Oxide-dispersed-strengthened (ODS) steels exhibit characteristics that may make them useful materials in future nuclear applications. It is clear that the next generation of reactor designs will operate at higher temperatures than current reactors. Increased temperatures induce higher stresses and require reactor designs with materials of greater strength. ODS steels have properties that would make them great fuel-cladding materials given these increased demands. They have been shown to have greater high-temperature creep strengths than current reactor materials while also controlling irradiation damage. These alloyed materials are made by introducing reinforcing oxides into the steel matrix by mechanical alloying processes including planetary ball milling and attritor ball milling.
Creep is mechanical deformation that occurs as a result of prolonged stresses that are less than the material's yield strength. It is typically a three-stage process consisting of a primary stage with a high but decreasing strain rate, a secondary phase caused by work hardening with an inflection point in the strain as a function of time, and finally a tertiary phase with exponentially increasing strain with the onset of necking. [1] Creep always increases at higher temperatures. Therefore, it is always a concern in nuclear reactor design.
Mechanical creep tests have shown that these alloys offer superior creep properties. For example, a Fe12Cr2.5W0.4Ti0.25Y2O3 (12YWT) alloy was shown to fail after 14,500 hours at a very high temperature of 800°C with a 2.3% elongation at a loading stress of 138 MPa. These properties were excellent when compared to a V4Cr4Ti alloy that failed after 4,029 hours and 52% elongation also tested at 800°C but at a significantly lower stress of 77 MPa. While the vanadium alloy exhibited tertiary creep, the 12YWT alloy did not. This may make imminent creep failure identification more difficult for the 12YWT alloy. [2] Furthermore, this is not an isolated case. Y or Y-Ti nanoprecipitates can reduce creep rates by six orders of magnitude at temperatures from 650°C-900°C. [3]
In addition to superior creep properties, ODS alloys have been shown to have high elevated temperature tensile properties. Microstructural analysis suggests that the tensile strength at high temperatures increases in alloys with smaller, more evenly distributed oxide particles. In these high-performing alloys, ductility remained high (reduction of area > 40%) with increases in strength to greater than 2 GPa.
Processing plays a large role in performance. During the alloying process, it is typical to accumulate oxygen and nitrogen interstitials. This can result in a high ductile-to-brittle transition temperature as well as poor mechanical impact properties. Thus, it is important to limit the milling time to reduce interstitial pick-up. Attritor milling is the preferred milling process because it creates a finer grain structure while reducing interstitial formation.
The superior mechanical properties of these materials are well established and have made them a hot area of research for more than a decade. However, the structure of the highly stabilized oxide nanoclusters is still not completely understood. Measurements conclude that these clusters are typically less than 5 nm in size. However, it was not until a recent study that utilized Cs-corrected transmission electron microscopy that reasons for the stability were confirmed. The research shows that these nanoclusters have highly defective NaCl structures with a high lattice coherency to the BCC-structured steel matrix. It is this high degree of point defects and the structural affinity of the nanoclusters to the steel matrix that account for the stability. [3] Moreover, this stability is extraordinary when combined with the material's mechanical properties. Even at temperatures of 1400°C (91% of the melting temperature) and in the presence of intense neutron irradiation fields, stability is maintained. [3]
Despite these promising results, challenges remain. First, steel-processing techniques for these highly-specialized alloys have not been perfected. Especially when compared to reduced-activation ferritic and martensitic steels, processes like fabrication and welding need to be perfected before fusion reactors can be constructed. Other problems persist. For example, producing these materials in large sizes is difficult. Additionally, a lot of the metal forming processes involved such as rolling and extrusion are directional and result in elongated grain structures. This causes anisotropic behavior that can have detrimental effects such as reduced mechanical properties along certain directions in the material. Finally, the structures of these materials are still not completely understood. As nanocharacterization techniques improve, it is possible that our understanding can lead to further structural manipulation and even better overall properties. As the world's energy demands increase and nuclear reactors play a larger role, higher-performing nuclear reactor materials will be required and ODS steels may be part of the solution.
© Max Quinn. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] W. D. Callister Jr., Fundamentals of Materials Science and Engineering (Wiley, 2005).
[2] R. L. Klueh et al., "Tensile and Creep Properties of an Oxide Dispersion-Strengthened Ferritic Steel," J. Nucl. Mat. 307-311, 773 (2002).
[3] A Hirata et al., "Atomic Structure of Nanoclusters in Oxide-Dispersion-Strengthened Steels," Nature Mat. 10, 922 (2011).