530.352 Materials Selection530.352 Materials Selection

Lecture #27: Superalloy Case StudyWednesday Nov 16th, 2005

The Boeing 777The Boeing 777

Powered by two GE 90’s

777’s are powered by 2 GE90’s:777’s are powered by 2 GE90’s:

Low Pressure Turbine

High Pressure Turbine

Combustor

High Pressure Compressor

Low Pressure Compressoror Booster

Fan

Fan AirCoreAir

GE 90 Jet Engine :GE 90 Jet Engine :

AEROTURBINE ENGINEAEROTURBINE ENGINE

Efficiency of a heat engineEfficiency of a heat engine

1

21

T

TTEfficiency

Three ways to increase efficiency:Three ways to increase efficiency:

TBC

Improvements in superalloy creep strength:Improvements in superalloy creep strength:

Improvements in creep strength:Improvements in creep strength:

Alloy development based on fundamental understanding of creep applied empirically.

Form stable precipitates (Ni3Al, Ni3Ti, M23C6, MoC, TaC)Have as many heavy atoms in solid solution as

possible (Hf, W, Co, Cr, Ta, etc.)Form a protective oxide surface (Cr2O3 or Al2O3)

Improvements in creep strength:Improvements in creep strength:

10 m10 m

1 m1 m

Solution Heat Treatment

Aging Treatment

Tem

pera

ture

Time

Tsolvus

Secondary ´

Tertiary ´Primary ´

• Supersolvus heat treatment (no “primary” ’)

• Secondary ’ develops upon cooling from supersolvus temperature

• Tertiary ’ forms upon cooling and subsequent aging

Microstructure development:Microstructure development:

Ref. Mills et alRef. Mills et al

Superalloy developments:Superalloy developments:

Improvements in creep strength:Improvements in creep strength:

Changing grain size to mitigate creep:Changing grain size to mitigate creep:

EquiaxedEquiaxed

Directional solidificationDirectional solidification

Single Single

crystal crystal turbine turbine bladeblade

Making single-crystalline turbine blades:Making single-crystalline turbine blades:

Investment casting:Investment casting:

http://www.alcoa.com/howmet/en/info_page/inv_cast.asp# http://www.alcoa.com/howmet/en/info_page/inv_cast.asp#

Section of a blade with cooling channels:Section of a blade with cooling channels:

Internal cooling channels: casting:Internal cooling channels: casting:

“TBCs are ‘bill-of-materials’ in many GEAE product lines: turbine blades, vanes, combustors.

Designers love it, and want more.”

Engine run without TBC

Engine run with TBC(150ºF reduction)

Ref. Ram Darolia, GE Aircraft Engines

Industrial perspective on the use of TBC’s:Industrial perspective on the use of TBC’s:

Strengthening of Gamma TiAl: Pros and ConsStrengthening of Gamma TiAl: Pros and Cons

Curt Austin and Jim Williams

TMS Annual Meeting

San Antonio, TX

2/17/98

Aircraft Engines

Outline of TalkOutline of Talk

Introduction to strengtheningDefinitionProperty correlations:

creep/yield strength; fatigue/yield strength; etc.Limitations, i.e. property trades & compromisesNeeds and applications

General discussion of strengthening of GammaMechanismsTrade-offsBenefits and “costs”

Consequences of strengthened GammaDuctilityFatigue strength

Summary and Conclusions

StrengtheningStrengthening

Narrow Definition: Alterations in the composition and/or microstructure that increase the time independent flow stress at small plastic strain (the yield stress)

Broader Definition: Alterations in the composition and/or microstructure that increase the time dependent flow stress at small plastic strain (the creep strength)

The available mechanisms and opportunities for these two strengthening modes are quite different

Effects Alloying on PropertiesEffects Alloying on Properties

DuctilityCr, Mn, V

Oxidation ResistanceCr, Nb, Ta, Zr

Tensile StrengthCr, Ta, W, B

Creep ResistanceCr, W, Ta, C, Si

Some alloying elements affect more than one property

Strength ImprovementStrength Improvement

Not difficult to improve strength:

- mechanisms avail.

