THE INFLUENCE OF TEMPERATURE AND PRE-AGING ON THE LOW
CYCLE FATIGUE BEHAVIOUR OF NICKEL COATINGS (Ni 200/201)
H. Koeberl, H. Leitner, W. Eichlseder
Chair of Mechanical Engineering
Montanuniversitaet Leoben
8700 Leoben, Austria
ABSTRACT
Generally Nickel alloys are used for thermal barrier coatings. To this purpose, it is necessary to study the low cycle fatigue
behaviour of this material. For thermal barrier coatings, temperature and oxidation are the most influential operating conditions.
In this publication the regions and limits of thermal application of Nickel are described in form of strain based S-N curves at
different temperatures. So the lifetime of these coatings can be very strongly affected by the temperature behaviour, described
by nodal temperatures as well as by their gradient. Therefore the tests were performed strain controlled with a servo-hydraulic
test rig at a strain rate of 1 % per second. The test temperatures and also the pre-ageing temperatures were 25°C, 225°C,
375°C and 575°C with ageing times of 0, 10, 100 and 500 h. For heating an inductive coil is used. Another influence factor for
thermally loaded Nickel coatings is the oxidation at high temperatures. The oxidation results in surface embrittlement. The
brittleness is also reflected in the lifetime behaviour. The effect of pre-ageing can also take an influence on the lifetime
behaviour. To study and characterise these influences for nearly perfect working conditions is fundamental. Optimised Nickel
operation conditions leads to a significant increase in service life which minimise the recoating costs.
Introduction
Cyclic thermal loading often takes place in the low cycle fatigue (LCF) regime [1]. Therefore it is necessary to study the low
cycle fatigue behaviour of pure nickel as shown in Figure 1, according to the Manson-Coffin-Basquin law [2, 4, 5]. For thermal
barrier coatings, studies of the influences of temperature and oxidation are a basic input [3]. In the recent research the regions
and the limits of the thermal application of nickel are principally described by strain based S-N curves at different
temperatures.
Figure 1. Strain S-N Curve at 25 °C
The oxidation leads to an embrittled fractured surface as shown in Figure 2. The brittleness is also reflected in the lifetime
behaviour. The higher the oxidation, in case of oxide (O2) on the fracture surface, the more the lifetime decreases. Optimised
Nickel operation conditions lead to a significant increase in service life thereby reducing the recoating costs.
Figure 2. Embrittled fracture surface at 375 °C
Pre-ageing takes a high influence on the lifetime behaviour. It increases or decreases the yield strength over the ageing time
and gives an overview about the temperature influence and the microstructural changes. The dominant mechanism is the
coarsening of the precipitations and so the diffusion is the main influence of this coarsening. So the yield strength is related to
the temperature. Pre-ageing is mainly described by Shercliff-Ashby [6, 7], as shown in Figure 10.
Experimental
The tests are performed strain controlled with a servo-hydraulic test rig at a strain rate of 1 % per second. The samples were
typical S/N samples with shank diameter 16 mm, shoulder radius 40 mm and waist-diameter 7 mm. For heating, an inductive
coil is used with a frequency generator of 10 kW power. Data analysis is done via double logarithmic strain S-N curve (see
Figure 1) based on the law of Manson-Coffin-Basquin (1),
ε A = ε Ael + ε Apl =
σ ′f
E
⋅ (2 ⋅ N ) + ε ′f ⋅ (2 ⋅ N )c
b
(1)
where ε Ael and ε Apl are the elastic and plastic parts of the strain amplitude ε A , σ ′f the dynamic strength coefficient, ε ′f the
(cyclic) ductility coefficient, b the dynamic strength exponent, c the (cyclic) ductility exponent, E Young’s modulus and N the
number of cycles to failure.
By using (2 ⋅ N ) reversals to failure in (1), at N = 1 / 2 (or (2 ⋅ N ) = 1 ) the intersection of the elastic and plastic S/N curve lies
at
σ ′f and
ε ′f
E
For the aim of measuring the oxygen concentration a scanning electron microscope (SEM) is used. With the integrated EDXanalysis it is rather easy to detect element traces. The EDX analysis confirms the assumption of embrittlement by oxidation at
high temperatures. It is a fact that the concentration of oxygen increases disproportionate by the temperature.
The pre-ageing is done with a temperature controlled electrical high temperature oven. The temperature steps were 225 °C,
375 °C and 575 °C. The time steps of the pre-ageing were about 10 h, 100 h and 500h. The data analysis is done with
Shercliff-Ashby (see Figure 10). The data analysis is done via tensile tests where the yield strength R p 0, 2 is the basic value.
Results
The results in Figure 3 show a high influence of the temperature on the cyclic behaviour of nickel, which gets more important
with lower strain amplitudes. For example, at 2 ‰ strain amplitude and 25 °C nickel tolerates over 50.000 cycles to failure, but
at 575 °C about 10.000 cycles, which is a factor of 5 in lifetime. But at strain amplitudes of 1 ‰ the sustained cycles decrease
rapidly from about 3.000.000 at 25 °C to about 100.000 at 575 °C, which is a factor of 30 in lifetime.
