Structure and Mechanical Properties of Oxidation Resistant Alloys

Structure and Mechanical
Properties of Oxidation
Resistant Alloys
Kevin Hemker
Johns Hopkins
hemker@jhu.edu

“The development of
ever more efficient gas
turbines has always been paced by the
results of research and development in
the concurrent fields of design and
materials technology.”
- G.W Goward
Surface and Coatings Tech, 108-109 (1998) 73-79

“Improved structural design
and cooling technology
applied to higher strengthat-temperature
alloys cast
by increasingly complex
methods, and coated with
steadily improved coating
systems, have lead to
remarkably efficient
turbine engines.”
- G.W Goward
Top coat
TGO
Bond coat
Substrate
A modern TBS
Surface and Coatings Tech, 108-109 (1998) 73-79

Drivers for bond coat design
Improve oxidation resistance and
slow TGO formation.
Increase bond coat strength.
Alloy to avoid phase transformations
and CTE mismatch with substrate.

Paradigm for materials development
Performance
- reduced weight
- increased temperature
Microstructure
- crystal structure
- grain size
- defects
- phases
Processing
- vapor deposition
- electrodeposition
-cast
- wrought
Properties
- modulus, strength, CTE
- temperature resistance

Outline
A brief history of oxidation resistant
alloys and coatings
Diffusion aluminide bond coats
• Microstructure
• Properties
• Factors influencing ratcheting
Overlay bond coats
• Microstructure
• Properties
• Factors influencing delamination

Delhi Iron Pillar
The famous iron pillar in Delhi is a
metallurgical wonder. This huge wrought iron
pillar, 24 feet in height 16.4 inches in
diameter at the bottom, and 6 1/2 tons in
weight has stood exposed to tropical sun and
rain for 1,600 years without showing any
visible signs of rusting or corrosion.
High concentrations of P catalyze the
formation Why? of δ-FeOOH (iron oxyhydroxide),
which is amorphous in nature and forms as
an adherent compact layer next to the
metal-scale How would interface, you Upon protect formation, the
corrosion resistance is significantly
enhanced this because iron pillor? δ-FeOOH forms a barrier
between the rust and the metal.”
- Balasubramaniam (IIT)
http://www.iupac.org/publications/ci/2005/2706/3_veazey.html#1

Paths to oxidation resistance
Alloying
• With select oxide formers
• Self healing
• Requires considerable quantities of solute
• Must not degrade other properties (e.g. creep)
Coatings
• Must be applied
• Must retard oxidation
• Must be adherent
• Cannot degrade properties of the substrate

Select oxide formers
Preferential oxidation
• Must form oxide or
protective layer before
base metal oxide
is formed
Protective
• Must form a dense, slow
growing, protective film
• E.g. Cr 2 O 3 , Al 2 O 3 , SiO 2
E for Oxide Formation (kJ/mol)

Alloying of steels
Low carbon steels
• Cheap, easily formed, and mechanically strong, but
rust at low T and oxidizes at high T.
Stainless steels
• Add significant quantities of Cr (~18%)
which forms a very protective oxide film
• Cuts down the rate of attack by 100 times
@ 900°C
• Affects microstructure and mechanical properties
• Other elements (e.g. Al, Si, P) also beneficial

Influence of Cr in superalloys
Metal loss (a la Ashby)
• Ni -> NiO
• Ni loss ~0.1mm in 600h at 0.7T m ~950°C
• Cr -> Cr 2 O 3
• Cr loss ~0.1mm in 1600h at 0.7T m ~1,231°C
• Cr loss ~0.1mm in 1,000,000h at 950°C
• Ni20% Cr -> Cr 2 O 3
• Ni20%Cr loss ~0.1mm in 6,000h at 950°C
– Foreign elements increase diffusion rates
• Commercial superalloys limited to

Alloying of Ni-base superalloys
Modern superalloys have been engineered
over decades and contain ~10 elements.
Composition of Single-crystalline René N5
Element: Ni Co Cr Ta Al W Re Mo Hf Ti C
Wt% 63.38 7.33 7.03 6.42 6.05 5.13 3.05 1.40 0.15 0.01 0.05
At% 64.92 7.48 8.13 2.13 13.48 1.68 0.99 0.88 0.05 0.01 0.25
The composition of most elements is
restricted by processing windows and
mechanical property requirements.

