Superalloy Dependent Stability ofβ-NiAl Phase in NiCoCrAlY Coatings

Emmanuel Perez, Travis Patterson

Yong-ho Sohn

Advanced Materials Processing and Analysis Center and

Mechanical, Materials and Aerospace Engineering Department

University of Central Florida, Orlando, FL

April 19, 2005

NIST Diffusion Workshop

NiCoCrAlY Coatings in Service

NiCoCrAlY coatings are employed in gas turbine engines toprotect hot-section components such as blades and vanesagainst oxidation and hot corrosion.

These coatings possess a two-phase microstructure consistingof high Al-content β-NiAl solid solution phase and low Al-contentγ-Ni solid solution phase.

The coatings are designed to form a continuous protective Al2O3

oxide scale that protects the coating and in turn the substrate.

Combustors

Air Intake

Compressor

High Pressure Turbine

Low Pressure Turbine

Exhaust

Shaft

Lifetime of NiCoCrAlY Coatings

The Al rich β-phase in the coating isdissolved from the top and the bottompseudo interfaces:

Al is depleted on the top by formation andmaintenance of a protective oxide layer, Al2O3.Al is depleted via interdiffusion with asuperalloy substrate.

As Al depletes, the β-phase dissolves intothe γ-phase (Ni Solid Solution).

The failure of NiCoCrAlY coatings may bedefined by the complete dissolution of β-phase.

Al2O3 scale loses continuity

As Coated

Depleted

BEI: As-Coated

BEI: 5 Cycles

BEI: 50 Cycles

BEI: 100 Cycles

BEI: 314 Cycles

BEI: 400 Cycles

BEI: As-Coated

BEI: 5 Cycles

BEI: 50 Cycles

BEI: 100 Cycles

BEI: 200 Cycles

BEI: 400 Cycles

Parabolic Growth of TGOKp = 6.3 x 10-3 µm·sec1/2

Depletion Zone: Deff = 3.4 x 10-15 m2/secInterdiffusion Zone: Deff = 9.3 x 10-15 m2/sec

Oxidation and Interdiffusion: Recession of (β+γ) in NiCoCrAlY

Y.H. Sohn et al., Surf. Coat. Technol., 146-147 (2001) pp. 70-78.

Interdiffusion and Lifetimeof Oxidation Resistant Coatings

3X in Lifetime (Measured by Stability of Al-Rich β-NiAl Phase)Can be Achieved by Appropriate Selection of SubstrateComposition (Given a Coating Composition).

Isothermal Exposure Time, t 3 x Isothermal Exposure Time, 3t

E. Perez, Y.H. Sohn, Unpublished Research.

Objectives

Determine/Predict the effective interdiffusion coefficientsof Al using solid-to-solid diffusion couples of β-NiAl vs.various superalloys (γ+γ’+others) by:

Direct determination of interdiffusion fluxes fromexperimental concentration profiles in single β-NiAlphase region.Calculation of effective interdiffusion coefficientsincorporating the multicomponent diffusionalinteractions.Prediction of effective interdiffusion coefficients inmultiphase superalloys based on mass balance.

Examine the composition-dependence of Al interdiffusioncoefficients as a function of initial superalloycompositions.

Experimental Details

Solid-to-solid diffusion couples.Equiaxed NiAl vs various commerciallyavailable Ni-superalloys.Encapsulated in quartz capsule in Ar(1 atm at 1050°C) after severalhydrogen flush.Diffusion anneal for 96 hours at 1050°Cusing Lindberg/Blue 3-Zone horizontaltube furnace.Diffusion structures examined byoptical and scanning electronmicroscopyConcentration profiles determine byElectron Probe Microanalysis (EPMA)using pure standards and ZAFcorrection.

Solid-to-Solid Diffusion Couples

Excellent Diffusion Bonding Between Alloys. Particles rich in refractory elements (e.g., Ta, W, Mo, Nb, etc) are

precipitating near the interface between NiAl and superalloys.

NiAl

CM247

NiAl

GTD111

NiAl

IN738

NiAl

IN939

NiAl

Waspalloy

Phenomenology of Isothermal Interdiffusionin Multicomponent System

Onsager’s formalism* for The Interdiffusion Flux of Componenti in a Multicomponent System :

* L. Onsager, Phys. Rev., 37 (1931) 405; 38 (1932) 2265; Ann. NY Acad. Sci., 46 (1965) 241.

