High-Temperature Creep Testing of a Superalloy Bond Coat

A superalloy is a material that has been specially designed to conserve high creep resistance and strength when used at high temperatures. Currently, these are made of complex materials which are composites of a dozen or more elements. To further improve their performance, they are often coated with a thermal barrier coating (TBC), composed of ceramic. This provides a protective capsule around the component which protects it against both harsh environments and heat.

The TBC-superalloy bond is via a bond coat that is produced by keeping the alloy at high temperature conditions over a long period in an oxygen atmosphere. When the bond is being formed, oxygen moves deeper into the alloy by diffusion with a corresponding outward diffusion of the various other elements that constitute the alloy, at different rates. As a result, the microstructure is multilayered and complex, producing a sturdy and resistant bond between the oxide TBC and the internal metal substrate.

All materials show a measurable alteration in mechanical behavior with temperature changes, which means that the properties of any bond coat layer must be evaluated using a wide set of temperature parameters. This is vital when generating models and designs of advanced superalloys with better mechanical properties. The bond coat layers are only a few microns in thickness in most cases, which makes it difficult or non-feasible to use the traditional creep tests on individual isolated layers.

This article describes the results of a study performed on the mechanical properties of bond coat layers in relation to temperature and time, intended to elucidate the complex interactions occurring between various superalloy components and the conditions under which they are used.

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.

Procedure

In this experiment, a bond coat graded with respect to composition was created on a commercially available super alloy sample based on nickel (CM-247LC) by heating it in air. It was cut through in cross-sections and polished so that the layers of the microstructure could be seen, as seen in the SPM image in Figure 1. Each zone has a different surface texture.

The first zone (Zone 1) is made up of an alloy comprising W and Cr precipitates within a NiAl matrix. Zone 2 consists of a solid solution of Pt, Ni, and Al. Both layers were subjected to creep testing by nanoindentation over various temperatures from 25 °C to 750 °C.

The specific technique was dynamic creep testing, using a small oscillation at a specified reference frequency, of 220 Hz in this situation, which is superimposed over the load function (which was quasi-static) so that the measurement of contact stiffness could be taken continuously through the test duration. The reference creep test results depend upon the relationship between the area of contact and the contact stiffness to give a value for the properties of the material tested over a long duration.

In this experiment, creep testing by nanoindentation was carried out at 25 °C, 500 °C, 650 °C, and 750 °C for 1500 seconds, on Zone 1 and Zone 2. The equipment comprised a Hysitron^® TI 980 TriboIndenter^® with an xSol® heating stage and a Berkovich probe for indentation.

Results

The in situ SPM imaging acquisition of the instrument was useful in choosing and confirming the locations of the creep testing nanoindentations. Figure 2 shows one instance of an SPM image at 650 °C. Data values obtained from the creep test at each temperature in both of the zones is shown in Figure 3 (top). They reveal the increase in depth of indentation with time when the quasi-static load is constant. Figure 3 (bottom) shows how hardness is reduced as the indent depth goes up.

Baseline measurements were obtained by doing the same tests at 25 °C, as room temperature creep is expected to be minimal. With increasing temperatures the hardness went down from the initial value and the creep increased rapidly. The equation below shows how creep changes in a steady-state regime:

Where ε• is the strain rate:

A is a proportionality constant

m is the stress exponent

Q is the activation energy

R is the gas constant

T is the absolute temperature.

The mechanism of creep shows changes in correlation with the stress exponent m and/or the activation energy Q stiffness values are continuously available when a reference creep test is performed. The strain rate is therefore determined on a continuous basis, as:

Where k is the contact stiffness.

The representative stress is assumed to be the hardness, or mean contact pressure. In order to calculate the stress exponent m, the slope of log ε• versus log H is used as Figure 4 (top) shows. In the initial period of the test, over the first 100-200 seconds, there was a brief period of non-linear behavior observed in the log ε• vs. log H curves. This is not shown in Figure 4 (top) as it is not a steady-state observation.

Figure 4 (bottom) shows how stress exponent m varies with temperature, indicating the possibility that the whole temperature range which was tested for in Zone 1 was predominantly characterized by the same creep mechanism, but in Zone 2 the m value showed large changes, which may suggest that more than one mechanisms were in competition in this zone.

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

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

Figure 4. (Top) Strain rate versus stress from each creep test, showing how stress exponents are calculated. (Bottom) The changing stress exponent for Zone 2 suggests a changing creep mechanism, while the consistent results for Zone 1 suggest a constant mechanism.

Conclusions

The use of a nanoindentation creep test measurement allows the creep properties of a material to be studied using very small volumes, as little as one layer in a complex system comprising of multiple layers. When nanoDMA® III testing is used with the xSol high-temperature stage on the Hysitron TI 980 device, measurements can be taken with accuracy on the layers selected for study, over a long period, and at temperatures of 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.