# Patent

## Title
Clad member and method of manufacturing same

## Abstract

A new nickel base superalloy suitable for compressor or turbine discs of gas turbine engines with fatigue crack propagation resistance equal to Waspaloy, tensile strength higher than Waspaloy and higher operating temperature than Waspaloy or UDIMET 720 family of alloys. The nickel base superalloy has a preferred composition by weight % of 14.0-19.0% cobalt, 14.35-15.15 Chromium, 4.25-5.25 Molybdenum, 1.35-2.15 tantalum, 3.45-4.15 titanium, 2.85-3.15 aluminium, 0.01-0.025 boron, 0.012-0.033 carbon, 0.05-0.07 zirconium, 0.5-1.0 hafnium, up to 1.0 rhenium, up to 2.0 tungsten, less than 0.5 niobium, up to 0.1 yttrium, up to 0.1 vanadium, up to 1.0 iron, up to 0.2 silicon, up to 0.15 manganese and balance nickel plus incidental impurities.

## Background
This invention is concerned with new nickel base superalloys, and with wrought and heat-treated products made from them e.g. compressor and turbine discs. The turbine disc, which may be up to one meter in diameter, is a critical part of a gas turbine e.g. a turbine. Failure of such a component in operation is usually catastrophic.

For more than thirty years there has been a continuing need for improved alloys to enable engine components such as turbine discs to be operated under more rigorous conditions. The nickel base superalloy known as Waspaloy was introduced in 1967, and is still used today despite its limitations of strength and maximum temperature of use. UDIMET 720, an alloy with improved strength, was introduced in 1986 (UDIMET is a Registered Trade Mark of Special Metals Corporation). However, UDIMET 720 was found to be unstable (with respect to the formation of deleterious Topologically Close Packed (TCP) phases) and was superseded in 1990 by powder processed UDIMET 720Li (low interstitial), an alloy with reduced chromium, carbon and boron. Improvements in cast and wrought (C+W) processing led to the introduction of C+W UDIMET 720Li in 1994. Cast and wrought UDIMET 720Li exhibits near equivalent properties to those of the powder variant. Although UDIMET 720Li has adequate strength, its resistance to fatigue crack propagation is somewhat lower than Waspaloy, and its maximum operating temperature is limited to approximately 650.degree. C.

There is a continuing need to define an alloy composition, microstructure, heat treatment and process route to meet the increasing demands of future civil and military turbine discs. It is an object of the present invention to meet that need. Nickel base superalloys are so complex, with generally about ten alloying components present, that optimisation of alloy composition is extremely difficult. Phase diagram modelling has been used extensively during the development of the invention to predict the component phases and their proportions.

The present invention provides a nickel base alloy comprising in weight percent 14.0% to 19.0% cobalt, 14.35% to 15.15% chromium, 4.25% to 5.25% molybdenum, 1.35% to 2.15% tantalum, 3.45% to 4.15% titanium, 2.85% to 3.15% aluminium, 0.01% to 0.025% boron, 0.012% to 0.033% carbon, 0.05% to 0.07% zirconium, 0.5% to 1.0% hafnium, up to 1.0% rhenium, up to 2.0% tungsten, less than 0.5% niobium, up to 0.1% yttrium, up to 0.1% vanadium, up to 1.0% iron, up to 0.2% silicon up to 0.15% manganese and the balance nickel plus incidental impurities.

One alloy may comprise in weight percent 18.5% cobalt, 15% chromium, 5% molybdenum, 2% tantalum, 3.6% titanium, 3% aluminium, 0.075% hafnium, 0.015% boron, 0.06% zirconium, 0.027% carbon and the balance nickel plus incidental impurities.

Another alloy may comprise in weight percent 15% cobalt, 14.5% chromium, 4.5% molybdenum, 1.5% tantalum, 4% titanium, 3% aluminium, 0.015% boron, 0.06% zirconium, 0.027% carbon and the balance nickel plus incidental impurities.

