GB2159174A - A nickel-base alloy suitable for making single-crystal castings - Google Patents

Abstract

An alloy suitable for making single crystal castings consisting essentially of the following constituents:- Chromium 7 to 12% by weight Aluminium 3 to 6% by weight Titanium 0 to 2% by weight Cobalt 0 to 10% by weight Tantalum 8 to 16% by weight Tungsten 8 to 12% by weight Molybdenum 0 to 2% by weight Niobium 0 to 5% by weight Rhenium 0 to 0.5% by weight Carbon by weight Balance Nickel plus impurities, provided that: the tantalum content plus four times the combined titanium and niobium content is equal to or less than 16% by weight, the combined weight of the tantalum plus tungsten plus molybdenum plus rhenium is equal to or less than 25% by weight and, the combined content of the aluminium plus titanium plus niobium plus tantalum is from 13.0 to 16.5 atomic %.

Description

SPECIFICATION A nickel-base alloy suitable for making single-crystal castings This invention relates to an alloy suitable for making single-crystal castings and to a casting made thereof.

Cast nickel-based alloys and in particular the nickel-based superalloys have been widely used in the past for applications in which resistance to high temperatures is required. Such applications are largely found in the hotter parts of gas turbine engines. It has been appreciated in recent years that an improvement in cast objects for operation in these extreme conditions may be made by casting the objects as single crystals rather than in the conventional multicrystalline form. In general, single crystal castings have better high temperature lives and strength than their equi-axed, multi-crystalline counterparts.

The nickel-based superalloys currently used represent highly developed formulations which have been specifically designed to make the best use of the equi-axed, multi-crystalline cast form in which they have been used. When these materials are used in standard form to produce single crystal castings, their properties are compromised by the presence and levels of a number of constituents whose major role is to overcome the deficiences of a multi-crystalline structure. It is possible, however, to design new alloys which are more accurately tailored to single crystal use.

In order to obtain the maximum material properties from a single crystal casting, it is necessary to carry out a solution and homogenisation heat treatment in order to refine the metallurgical structure. The temperature at which this heat treatment is carried out must be above the gamma prime solvus of the alloy and below the alloy solidus. This temperature difference is generally referred to as the heat treatment window of the alloy.

However with many single crystal alloys, the heat treatment window is so small as to provide serious manufacturing problems in achieving the correct heat treatment temperature.

It is an object of the present invention to provide a nickel-based superalloy suitable for making single crystal castings which has a high creep strength and an acceptably wide heat treatment window.

According to the present invention, an alloy suitable for making single-crystal castings consists essentially of the following constituents: Chromium 7 to 12% by weight Aluminium 3 to 6% by weight Titanium 0 to 2% by weight Cobalt 0 to 10% by weight Tantalum 8 to 16% by weight Tungsten 8 to 12% by weight Molybdenum 0 to 5% by weight Niobium 0 to 2% by weight Rhenium 0 to 5% by weight Carbon 0 to 0.5% by weight Balance Nickel plus impurities provided that The tantalum content plus four times the combined titanium and niobium content is equal to or less than 16% by weight.

The combined weight of the tantalum plus tungsten plus molybdenum plus rhenium is equal to or less than 25% by weight and the combined content of the aluminium plus titanium plus niobium plus tantalum is from 13.0 to 16.5 atomic /O.

The present invention also includes a cast single crystal object made from an alloy falling within the ranges set out above.

The invention will now be described, by way of example, with reference to the accompanying drawings in which Figure 1 is a plot of stress versus temperature.

Figure 2 is a plot of specific stress versus temperature.

An alloy A in accordance with the present invention and containing the following constituents was prepared: Chromium 9% by weight Aluminium 4% by weight Tantalum 15% by weight Tungsten 10% by weight Balance Nickel plus impurities.

The stress-rupture properties of this alloy were compared with those of a typical first generation single crystal alloy (alloy B) and the results of this comparison are shown in Figure 1. Figure 2 illustrates the properties of the alloys based on a comparison of specific stress (stressidensity) - rupture properties (alloy A has a density of 9.3 Kg L-1).

