Nanoindentation of Duplex Stainless Steel

Duplex 2205 is a two-phase, austenitic (γ) and ferritic (α) stainless steel alloyed with 5 to 6% Ni, 3% Mo and 22% Cr. It is characterized by general corrosion in harsh environments, excellent resistance to stress corrosion cracking and good fatigue strength.

Compared to the standard austenitic stainless steel, duplex steel provides very high yield strength. Heat exchangers, oil field piping, tanks, pressure vessels, storage, transport, and chemical processing are the main applications for duplex steel. When these steels are subjected to high temperatures, deleterious intermetallic phases like Sigma and Chi can form resulting in decreased quality of corrosion and mechanical properties.

This article describes research conducted to substantiate the presence of Sigma and Chi phases and their distribution in the Ferrite/ Austenite matrix of an annealed duplex steel employing electron backscatter diffraction (EBSD). Bruker's Hysitron^® PI 88 SEM PicoIndenter^® was used to measure the mechanical properties of individual Ferrite/Austenite phases through in-situ nanoindentation.

Figure 1. - Hysitron PI 88 SEM PicoIndenter.

Hysitron PI 88 SEM PicoIndenter

As a depth-sensing nanomechanical test instrument, the Hysitron PI 88 SEM PicoIndenter is exclusively designed to leverage the sophisticated imaging abilities of contemporary scanning electron microscopes (SEM, FIB/SEM). When fitted with optional rotation and tilt stages, the PI 88 features flexible sample positioning with five degrees of freedom (X, Y, Z, tilt, rotation) allowing the user to align the sample with an ion beam for sample preparation or detectors such as WDS, EDS or EBSD to achieve a deeper understanding of a material’s mechanical response.

Figure 2. - A schematic of the optional rotation and tilt stages for the Hysitron PI 88 SEM PicoIndenter

e-Flash^FS EBSD Detector™

The EBSD results demonstrated in this article were obtained with the e-Flash^FS high-speed detector. Its exceptional signal sensitivity and speed together with novel features like the in-situ tilting and ARGUS FSE/BSE imaging system make e-Flash^FS - the ideal choice for in-situ experiments and a great complement to high-resolution mechanical testing such as nanoindentation.

Figure 3. - Color coded ARGUS FSE image showing orientation contrast in one of the indented areas; EBSD phase map (top left) with Ferrite (red), Austenite (blue), Sigma (yellow) and Chi (aqua) phases; grain orientation spread (GOS) map (top-right) indicating that the plastic strain field developed by the indent does not cross from the Ferrite grain into the much harder Sigma phase grain.

Experimental Procedure

Using a conductive adhesive, a polished sample of duplex steel was attached to a microscopy stub and then mechanically secured in the staging of the Hysitron PI 88. The optional rotation and tilt abilities of the Hysitron PI 88 were used to align the sample with the e-Flash detector for phase and grain mapping with ESPRIT 2.1 software. The sample was re-oriented with the indentation probe after mapping, and load-controlled nanoindentation tests were performed to peak loads of 1, 5, and 20 mN. To map the regions where the indentations were carried out, the sample was again aligned with the EBSD detector after the mechanical tests. (See FSE image in Figure 3 and EBSD results in Figure 4).

Figure 4. (a) Pattern quality map, (b) phase map, (c) orientation (IPFy) map, and (d) GOS map. Colors indicate orientation spread in degrees from 0° (blue) up to 15° (red) EBSD results.

Figure 5. Typical P-h curves from austenitic (γ) and ferritic (α) phases (top), elastic modulus and hardness variation in ferrite and austenite (bottom).

EBSD-Enhanced Nanoindentation Results

Shown in Figure 4 are the EBSD results obtained from an area consisting of an indent made at the boundary between an austenite (γ) and a ferrite (α) grain. From the results, it can be seen that the amount of plastic strain field developed by the indent is relatively larger in the austenite, that is, in the softer phase.

It was possible to determine the local plastic and elastic properties of the phase domains from load-displacement curves by analyzing 5 to 8 indentations from each phase. The elastic moduli (E) were measured to be 186 ± 1.4 GPa for the austenitic phase and 215 ± 7.2 GPa for the ferritic phase. The hardness values (H) display a similar trend with values of 3.2 ± 0.07 GPa and 3.6 ± 0.05 GPa for austenite and ferrite, respectively. Despite the actual numbers slightly differing with the results reported by Guo et al., Campos et al., and Gadelrab et al., the relative difference in the mechanical properties of the two phases corresponds well with the trends reported in the earlier studies. In addition, the results confirm the estimated improvement offered by solid solution hardening of Mo and Ni in the ferrite phase.

Employing the - Hysitron^® PI 88 SEM PicoIndenter^® - fitted with rotation and tilt stages in combination with QUANTAX EBSD system allows a more robust characterization of metallic materials by integrating grain-orientation mapping abilities and high-resolution phase with targeted nanomechanical property measurements. This combination could also be employed to extend the scope of research associated with other sophisticated multi-phase, anisotropic or textured materials.

References

1. K. Gadelrab, G. Li, M. Chiesa, and T. Souier, J. Mater. Res., 27 (2012), 1573.
2. M. Campos, A. Bautista, D.Caceres, J. Abenojar, J.M. Torralba, J. of the European Ceramic Society 23 (2003) 2813.
3. L. Q. Guo, M. C. Lin, L. J. Qiao, A. A. Volinsky, Applied Surface Sci., 287 (2013) 499.

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.