Diffusion aluminide coatings
Pack cementation aluminizing
• Parts packed in Al powder, sal ammoniac (NH 4 Cl),
graphite and alumina filler and heated to 450°C
for 2h.
• First used for Fe wire or ribbon heating elements,
Cu condenser tubes, etc.
First patent
• Van Aller, Allison, Hawkins (GE) 1911
• Way to ‘calorize’ metals to render then ‘inoxidizable’
• Attributed to selective formation of alumina scales

Schematic of the pack cementation
process

Pack cementation reactions
Pack Components
Source (Cr, Al, Si or their alloys)
Activator (NaCl, NH 4
Cl or other halide)
Inert Filler (often alumina)
Reactions between Source and Activator (Aluminizing)
3NaCl(g) + Al(l) = AlCl 3
(g) + 3Na(g)
2AlCl 3
(g) + Al(l) = 3AlCl 2
(g)
AlCl 2
(g) + Al(l) = 2AlCl(g)
Deposition on Substrate (Aluminizing)
Disproportionation 2AlCl(g) = Al + AlCl 2
(g)
3AlCl 2
(g) = Al + 2AlCl 3
(g)
Displacement
AlCl 2
(g) + Ni = Al + NiCl 2
(g)
Direct Reduction AlCl 3
(g) = Al + 3/2Cl 2
(g)
And, for activators which contain hydrogen e.g. NH 4
Cl,
Hydrogen Reduction AlCl 3
(g) + 3/2H 2
(g) = Al + 3HCl(g)
(The underlined symbols refer to species in the solid substrate.)

Pack cementation coatings
High-activity diffusion coating
on a Ni-base superalloy
- Al diffuses in -
Low-activity diffusion coating
on a Ni-base superalloy
- Ni diffuses out -

Schematic of CVD processing

Aero history of diffusion aluminides
1950’s
• Allison and Curtiss Wright – hot dipping of Ni blades
1960’s
• P&W – aluminizing of Ni blades by slurry processing
1970’s
• Most vane and blade coatings applied by “pack
cementation” and more recently by CVD
1990’s
• Recognized as useful TBC bond coats
• Forms an adherent α-Al 2 O 3 TGO

Making a diffusion aluminide
coating
NiAl
30-40 µm
Ni substrate
• Ni and Al diffuse together and form an
intermetallic coating

Ni-Al binary phase diagram:

Ni-Al binary phase diagram:
γ
FCC

Ni-Al binary phase diagram:
γ’
L1 2

Ni-Al binary phase diagram:
β
B2

Commercial Pt aluminide coatings
Pt
(Pt,Ni)Al + X i
30-40 µm
Superalloy substrate
• Bond coat chemistry and microstructure effected
by interdiffusion with substrate … reactive elements
appear to be good!!

TEM observations of diffusion
alumined coating
As-deposited
Thermal cycled
SADP
SADP
{110} T
100 nm
Micro-beam DP
B2
L1 0 Martensite

Compositional changes:
Concentration (at. %)
BC IDZ
60
40
(a) As-TBC'd
Ni
20
0
Al
Pt
60 (b) 28% FCT life
Ni
40
20
Al
0
Pt
60 (c) 100% FCT life
Ni
40
20
Al
0
Pt
0 20 40 60 80 100
Distance from surface of bond coat (µm)
β
β
M M +γ’ +
M +γ’ +
γ’+γ
γ’
γ’+γ
As-TBC’d
28% FCT life
γ’+γ 100% FCT life
Sample Ni Al Pt Cr Co Ta Re W
0% 36.72 43.50 12.10 3.9 3.78 0 0 0
28% 46.01 35.92 8.09 4.45 5.11 0.28 0.049 0.057
100% 51.32 34.44 4.08 4.21 5.17 0.57 0.08 0.13
Ni diffuses outward

Bond coat evolution …
Martensite transformation has been
reported in this composition range.
For example:
Smileck, et al, , 1973, Metall. . Trans.
Khadkikar, et al, , 1993, Metall. . Trans.