Alloy B contained the following constituents: Chromium 8.5% by weight Aluminium 5.5% by weight Titanium 2.2% by weight Cobalt 5.0% by weight Tantalum 2.8% by weight Tungsten 10.0% by weight Carbon 0.015% by weight The alloy had a density of 8.5 Kg L-.

Figures 1 and 2 both demonstrate the superior stress rupture properties of alloy A in accordance with the present invention over a wide range of temperatures. Thus alloy A offers significant advantages in those applications in which very high creep strength is required but where density is of secondary significance. Thus the alloy is, for example, particularly suitable for the manufacture of small rotary turbine aerofoil blades for helicopter gas turbine propulsion engines.

In its broadest aspect, the present invention encompasses alloys which contain the following constituents :- Chromium 7 to 12% by weight Aluminium 3 to 6% by weight Titanium 0 to 2% by weight Cobalt 0 to 10% by weight Tantalum 8 to 16% by weight Tungsten 8 to 12% by weight Molybdenum 0 to 5% by weight Niobium 0 to 2% by weight Rhenium 0 to 5% by weight Carbon 0 to 0.5% by weight Balance Nickel plus impurities Chromium is present in the alloy in order to confer both oxidation and corrosion resistance. Additions of chromium would, if too large, severely limit the amount of strenghthening elements that can be added.Thus the chromium level must be sufficient to confer satisfactory oxidation and corrosion resistance to prevent catastrophic degradation following failure of a protective coating (if present) but permit alloy strengthening by the addition of further alloying elements. Generally speaking a chromium range of 7 to 12% by weight is sufficient to achieve those ends although it is preferred to have a chromium range of 8 to 10% by weight.

The role of cobalt in alloys in accordance with the present invention and indeed in all superalloys is not completely understood. it is believed, however, that it changes stacking fault energies and hence has some influence upon deformation mechanisms. A cobalt range of from 0 to 10% by weight is envisaged in alloys in accordance with the present invention although the preferred cobalt range is from 0 to 5% by weight Aluminium is added to the alloy of the present invention in order to promote the formation of the precipitation hardening phase gamma prime (Ni3AL). Additionally aluminium promotes good high temperature oxidation resistance. The level of aluminium chosen is such that when combined with other gamma prime partitioning elements such as tantalum, titanium and niobium, a gamma prime volume fraction of 65-75% results. A suitable aluminium level is from 3 to 6% by weight although we prefer a level of 3-5% by weight.

Titanium partitions to the gamma prime phase and modifies the deformation mechanism. Unfortunately, however, it also lowers the incipient melting point of the alloy. There is a danger therefore that too high a level of titanium will result in narrowing of the alloy heat treatment window i.e. the temperature interval between the solvus of the precipitation hardening phase gamma prime and the incipient alloy melting point.

In the light of these problems, titanium can be in part or totally replaced by additions of tantalum whilst maintaining an acceptable heat treatment window. Unfortunately low titanium content alloys have poor hot corrosion resistance and consequently alloys of this type usually have to be provided with some form of protective coating.

In view of the above points, the titanium content of the alloy is within the range 0 to 2% by weight although we prefer that the range should be 0 to 1% by weight. Alloy A in accordance with the present invention has a gamma prime solvus of 1280 C and an incipient melting point of 1300"C to give an acceptable heat treatment win down of 20"C.

Tantalum partitions primarily to the gamma prime phase so imparting strength by solid solution hardening and high temperature stability by hindering diffusional precipitation growth (overageing). It additionally promotes good oxidation resistance but decreases the size of the alloy's heat treatment window as referred to above.

Alloys in accordance with the present invention may contain from 8 to 16% by weight tantalum and preferably from 13 to 16% by weight.

Tungsten may partition to either gamma or gamma prime and solid solution strenghens both phases.

It is desireable to include significant quantities of both tantalum and tungsten in alloys in accordance with the present invention in order to promote good high temperature strength. Tungsten levels of 8 to 12% by weight are present in alloys in accordance with the present invention although we prefer that tungsten is present in the range 9 to 11% by weight.

Molybdenum may be present as a solid solution strengthener although it is not as effective in this role as is tungsten. It partitions primarily to the gamma phase and is useful in controlling the misfit strain between the precipitate and matrix. Molybdenum may be present in the range 0 to 5% by weight although we prefer that it is present in the range 0 to 3% by weight.

