromium may be limited to 11.0 wt% to 12.0 wt%; alternatively or additionally, the amount of iron may be limited to 7.5 wt% to 8.5 wt%; alternatively or additionally, the amount of aluminium may be limited to 6.0 wt% to 7.0 wt%; alternatively or additionally, the amount of molybdenum may be limited to 3.0 wt% to 4.0 wt%; alternatively or additionally, the amount of titanium may be limited to 1.35 wt% to 1.85 wt%; alternatively or additionally, the amount of niobium may be limited to 1.0 wt% to 2.0 wt%.

[0010] This brief summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description and the drawings. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.BRIEF DESCRIPTION OF THE DRAWING

[0011] The various embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: FIGURE 1 illustrates a system view of an exemplary internal combustion engine including a turbocharger having a turbine wheel in accordance with some embodiments of the present disclosure; FIGURE 2 illustrates a graphical representation of the materials phase volume fractions across temperature for an alloy composition falling within the prior art GMR-235 composition range calculated using Thermo-Calc ®< ; FIGURE 3 illustrates a graphical representation of the materials phase volume fractions across temperature for an alloy composition falling within the inventive Alloy A composition range (designated G235+A) calculated using Thermo-Calc ®< ; FIGURE 4 illustrates a prior art plot of volume fraction γ' phase versus rupture life for two prior art Ni-based superalloys - IN-713C and single crystal TMS-75; FIGURE 5 shows micrographs of the as cast alloy with the GMR-235 composition; FIGURE 6 shows micrographs of an as cast alloy with the G235+A composition; FIGURE 7 illustrates a plot of time versus mass change at 850°C for the GMR-235 and G235+A alloys; FIGURE 8 illustrates a plot of temperature versus Cr content of the γ phase in the GMR-235 and G235+A alloys; FIGURE 9 illustrates a plot of temperature versus Young's modulus for the GMR-235 and G235+A alloys; FIGURE 10 illustrates a plot of temperature versus yield stress for the GMR-235 and G235+A alloys; FIGURE 11 illustrates a plot of temperature versus ultimate tensile strength for the GMR-235 and G235+A alloys; FIGURE 12 illustrates a plot of the number of cycles versus maximum stress at 700°C for the GMR-235 and G235+A alloys; FIGURE 13 illustrates a plot of creep rupture life versus stress at 800°C for the GMR-235 and G235+A alloys; FIGURE 14 illustrates a plot of density versus temperature for the GMR-235 and G235+A alloys; FIGURE 15 illustrates a plot of temperature versus thermal expansion coefficient for the GMR-235 and G235+A alloys; FIGURE 16 illustrates a plot of temperature versus heat capacity for the GMR-235 and G235+A alloys; and FIGURE 17 illustrates a plot of temperature versus thermal conductivity for the GMR-235 and G235+A alloys. DETAILED DESCRIPTION

[0012] The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. As used herein, the word "exemplary" means "serving as an example, instance, or illustration." Thus, any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. All of the embodiments described herein are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description.

[0013] Embodiments of the present disclosure provide an improved nickel-based alloy and turbocharger turbine wheels made with this nickel-based alloy. The disclosed embodiments detail an improved nickel-based alloy chemistry that is less expensive than IN-713C yet has properties which are more comparable with IN-713C at temperatures of between 770°C and 850°C, or which at least render the alloy suitable for use as a replacement for IN-713C in turbocharger turbine wheels which operate at temperatures of between 770°C and 850°C.

[0014] With reference now to FIG. 1, illustrated is a turbocharger 101 in accordance with the present disclosure having a radial turbine and that includes a turbocharger housing, and a rotor configured to rotate within the turbocharger housing along an axis of rotor rotation 103 on thrust bearings and two sets of journal bearings (one for each respective rotor wheel), or alternatively, other similarly supportive bearings. The turbocharger housing includes a turbine housing 105, a compressor housing 107, and a bearing housing 109 (i.e., a centre housing that contains the bearings) that connects the turbine housing 105 to the compressor housing 107. The rotor includes a turbine wheel 111 located substantially within the turbine housing 105, a compressor wheel 113 located substantially within the compressor housing 107, and a shaft 115 extending along the axis of rotor rotation 103, through the bearing housing 109, to connect the turbine wheel 111 to the compressor wheel 113.

