Oxide Dispersion Strengthened Steel Tested up to 700 °C

The term oxide dispersion strengthened (ODS) alloys refers to materials which are marked by extreme creep resistance at high temperatures. Their superior mechanical behavior in this harsh environment has led to their widespread use in challenging situations such as the production of energy, turbine blades, or tubing of heat exchangers. This experiment explores the use of a specific ODS FeCrAl alloy, which contains oxides of YAlO perovskite (YAP), Y₂Al₅O₁₂ garnet (YAG) and Y₄Al₂O₉ monoclinic (YAM).

The mechanism by which these alloys are strengthened is a function of grain size, of 1 μm, as well as dispersed oxide particles between about 10 nm to 30 nm, since these regulate the growth of the grains at higher temperatures. The movement of dislocations comes to a halt at the interfaces formed by the particles and the matrix, resulting in elevated yield stress, but only up to 60% of the melting point of the specific alloy.

Above this temperature, the early diffusion motion of voids makes room for dislocations to climb around the oxide particles and renders ineffective the above strengthening phenomenon.

Procedure

In order to characterize the mechanical properties of the ODS alloy at high temperatures, the Hysitron^® TI 950 TriboIndenter^® equipped with the xSol® High-Temperature Stage was used to conduct nanoindentation and creep testing using a sapphire Berkovich indenter.

To avoid oxidation occurring at the high temperature conditions, a shield gas consisting of a mix of 5% hydrogen and 95% nitrogen was employed. By ensuring that the xSol was tightly regulated for temperature and providing a controlled experimental environment, the testing was carried out under stable conditions.

Figure 1. Force-displacement indentation curves obtained for temperatures up to 700 °C from the quasi-static indentation performed with 10s of loading, 5s of hold, and 1s unloading.

Results

As seen in Figure 1, the quasi-static indentation generated force-displacement curves which show regular mechanical behavior and an increase in plasticity values as the temperature rises. Once it reaches 600 °C, the modulus shows a 20% fall and the hardness decreases by over 50% from the values determined at room temperature. This pattern conforms to previously published values in the literature.

The nature of the creep is exploredin relation to temperature, and the temperature secondary creep affects the mechanical behavior as shown in the equation below:

ε = Aσ^me^{-Q ⁄RT}

Where A is a constant factor:

            σ represents stress

            m is a stress exponent

            Q is an activation energy for the deformation process at the temperature

R represents gas constant

and T is the absolute temperature.

There is a continuous change in the strain rate with constant loading during the indentation procedure. In just one experiment it is possible to find a link between the change in strain rates and the applied stress. By plotting strain rates against hardness or mean pressure, as seen in Figure 3, the stress exponent m can be determined. This then acts as an identifier for the deformation mechanism underlying the change, and which occurs during the specified rate of strain.

When the stress exponent is high, such as m=78.5 for 300 °C, it is considered normative for ODS alloys. However, it is much lower (m=8.2 for 600 °C) when dislocation creep or any other mechanism activated by rising temperatures is present.

Figure 2. Hardness (top) and Young’s Modulus (bottom) in a function of temperature and comparison to tensile test data.

Figure 3. Strain rate in a function of hardness. The stress exponent, m, calculated for the different creep experiments is shown next to the relevant data.

Conclusions

Nanoindentation testing in combination with the xSol High Temperature Stage is successfully used to characterize the basic parameters of alloy performance. It identified two separate deformations occurring in an ODS alloy sample. When the temperatures were lower than 500 °C, the mechanism of deformation was the dispersion strengthening mechanism, while at temperatures higher than this, creep mechanisms were activated.

References

1. Chen, C.-L., A. Richter, and R. Kögler, JALCOM 586S173- S179, 2014.
2. Hangen, U.D., C.-L. Chen, and A. Richter, Nanomechanical characterization of ODS alloys at elevated temperatures; submitted.
3. Schneibel; Act a Materialia 59, 1300–08, 2011.
4. Beitz, W., K.H. Grote: Dubbel – Springer ISBN3-540-67777-1.

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.