Rhenium may be present in alloys in accordance with the present invention since it acts in a similar manner to molybdenum but is considerably more effective. However its usefulness must be countered against its cost. It may be present in the range 0 to 5% by weight, although we prefer that it should be present in the range 0 to 3% by weight.

Niobium may be present as a substitute for titanium, again partitioning primarily to gamma prime. It may be present in the range 0 to 2% by weight although we prefer that it is present in the range 0 to 1% by weight.

The refractory element (tantalum, tungsten, molybdenum and rhenium) content of alloys in accordance with the present invention is high and provides the alloys with good high temperature strength. There is, however, a limit on the alloy composition in terms of stability since some alloys could precipitate tungsten after exposure to high temperatures. This may be remedied by a reduction in tungsten or chromium or both towards their lower limits.

In addition to the ranges of the various constituents of alloys in accordance with the present invention, there are three constraints on the relationships between various alloy consti- tuents which must be observed in order to arrive at alloys with acceptable properties: a) In order to maintain an acceptable heat treatment window, the tantalum content of the alloy plus four times the combined titanium and niobium content must be equal to or less than 16% by weight of the total alloy weight. This value is 15% by weight for Alloy A. Titanium, niobium and tantalum all cause a reduction in the heat treatment window; titanium and niobium both depress the incipient melting point significantly whereas tantalum is more effective at raising the gamma prime solvus temperature.Consequently, additions of tantalum must be balanced by a corresponding decrease in the levels of titanium and niobium.

b) In order to ensure metallurgical stability, the combined tantalum, tungsten, molybdenum and rhenium content of the alloy must be equal to or less than 25% by weight of the total alloy weight. This value is 25% by weight for Alloy A.

c) Alloys in accordance with the present invention depend upon a high gamma prime volume fraction for their properties. In order to generate the necessary high volume fraction of gamma prime, the combined aluminium, titanium, niobium and tantalum content of the alloy must be from 13.0 to 16.5 atomic %. This value is 15.3 atomic % for Alloy A.

Alloys in accordance with the present invention are solution heat treatable so that a homogeneous microstructure of reprecipitated gamma-prime can be developed, and this, together with their high refractory metal content, ensures that castings made from the alloys will be highly amenable to coating with suitable protective coatings such as aluminide coatings, and are stable when exposed to high tem peratu res.

Claims (7)

1. An alloy suitable for making single crystal castings consisting essentially of the following constituents: Chromium 7 to 12% by weight Aluminium 3 to 6% by weight Titanium O to 2% by weight Cobalt O to 10% by weight Tantalum 8to 16% by weight Tungsten 8 to 12% by weight Molybdenum O to 5% by weight Niobium O to 2% by weight Rhenium O to 5% by weight Carbon O to 0.5% by weight Balance Nickel plus impurities. provided that: the tantalum content plus four times the combined titanium and niobium content is equal to or less than 16% by weight, the combined weight of the tantalum plus tungsten plus moly- bdenum plus rhenium is equal to or less than 25% by weight and the combined content of the aluminium plus titanium plus niobium plus tantalum is from 13.0 to 16.5 atomic %.

2. An alloy as claimed in claim 1 wherein said alloy contains the following constituents: Chromium 8 to 10% by weight Aluminium 3 to 5% by weight Titanium O to 1% by weight Cobalt O to 5% by weight Tantalum 13 to 16% by weight Tungsten 9 to 11% byweight Molybdenum O to 3% by weight Niobium O to 1% by weight Rhenium O to 3% by weight Carbon O to 0.05% by weight Balance Nickel plus impurities.

3. An alloy as claimed in claim 1 or claim 2 wherein said alloy contains the following constituents: Chromium 9% by weight Aluminium 4% by weight Tantalum 15% by weight Tungsten 10% by weight Balance Nickel plus impurities.

4. An alloy as claimed in any one preceding claim wherein said alloy has a gamma prime volume fraction of from 65-75%.

5. An alloy as claimed in any one of claims 1 to 4 wherein said alloy has been solution heat treated.

6. A casting made from an alloy in accordance with any one of claims 1 to 4.

7. An alloy substantially as hereinbefore described and in accordance with the present invention.