Requires Knowledge of (n-1) Independent Concentrations and(n-1)2 Interdiffusion Coefficients.

For a Ternary Systems:

€

˜ J 1 = − ˜ D 113 ∂C1∂x

− ˜ D 123 ∂C2

∂x and ˜ J 2 = − ˜ D 21

3 ∂C1∂x

− ˜ D 223 ∂C2

∂x

€

˜ J i = − ˜ D ijn ∂C j

∂xj=1

n−1

∑ (i = 1,2,...,n -1)

where ∂C j ∂x is the n -1( ) independent concentration gradients

˜ D ijn is the n -1( )2 interdiffusion coefficients

Determination of Ternary Interdiffusion Coefficients byExtension of Boltzmann-Matano Analysis*

* J. Kirkaldy, Can. J. Phys., 35 (1957) 435.

1

23

A B

CD

Distance, x

Co

nce

ntr

atio

n,

Ci

A

B D

C

Requires two independent diffusion couples intersecting at a commoncomposition.

Requires a significant number of diffusion couple experiment toassess compositional dependence of interdiffusion coefficients.

Determination of Interdiffusion FluxesInterdiffusion fluxes of allcomponents can be determineddirectly from their concentrationprofiles without the need of theinterdiffusion coefficients:

€

˜ J i =12t

(x - xo )dCiCi

− or Ci+

Ci x( )

∫ (i = 1,2,..,n)

where t is time

M. A. Dayananda, C. W. Kim, Metall. Trans., 10A (1979) 1333.

No Need for Interdiffusion Coefficientto Assess Diffusional Bahavior ofIndividual Components.

Profiles of experimentalconcentration and the correspondinginterdiffusion fluxes of Cu-Ni-Zncouple, α5 (Cu-43.5at.%-25.0at.%Zn)vs. α12 (Cu-17.5at.%Ni), annealed at775°C for 48 hours.

Integrated and Effective Interdiffusion

€

Di,NiAlint = ˜ J i x( )dx−∞

xo∫ and Di,SAint = ˜ J i x( )dxxo

+∞∫

€

€

˜ D i,NiAleff =

˜ D i,NiAlint

Ci− −Ci

o =

˜ J i−∞

xo∫ dx

Ci− −Ci

o and ˜ D i,SAeff =

˜ D i,SAint

Cio −Ci

+=

˜ J ixo

+∞

∫ dx

Cio −Ci

+

Integrated interdiffusion coefficients for a component iin NiAl and superalloy sides can be defined as:

Effective interdiffusion coefficients for a component iin NiAl and superalloy sides can be defined as:

Effective interdiffusion coefficients incorporatesmulticomponent diffusional interactions:

€

˜ D ieff = ˜ D ii

n +˜ D ij

n ∂C j ∂x∂Ci ∂xj

∑ (j≠ i)

M. A. Dayananda, Y.H. Sohn, Scripta Mater., 35 (1996) 683.

Correlation in Interdiffusion Coefficientswith Concentrations

€

Ci− −Ci

o

Cio −Ci

+=αi,SAαi,NiAl

˜ D i,NiAleff

˜ D i,SAeff

and Ci− −Ci

o

Cio −Ci

+=

˜ D i,NiAlapp

˜ D i,SAapp

where Diapp =

˜ D ieff

αi

Effective interdiffusion coefficients on either side ofthe analysis can be related to compositions:

Therefore, interdiffusion coefficients calculated fromsingle-phase region (e.g., NiAl) can be employed topredict those of multiphase regions (e.g., superalloys).