[0015] The turbine housing 105 and turbine wheel 111 form a turbine configured to circumferentially receive a high-pressure and high-temperature exhaust gas stream 121 from an engine, e.g., from an exhaust manifold 123 of an internal combustion engine 125. The turbine wheel 111 (and thus the rotor) is driven in rotation around the axis of rotor rotation 103 by the high-pressure and high-temperature exhaust gas stream 121, which becomes a lower-pressure and lower-temperature exhaust gas stream 127 and is axially released into an exhaust system (not shown).

[0016] The compressor housing 107 and compressor wheel 113 form a compressor stage. The compressor wheel 113, being driven in rotation by the exhaust-gas driven turbine wheel 111, is configured to compress axially received input air (e.g., ambient air 131, or already-pressurized air from a previous-stage in a multi-stage compressor) into a pressurized air stream 133 that is ejected circumferentially from the compressor. Due to the compression process, the pressurized air stream is characterized by an increased temperature over that of the input air.

[0017] Optionally, the pressurized air stream may be channelled through a convectively cooled charge air cooler 135 configured to dissipate heat from the pressurized air stream 133, increasing its density. The resulting cooled and pressurized output air stream 137 is channelled into an intake manifold 139 on the internal combustion engine, or alternatively, into a subsequent-stage, in-series compressor. The operation of the system is controlled by an engine control unit (ECU) 151 that connects to the remainder of the system via communication connections 153.

[0018] The constituent elements of nickel-based alloy for use making the turbine wheel 111 for such a turbocharger 101 must be carefully balanced to achieve the required mechanical properties (in particular high cycle fatigue resistance), and oxidation resistance, whilst reducing the cost of the alloy. Turbine wheels are made by investment casting, typically vacuum casting, and may need to be joined to other parts of the turbocharger 101 by welding. As such, the castability and weldability of the alloy must also be considered.

[0019] The strength and fatigue resistance of nickel-based alloys such as INC-713C are obtained by precipitation hardening derived from the presence of a gamma-prime (γ') phase which is coherent with the face-centred cubic (FCC) gamma (γ) matrix phase. Gamma-prime phase volume fraction and its thermal stability at high temperature is key for alloys for higher mechanical properties, creep and thermo-mechanical fatigue resistance during severe thermal-shock loading conditions. Elements such as aluminium (Al), titanium (Ti), tantalum (Ta) and niobium (Nb) promote the formation of the γ' phase.

[0020] The oxidation and corrosion resistance of the alloy is enhanced by the addition of Al and Cr, whilst elements such as molybdenum (Mo), tungsten (W), and cobalt (Co) can be introduced to provide creep resistance.

[0021] These elements may, however, have other effects, and / or interact in other ways which are detrimental to the properties of the alloy. For example, metals such as Ti, Nb, Mo, W and Ta can combine with carbon (C) to form carbides. These carbides can play an important role in strengthening the alloy by hindering the movement of dislocations, thus improving the high-temperature properties of the material, and aiding microstructural refinement. On the other hand, the carbides can be detrimental to the fatigue life of the alloy, by acting as an initiation point for crack formation. Mo, W, Co and Nb can provide solid solution strengthening, but Cr, Mo, Co and W can also combine to form undesirable, brittle topologically close packed (TCP) phases such as the sigma phase. Moreover, there is evidence that Mo can be detrimental to hot corrosion or oxidation resistance if it combines with Cr to promote precipitation of TCP phases. Aluminium, as mentioned above, promotes the formation of the γ' phase, but also promotes the formation of the sigma phase, by reducing the volume of the γ gamma phase available for retaining Cr, Mo and W in solid solution. TCP phases can decrease the ductility, notch strength and stress-rupture strength of the alloy.

[0022] The composition of an exemplary nickel-based alloy is now provided below with respect to its constituent elements (all percentages being provided on a weight basis of the overall alloy composition, unless otherwise noted).

[0023] In one embodiment, elements that are associated with grain boundary cracking and embrittlement should be minimized. For example, in this embodiment, the content of silicon (Si) is maintained below or equal to 0.2 wt%. The content of phosphorous (P) is maintained below or equal to 0.005 wt%. Further, the content of sulphur (S) is maintained below or equal to 0.003 wt%. As an additional matter, to reduce cracking, the master heat alloy that is used to process the alloy to ingot form desirably does not contain any casting revert or scrap having detrimental tramp or trace elements.

