On the complex synthesis reaction mechanisms of the MAB phases: High-speed in-situ neutron diffraction and ex-situ X-ray diffraction studies of MoAlB

Abstract

High-intensity in-situ neutron diffraction was used in this work to characterise the reaction mechanisms of the MAB phase ceramic, MoAlB, synthesised using both standard pressureless sintering in a vacuum furnace and induction-assisted self-propagating high-temperature synthesis (SHS). The high-speed diffractogram capture rate of 0.1–20 Hz (0.05–10 s) allowed the identification of intermediate phases with lifespans of 1–30 s. It was found that MoAlB formed during pressureless sintering through a multi-stage reaction beginning at ∼510 °C with the formation of MoAl12. An SHS reaction occurred at ∼578 °C, producing metastable β-MoB as an intermediate phase, followed by MoAlB and Mo3Al8. MoAlB was also found to be present during two separate temperature windows, initially following the SHS reaction at ∼578 °C, decomposing almost completely by ⁓1020 °C, then re-forming at ⁓1200 °C. For the induction-assisted SHS process, a reaction between the elemental powders at ∼660 °C lead to the primary formation of β-MoB as an intermediate phase, followed by a transition to α-MoB and intercalation with Al, forming MoAlB in ∼3 s from the beginning of the SHS reaction. Neutron diffraction (ND), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used post-reaction to confirm the compositions and analyse the microstructure of samples produced using both methods.