Superalloys Dra. A Salas Zamarripa

“Superalloys as a class constitute

the currently reigning aristocrats

of the metallurgical world. They

are the alloys which have made

jet flight possible, and they show

what can be achieved by

drawing together and exploiting

all the resources of modern

physical and process metallurgy

in the pursuit of a very challenging

objective.”

from R.W. Cahn The coming of materials science, 2001

Introduction Certain classes of material possess a

remarkable ability to maintain their properties at elevated temperatures. These are the high-temperature materials.

Their uses are many and varied, but good examples include the components for turbines, rockets and heat exchangers.

For these applications, the performance characteristics are limited by the operating conditions which can be tolerated by the materials used.

What are the desirable characteristics

of a high-temperature material?

The first is an ability to withstand loading at an operating temperature close to its melting point.

A second characteristic is a substantial resistance to mechanical degradation over extended periods of time.

A final characteristic is tolerance of severe operating environments.

Superalloys

A superalloy is a metallic alloy which can be used at high temperatures, often in excess of 0.7 of the absolute melting temperature.

Creep and oxidation resistance are the prime design criteria.

Superalloys can be based on iron, cobalt or nickel, the latter being best suited for aeroengine applications.

Superalloys as high-

temperature materials

Resistance to loading under static, fatigue and creep conditions is required, the nickel-base superalloys have emerged as the materials of choice for high temperature applications (800 ◦C).

This is the case for gas turbines used for jet propulsion: 100,000 lb thrust engines used for the Rolls-Royce Trent 800 and General Electric GE90 which power the Boeing 777 , but also the smaller 1000 lb engines used for helicopter applications.

Superalloys as high-temperature

materials

When designing a gas turbine engine, great emphasis is

placed on the choice of the turbine entry temperature (TET): the temperature of the hot gases entering the turbine

arrangement

Evolution of the turbine

entry temperature (TET)

capability of Rolls-Royce’s

civil aeroengines, from

1940 to the present day.

Diagram illustrating the basic

features of a very basic gas

turbine engine: the turbojet.

(Courtesy of Rolls-Royce.)

Illustration of material usage in the Trent

800 engine. Note the extensive use of

nickel-based superalloys in the

combustor and turbine sections.

(Courtesy of Rolls-Royce.)

What are the operating conditions inside a

gas turbine ?

Turbine blades in a jet engine experience:

Mechanical forces

• creep

• fatigue

• thermomechanical fatigue

•High temperature environment

• oxidation

• hot corrosion

Superalloys as high-temperature

materials

Variation of the turbine entry temperature (TET) during a

typical flight cycle of a civil aircraft.

Historical development of the

superalloys America 1930, Ti and Al added to the classic heating

element alloy nichrome (Ni-20Cr), resulted in significant increase in creep resistance

1940s first superalloy Nimonic 75 made high creep resistance thought to be due to precipitation

hardening confirmed by Taylor and Floyd (1951-2) with their work on

phase diagrams [time of quantitative revolution in metallurgy]: age hardening was due to an ordered intermetallic phase Ni3Al and Ni3Ti (or rather Ni3(Al, Ti) (γ‘) dispersed in a more Ni rich disordered matrix (γ)

Both γ and γ’ phases are cubic, with their cube axes parallel; structure extremely fine in scale (γ’ cuboids <0.5μm)

Historical development of the

superalloys Westbrook 1957 discovered the highly unusual characteristic of γ’ of

becoming stronger with increasing temperature (reason ? … to do with the geometry of dislocations in the phase…)

Maximum creep rupture life when the mismatch in lattice parameters of γ and γ’ is a small fraction of 1% and when volume fraction of γ’ is as high as possible. (Decreasing the mismatch from 0.2% to zero led to a 50x increase in creep rupture life!)

The microstructure is also unusually stable - the γ’ precipitates coarsen (Ostwald ripening) very very slowly, because of the low interfacial energy between the γ and γ’

Historical development of the

superalloys

Evolution of the high-temperature capability of the superalloys over a 60

year period since their emergence in the 1940s.

Historical development of the

superalloys

Turbine blading in the (a) equiaxed, (b) columnar and (c)

single-crystal forms

Why nickel-based alloys

Nickel displays the FCC crystal structure and is thus both tough and ductile, due to a considerable cohesive energy arising from the bonding provided by the outer d electrons.

Nickel is stable in the FCC form from room temperature to its melting point, so that there are no phase transformations to cause expansions and contractions which might complicate its use for high-temperature components.

Other metals in the transition metal series which display this crystal structure, i.e. the platinum group metals (PGMs), are dense and very expensive.

Low rates of thermally activated creep require low rates of diffusion.

Why nickel-based alloys

Diffusion rates for FCC metals such as Ni are low; hence, considerable microstructural stability is imparted at elevated temperatures.

Finally, consider other elements which possess different crystal structures.

HCP metals: Co displays an acceptable density and cost. Re and Ru are PGMs and are therefore expensive. Os has an oxide which is poisonous. Tc is radioactive.