[0024] Elements that are associated with grain boundary strengthening, including carbon (C), boron (B), and zirconium (Zr) are melting point depressants which can create hot cracks during solidification in a casting process (hot tearing phenomenon) and welding (solidification cracking) of alloys. Grain boundary liquation during welding of alloys is linked to carbides and borides. Since C (but not B) achieves a "carbon boil" during master alloy refining, embodiments of the nickel-based alloy retain some carbon, likely in the form of carbides (as described below) but not a significant content of borides. Zr or B does not influence the castability when added individually. However, when both Zr and B are present in the alloy, high hot tearing susceptibility has been found, the effect being particularly strong if the Zr concentration is high. Accordingly, the content of B is maintained in a range of 0.01 wt% to 0.02 wt%, the content of carbon is maintained in a range of 0.1 wt% to 0.2 wt%, and the content of zirconium is maintained in a range of 0.03 wt% to 0.07 wt%.

[0025] A reduction in the cost of the alloy compared to IN-713C is primarily achieved through the addition of iron (Fe), since Fe is significantly cheaper than the other metallic constituents of IN-713C. Fe is a γ' depressant, however, and the presence of Fe in the GMR-235 alloy may contribute to the low γ' volume fraction and promote the formation of TCP phases in this alloy, and therefore contribute to its inferior mechanical properties.

[0026] In some embodiments, the content of Fe is from 5.2 wt% to 8.5 wt%. In one embodiment, the content of Fe is provided in a range of 7.5 wt% to 8.5 wt%.

[0027] Al is included in the alloy composition of the present disclosure as the prime γ' phase forming element. In some embodiments, the content of Al is from 6.0 wt% to 7.2 wt%. In one embodiment, the content of Al is from 6.00 wt% to 7.00 wt%.

[0028] Cr enhances the oxidation resistance of the alloy, but is a γ' depressant, and at the aforementioned relatively low C and B contents, Cr tends to combine with Mo to form brittle TCP phases. In some embodiments, the content of Cr is maintained within a range of 11.0 wt% to 13.5 wt%. In one embodiment, the content of Cr is maintained in a range of 11.0 wt% to 12.0 wt%.

[0029] Mo is a solid solution strengthening element and increases the Al partitioning to the gamma-prime phase by reducing the solubility of Al in the γ matrix which increases gamma-prime volume fraction. Moreover, Mo is known to be an element that forms primary MC carbides and that forms detrimental TCP phases.

[0030] Accordingly, in some embodiments, the content of Mo is maintained within a range of 3.0 wt% to 4.4 wt%. In one embodiment, the content of Mo is maintained with a range of 3.0 wt% to 4.0 wt%.

[0031] Titanium (Ti) is both a carbide and gamma-prime phase forming element. In some embodiments, titanium is provided in a range of 1.15 wt% to 2.00 wt%. In one embodiment, titanium is provided in a range of 1.35 wt% to 1.85 wt%.

[0032] Continuing with the description of an exemplary embodiment of the nickel-based alloy, niobium (Nb) is known to be an element that forms carbides and a solid solution strengthening element that also partitions to the gamma-prime phase. The phase fraction and stability of γ' increases with niobium content. Allowing a higher content of Nb will favour Nb-rich carbides and make up for the absence of Ti in the gamma-prime phase while contributing to solid solution strengthening. Moreover, Nb is preferred as a MC carbide former over Ti because titanium-rich MC carbides are less stable and can cause the precipitation of secondary Cr rich carbides. The formation of secondary Cr rich carbides (M23C6 and M6C) can be detrimental to the oxidation resistance as they remove free Cr from solid solution. They can cause the formation of Cr depleted zones which, if closed to the material surface or grain boundaries, can act as initiation sites for dwell-fatigue or thermo-mechanical fatigue failure.

[0033] Nb is, however, a relatively expensive element. Accordingly, Nb may optionally be included up to 2.0 wt%. In one embodiment, the content of Nb is from 1.0 wt% to 2.0 wt%.

[0034] Other elements that may be present in small amounts (or possibly 0 wt%) in some embodiments include manganese (Mn) in an amount of up to 0.02 wt% and copper (Cu) in an amount of up to 0.03 wt%.

[0035] Nickel (Ni) accounts for the balance of the content not otherwise described above, while accounting for inevitable / unavoidable impurities not otherwise set forth above as are commonly understood in the art. Ni therefore forms a majority of the content (i.e., greater than 50%) of the described alloy.

