Superalloys

Engineering of Superalloys • • • •

Gas turbines - how they work, examples Operating conditions in a gas turbine Materials in turbines and turbine blades Evolution of Ni-base superalloy turbine blades – alloy development – processing (equiaxed, DS, SC) – blade cooling – coatings – HIPing, liquid metal cooling • The future ? Jane Blackford

“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.

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How does a gas turbine work ? Newton’s 3rd Law

MV

Equilibrium

Reaction

Action

Thrust = Mass x Velocity (MV)

From Cervenka, Rolls Royce, 2000

Gas turbines • Aircraft – civil – military

• Industrial – for power generation • based on the same principles

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Propeller versus Jet Propulsion Propeller - moves LARGE MASS of air at low velocity

Mvjet

Mvaircraft

Thrust = M(vaircraft - vjet)

Thrust = m(Vaircraft - Vjet)

mVaircraft

mVjet

Jet - moves small mass of gas at HIGH VELOCITY From Cervenka, Rolls Royce, 2000

Modern civil aircraft engines

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Jet Engine Layout Compressor

Combustion Chamber

Exhaust Nozzle

mVaircraft

mVjet

Shaft

Turbine


What are the operating conditions inside a gas turbine ? In particular think about the forces and environment turbine blades are subjected to….

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Turbine blades in a jet engine experience: Mechanical forces • creep • fatigue • thermomechanical fatigue •High temperature environment • oxidation • hot corrosion

Pressure and Temperature

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Pressure (atmospheres) 0 1500

Temperature (degrees C) 0 From Cervenka, Rolls Royce, 2000

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Design of modern aircraft turbine engines and the materials used NB drive to increase operating temperatures as this increases maximum speed and improves efficiency (lower fuel costs)

Multiple Shafts - Trent 95,000 lbs Thrust LP System 1 Fan stage IP System 5 Turbine stages 8 Compressor stages >3,000 rpm 1 Turbine stage >7,500 rpm

HP System 6 Compressor stages 1 Turbine stage >10,000 rpm


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Material Strength Specific Strength Titanium Alloy Nickel Alloy

Steel Aluminium Alloy

Temperature From Cervenka, Rolls Royce, 2000

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Ni-based superalloys are used for turbine blades Now we will consider their evolution

Evolution of Ni-based superalloy turbine blades 1. Alloy development (historical) 2. Processing (equiaxed, DS, SC) 3. Blade cooling 4. Alloy development (modern) 5. Coatings 6. Novel processing

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1. Alloy development (historical) - 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)

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1. Alloy development (historical) - 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’


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Investment casting • Vacuum process, reduces oxide contamination • Huge advances in process cleanliness made about 20-30 years ago - led to considerable improvements in blade properties

• Controlled cooling (directional solidification) enables microstructural control • Cooling channels can be cast into the blade (using a ceramic cored mould) • Blades heat treated (solution treatment + aged)

Improvements in blade microstructure

Equiaxed Directionally Single Crystal Crystal Structure Solidified Structure


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Turbine blade heat treatment

As-cast dendritic microstructure

longitudinal section

transverse section

micrographs of DS as-cast

simulation of dendrite growth

snow crystal dendrite

superalloy IN792

Turbine blade heat treatment As-cast dendritic microstructure

precipitation hardened: γ’ in γ matrix precipitation hardening (solution treatment + ageing)

γ’

γ

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Turbine Cooling

Single pass

Cooling air Multi-pass

Thermal Barrier Coating


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4. Alloy development (modern) Modern Ni-based superalloys contain 10+ alloying additions (plus impurities) Solid solution strengthening (Co, Cr, Fe, Mo, W, Ta,Re) Grain boundary strengthening with carbides (W, Ta, Ti, Mo, Nb, Hf, Cr) and other precipitates (e.g. carbonitrides) γ’ formers Al, Ti Improve oxidation resistance (Al, Cr, Y, La, Ce) Improve hot corrsion resistance (Cr, Co, Si, La, Th) Grain boundary refiners (B, C, Zr, Hf) Development of new alloys is a combination of experimental work, modelling and black art see Sims for microstructural developments

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Coatings coating

substrate

environment

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5. Coatings (i) for oxidation (and hot corrosion) resistance Aluminide diffusion coatings (used on ca. 80% HP blades) by pack aluminising process (alternative IVD+HIP) based on NiAl (50µm thick) MCrAlY overlay coatings by physical vapour deposition (PVD) process based on MCrAlY (100-200µm thick) (ii) thermal barrier coatings (see slide on blade cooling) ceramic materials - e.g. zirconia (0.3-0.4mm thick) used to insulate blade deposited on top of MCrAlY overlay coating see Nicholls and Stephenson 1991


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6. Novel processing • Hot isostatic pressing (HIPping) • Liquid metal cooling (LMC)

Improvement of cast superalloy turbine blade properties by hot isostatic pressing (HIPping) Cast alloys often contain pores, these are detrimental to the mechanical properties HIP = simultaneous application of high temperature (up to 2000°C) and pressure (up to 200MPa) via inert (argon) gas HIPping can remove sealed porosity from castings (cast +HIP = forged) (NB the casting remains solid you don’t want it to melt....) 90+% of high pressure turbine blades are HIPped Blades can be “rejuvenated” by HIPping

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Improvement of cast superalloy turbine blade properties by hot isostatic pressing (HIPping) To illustrate the effectiveness of the HIP process a 25mm diameter hole was machined in two halves of a stainless steel block 75mm square. The edges of the block were welded together, the air evacuated from the hole and the evacuation pipe sealed to create a subsurface pore. The block was HIPped and subsequently cut in half to reveal fully dense material and complete absence of any pore.

Liquid metal cooling • Improved microstructures • Faster solidification - higher throughput and reduced costs

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The 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

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Process development Larger blades Process cleanliness LMC? Reduce hot tearing in DS blades Higher production yield of single crystal blades

Computer modelling of single crystal superalloy solidification illustrates how a single grain is selected using the “spiral selector” the grain has an <001> orientation oriented vertically

from Bhadeshia

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Superalloys

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