Creep Testing of Superalloys at High Temperature

Superalloys are materials that have been developed to retain high strength and creep resistance even when operated at high temperatures.

Superalloy performance may be further improved by applying a thermal barrier coating or a TBC, which is a ceramic coating encapsulating the component and offering environmental protection and high thermal resistance. A bond coat is used to bond the super alloy to the TBC. Holding the alloy at a high temperature in the presence of oxygen for an extended time period causes formation of the bond coat. At the time of formation, oxygen diffuses inwards while various constituent elements diffuse outward toward the surface at varying rates.

Since the mechanical properties of all the materials change with temperature, measurement of the behavior of the various bond coat layers over a wide temperature range is important for designing and modelling super alloys with enhanced properties. Nanoindentation offers a suitable method for studying the time and temperature dependent mechanical properties of the layers to understand the complex mechanical interactions that take place when superalloy components are used under service conditions.

Procedure

The procedure used is:

• A sample of a commercially available CM-247LC nickel based superalloy was heated in air to produce a compositionally graded bond coat.
• Cross-sectioning and polishing of the sample is done to reveal the microstructural layers
• The SPM image in Figure 1 shows various microstructural zones which were distinguishable by their differing surface textures.

Figure 1. SPM image of the cross-sectioned sample surface showing the bond coat layers that were characterized. Images are collected with the same probe tip used to perform the tests, permitting very precise test positioning.

• Zone 1 was made up of a NiAl matrix with W and Cr precipitates while Zone 2 was composed of a Pt, Ni, Al solid solution.
• At temperatures ranging from 25 to 750°C, nanoindentation creep tests were done. A dynamic creep testing technique was used by which a small oscillation at a particular reference frequency was superimposed onto the quasi-static load function allowing contact stiffness to be measured continuously throughout the test.
• The reference creep test is based on the relationship between contact area and contact stiffness to determine the material’s properties over long time periods.
• Nanoindentation creep tests lasting 1500s were done at 25, 500, 650, and 750°C on Zone 1 and Zone 2 using a Hysitron TI 950 nanomechanical test system with a Berkovich indenter probe.

Results

With the help of the instrument’s in situ SPM imaging capability, the locations of the nanoindentation creep tests were selected and confirmed. Figure 2 shows an example of an SPM image collected at 650°C. Figure 3 (left) shows data from a creep test at each temperature in each zone, showing how the indent depth increased over time at constant quasi-static load.

Figure 2. SPM image of indent impressions in Zone 2 collected at 650 °C.

Figure 3. (Left) Creep data from each temperature showing the evolution of indent depth during the test. (Right) Decaying hardness over time at each temperature associated with increasing indent depth.

Figure 4. (Left) Strain Rate Vs. Stress from each creep test, showing how stress exponents are calculated. (Right) The changing stress exponent for Zone 2 suggests a changing creep mechanism while the consistent results for Zone 1 suggest a constant mechanism.

In relation to the increasing indent depth, Figure 3 (right) shows the corresponding decrease in hardness. Tests done at 25°C served as a baseline since very little creep was expected at room temperature. With the increase in temperature, the initial hardness decreased and the creep rates accelerated.

Creep in the steady-state regime is described according to the equation:

    ε = Aσ^me^-Q/RT

Where e is the strain rate, A is a proportionality constant, m is the stress exponent, Q is the activation energy, R is the gas constant, and T is the absolute temperature.

The strain rate is determined as:

    ε = Κ/Κ

Where k is the contact stiffness.

The stress components are determined by taking the slope of log e vs. log H, as shown in Figure 4 (left).

The stress exponent as a function of temperature is shown in Figure 4(right) and it shows that a single creep mechanism dominated over the temperature range tested for Zone 1 while the large changes in stress exponent for Zone 2 show that there were competing mechanisms at work.

Conclusions

Nanoindentation creep testing helps in measuring the creep behaviour of small volumes of material, including individual thin layers in a complex multilayer system. The use of nanoDMA^® III along with the xSol High Temperature Stage on the Hysitron TI 950 platform enables measurements to be accurately positioned on the layers of interest and performed over extended time durations at temperatures up to 800°C.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.