[0036] Table 1, set forth below, provides the elemental content (in wt %) of a nickel-based alloy of the present disclosure in accordance with the description provided above (hereinafter referred to as G235+), while also specifying the maximum content of additional detrimental tramp or trace elements commonly encountered in nickel-based alloys. Each weight percentage included in Tables set out in this application should be understood to be determined to the accuracy of measuring equipment typically used in the art, with normal rounding conventions applied. It should be acknowledged that trace amounts of inevitable / unavoidable impurities may be present (in trace amounts), although not explicitly listed in Table 1, as is known in the art. For comparison, the compositions of the prior art alloy compositions mentioned above are also shown in this table. TABLE 1G235+ Mar-M246 Mar-M247 IN-713C GMR-235 GMR-235D Cr 11.0-13.58.0 - 10.08.0 - 8.811.0 - 14.014.00 - 17.0014.00 - 17.00Fe 5.2 - 8.5≤ 1.0≤ 0.5≤ 5.08.00 - 12.003.50 - 5.00Al 6.0 - 7.25.25 - 5.755.3 - 5.75.5 - 6.52.50 - 3.503.25 - 4.00Mo 3.0 - 4.42.25 - 2.750.5 - 0.83.5 - 4.54.5 - 6.04.5 - 6.0Ti 1.15 - 2.001.25 - 1.750.9 - 1.20.25 - 1.251.50 - 2.502.00 - 3.00C 0.1 - 0.20.13 - 0.170.13 - 0.17≤ 0.20.1 - 0.20.1 - 0.2Zr 0.03 - 0.070.03 - 0.080.03 - 0.08B 0.01 - 0.020.01 - 0.020.01 - 0.020.025 - 0.100.05 - 0.1Nb 0 - 2.01.0 - 3.0Si 0 - 0.2≤ 0.2≤ 1.0≤ 0.6≤ 0.30Cu 0 - 0.03≤ 0.1≤ 0.1Mn 0 - 0.02≤ 0.2≤ 0.2≤ 1.0≤ 0.25≤ 0.1P 0 - 0.005≤ 0.01S 0 - 0.003≤ 0.015≤ 0.015Co -9.0 - 11.09.0 - 11.0Ta -1.25-1.752.8-3.3W -9.0-11.09.5-10.5Hf -1.2-1.6Ni + OET BalanceBalanceBalanceBalanceBalanceBalance

[0037] As such, described herein are embodiments of improved nickel-based alloys and turbocharger turbine wheels made with such nickel-based alloys. The described embodiments provide an alloy which is cheaper than Mar-M246, Mar-M247 and IN-713, but which has better oxidation, high cycle fatigue and creep resistance than GMR-235 and GMR-235D. This is achieved, in part, by the increased iron content along with careful adjustment of the chromium, aluminium, molybdenum, titanium and niobium content to increase the volume fraction of γ' phase to a level which is significantly higher than the volume fraction of γ' present in GMR-235 and GMR-235D. The resultant alloy has a reduced cost, wherein the raw material cost is less than IN-713C. The alloy of the present disclosure can be used to make turbine wheels for applications reaching temperatures from 770°C to 850°C at a lower cost than existing commercial alloys.ILLUSTRATIVE EXAMPLES

[0038] The present disclosure is now illustrated by the following non-limiting examples. It should be noted that various changes and modifications can be applied to the following examples and processes without departing from the scope of this invention, which is defined in the appended claims. Therefore, it should be noted that the following examples should be interpreted as illustrative only and not limiting in any sense.

[0039] The composition of the new and inventive nickel-based alloy was derived using Thermo-Calc ®< and JMatPro ®< software. The percentage by weight of each of carbon, manganese, sulphur, silicon, and boron content was fixed at a level within the ranges present in the prior art GMR-235 and GMR235D alloy compositions, and a percentage by weight was fixed for phosphorous, zirconium and copper based on past experience of the content typically required to achieve good castability and weldability. The levels of chromium, iron, aluminium, molybdenum, titanium, and niobium were targeted for optimisation using the Thermo-Calc ®< software, the limits for the percentage by weight of each of these elements being expanded beyond the limits in the known GMR-235 and GMR-235D alloys, with the particular aim of increasing the percentage of γ' phase in the resultant alloy. In particular, the lower end of the chromium and molybdenum content ranges was decreased over the ranges present in both the GMR-235 and GMR-235D alloys, the titanium content range was increased at both its upper and lower limits, the upper limit of the aluminium content was increased, the lower end of the iron content range was decreased relative to the GMR-235 range, and niobium was introduced as an optional addition to the alloy.