Failure by ratcheting and spallation
Key observations:
Isothermal exposure is not
the same as thermal cycling.
TGO thickens, lengthens and
roughens as a result of
thermal cycling.
Bond coat must creep to allow
TGO to ratchet.
Top coat cannot creep;
it cracks instead
Mumm, Evans et al. (2001)

Advanced model of TGO ratcheting:
Balint-Hutchinson model
A special purpose multilayer code that takes input from
micromechanical parameters and predicts the progressive
development of undulations of the TGO layer upon thermal cycling.
Key microstructural parameters:
E sub , α sub , E bc , α bc , YS bc , (creep rate) bc , t bc , ε mart , M s , A s ,
α tgo , E tgo , (growth rate) tgo , (creep rate) tgo , YS tgo , t tgo ,
(shape) tgo , E tbc , α tbc , (creep rate) tbc , YS tbc , …
Note: property measurements and microstructural
observations of bond coat are crucial to model
development.
Balint and Hutchinson, Acta 2003, JMPS 2005

Measuring the mechanical properties
of diffusion aluminide bond coats

Bond coat microsample preparation:
Bond coat
Substrate
600 µm slice is scalped from 1 inch diameter button using
a wire EDM.
A lapping machine is employed to grind the superalloy side
of the slice to get a uniform thickness of ~150µm.
500 µm
A sinking EDM is then used to punch microsamples from
each slice.
Faces of the microsample are polished to a mirror finish
and a final thickness of 30~50 µm with TEM ‘dimpler’.
100µm
Pt lines are deposited on both sides of the microsample
using a Focused Ion Beam (FIB).

Microsample testing
• Microsamples:
3.5 mm x 1.2 mm x 25 µm
(overall)
250 µm x 500 µm x 25 µm
(in gage)
Fringe
Detector
Laser
Fringe
Detector
Linear Air
Bearing
Load Cell
Piezoelectric
actuator
Grips
Manual
turnbuckle
• Temperature range of 25-1200°C can be
achieved by resistive heating.
• ISDG provides a strain resolution of
better than 5 µstrain.

Measuring bond coat CTE (T):
Engineering Strain (%)
1.5
1
0.5
y = -0.15515 + 0.0015501x R 2 = 0.98857
As-bond-coated
0
400 500 600 700 800 900
Temperature ( o C)
σ = ~2 MPa
• CTE of as-coated
bond coat has been
determined to be
15.5 ppm o C -1
between 400 o C and
850 o C.
•CTE measurement
above 850 o C was
hindered by creep.

Optical CTE measurements
Rene N5
dε
xx
( T )
CTE =
dT
=
d
dT
⎛
⎜
⎝
∂u
∂x
⎞
⎟
⎠

Optical CTE measurements
?

Measuring bond coat E(T):
400
200
350
Stress (MPa)
300
250
200
150
100
50
0
0.2
T = 25 o C
x
E
Strain (%)
T = 650 o C
Young's Modulus (GPa)
150
100
50
BC-1, as-coated
BC-2, as-coated
BC-1, cycled
NiAl literature values
0
0 200 400 600 800 1000
Temperature ( o C)

Microsample tests of Rene N5 :
1200
Engineering Stress (MPa)
1000
800
600
400
200
0
E = 129 GPa
E = 126 GPa
E = 126 GPa
T = 25 o C
0 0.01 0.02 0.03 0.04
Engineering Strain
• Modulus of [100] oriented
single-crystalline Rene N5
microsamples measured to be
126 ± 3 GPa (8 measurements).
• GE reported modulus to be
E = 129 GPa.

Texture effects on E:
Young's Modulus
300
250
200
150
100
The Elastic Constants of NiAl
(Ref. Rusovic et al, 1977)
C 11 C 12 C 44
199 GPa 137 GPa 116 GPa
50
0 50 100 150
In-plane Angle (Degree)
(001) plane
(011) plane
(111) plane
The Average E in Certain Texture of NiAl
E(GPa) (001)
plane
(011)
plane
(111)
plane
Voigt 125 184 181
Reuss 117 159 181
• Each angle corresponds to one certain
orientation in that plane.
• The measured value of E is comparable to
that of NiAl with a [001] texture.
• X-ray texture determination did not
evidence any obvious out-of-plane texture
of bond coats.

