ACKNOWLEDGEMENTS

Thanks to Mike Boskwick and Trevor Smith for technical

assistance and NSERC for financial assistance.

FUTURE WORK

OBJECTIVE

INTRODUCTION MATERIALS AND METHODOLOGY CONCLUSIONS

Isothermal Oxidation Comparison of Three Ni-based

Superalloys

Mallikarjuna H.T, W. F. Caley, N. L. Richards

University of Manitoba, Winnipeg, Canada - R3T 2N2

Air

Compressor section

Air inlet Combustor section

Turbine section

Exhaust gas

Low temperature zone High temperature zone

(750°C to 1100°C)

Fig.1 Lycoming ALF502R gas turbine engine

The objective is to compare the high temperature isothermal

oxidation resistance of three Ni-based superalloys;

polycrystalline cast IN738LC, single-crystal N5 and a powder

metallurgy Ni-Cr-Fe alloy (with 6% Al and 0.5% Si –

“TAS”); their microstructures are given below in Fig. 2.

Effect of tantalum on single crystal N5 during earlystages of oxidation.

Determine the relative oxidation resistance withrespect to various precipitate sizes.

REFERENCES

An isothermal air oxidation study has been conducted at 900°C for

times up to 1000h, on the Ni-base alloys IN738LC, N5 and TAS.

Both the oxidation kinetics and formation of the oxidation layer of

all three alloys were determined to follow the parabolic rate law .

The magnitude of the kp value, for N5, 1.42 x 10-7 mg2 cm-4 s-1

and for TAS, 1.64 x 10-7 mg2 cm-4 s-1 were much lower than the

IN738LC, 2.79 x 10-6 mg2 cm-4 s-1. Hence, N5 and TAS (Ni-Cr-

Fe) alloys are more oxidation resistant than IN738LC Superalloy

under these conditions.

The protective oxide layer developed on IN738LC superalloy

during isothermal oxidation principally showed Type II oxidation

behaviour with an outer chromia layer and internal alumina layer.

However, alumina formed as an outer layer in N5 and PM-TAS

alloys, which confirms Type III oxidation.

The presence of titanium in IN738LC increases the rate of

oxidation compared to other two alloys N5 and PM-TAS which

had no titanium.

R. C. Reed (2006): The Superalloys Fundamentals and Applications,

Cambridge University Press, Cambridge, P. 1-8.

Cade B.G., Caley W.F., Richards N.L. (2014): Comparison Of Oxidation

Performance of Two Nickel Base Superalloys For Turbine Applications,

CMQ, V 53, N 4, P 460-468.

Alloy Cr Co Mo W Ta Re Nb Al Ti Hf Si Fe

IN738LC 16.0 8.5 1.8 2.6 1.7 - 0.9 3.4 3.4 - 0.04 0.03

N5 7.1 7.4 1.4 5.0 6.4 2.9 - 6.1 0.02 0.2 0.1 0.1

TAS 11.2 - - - - - - 6.0 - - 0.5 8.4

RESULTS AND DISCUSSION

As-received alloy compositions (weight %, balance Ni)

After oxidation, all specimens exhibited a parabolic oxidation

behaviour. The parabolic rate constants, kp were calculated from

the plot as shown in Fig. 4.

Depending on the alloy compositions three primary regions

of oxidation (Type I, II and III) can be seen corresponding to

(I) NiO external scale + Al2O3/Cr2O3 internal oxides, (II)

Cr2O3 external scale + Al2O3 internal oxides, and (III)

external scales of only Al2O3. SEM-microstructures of the

oxidized specimens are shown in Fig. 5 including precipitate

free zones (PFZ).

The effect of vacancies appears to be that of decreasing the

contact area between the oxide scale and metal by void

formation as shown in Fig. 6. Also, the deleterious

chromium rich σ-phase was found in TAS which is shown

in Fig. 7

Fig. 7 Chromium rich σ-phase below PFZ in TAS,

a) 15h of oxidation and b) 420h of oxidation

To reveal the γ' (gamma prime) precipitates each cast alloy

was age hardened at temperatures 845°C (IN738LC),

1079°C (N5) and the TAS alloy was sintered (1200°C); the

average precipitate sizes were calculated to be 390nm,

430nm and 430nm respectively as shown in Fig. 3.

The use of superalloys is dictated primarily by high

temperature aerospace requirements. The ability of these

alloys to resist oxidation, fatigue and creep under such

extreme conditions of stress and temperature is due to their

microstructure, which is a product of their constituents and

processing. The oxidation resistance of a superalloy is

achieved primarily through the formation of dense Al2O3

and/or Cr2O3 layers including spinels. An example of an aero

engine is given above in Fig. 1.

Fig. 2 SEM-microstructure of the Ni-based superalloys a) Polycrystalline IN738LC

b) Single-crystal N5 c) powder metallurgy ternary Ni-Cr-Fe

a) b) c)

5µm5µm5µm

Fig. 3 SEM-microstructure of age hardened a) IN738LC, b) N5

and sintered c) TAS(Ni-Fe-Cr)

a) b) c)

1µm1µm1µm

Chromium rich σ-phase

5µm1µm

a) b)

All three alloys were subsequently oxidized in 1atm air at

900°C for various times up to 1000h.

Polycrystalline cast IN738LC

200h 1000h7h

Single-crystal N5

Powder metallurgy TAS

200h 1000h7h

200h 1000h7h

Al2O3

PFZ

Void

Al2O3

Cr2O3

PFZ

Al2O3

PFZ

Spinel

2µm 10µm 10µm

2µm 2µm 2µm

2µm2µm2µm

Fig. 5 SEM-microstructure of oxidized specimens a) IN738LC, b) N5 and

c) TAS(Ni-Fe-Cr)

Fig.6 SEM- micrographs showing interface voids in N5 by vacancy condensation

during oxidation at 900°C a) 15h of oxidation and b) 420h of oxidation

a) b)Void formation

Al2O3

Voids

1µm1µm

PFZ

Fig. 4 Parabolic rate plot for oxidation of IN738LC, N5, and TAS (Ni-Fe-Cr), in

1atm air at 900°C for 1000h

0

2

4

6

8

10

12

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1000000 2000000 3000000 4000000

Pa

rab

oli

c w

eig

ht

ga

in (

mg

2.

cm-4

)

Oxidation duration at 900°C (s)

N5 TAS IN738LCN5 &TAS IN738LC

IN738LC

1.42E-07

1.64E-07

2.79E-06

N5

TAS

Alloy kp