On the origin of extraordinary cyclic strengthening of the austenitic stainless steel Sanicro 25 during fatigue at 700 °C

• Published in: Journal of materials research Vol. 32; no. 23; pp. 4342 - 4353
• Main Authors: Heczko, Milan, Esser, Bryan D., Smith, Timothy M., Beran, Přemysl, Mazánová, Veronika, Kruml, Tomáš, Polák, Jaroslav, Mills, Michael J.
• Format: Journal Article
• Language: English
• Published: New York, USA Cambridge University Press 14.12.2017; Springer International Publishing; Springer Nature B.V
• Subjects: Alloy steels; Alloys; Applied and Technical Physics; Atomic structure; Austenitic stainless steels; Biomaterials; Cellular structure; Coal-fired power plants; Corrosion resistance; Crack initiation; Cyclic loads; Dislocation density; Dislocation pinning; Electron energy; Electron energy loss spectroscopy; Energy consumption; Grain boundaries; Inorganic Chemistry; Materials Engineering; Materials fatigue; Materials research; Materials Science; Metal fatigue; Microstructure; Movement; Nanoparticles; Nanotechnology; Niobium carbide; Nitrogen; Nuclear electric power generation; Nucleation; Physics; Scanning transmission electron microscopy; Spatial resolution; Spectrum analysis; Stainless steel; Temperature; Transmission electron microscopy; steel; scanning transmission electron microscopy (STEM); fatigue
• ISSN: 0884-2914; 2044-5326
• DOI: 10.1557/jmr.2017.311

The origin of the extraordinary strengthening of the highly alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700 °C was investigated by the use of advanced scanning transmission electron microscopy (STEM). Along with substantial change of the dislocation structure, nucleation of two distinct populations of nanoparticles was revealed. Fully coherent Cu-rich nanoparticles were observed to be homogeneously dispersed with high number density along with nanometer-sized incoherent NbC carbides precipitating on dislocations during cyclic loading. Probe-corrected high-angle annular dark-field STEM imaging was used to characterize the atomic structure of nanoparticles. Compositional analysis was conducted using both electron energy loss spectroscopy and high spatial resolution energy dispersive X-ray spectroscopy. High-temperature exposure-induced precipitation of spatially dense coherent Cu-rich nanoparticles and strain-induced nucleation of incoherent NbC nanoparticles leads to retardation of dislocation movement. The pinning effects and associated obstacles to the dislocation motion prevent recovery and formation of the localized low-energy cellular structures. As a consequence, the alloy exhibits remarkable cyclic hardening at elevated temperatures.