A further alloy may comprise in weight percent 15% cobalt, 14.5% chromium, 4.5% molybdenum, 1.5% tantalum, 4% titanium, 3% aluminium, 0.75% hafnium, 0.015% boron, 0.06% zirconium, 0.027% carbon and the balance nickel plus incidental impurities.

The Ni level is often 40-60 wt %.

Preferred alloys should have the following characteristics Waspaloy. This key property is achieved without loss of overall property balance. Strength (UTS) of at least 1400 MPa at a temperature of 550.degree. C. hours at a temperature of 725.degree. C. with an applied stress of 500 MPa. Increasing the gamma prime volume fraction improves the tensile strength. Controlling the gamma prime weight fraction to these levels retains the balance between tensile strength and fatigue crack propagation resistance. ! the M.sub.6 C and/or M.sub.23 C.sub.6 types. Our work leading up to this invention has indicated that less stable alloys exhibit greater resistance to fatigue crack propagation.! the proportion of TCP phases (in the wrought and heat treated product) is less than 7.0 wt % at a temperature of 725.degree. C. We have found that excessive precipitation of sigma and mu phases degrade the creep properties of these superalloys.! than the solvus of the M.sub.6 C or M.sub.23 C.sub.6 phases. family of alloys.

The following Table I recites the compositions of three preferred alloys according to the invention, together with the compositions of four alloys from the prior art. It can be seen that the preferred alloys of the present invention are characterised by the inclusion of tantalum, and by the combination of ranges of chromium, molybdenum, titanium and aluminium.

Various components are identified as having particular effects on the chemistry/mechanical property relationships of the alloy: tensile or creep strength of the alloys. The presence of 15 wt % cobalt generates a minimum Stacking Fault Energy (SFE) which promotes planar deformation and potentially improved fatigue crack propagation resistance. resistance without excessive formation of TCP phases. high temperatures, but levels have been controlled to balance the high chromium with respect to TCP phase formation. tantalum carbide (MC carbide). The tantalum concentration has been controlled to allow the MC carbide to breakdown and promote the formation of grain boundary carbides. the greatest effect on the gamma prime solvus. The titanium content has been increased to balance the reduced tantalum levels in order to maintain tensile strength, whilst also controlling the gamma prime weight fraction and TCP phase formation. the gamma prime weight fraction. The aluminium concentration has also been limited in order to reduce the propensity for TCP phase formation. crack propagation resistance and tensile strength. temperature creep resistance. stress rupture and creep resistance. addition of hafnium improves all properties. usefully be included. __________________________________________________________________________ SUPERALLOY COMPOSITION COMPARISON Alloy Co Cr Mo Ta Ti Al Hf W  Nb B   Zr C __________________________________________________________________________ 1    18.5 15 5  2 3.6 3  0.75 -- -- 0.015 0.06 0.027 2    15 14.5 4.5 1.5 4  3  -- -- -- 0.015 0.06 0.027 3    15 14.5 4.5 1.5 4  3  0.75 -- -- 0.015 0.06 0.027 Waspaloy 13.5 19.5 4.25 -- 3.05 1.4 -- -- -- 0.0065 0.05 0.06 U720Li 15 16 3  -- 5  2.5 -- 1.25 -- 0.015 0.035 0.015 Rene 88 13 16 4  -- 3.7 2.1 -- 4  0.75 0.02 0.04 0.04 N18  15.7 11.5 6.5 -- 4.35 4.35 0.5 -- -- 0.015 0.03 0.015 __________________________________________________________________________ All values are expressed in weight percent. Balance nickel.

In order to take advantage of the potential of the new alloys of this invention, the following processing steps are preferred for manufacture of an article: techniques; either partial or full solution treatment, controlled cooling and ageing.