Co-based superalloys are, in fact, used for high-temperature applications; however, they tend to be more expensive than the nickel-based superalloys.

BCC metals such as Cr are prone to brittleness, and there is a ductile/brittle transition which means that the toughness decreases significantly with decreasing temperature.

Ni base Superalloys

The essential solutes in nickel based superalloys are aluminium and/or titanium, with a total concentration which is typically less than 10 atomic percent.

This generates a two-phase equilibrium microstructure, consisting of gamma (γ) and gamma-prime (γ').

It is the γ' which is largely responsible for the elevated-temperature strength of the material and its incredible resistance to creep deformation.

The amount of γ' depends on the chemical composition and temperature

Superalloys

Transmission electron micrograph

showing a large fraction of cuboidal

γ' particles in a γ matrix. Ni-9.7Al-

1.7Ti-17.1Cr-6.3Co-2.3W at%. Hillier,

Ph.D. Thesis, University of

Cambridge, 1984.

Transmission electron micrograph

showing a small fraction of

spheroidal γ' prime particles in a γ

matrix. Ni-20Cr-2.3Al-2.1Ti-5Fe-0.07C-

0.005 B wt%. Also illustrated are

M23C6 carbide particles at the grain

boundary running diagonally from

bottom left to top right.

Alloy Compositions

Commercial superalloys contain more than just Ni, Al and Ti.

Cr and Al are essential for oxidation resistance small quantities of yttrium help the oxide scale to cohere to the substrate.

Polycrystalline superalloys contain grain boundary strengthening elements such as boron and zirconium.

There are also the carbide formers (C, Cr, Mo, W, C, Nb, Ta, Ti and Hf). The carbides tend to precipitate at grain boundaries and hence reduce the tendency for grain boundary sliding.

Elements such as Co, Fe, Cr, Nb, Ta, Mo, W, V, Ti and Al are also solid-solution strengtheners, both in γ and γ'.

There are, naturally, limits to the concentrations that can be added without inducing precipitation. It is particularly important to avoid certain embrittling phases such as Laves and Sigma. There are no simple rules governing the critical concentrations; it is best to calculate or measure the appropriate part of a phase diagram.

Alloying element effects

in nickel based superalloys. The "M" in

M23C6 stands for a

mixture of metal atoms.

(Cr,Fe)23C6

Alloy Cr Co Mo W Ta Nb Al Ti Fe C B Zr Re Hf Others

Astroloy PM 14.9 17.2 5.1 4 3.5 0.03 0.04

CMSX2 SX 8 4.6 0.6 7.9 5.8 5.6 0.9

CMSX4 SX 5.7 11 0.42 5.2 5.6 5.2 0.74 3 0.1

CMSX6 SX 9.8 5 3 2.1 4.8 4.7

CMSX10 SX 2 3 0.4 5 8 0.1 5.7 0.2 6 0.03

FT750DC wrought 20 3.5 2.3 2.1 5 0.07 0.005 0.4 Si

Hastelloy X wrought 22 1.5 9 6 18.5 0.1 0.5Mn, 0.5Si

Hastelloy S wrought 15.5 14.5 0.2 1 0.02 0.009 0.02 La

Inconel 600 wrought 15.8 7.2 0.04 0.2Mn, 0.2 Si

Inconel 718 wrought 18.6 3.1 5 0.4 0.9 18.5 0.04 0.2Mn, 0.3Si

MA758 MA/ODS 30 0.5 0.3 0.05 0.6 yttria

MA760 MA/ODS 19.5 3.4 6 1.2 0.06 1.0 yttria

MA6000 MA/ODS 15 3.9 4.5 2.3 1.5 0.06 1.1 yttria

MAR-M200 cast 9 10 12 1 5 2 0.15 0.015 0.05

Nimonic 80A wrought 19.5 1.1 1.3 2.5 0.06

Nimonic 105 wrought 14.5 20 5 1.2 4.5 0.2

PM1000 MA/ODS 20 0.3 0.5 3 0.6 yttria

Rene N5 SX 7 8 2 5 7 6.2 3 0.2

Rene N6 SX 4.2 12.5 1.4 6 7.2 5.75 5 0.15

Rene 41 wrought 19 11 10 1.5 3.1 0.09 0.05

RR2000 SX 10 15 3 0.05 4 1 V

SRR99 SX 8.5 5 9.5 2.8 5.5 2.2

TMS 63 SX 6.9 7.5 8.4 5.8 0

Udimet 500 wrought 18 18.5 4 2.9 2.9 0.08 0.006 0.05

Udimet 700 wrought 15 18.5 5.2 4.3 3.5 0.08 0.03

Waspaloy wrought 19.5 13.5 4.3 1.3 3 0.08 0.006 0.06

Nominal chemical compositions, wt%. MA/ODS ≡ mechanically alloyed,

oxide dispersion-strengthened.

PM ≡ powder metallurgical origin. The alloy names are proprietary. SX ≡

single crystal.

Alloy Composition The single-crystal superalloys are often classified into first,

second and third generation alloys.

The second and third generations contain about 3 wt% and 6wt% of rhenium respectively.