M. A. Dayananda, Y.H. Sohn, Scripta Mater., 35 (1996) 683.

IN93

9 C

on

cen

trat

ion

(at

%)

Ni

Co

Cr

Al

Ti

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Position (µm)

Matano plane X0=154µm

IN93

9 C

on

cen

trat

ion

(at

%)

Ni

Co

Cr

Al

Ti

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Position (µm)

Matano plane X0=154µm

IN939

-1.0

-0.5

0

0.5

1.0

1.5

2.0

50 100 150Position (µm)

Al

Co

Cr

Ni

Ti

Matano plane: 154µm

Inte

rdif

fusi

on

Flu

x Ji

(10- 1

1m

/sec

)

IN939

-1.0

-0.5

0

0.5

1.0

1.5

2.0

50 100 150Position (µm)

Al

Co

Cr

Ni

Ti

Matano plane: 154µm

Inte

rdif

fusi

on

Flu

x Ji

(10- 1

1m

/sec

)

Profiles of Concentration and Interdiffusion Flux(NiAl vs. IN939 Annealed at 1050°C for 96 Hours)

The interdiffusion flux was calculated only on the NiAl side (i.e.,single-phase region) of the couple using estimated mass-balanceframe of reference (e.g., Matano plane determined byconcentration profiles and microscopy).

0

10

20

30

40

50

60

70

0 100 200 300 400

Position (µm)

IN73

8 C

on

cen

trat

ion

(at

%)

Al

Co

Cr

Ni

TiMatano plane X0=187µm

0

10

20

30

40

50

60

70

0 100 200 300 400

Position (µm)

IN73

8 C

on

cen

trat

ion

(at

%)

Al

Co

Cr

Ni

TiMatano plane X0=187µm

-1.0

-0.5

0

0.5

1.0

1.5

2.0

0 100 200Position (µm)

Inte

rdif

fusi

on

Flu

x Ji

(10-1

1m

/sec

)

IN738

Al

CoCr

Ni

Ti

Matano plane: 187µm

-1.0

-0.5

0

0.5

1.0

1.5

2.0

0 100 200Position (µm)

Inte

rdif

fusi

on

Flu

x Ji

(10-1

1m

/sec

)In

terd

iffu

sio

n F

lux

Ji(1

0-11

m/s

ec)

IN738

Al

CoCr

Ni

Ti

Matano plane: 187µm

Profiles of Concentration and Interdiffusion Flux(NiAl vs. IN738 Annealed at 1050°C for 96 Hours)

The interdiffusion flux was calculated only on the NiAl side (i.e.,single-phase region) of the couple using estimated mass-balanceframe of reference (e.g., Matano plane determined byconcentration profiles and microscopy).

0

10

20

30

40

50

60

70

0 100 200 300Position (µm)

CM

247

Co

nce

ntr

atio

n (

at%

)

Al

Ni

Co

Cr

TiMatano plane X0=214µm

0

10

20

30

40

50

60

70

0 100 200 300Position (µm)

CM

247

Co

nce

ntr

atio

n (

at%

)

Al

Ni

Co

Cr

TiMatano plane X0=214µm

CM247Al

Co

Cr

Ni

Ti

-1.0

-0.5

0

0.5

1.0

1.5

100 200

Position (µm)In

terd

iffu

sio

n F

lux

Ji(1

0- 11

m/s

ec)

Matano plane: 214µm

CM247Al

Co

Cr

Ni

Ti

-1.0

-0.5

0

0.5

1.0

1.5

100 200

Position (µm)In

terd

iffu

sio

n F

lux

Ji(1

0- 11

m/s

ec)

Matano plane: 214µm

Profiles of Concentration and Interdiffusion Flux(NiAl vs. CM247 Annealed at 1050°C for 96 Hours)

The interdiffusion flux was calculated only on the NiAl side (i.e.,single-phase region) of the couple using estimated mass-balanceframe of reference (e.g., Matano plane determined byconcentration profiles and microscopy).

DeffNiAlDappNiAlDIntNiAlAluminum

14.45.452.16Waspalloy

8.473.021.50IN939

11.24.061.58IN738

10.23.651.56GTD111

11.24.151.31CM247

10-15(m2/sec)10-15(m2/sec)at%Alloy

DeffSADappSADIntSAAluminum

3.541.130.98Waspalloy

3.591.140.92IN939

3.491.110.83IN738

3.481.110.86GTD111

2.890.920.62CM247

10-15(m2/sec)10-15(m2/sec)at%Alloy

Interdiffusion Coefficients of AlCalculated for NiAl and Predicted* for Superalloy

NiAl Side Calculatedwith Concentration Profiles

Superalloy Side Predicted withCorrelations* in Interdiffusion Coefficients

€

Ci− −Ci

o

Cio −Ci

+=αi,SAαi,NiAl

˜ D i,NiAleff

˜ D i,SAeff

and Ci− −Ci

o

Cio −Ci

+=

˜ D i,NiAlapp

˜ D i,SAapp

Correlations* Integrated, apparent and effectiveinterdiffusion coefficients formultiphase region (i.e.,superalloys and precipitates) canbe predicted.

