Fabrication and Characterization of Naturally Selected Epitaxial Fe-{111} Y2Ti2O7 Mesoscopic Interfaces: Some Potential Implications to Nano-Oxide Dispersion-Strengthened Steels

Abstract

The smallest features of ≈2 to 3 nm in nanostructured ferritic alloys (NFA), a variant of oxide dispersion-strengthened steels, include the Y2Ti2O7 complex oxide cubic pyrochlore phase. The interface between the bcc Fe-Cr ferrite matrix and the fcc nanometer-scale Y2Ti2O7 plays a critical role in the stability, strength, and damage tolerance of NFA. To complement other characterization studies of the actual nanofeatures (NF) themselves, mesoscopic interfaces were created by electron beam deposition of a thin Fe layer on a 5 deg miscut {111} Y2Ti2O7 bulk single crystal surface. While the mesoscopic interfaces may differ from those of the embedded NF, the former facilitate characterization of controlled interfaces, such as interactions with point defects and helium. The Fe-Y2Ti2O7 interfaces were studied using scanning electron microscopy, including electron backscatter diffraction, atomic force microscopy, X-ray diffraction, and transmission electron microscopy (TEM). The polycrystalline Fe layer has two general orientation relationships (OR) that are close to (a) the Nishiyama–Wasserman (NW) OR \( \left\{ {110} \right\}_{\text{Fe}} ||\left\{ {111} \right\}_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} \) and \( \left\langle {100} \right\rangle_{\text{Fe}} ||\left\langle {110} \right\rangle_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} \) and (b) \( \left\{ {100} \right\}_{\text{Fe}} ||\left\{ {111} \right\}_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} \) and \( \left\langle {100} \right\rangle_{\text{Fe}} ||\left\langle {110} \right\rangle_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} \). High-resolution TEM shows that the NW interface is near-atomically flat, while the {100}Fe grains are an artifact associated with a thin oxide layer. However, the fact that there is still a Fe-Y2Ti2O7 OR is significant. No OR is observed in the presence of a thicker oxide layer.