Measuring bond coat σ yield (T):
300
700
Engineering Stress (MPa)
250
200
150
100
50
E
T = 650 o C
T = 890 o C
T = 980 o C
Yielding Strength (MPa)
600
500
400
300
200
100
As-BC'd -1
As-BC'd -2
TC100
0
0.2
Engineering Strain(%)
0
600 700 800 900 1000 1100 1200
Temperature ( o C)

Effect of thermal cycling
600
BC1,as-coated
Yield Strength (MPa)
500
400
300
200
100
BC1, cycled
BC2, as-coated
BC2, cycled
BC3, as-coated
BC3, cycled
Deng Pan et al., Acta 2003
0
600 700 800 900 1000 1100 1200
Temperature (C)

Effect of thermal cycling
600
BC1,as-coated
500
BC1, cycled
BC2, as-coated
Yield Strength (MPa)
L1 0
Martensite
400
300
200
100
BC2, cycled
BC3, as-coated
BC3, cycled
100 nm
B2
Deng Pan et al., Acta 2003
0
600 700 800 900 1000 1100 1200
Temperature (C)

High T deformation

Stress relaxation behavior:
Stress (MPa)
300
250
200
150
BC-1, as-coated
400 o C -experimental
650 o C -experimental
900 o C -experimental
1150 o C -experimental
100
50
0
0 200 400 600 800 1000
Time (s)
Strain rate*exp(Q/RT) (1/s)
.
ε pl =A(σ/E sample ) n exp(-Q/RT)
10 4
10 10 10 3
650 o C
10 82
n = 4
10 1
10 6 n = 4
900 o C
10 0
10 4
10 -1 1150 o C
Q = 250kJ/mol 125kJ/mol
10 -2 2
10 -4 10 -3 -3 10 -2 -2
•Empirical fit suggests that Q creep is only half of Q D for NiAl and n = 4.
σ/E

Stress relaxation behavior
300
250
as-deposited
Stress (MPa)
200
150
100
400 o C -experimental
650 o C -experimental
900 o C -experimental
1150 o C -experimental
Power law
50
Empirical fit:
. .
0
0 200 400 600 800 1000
Time (s)
ε pl =-σ/E machine = 1.2 x 10 14 (σ/E sample
) 4 exp(-125(kJ/mol)/RT)

Characterizing the
microstructural changes in
diffusion aluminide bond coats

In-situ TEM observations:
25°C -> 900°C
L1 0
B2
Transformation from martensitic L1 0 to B2 observed
at approximately 800°C.

n situ heating X-ray diffraction:
Reversible transformation
Crystallographic relationship
c 25°C, cooled from 1150°C
between B 2 and L1 0
b
1150°C
Intensity
25°C
(100)
(110) B2
L1o
(110) L12
(111) L12
(111) L1o
(110) B2
(200) L1o
(200) L12
(200) L12
(111) B2
(201) L1o
(200) B2
(220) L1o
(210) B2
(202) L12
(221) L1o
(211) B2
a
L1 B 2
0
(011)B2//(11-1)L1 0
(311) L1o
30 35 40 45 50 55 60 65 70 75 80 85 90
2θ (degree)

Transformation to B2:
Intensity (counts)
7500
7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
(111)L10
(110)B2
0
-500
41.0 41.5 42.0 42.5 43.0 43.5 44.0 44.5 45.0 45.5 46.0 46.5 47.0 47.5 48.0 48.5
2θ (degree)
450 ºC
500 ºC
550 ºC
600 ºC
625 ºC
650 ºC
675 ºC
685 ºC
700 ºC
(200)L10
Volume Ratio(B2/ M)
60
50
40
30
20
10
0
Volume ratio between B2 and
Martensite calculated from
the intensities of (110)B2 and
(111)L10
450 500 550 600 650 700
Temperature (ºC)
As
The A s temperature is approximately 620ºC.