1. Billet

Billet can be produced by either powder or cast & wrought routes. consolidation by routes such as HIP+extrude or HIP+cog. Consolidation takes place at a temperature below the gamma prime solvus of the alloy. conversion route defined to give a suitably homogeneous product.

Step 1(a) is preferred for larger forgings, with cast & wrought potentially more suitable for smaller items.

The option exists to precondition the billet prior to forging, by soaking at a temperature up to gamma prime solvus minus 100.degree. C., for times between 2 and 24 hours.

2. Forging

Forging the billet near to shape under either isothermal or hot die conditions, eg: at a billet temperature up to gamma prime solvus minus 60.degree. C., at a strain rate between 1.times.10.sup.-4 and 1.times.10.sup.-2 s.sup.-1 ; or at a temperature up to gamma prime solvus minus 120.degree. C. at a strain rate between 1.times.10.sup.-2 and 5.times.10.sup.-1 s.sup.-1.

3. Heat Treatment

Partially or fully solution treating the item at a temperature in the range: gamma prime solvus minus 40.degree. C. up to gamma prime solvus plus 20.degree. C., for times between 0.5 and 8 hours. Cooling from solution temperature at a rate suitable to avoid cracking, whilst maintaining the alloy tensile response, eg: between 0.2 and 10.degree. C./s. Finally, ageing at temperatures between 650 and 900.degree. C. for between 10 and 30 hours.

A relatively coarse grain size is associated with good fatigue crack growth resistance. An aim of the overall processing conditions of the current invention is therefore to achieve a fairly coarse grain size in the wrought and heat treated product, preferably within the range 6 to 45 .mu.m. A uniform grain size in the range 25 to 35 .mu.m is particularly preferred, but a non-uniform grain size, including a duplex structure may be satisfactory.

The following Table II provides information about the gamma prime and sigma phases in the alloys of the present invention, the prior alloy UDIMET 720Li being included for comparison. It can be noted that the weight percent and the solvus of the sigma phase in alloys 2 and 3 have been reduced below the levels for UDIMET 720Li. ______________________________________ gamma gamma prime prime weight percent solvus   weight percent sigma solvus Alloy (725.degree. C.) (.degree. C.) sigma (725.degree. C.) (.degree. C.) ______________________________________ 1     44.4       1165     6.87      888 2 & 3 45.7       1157     4.12      827 Udimet 43.7       1163     4.67      832 720Li ______________________________________

The following Table III reports on certain mechanical properties, creep and ultimate tensile strength, of the alloys of the invention compared to some known alloys. ______________________________________ Superalloy Mechanical Properties Time to 0.1% TPS (hours) UTS (MPa) Alloy       500 MPa/725.degree. C. typical 600.degree. C. ______________________________________ 1           35-40           1500+ 2 & 3       40-45           1550+ Waspaloy    <2              1143 UDIMET 720Li 15             1510 ______________________________________

Reference is directed to the accompanying FIGS. 1, 2 and 3, each of which is a phase diagram model prediction for alloy 2.

FIG. 1 shows phase mass from 0-100 wt % against temperature.

FIG. 2 is an enlarged version of part of FIG. 1 and shows phase mass from 0-2 wt % against temperature.

FIG. 3 is an enlarged version of part of FIGS. 1 and 2 and shows phase mass from 0-1 wt %, and temperature from 1000-1200K.

The following key applies to these figures:

The sigma phase (7) has a solvus at 1100K (827.degree. C.) The M.sub.23 C.sub.6 phase (6) has a solvus around 1170K (897.degree. C.). An ageing heat treatment lying between these temperatures (ie: applicable heat treatment window) encourages formation of a desired M.sub.23 C.sub.6 phase.

It should be noted that there is no heat treatment `window` for alloy 1. This alloy exhibits a sigma solvus temperature which is above that of the M.sub.23 C.sub.6 solvus.

It is preferred that there is less than 0.5% niobium added to these alloys, more preferably there is no niobium added to these alloys.

