High-temperature low-cycle fatigue behavior and microstructural evolution of an improved austenitic ODS steel
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In this work, a high-temperature low-cycle fatigue (LCF) behavior of a newly developed austenitic oxide dispersion strengthened (ODS) steel is investigated. The LCF tests were performed in air at 650 °C under three different strain amplitudes (60.4, 60.5, and 60.7%) with a nominal strain rate of 103 s1. The measured cyclic stress response showed four distinct stages which include short initial stable cyclic response followed by a prolonged hardening with subsequent short saturation and finally crack initiation and growth stage. The rate of hardening and the duration of stages are a function of applied strain amplitude. Microstructural investigations were carried out to shed light on the deformation mechanisms. After cycling, the overall microstructure appears stable without any modifications in grain shape and size. In addition, twinning and stacking fault fractions remain unchanged. However, cyclic hardening is an aftermath of dislocation multiplication whose rate is also a function of applied strain amplitude. Furthermore, oxide particles, as well as fine grains, inhibit strain localization by restricting three-dimensional dislocation structure formation that are associated with the development of extrusions and intrusions and are readily observed in conventional austenitic non-ODS steels.
I. INTRODUCTION
Oxide dispersion strengthened (ODS) steels are the promising candidate structural materials for future fusion or enhanced fission reactors. These steels are currently under investigations in comprehensive research programs worldwide. Investigations on ferritic and ferritic– martensitic ODS versions already revealed improved mechanical properties at elevated temperatures.1–5 Further improvements are also shown by introducing oxide particles in austenite,6 a phase with superior strength, creep, corrosion, and oxidation resistance in comparison with ferrite. ODS steels are generally produced via powder metallurgy that includes mechanical alloying (MA). MA enables homogeneous distribution of oxide particles throughout the matrix.7 However, the production process of austenitic ODS steels is more difficult in comparison to the ferritic versions. Because of its higher ductility, austenite powder during milling adheres to the wall of the milling container and to the milling media, which a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218, USA. c) Present address: Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. DOI: 10.1557/jmr.2018.136
decreases the production yield and alternates the chemical composition.8 Recently, Gräning et al. developed a two-step MA process allowing them to fabricate large batches with sufficient yield.9 These batches (namely, MS VI and MS VIII: MS stands for milling study) were produced using ZrO2 milling balls which led to an increased production yield (75–85%).9 Moreover, the abrasion of ZrO2 balls during milling was conceptualized as an approach to facilita
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