Martensite transformation temperatures:
Heat Flow Endo Down (mW)
80
60
40
20
0
-20
-40
DTA (Differential Thermal Analyzer)
Peak T of B2 to M
Cooling at ~25 ° C/min
Peak T of M to B2
Heating at 40C/min
°
0 200 400 600 800 1000 1200
‣The M to B2
transformation occurs
at 600-700ºC
‣The B2 to M
transformation occurs
at 550-650 ºC
‣M s is approximately
40-50ºC less than A s
Temperature(C)

Thermal and transformation strains:
2.2
2.0
1.8
Thermal strain of B2
1.6
1.4
Transformation
M s
Strain (%)
1.2
1.0
0.8
0.6
strain
cooling
A s
heating
0.4
0.2
Thermal strain of L1 0
(° C)
0.0
0 200 400 600 800 1000
Temperature

Effect of martensitic transformation
Balint and Hutchinson, JMPS 2005

Strategies for bond coat design
Improve oxidation resistance and
slow TGO formation.
Increase bond coat strength.
Alloy to avoid martensitic and
other phase transformations.

Overlay coatings for superalloys
Developed to allow for greater
flexibility in bond coat compositions.
• Addition of reactive elements known to be
beneficial
• Peg formation, S gettering, increased TGO adhesion ???
• Can tailor the phases present in the bond coat
• β + γ, β + γ ’, γ + γ ’, etc.
Requires deposition
• Initially PVD
• More commonly now LPPS

Schematic of an EBPVD process

EBPVD coatings
CoCrAlY coating as-deposited
CoCrAlY coating after peening
and heat treatment

Schematic of a plasma deposition
system

NiCoCrAlY argon-shrouded
plasma sprayed coating
NiCoCrAlY coating after peening and heat treatment

Aero history of overlay coatings
FeCrAlY
• Alloy merged from GE nuclear programs (1964)
• P&W applied (EBPVD) as a coating (1970)
• Limited at high T by formation of a brittle NiAl layer
CoCrAlY (1972)
• Useful oxidation resistance but too brittle
NiCrAlY (1973)
• Limited hot corrosion resistance but ductile
NiCoCrAlY (1975; 1986)
• Good compromise; 40+ patented MCrAlY’s

Microstructural observations of
Pratt & Whitney’s “two-phase”
NiCoCrAlY(Hf,Si) bond coat

Effect of thermal cycling
As-received 100 hrs @ 1100 o C
Near-TGO γ
100
80
60
100 hrs @ T i
Average across entire thickness
γ
Vf [%]
40
20
β
0
0 20 40 900 1000 1100 1200
Temperature [C]
“two-phase bond coat”
(β-NiAl + γ-Ni)

TEM observations of the bond coat
1750
1500
β-phase
Ni Ka
1250
Counts
1000
750
Co L/ Ni L
500
Al Ka
250
0
Co Ka
Cu Ka
Cr Ka
Ni Kb
Cr Kb
Cu Kb
0 1 2 3 4 5 6 7 8 9 10
Energy (keV)
1750
1500
γ-phase
Ni Ka
1 µm
Counts
1250
1000
750
Cr Ka
Co Ka
As–received NiCoCrAlY
500
250
0
Co L/ Ni L
Cr L
C K
Al K
Si K
Cr Kb
0 2 4 6 8 10
Energy (keV)
Cu Ka
Co Kb
Cu Kb

TEM observations of the γ grains:
As-received
BF
1 10
DF
110
100 nm
200 nm
ELTB
BF
500 nm
DF
1 10
110
γ grains filled with
very fine γ’
precipitates
modest coarsening
suggests the γ’
dissolves at T.

EFTEM of γ′ in the γ grains:
200nm
Elastic image Thickness map Cr L (574 eV) Co L (779 eV)
The γ’ precipitates are
rich in Ni and Al while
the matrix is rich in Cr
and Co.
Ni L (854 eV)
Al L (73 eV)

TEM observations of the β grains
As-received
ELTB
100 hrs @ 1100 o C
750 nm
g 110
g 110
250 nm
-Mottled-
250 nm
-Tweed-
-Lath-
[001] [001]

Do β-grains contain martensite?
As-received
M s

Microstructural evolution of NiCoCrAlY
Beta Gamma
As-received
Thermal cycled

Precipitates found in an ELTB
8000
6000
Cr
Counts
4000
?
2000
Energy (keV)
Co Ka
Ni Ka
Cr
Cr L Co Kb
Co Kb
Si Ka
Ni
0
0 1 2 3 4 5 6 7 8 9 10
500 nm
Cr-rich intergranular precipitates
widely dispersed in the ELTB
specimen but not observed in the
as–received samples.
Composition is 55.5 at% Cr, 32.1 %
Co, 11.0 % Ni and 1.4 % Si.

