C.A. Yablinsky, M.J. Mills, K.M. Flores, J.C. Williams

Joe Rigney- GE

MEANS2 Meeting

Fatigue Behavior in Ni-base Superalloys for blade applications

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Introduction- Modern Designs

• Modern designs have cooling channels. • Experience and experimental studies show

fatigue is the life-limiting factor

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Introduction: Turbine blade fatigue• Sources of Fatigue:

– Mechanical stress due to centrifugal motion and stress concentrations

– Thermal-mechanical fatigue (TMF) due to local hot spots constrained by cooler surrounding regions

• Isothermal Sustained Peak Low Cycle Fatigue (SPLCF) Tests:– Mimic TMF cycles and behavior– Each cycle has hold time

• Comprehensive fatigue mechanism investigations have not previously been done

High Temperature

Low Temperature

Temperatures within an airfoil

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Experimental Details• Material

– Monocrystalline Ni-based superalloy René N5– Coated, cylindrical specimens

• Sustained Peak Low Cycle Fatigue Testing– Temperature: 980°C-1090°C– Total Strain Control– Held in compression or tension

Ni Cr Co Mo W Al5 6.2

Ta Re7 3

Hf61.8 7 8 2 0.2

Durand-Charre, Madeleine. The Microstructure of Superalloys.Amsterdam: Gordon and Breach Science, 1997. p11.

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Stress State Changes in SPLCF CyclesCompression Hold• Yield possible, creep relaxation• Mean stress shift upward• Positive mean stress develops

Tension Hold• Creep relaxation• Mean stress shifts down

(+)

σ

(+)

(-)

1st cycle 2nd cycle 3rd cycle

t

tε

(-)

σ

(+)

(-)

1st cycle 2nd cycle 3rd cycle

t

t

ε

(-)

(+)

6

2mm 2mm2mm

SEM Fractography- Compression

• Crack initiation at surface• Multiple initiation sites• Crack propagation depends on temperature

– Environmental effects

1090°C 980°C 1040°C

Primary Initiation SitesSecondary Initiation Sites

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SEM Fractography- 1040°C, 0.9%

• Striations• Change in crack plane• Surface is oxidized along

crack wake

2mm2mm

Compression hold Tension hold

• Creep deformation observed; initiating at casting pores and carbides

• Macroscopically flat fracture surface

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SEM γ’ Characterization- 1040°C

• Cuboidal γ’• No preferential alignment (no rafting)• Note: γ phase is the lighter phase in all pictures

Thermally Exposed Material- No StressParallel Transverse

5μm 5μm

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SEM γ’ Characterization- 1040°C, 0.9%

• N-type– Rafted perpendicular to tensile

loading direction• γ phase shorter than seen in

creep alone

• P-type – Rafted parallel to tensile

loading direction• Rafted domains in

orthogonal orientation

Compression hold Tension holdParallel Transverse Parallel Transverse

γ’ phase is continuous phaseThese types of rafted structures are seen for negative misfit

5μm 5μm

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TEM- Thermally Exposed Material- 1040°C

• Dislocations at γ/γ’interface

• Dislocation looping around γ’ phase

g020

500nm

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SPLCF Results- 1040°C, Compression hold

• Extensive dislocation networks in γ and at γ/γ’ interfaces

• Dislocations in γ’

γ

γγ’

g020

g200

500nm200nm

12

500nm

γ’

γ

g200

3au

SPLCF Results- 1040°C, Tension hold• Dislocation network in γ

phase and at γ/γ’ interface• 001 superdislocations in γ’

phase

g020

200nm

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Conclusions• Fracture is very different

– Compression hold• Crack propagation from surface• Oxidation of fracture surface in crack wake

– Tension hold• Crack propagation from casting pores• Surface macroscopically flat

• Rafted microstructure from cuboidal structure under stress

• P-type rafting with compression hold• N-type rafting with tension hold

– Broken microstructure due to influence of fatigue compared to rafted creep structures

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Conclusions- cont.• TEM

– Thermally exposed• Dislocations along γ/γ’ interfaces, which have begun

looping around γ’ phase– Compression hold

• Extensive dislocation networks in γ• Dislocations in γ’

– Tension hold• Dislocation networks at γ/γ’ interfaces; • 001 superdislocations in γ’

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Future Test Plans

• Test cylindrical specimens under low cycle fatigue conditions in order to study deformation mechanisms that lead to damage accumulation and crack initiation.

–Use the FIB/TEM technique to examine deformation at the initiation site

–Determine the crystallography of the initiation site

–Determine the crystallographic plane(s) of crack propagation

• Existing LCF and SPLCF samples (Coated)–New tests on uncoated specimens in air

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Future Test Plans

• Test compact tension specimens under cyclic loading conditions to determine the influence of microstructure on crack propagation and fatigue failure. – Test effects of load ratio, environment, and

crystallographic orientation– Use the FIB/TEM technique to examine deformation in

the plastic zone ahead of the crack tip and in wake of the crack.

• Analyze and compare deformation mechanisms at the crack initiation site, ahead of the crack tip, in the plastic wake of the crack, and in the bulk material.

• Compare results to SPLCF results.