Deff determined based on

€

αi = π

DeffSADappSADIntSAAluminum

2.971.130.98Waspalloy

2.951.140.85IN939

3.572.150.96IN738

2.301.110.64GTD111

2.080.920.52CM247

10-15(m2/sec)10-15(m2/sec)at%Alloy

DeffSADappSADIntSAAluminum

3.541.130.98Waspalloy

3.591.140.92IN939

3.491.110.83IN738

3.481.110.86GTD111

2.890.920.62CM247

10-15(m2/sec)10-15(m2/sec)at%Alloy

Interdiffusion Coefficients of AlPredicted and Estimated* for Superalloy

Predicted InterdiffusionCoefficients for Superalloys

Estimated* InterdiffusionCoefficients for Superalloys

*Estimation using spline-fittedconcentration profile through thescatter in multiphase region insuperalloys

Predicted using correlations ininterdiffusion coefficients.

Deff determined based on

€

αi = π

Integrated Diffusion Coefficient (i.e., TotalInterdiffusion Flux) of Al in Various Superalloys

Aluminum

3.14Waspalloy

2.34IN939

2.54IN738

2.20GTD111

1.83CM247

10-15(m2/sec)at%Alloy

€

˜ D Al, TotalInt The integrated interdiffusion

coefficient for the entireprofile employs thatcalculated from the NiAlside (i.e., single-phaseregion) of the couple andthat predicted for thesuperalloy side (i.e.,multiphase-region).

The integrated interdiffusioncoefficient indicates theoverall interdiffusion flux foreach diffusion couples.

Variation of Apparent Diffusion Coefficientswith Initial Superalloy Composition

0.80

1.00

1.20

0 5 10 15 20 25 300.80

1.00

1.20

0 5 10 15 20 25 30

Cr (at%)

0.80

1.00

1.20

0 1 2 3 4 5 6 70.80

1.00

1.20

0 1 2 3 4 5 6 70.80

1.00

1.20

0 1 2 3 4 5 6 7

˜ D A

l, SA

App

x10

15(

) m

2se

c

Ti (at%)

˜ D A

l, SA

App

x10

15(

) m

2se

c

0.80

1.00

1.20

0 2 4 6 8 10 12 14

Al (at%)

˜ D A

l, SA

App

x10

15(

) m

2se

c

Variation of Apparent Diffusion Coefficientswith Initial Superalloy Composition

0.80

1.00

1.20

0.0 0.5 1.0 1.50.80

1.00

1.20

0.0 0.5 1.0 1.5

Ta (at%)

˜ D A

l, SA

App

x10

15(

) m

2se

c

0.80

1.00

1.20

0.0 1.0 2.0 3.0 4.00.80

1.00

1.20

0.0 1.0 2.0 3.0 4.0

W (at%)

˜ D A

l, SA

App

x10

15(

) m

2se

cApparent Al interdiffusion coefficients in superalloysincreased with increases in Cr and Ti concentrations,but decreased with increases in Al, Ta and Wconcentrations in the superalloys.

Summary

Solid-to-solid diffusion couples studies using β-NiAl vs.CM247, GTD111, IN738, IN939 and Waspalloy were carriedout.Integrated, effective and apparent interdiffusioncoefficients from single-phase region (β-NiAl) werecalculated based on concentration profiles determined byEPMA.Integrated, effective and apparent interdiffusioncoefficients in the multiphase phase region (superalloys)were predicted.Apparent Al interdiffusion coefficients in superalloysincreased with increases in Cr and Ti concentrations, butdecreased with increases in Al, Ta and W concentrationsin superalloys.Experimental work is in progress to determine themagnitude of compositional dependence (I.e., crossinterdiffusion coefficients).