[0040] The selected composition ranges as entered into the Thermo-Calc ®< software is shown in Table 2 below. TABLE 2Cr Fe Al Mo Ti C Zr B Nb Si Cu Mn P S Ni Wt% 10.00-17.005.00-12.003.00-7.003.00-6.001.00-4.000.150.050.0150.00-3.000.10.010.010.0050.0015Bal

[0041] The Thermo-Calc ®< software was used to determine the volume fraction, composition and stability of the various phases (including γ', TCPs including sigma, M23C6, M6C, & carbides) in the alloy as a function of temperature. Thermophysical parameters such as density, liquidus, solidus and freezing range, were also obtained. Multiple simulations were iteratively run, each using hundreds of potential alloy compositions derived using randomized and fractional factorial designs. Kinetic simulations of the most promising compositions were evaluated using JMatPro ®< in order to understand the long-term stability of the primary phases and the potential precipitation of secondary and TCP / carbide phases over time. Finally, the shortlisted compositions were ranked based on various weighted key performance indicators (KPIs) which included density, yield strength, Young's modulus, volume fraction of γ', carbide type and stability, TCP risk, castability, weldability and % cost saving. This analysis revealed the five most suitable chemical compositions, shown in Table 3 below. TABLE 3Cr Fe Al Mo Ti C Zr B Nb Si Cu Mn P S Ni A1 11.008.006.504.001.600.150.050.0151.000.10.010.010.0050.0015BalA2 12.008.006.503.001.600.150.050.0152.000.10.010.010.0050.0015BalB1 13.005.706.703.501.400.150.050.0151.000.10.010.010.0050.0015BalC1 12.005.806.404.002.000.150.050.015-0.10.010.010.0050.0015BalC2 13.005.706.704.001.400.150.050.015-0.10.010.010.0050.0015Bal

[0042] These five compositions were used to generate the element ranges for three alloy composition ranges, A, B & C, set out in Table 4 below. TABLE 4Cr Fe Al Mo Ti C Zr B Nb Si Cu Mn P S Ni A 11.0 to 12.07.5 to 8.56.0 to 7.03.0 to 4.01.35 to 1.850.1 to 0.20.03 to 0.070.01 to 0.021.0 to 2.0≤0.2≤0.03≤0.02≤0.005≤0.003BalB 12.5 to 13.55.2 to 6.26.2 to 7.23.0 to 4.01.15 to 1.650.1 to 0.20.03 to 0.070.01 to 0.020.5 to 1.5≤0.2≤0.03≤0.02≤0.005≤0.003BalC 12.0 to 13.05.25 to 6.256.0 to 7.03.4 to 4.41.4 to 2.00.1 to 0.20.03 to 0.070.01 to 0.02-≤0.2≤0.03≤0.02≤0.005≤0.003Bal

[0043] Alloys A, B and C fall within the broad composition range for the G235+ alloy disclosed in table 1. Whilst all would be suitable for use in a turbine for a turbocharger to operate within a temperature range of 770°C and 850°C, alloy A was determined to be the most preferred composition range to achieve the best balance between cost and properties based on the KPI ranking described above.

[0044] Figure 2 shows a graphical representation of the materials phase volume fractions across temperature calculated using Thermo-Calc ®< for a typical prior art GMR-235 composition. Figure 3 shows a graphical representation of the materials phase volume fractions across temperature for an alloy composition falling within the inventive Alloy A composition range (designated G235+A in the description below), also calculated using Thermo-Calc ®< . The specific compositions (in wt%) used in the calculations are set out in Table 5 below. TABLE 5Cr Fe Al Mo Ti C Zr B Nb Si Cu Mn Co Ni GMR235 15.1010.603.424.801.800.150.020.060.100.350.010.350.30BalG235+A 11.508.006.503.501.600.150.050.021.500.100.010.010.01Bal

[0045] Co was included in the G235+A composition at impurity / trace element level for the purposes of these calculations.