Improved strength with constant ductility very difficult (complex issue)

Kth increase also req’d. to realize full benefit

200

300

400

500

600

Yie

ld S

tren

gth

, M

Pa

0 0.5 1 1.5 2 2.5 3

Elongation

Exp Alloys48-2-2

Available Strengthening MechanismsAvailable Strengthening Mechanisms

Solid solution Generally low solubility, second phases can be harmful Not a major contributor

Boundary Strongest contributor Can use heat treatment to tailor strength (within limits)

Texture (preferred orientation) Possible in principle for wrought products Difficult to control cost? Magnitude of effect not well-characterized

Second phase particle strengthening Relatively low Gfine dispersion for much strengthening Coarsening could be an issue

The real “player” is boundary strengthening

Ductility Vs. Yield StrengthDuctility Vs. Yield Strength

Increasing Yield Strength has a large effect on ductility

f =

0

f

f

00

f

Str

ess

Strain

y

y

y

Fracture Surfaces in Ti- 48.9% AlFracture Surfaces in Ti- 48.9% Al

Transgranular, interdendritic fracture mode

with B2(1.2 % elong.)

without B2(1.5% elong.)

B2

Ductility of Gamma - IssuesDuctility of Gamma - Issues

Major negative distinction from conventional alloys Careful component selection eliminates this technical concern Still a psychological issue with designers

Requirement often not explicitly known See component selection above Useful for “margin” and “robustness”

Often implicitly required at stress concentrations Need enough to relax stress without crack initiation Very helpful in avoiding handling and impact damage

Explicitly required in a few components rotor overspeed relatively small amount required (but > 0 !!)

containment, applies to casing applications

Ductility is a bigger perceived than real issue

Ductility as f(Al, dT/dt)Ductility as f(Al, dT/dt)

0.5

1

1.5

2

2.5

3%

Elo

ng

.

45 46 47 48 49

Al level, a/o

witness bars, fastest dT/dt

slabs, slower

slabs, faster

Fracture as f(Al, dT/dt)Fracture as f(Al, dT/dt)

Cr-Nb-Ta assessed effects on ductilityCr-Nb-Ta assessed effects on ductility

-0.3

-0.2

-0.1

0.0

0.1

0.2 D

elta

lo

g e

lon

g.

0 1 2 3 4Atomic percent

Cr

Ta

Nb

B2 ppts. interdendritic embritt.Interdendritic strengthening

Two distinct domains in cast alloys

Yield Strength vs. HCF StrengthYield Strength vs. HCF Strength

Data for several alloys and several temperatures

0

20

40

60

80

0 20 40 60 80 100

0.2% Yield Strength, ksi

R = 0Alt

ern

atin

g s

tre

ss

(k

si)

@ 1

0 7 c

yc

les

R = -1

Limiting Property Often KthLimiting Property Often Kth

Material 1: Lo fatigue strength, Baseline FCG, baseline Kth (4.35)Material 2: Hi fatigue strength, Baseline FCG, baseline Kth Material 3: Hi fatigue strength, Lower FCG (0.4x) , baseline Kth Material 4: Hi fatigue strength, Lower FCG (0.4x) , higher Kth (5.0)

Life (Cycles)

101

102

103

104

105

106

107

20

30

40

50

60

70M

ax. S

tres

s (k

si)

Material 4

th80

Materials 1,2, & 3

0.015” surface flaw

a

from Ken Wright, GE

Fat. Strength:Material 1Materials 2, 3 & 4

FCG:Material 1,2Material 3,4

As-cast segregation patternAs-cast segregation pattern

Sutliffe, Huang and Sitzman

NCG 359ENCG 359E

0.15% elong

47Al-1.8Cr-0.8Nb-1.7Ta 0.6% elong

High impurity heat

Low Al heat

Speculation Regarding CreepSpeculation Regarding Creep

Alloying effects:Cr is potent strengthener

Strengthens gammaVery fine B2 ppts may be good

NbHeavy solute atom effect outweighed by reduced grain size

TaHeavy solute effect outweighs reduced grain size

Microstructure effects:Grain size is importantGrain aspect ratio may be importantSecond phase coarsening not well understood

Hard to separate composition and microstructure contributions

Some Property CorrelationsSome Property Correlations

Direct correlations:Strength - HCF strength Strength -Partcost

Particularly true for wrought processing

Inverse correlations:Strength - Ductility Ductility -Crack growth Creep -Ductility

Caveat: Above observations colored by GE focus on

cast material, but: some very interesting wrought properties obtained recently GE following closely those related to components for which

wrought processing is amenable and affordable.

Blade StatusBlade Status

Design releasedCasting trials nearing completionRisk issues under studyEconomical production is major

concern

Summary and Take-awaysSummary and Take-aways

Strengthening of Gamma is possible: Standard mechanisms pertain (to varying degrees)

Increased strength hurts ductility: Component selection to minimize risk is essential Need to deal with perceived risk by designers Situation for cast and for wrought alloys may be different

Increased strength helps smooth fatigue but: Life benefit limited unless FCG rate and Kth can be

improved

Creep less well understood but relation to yield strength appears limited

Cast Gamma still most likely to be affordable Need to get one or two real service applications