Rhenium is a very expensive addition but leads to an

improvement in the creep strength.

It is also claimed that rhenium reduces the overall diffusion rate in nickel based superalloys.

Applications of nickel based

superalloys

Turbine Blades

A major use of nickel based superalloys is in the manufacture of aeroengine turbine blades.

A single-crystal blade is free from γ/γ grain boundaries. Boundaries are easy diffusion paths and therefore reduce the resistance of the material to creep deformation.

The directionally solidified columnar grain structure has many γ grains, but the boundaries are mostly parallel to the major stress axis; the performance of such blades is not as good as the single-crystal blades.

However, they are much better than the blade with the equiaxed grain structure which has the worst creep life.

Applications of nickel based

superalloys

Applications of nickel based

superalloys

One big advantage of the single-crystal alloys is that many of the grain boundary strengthening solutes are removed.

This results in an increase in the incipient melting temperature (i.e., localized melting due to chemical segregation).

The single-crystal alloys can therefore be heat treated to at temperatures in the range 1240-1330°C, allowing the dissolution of coarse γ' which is a remanent of the solidification process.

Applications of nickel based

superalloys

Subsequent heat treatment can therefore be used to achieve a controlled and fine-scale precipitation of γ'.

The primary reason why the first generation of single-crystal superalloys could be used at higher temperatures than the directionally solidified ones, was because of the ability to heat-treat the alloys at a higher temperature rather than any advantage due to the removal of grain boundaries.

A higher heat-treatment temperature allows all the γ' to be taken into solution and then by aging, to precipitate in a finer form.

Applications of nickel based

superalloys

Superalloy blades are used in aeroengines and

gas turbines in regions where the temperature is

in excess of about 400oC, with titanium blades

in the colder regions.

This is because there is a danger of titanium

igniting in special circumstances if its

temperature exceeds 400oC.

Applications of nickel based

superalloys

Turbine blades are attached to a disc which in turn is connected to the turbine shaft.

The properties required for an aeroengine discs are different from that of a turbine, because the metal experiences a lower temperature.

The discs must resist fracture by fatigue.

Discs are usually cast and then forged into shape. They are polycrystalline.

Applications of nickel based

superalloys

One difficulty is that cast alloys have a large columnar grain structure and contain significant chemical segregation; the latter is not completely eliminated in the final product. This can lead to scatter in mechanical properties.

One way to overcome this is to begin with fine, clean powder which is then consolidated. The powder is made by atomisation in an inert gas; the extent of chemical segregation cannot exceed the size of the powder.

After atomisation, some discs are made from powder which is hot-isostatically pressed, extruded and then forged into the required shape.

The process is difficult because of the need to avoid undesired particles introduced, for example, from the refractories used in the atomisation process, or impurities picked up during solidification. Such particles initiate fatigue; the failure of an aeroengine turbine disc can be catastrophic.

Applications of nickel based

superalloy

Turbochargers An internal combustion engine generally uses a

stoichiometric ratio of air to fuel. A turbocharger is a device to force more air into the engine, allowing a correspondingly greater quantity of fuel to be burned in each stroke. This boosts the power output of the engine.

The turbocharger consists of two components, a turbine which is driven by exhaust gases from the engine. This in turn drives an air pump which forces more air into the engine. The typical rate of spin is 100-150,000 rotations per minute. Because the turbocharger is driven by exhaust gasses, it gets very hot and needs to be oxidation resistant and strong.

Failure in Superalloys

Case: Boeing 737

On13 October 2000, experienced an in-flight engine failure that resulted in the engine being shut down and the aircraft returning safely to Hobart airport in Tasmania, Australia.

Powered by two General Electric CFM56 turbofan engines,.

After disassembly and inspection of the engine, failure was attributed to the loss of a 15mm by 20mm segment from the trailing edge of a single high-pressure (HP) turbine blade fabricated from the Rene 125 alloy.

This passed into the low-pressure turbine stages, where it caused overloading and collapse of the entire blade array.

Metallographic examination of the HP blades indicated that radial cracks near the blade tips were common, as a result of severe thermal cycling, high thermal gradients and thermal fatigue.

Case: Boeing 737 Many of the tips had been weld-repaired using the

Rene 80 alloy.

In the failed blade, tip cracks had grown into a v-shaped notch because of oxidation and corrosion effects, and had intercepted a deep underlying repair weld fabricated from the lower strength superalloy Inconel 625.

On reaching the base of the repair weld, the resultant fatigue stresses were sufficient to propagate the cracking to the point of final failure.

The failed blade had completed 17,928 flight cycles, and had flown 5,332 cycles since its repair and overhaul.

(a) General view of the high-pressure turbine; (b) high-pressure turbine

rotor with the failed blade indicated; (c) failed high-pressure turbine

blade; (d) tip notch damage observed on the remaining blades.

FUTURE: ALLOY Development

• Development of new alloys is a combination of

experimental work, modelling and black art.

• Further improvements - YES, but the scope is

limited as superalloys now operate at 85-90% of

their melting temperature.

• Cheaper alloys

• Develop concurrently with coating systems