High temperature oxidation of ultrafine-grained materials: evolution of composition, microstructure, and mechanical properties
Ultrafine-grained (UFG) materials processed by severe plastic deformation (SPD) technique exhibit superior mechanical properties and excellent plasticity. As an SPD technique, high-pressure torsion (HPT) method receives much attention for processing UFG materials due to its high efficiency in developing nanostructures with high angle grain boundaries. UFG metals with grain sizes of <1 µm have a potential for superplastic forming at elevated temperatures. However, for high-temperature applications, oxidation has been a long-standing issue. During oxidation, materials exhibit complex evolution in composition, microstructure, and morphology in the near-surface region, leading to degradation of mechanical properties. In most cases, oxidation process is diffusion-controlled, while the large number of grain boundaries (GBs), as well as the high density dislocations after HPT process, could serve as “short-circuit” diffusion paths for oxygen and hence accelerate the oxidation process. Oxidation along grain boundary reduced the mechanical properties severely, causing grain boundary cracking. Understanding the interplay between grain boundaries and oxidation helps better understand the failure of the materials. To this end, we studied the high-temperature oxidation of Fe-15wt%Cr (BCC, ferrite) processed by HPT. The thermal stability of UFG materials at various elevated temperatures for different time was studied. The composition and microstructure of the substrate and the oxide scale with and without the HPT process were investigated using Energy-Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). The Young’s modulus and hardness were also compared before and after oxidation using nanoindentation tests.