It is well know that the fatigue crack propagation resistance and the creep resistance of the majority of nickel base superalloys may be improved by increasing the grain size. Nickel base superalloys are composed of two principal phases, a gamma matrix and an ordered strengthening gamma prime phase (Ni.sub.3 Al/Ti). At the gamma prime solvus temperature of the alloy, the gamma prime phase is taken completely into solution in the gamma matrix. The gamma prime phase exists as two principal sizes, the primary gamma prime and the secondary gamma prime. The primary gamma prime is the larger of the two and is located on the grain boundaries. The primary gamma prime is retained throughout the manufacturing process to prevent the migration of the grain boundaries and hence to control grain size. If the primary gamma prime volume fraction is reduced the grain size is increased, even at temperatures below the gamma prime solvus temperature. The secondary gamma prime is precipitated uniformly throughout the gamma matrix on cooling during heat treatment processes.

Heat treatment at temperatures greater than the gamma prime solvus temperature, super solvus heat treatment, usually results in non-uniform grain growth, and thus it is difficult to generate reproducible structures using supersolvus heat treatments. Heat treatments at a temperature near, but less than, the gamma prime solvus temperature may be used to generate controlled and reproducible uniform grain growth.

The alloys of the present invention have a fine grain microstructure/size and it has been found that they inherently have good fatigue crack propagation resistance. The creep resistance and fatigue crack propagation resistance of the alloys of the present invention may be improved by increasing the grain size. Thus the alloys of the present invention do not require a supersolvus heat treatment, or other heat treatments, to generate a coarser grained microstructure in order to obtain good fatigue crack propagation resistance. Thus it can be seen that the alloys of the present invention make it possible to dispense with the expensive super solvus, or other heat treatments. The fine grains are normally 6-12 .mu.m, medium grains are 12-30 .mu.m and coarse grains are greater than 30 .mu.m.

## Claims

1. A nickel base alloy consisting essentially of in weight percent 18.5% cobalt, 15% chromium, 5% molybdenum, 2% tantalum, 3.6% titanium, 3% aluminum, 0.015% boron, 0.027% carbon, 0.06% zirconium, 0.75% hafnium, up to 0.1% iron, up to 0.2% silicon, up to 0.15% manganese and the balance nickel plus incidental impurities.

2. An alloy as claimed in claim 1, wherein the solvus of a topologically close packed phase is less than the solvus of an M.sub.23 C.sub.6 or M.sub.6 C phase.

3. A wrought and heat treated product which consists essentially of an alloy according to claim 1.

4. A product as claimed in claim 3, which is a disc for a turbine.

5. A product as claimed in claim 4 wherein the grain size is 6-45 .mu.m.

6. A product as claimed in claim 3, wherein the proportion of topologically close packed phase is less than 7.0% at a temperature of 725.degree. C.

7. A product as claimed in claim 3, wherein a M.sub.23 C.sub.6 or M.sub.6 C phase is present at grain boundaries.

8. A product as claimed in claim 3, wherein the gamma prime weight fraction at a temperature of 725.degree. C. is 45.+-.2%.

9. A nickel based alloy comprising in weight percent 18.5% cobalt, 15% chromium, 5% molybdenum, 2% tantalum, 3.6% titanium, 3% aluminum, 0.75% hafnium, 0.015% boron, 0.06% zirconium, 0.027% carbon and the balance nickel plus incidental impurities.

10. A nickel based alloy consisting essentially of, in weight percent, 18.5% cobalt, 14.35%-15.15% chromium, 4.25%-5.25% molybdenum, 1.35% to 2.15% tantalum, 3.45% to 4.15% titanium, 2.85% to 3.15% aluminum, 0.5% to 1.0% hafnium, 0.01% to 0.025% boron, 0.05% to 0.07% zirconium, 0.012% to 0.033% carbon, up to 1.0% iron, up 0.2% silicon, up to 0.15% manganese, and the balance nickel plus incidental impurities.