Comparing 25 °C and 375 °C, the influence of temperature still exists, but the difference in the lifetime behaviour is not as
serious as above. At 2 ‰ the lifetime factor is about 3 (more than 20.000 cycles at 375 °C), and at 1 ‰ the factor is 6 (more
than 500.000 cycles at 375 °C).
Figure 3. Comparison of strain S-N curves at 25, 225, 375 and 575 °C
At 25 °C nickel shows a very ductile fracture surface, but with higher temperatures the fracture surface gets more and more
embrittled (see Figure 4.). This brittleness indicates that the oxidation of nickel at higher temperatures is a fatigue effect. So it
will be of interest to detect the concentration of oxygen on the fracture surface.
Figure 4. Fracture surface at 25 °C (left), 375 °C (middle) and 575 °C (right)
The EDX analysis confirms the assumption of embrittlement by oxidation at high temperatures. It is a fact that the
concentration of oxygen increases disproportionately with the temperature. That means at temperatures up to 250-300 °C the
influence of the oxidation and further of the embrittlement is not deciding. As shown in Figure 5, the oxygen ratio at 375 °C is
at least the half as the ratio at 575 °C. Further the cycles to failure are decreasing rapidly with the temperature and therefore
with the oxidation of the fracture surface.
Figure 5. Oxidation (oxygen ratio [w%])
Analysis of pre-ageing was based on tensile tests Figure 6. To this purpose, lots of pre-ageing steps, at different temperatures
and times, had do be done. In this study the time steps were 0, 10, 100 and 500 h and the temperature steps were 25, 225,
375 and 575 °C.
Figure 6. Tensile tests after 500 h pre-ageing at different temperatures
Figure 6 shows the tensile tests after an ageing time of 500 h and different temperatures. Further it appears that the ultimate
strength will increase by about 20% at an ageing temperature of 225 °C compared to 25 °C. But at higher ageing temperatures
the ultimate strength decreases again. On the other hand, the yield strength, in case of PA 500 h, is at room temperature the
highest and degrease slowly with PA (pre-ageing) 225 °C and PA 375 °C, but with PA 575 °C the yield strength degrease
rapidly by about 27 % from the room temperature value.
A similar effect is shown by the LCF (low cycle fatigue) behaviour of Nickel. In case of a pre-ageing temperature of about
225 °C, the lifetime behaviour exhibits a small scatter band (Figure 7), where the pre-ageing times (10, 100, 620 h) give nearly
the same results. The lifetime factor is between 0 and 4.
Figure 7. Pre-Ageing 225 °C
If the pre-ageing temperature is 575 °C, the lifetime behaviour will develop a wider scatter band as shown in Figure 8. Here the
ageing time gives also a high influence to the lifetime behaviour. Between 10 and 100 h, the scatter band is as small as at a
pre-ageing temperature of about 225 °C. But when the time is rising up to 500 h, the lifetime decreases. In case of a preageing temperature of about 575 °C and different pre-ageing times, the lifetime behaviour has a factor between 0 and 15
compared to the lifetime at room temperature.
Figure 8. Pre-Ageing 575°C
By comparing different pre-ageing temperatures at the same ageing time, in case of about 500 h (see Figure 9), a coupled
effect, of oxidation and fatigue is discovered. 225 °C and 375 °C have at least still the same characteristics, just a little bit
lower compared to room temperature. But if the pre-ageing temperature is increased to 575 °C, the characteristics of the LCF
behaviour fall rapidly to lower strain amplitudes, with a factor 0 to 10 (Figure 9).
Figure 9. Pre-Ageing at 500h
For the ageing model depending on Shercliff-Ahsby, a series of tensile tests at all ageing temperatures and times were done.
With the evaluated yield strength and the corresponding times and temperatures, the model is calibrated (see Figure 10).
Quite previsibly, the behaviour at 225 °C and 375 °C is well predicted by nearly identical model parameters, but not the 575 °C
results. A reason for this behaviour is that at 500 °C Nickel develops a high affinity to oxygen and also becomes softer, as
shown in Figure 6.
Figure 10. Pre-Ageing Model (Shercliff-Ashby)
Conclusions
The oxidation of Nickel in the fractured surface is an important factor in fatigue life calculation. So it will be interesting to get
more information of the behaviour of Nickel at high temperatures in anti-oxidative atmospheres. Also the oxygen behaviour at
temperatures above 500 °C in such atmospheres will be of interest for the pre-ageing conditions. For the Shercliff-Ashby preageing model, more temperatures and time steps are required to predict the transition between low-temperature-aging and
high-temperature-aging. Further the processing method of nickel might be of interest for an increase of the lifetime.
Acknowledgements
The research work of this paper was performed at the Chair of Mechanical Engineering (Leoben, Austria) within the framework
of the Bridge Project “Highly wear-resistant coatings for thin-band casting-rollers” of the Austrian Research Council (FFG).
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