Characterization of a Cr2O3 scale formed on chromium with implanted yttrium ions by advanced electron microscopy

Although the advantageous effect of reactive elements on the oxidation resistance of alloys that form protective Cr2O3 scales was discovered 80 years ago, the underlying mechanism has still not been fully explained. One of its aspects, which was investigated in depth in the presented paper, is the structure of grain boundaries in the oxidation product. Atomic-resolution scanning transmission electron microscopy ((S)TEM) was used to examine the micro- and nanostructures of Cr2O3 formed on chromium implanted with yttrium ions. Oxidation was performed in air at either 1073 K or 1273 K. A FEI (S)TEM Titan Cubed G2 60–300 equipped with a ChemiSTEM was used to examine scale's nanostructure at a resolution down to 70 picometres. Lamellae were cut using a ZEISS NEON 40 EsBCrossBeam, a scanning electron microscope with focused ion beam (FIB). Phase composition was identified via selected area diffraction (SAED), and the chemical composition was determined via energy-dispersive X-ray spectroscopy (STEM-EDS). For both oxidation temperatures, chromium was found in the interior of the Cr2O3 scale in the form of the YCrO3 perovskite. The STEM-HAADF atomic-resolution microscopic image was found to be fully consistent with a computer-simulated crystal structure of the perovskite structure obtained by means of the JEMS software. If the layer containing yttrium is treated as a marker, then it may be assumed that the thinner outer layer grew due to the outward diffusion of metal via point defects in the cation sublattice, while the thicker inner layer formed as a result of the inward diffusion of oxygen via point defects in the anion sublattice. For the higher oxidation temperature, yttrium enrichment was also observed in the grain boundary area in chromia. However, STEM observations did not reveal the presence of an intermediate layer with a perovskite structure between Cr2O3 grains. Sulfur segregation at the chromia/metal interface was observed.