[0046] As illustrated in Figure 3, G235+A has a significantly higher calculated volume fraction of the γ' phase at to GMR-235 - 56% compared to 22% at 850°C and 59% compared to 26% at 800°C.

[0047] Figure 4 shows a prior art plot of volume fraction γ' phase versus rupture life for two prior art Ni-based superalloys - IN-713C and single crystal TMS-75, [Source: T. Murakumo et al. / Acta Materialia 52 (2004) 3737-3744] which shows that the optimum γ' volume fraction is between 55 and 75%. Thus, the calculated γ' volume fraction for the G235+A alloy falls within this optimum range.

[0048] Figure 5 shows micrographs of the as cast alloy with the GMR-235 composition, and Figure 6 shows micrographs of an as cast alloy with the G235+A composition, and these show that the phases present correspond to those expected from the graphs illustrated in Figures 2 & 3. No abnormal microstructures or TCP phases were observed in the as cast condition.

[0049] Figure 7 shows a plot of time versus mass change at 850°C for the GMR-235 and G235+A alloys. This shows that G235+A demonstrates better oxidation resistance than GMR-235, despite the lower Cr content. It is believed that this is achieved by the re-balancing of the other alloying elements, which allows for more Cr to be retained in solid solution in the γ phase. Figure 8 shows a plot of temperature versus the Cr content of the γ phase in the GMR-235 and G235+A alloys.

[0050] As expected from the increased γ' volume fraction, and the increased amount of Cr in solid solution, the mechanical properties of the G235+A alloy are superior to those of GMR-235.

[0051] Figures 9, 10 and 11 show plots of temperature versus Young's modulus, yield strength and ultimate tensile strength for the GMR-235 and G235+A alloys. G235+A has a lower modulus, and higher yield strength and ultimate tensile strength than GMR-235.

[0052] Figure 12 shows a plot of the number of cycles versus maximum stress at 700°C for the GMR-235 and G235+A alloys. G235+A exhibits a much superior low cycle fatigue life to GMR-235 (around 50 times better).

[0053] Figure 13 shows a plot of creep rupture life versus stress at 800°C for the GMR-235 and G235+A alloys. G235+A exhibits much superior creep properties to GMR-235 (around 6 times higher).

[0054] Figure 14 shows a plot of density versus temperature for the GMR-235 and G235+A alloys. This shows that G235+A has a lower density than GMR-235. Use of a lower density alloy in a turbine wheel is advantageous as it reduces the inertia of the turbine wheel, and lower density may also improve the castability of the alloy.

[0055] Turning now to the thermal properties of the inventive alloy, Figures 15, 16 and 17 show plots of temperature versus thermal expansion coefficient, heat capacity and thermal conductivity for the GMR-235 and G235+A alloys. The G235+A alloy has a lower thermal expansion coefficient, and a higher heat capacity and thermal conductivity.

[0056] While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

Claims

1. A nickel-based alloy composition consisting, on a weight basis, of: 11.0% to 13.5% chromium, 5.2% to 8.5% iron, 6.0% to 7.2% aluminium, 3.0% to 4.4% molybdenum, 1.15% to 2.00% titanium, 0.1% to 0.2% carbon, 0.03% to 0.07% zirconium, 0.01% to 0.02% boron, 0% to 2.0% niobium, 0% to 0.2% silicon, 0% to 0.03% copper 0% to 0.02% manganese 0% to 0.005% phosphorous 0% to 0.003% sulphur, with the balance being nickel and incidental / inevitable / unavoidable impurities.

2. The nickel-based alloy composition according to claim 1 wherein the amount of chromium is limited to 11.0% to 12.0% by weight.

3. The nickel-based alloy composition according to any preceding claim wherein the amount of iron is limited to 7.5% to 8.5% by weight.

4. The nickel-based alloy composition according to any preceding claim wherein the amount of aluminium is limited to 6.0% to 7.0% by weight.

5. The nickel-based alloy composition according to any preceding claim wherein the amount of molybdenum is limited to 3.0% to 4.0% by weight.

6. The nickel-based alloy composition according to any preceding claim wherein the amount of titanium is limited to 1.35% to 1.85% by weight.

7. The nickel-based alloy composition according to any preceding claim wherein the amount of niobium is limited to 1.0% to 2.0% by weight.

8. A turbocharger turbine wheel comprising of a nickel-based alloy according to any preceding claim.

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