Amorphous/nanocrystalline composite structure strategy for MoAlB: Achieving rapid formation and gradual growth of α-Al2O3 scale at 1200 °C

Abstract

MoAlB, with a blend of metallic and ceramic attributes, receives special attention for high-temperature protective applications. Nevertheless, the formation of volatile oxides resulting from Al depletion accelerates the degradation of MoAlB. In this study, we adopted a configurational design strategy specifically targeting the in-situ precipitation of MoAlB nanocrystals within amorphous composites to achieve an amorphous/nanocrystalline composite structure of MoAlB (ACCS-MoAlB), for enhancing its oxidation resistance and elucidating the atomic-level oxidation mechanism of MoAlB. The obtained ACCS-MoAlB sample exhibited remarkable resistance to oxidation in ambient air at 1200 °C. This is because the rapid formation of a protective α-Al₂O₃ scale generated by the synergistic interaction between metastable amorphous composites (resistant to oxygen permeation) and MoAlB nanocrystals (exhibiting selective oxidation behavior), as well as the slow growth characteristics of the scale due to the nucleation and growth of both these structural units exposed to thermal conditions during the oxidation process. Among them, the selective oxidation behavior of MoAlB was observed as a result of vacancy-mediated preferential outward migration of Al along the [100] direction. Furthermore, the good adhesion between the oxide scale and the Al-depleted MoAlB matrix was found to originate from the dislocation-free coherent epitaxial growth of α-Al₂O₃.