thermal-mechanical fatigue behavior of cmsx-4 in virgin and...

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Thermal-Mechanical Fatigue behavior of CMSX-4 in virgin and long-term aged condition

Johan MoverareMaterials TechnologySiemens Industrial Turbomachinery AB, Finspång

Sten JohanssonEngineering MaterialsLinköpings universitet


Introduction

Thermal-mechanical fatigue (TMF)Cyclic mechanical strain and temperatureMore component near testing than LCF or other creep-fatigue experimentsDifferent damage mechanismsWe have tested more then 15 materials and have found no good correlation between LCF and TMF test dataLife predictions (done by Siemens in Finspång) of high temperature components are today based on TMF data and not LCF data, if the operation temperature is >600ºC


Finspång’s TMF systems

2 almost identical system1:st system installed 20022:nd system installed 2005

MTS 810 system100kN servo-hydraulic system

Induction heatingSolid cylindrical specimens, diameter 6 or 10 mm

Forced air coolingCompressed air


TMF cycle

0

200400

600800

1000

0 200 400 600 800Time

Tem

pera

ture

, C

-0.005

-0.004

-0.003

-0.002

-0.001

00 200 400 600 800

Time

Mec

h. S

trai

n, %

Out-of-phase (OP) TMF100 - 1000°C

10°C/s up to 600°Cafter that 1.5°C/s

R= -∞

20 hour dwell time in 1:st cycle and 5 min in the following cycles

Cooling rate: 2.5 - 10°C


Intermediate ageing

Testing interrupted after 25 cyclesAgeing in furnace

1000°C for 4000 hours

Test restarted


Material

CMSX-4Single crystal alloyTested along the <001> direction

Al Co Cr Hf Mo 5.65 9.6 6.4 0.11 0.61 Ni Re Ta Ti W Bal 2.9 6.6 1.02 6.4


Results – Mechanical Strain vs. Life Ni

0,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

1,2

1 10 100 1000 10000

Cycles to crack initiation, Ni

Mec

h. S

trai

n ra

nge,

%

CMSX-4 <001> virgin OP TMF 100-1000degC

CMSX-4 <001> aged OP TMF 100-1000degC

Virgin

Aged

(a)


Results – In-elastic Strain vs. Life Ni

0,001

0,010

0,100

1,000

1 10 100 1000 10000

Cycles to crack initiation, Ni

In-e

last

ic S

trai

n ra

nge,

%

Virgin

Aged

(b)


Stress-strain hysteresis loop shape

-400

-200

0

200

400

600

800

-1 -0,8 -0,6 -0,4 -0,2 0

Strain, %

Stre

ss, M

Pa

VIRGIN material

(a)-400

-200

0

200

400

600

800

-1 -0,8 -0,6 -0,4 -0,2 0

Strain, %

Stre

ss, M

Pa

AGED material

(b)


Fracture appearance

Virgin Aged


Microstructure instability

Rafting occur during the testing of the virgin material

Un-tested virgin material

Virgin material – after testing

Longitudinal section

Virgin material – after testing

Transverse section


“Lab-tests” versus “real” service conditions

Even if the TMF tests is a more component near test compared to isothermal LCF tests it differs from real service conditions in terms of

Longer hold timesEnvironmental effects (corrosive elements, moisture, etc.)Gradients (stress and temperature)More complex cycles

To fully utilize the results from the “lab-scale” tests on real components, a better understanding of deformation and damage mechanisms are very important.


Localized deformation in tests on virgin material

Virgin material


Deformation induced TCP-phases

Virgin material


Localized deformation by twinning

Distance (μm)

Mis

orie

ntat

ion

(deg

ree)

Misorientation Profile

SEM

EBSD

Distance (μm)

Mis

orie

ntat

ion

(deg

ree)

Misorientation Profile

SEM

EBSDVirgin material


Crack propagation along twin boundaries

The cracks preferably propagate along the twin boundaries on the {111}planes

Virgin material


Re-crystallization occur inside the twins

Virgin material


TCP formation during long term ageing

Aged material

Uneven distribution of TCP-phases due to dendrite structure


TCP-phases interact with the deformation

Aged material

The TCP-phases forces the material to deform with a lower degree of localization


Transmission Electron Microscopy

Aged materialVirgin material

More even distribution of dislocations between γ and γ’ in the aged material


Summary and Conclusions

The deformation and damage mechanisms seen in the TMF tests can only be generated during un-isothermal deformation

TMF testing is therefore vital for reliable life time predictions of gas turbine components

thermal-mechanical fatigue behavior of cmsx-4 in virgin and...

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