Crystal structure of Cr-rich precipitate
110/-1-10 110/-1-10
10 o (10 o ) 10 o (7 o )
B = [-110] B = [-221]
B = [-111]
30 o (31 o )
001/00-1
413/-4-1-3
B = [-140]
The precipitate is a (CrCoNi)
σ-phase (P4 2 /mnm space group).
Tetraganol unit cell with lattice parameters
a = 8.37 Å and c = 4.30 Å.

Elemental maps of the σ-precipitates
Mendis
Backscattered image
Al map
Cr map
Tryon
Backscattered image Cr map Si map

Potential impact of σ-phase …
Notoriously brittle but bond coat is not a load
carrying structure.
Appears to trap Si, which would mitigate its effect.
Is observed at the TGO interface and may affect
adhesion.
Bond coat
TBC
TGO
Elastic image
Cr L (574 eV)
Note: only observed in ELTB;
not as-received or after 100hrs at 1100C!

Predicting phase stability…
Achar, Munoz, Arroyo, Singheiser, Quadakkers, Modeling of phase
equilibria in MCrAlY coating systems, Surface and Coating Tech (2004)

TGO and PEG
Chemistry
3500
3000
2500
O/ Cr
Al
EDX of TGO peg
columnar TGO
Counts
2000
1500
1000
500
0
Y
Co
Co/ Ni
Ni
Hf
Cr
Hf/ Cu
S
Cr
Ni Hf
0 2 4 6 8 10
Energy (kV)
1 µm
4000
3500
Al
Counts
3000
2500
2000
1500
O
EDX of columnar TGO
peg
1000
500
0
Y
0 2 4 6 8 10
Energy (kV)
The peg contains a large number of elements
including sulfur, although the neighboring columnar
TGO is nearly pure Al 2 O 3 .

STEM shows S in pegs but not
the TGO
Bond coat
Al
O
Ni
Co
S
Hf
Y
Cr

Mechanical behavior of P&W’s
NiCoCrAlY(Hf,Si) bond coat

RT strength of NiCoCrAlY
1600
Tensile test of NiCoCrAlY at RT
Eng. Stress [MPa]
1400
1200
1000
800
600
400
E=155 GPa
as received
thermally cycled
E=130 GPa
1.4 GPa
with RT
ductility!
200
E=130 GPa
0
0.0 0.5 1.0 1.5 2.0 2.5
True Strain [%]

Effect of thermal cycling
Diffusion aluminde
NiCoCrAlY
- strength increases - strength decreases
Yield Strength (MPa)
600
500
400
300
200
100
Deng Pan et al., Acta 2003
BC1,as-coated
BC1, cycled
BC2, as-coated
BC2, cycled
BC3, as-coated
BC3, cycled
Eng. Stress [MPa]
1600
1400
1200
1000
800
600
400
200
E=155 GPa
E=130 GPa
as received
thermally cycled
E=130 GPa
0
600 700 800 900 1000 1100 1200
Temperature (C)
0
0.0 0.5 1.0 1.5 2.0 2.5
True Strain [%]

High temperature deformation:
Strain [%]
12
10
8
6
4
2
0
250
800
Stress [MPa]
200
150
100
50
0
400 450 500 550 600 650 700
700
600
500
400
300
Temperature [C]
T im e [s ]

High temperature deformation:
Eng. Stress [MPa]
Constant strainrate Constant strainrate
225
Cross head fixed Cross head fixed
200
175
150
125
100
75
3.0
2.5
2.0
1.5
1.0
True Strain [%]
50
25
0
0.5
0.0
0 50 100 150 200 250
Time [s]
500ºC

Comparison of high T strength:
500
Flow stress [MPa]
400
300
200
100
MCrAlY
(Ni,Pt)Al corr
(Ni,Pt)Al
0
400 600 800 1000 1200
Temperature [C]

Failure of NiCoCrAlY TBC systems
e.g. NiCoCrAlY bond coat systems
Salient observations:
No TGO ratcheting.
Cracks occur below the
TGO.
Crack growth is mode-
II and effected by
interfacial energy and
plasticity in the
adjacent layers.
Cracks must be initiated
before they can grow.
Xu, Evans et al., Acta 2004

Proposed failure mechanism map:
No !!