Microstructure characterization and formation mechanism of lanthanum phosphide in FeCrAlRE alloy with ultra-low oxygen

Rare earth (RE) could be added to steels to improve their properties, but there is still controversy on its presence form. Taking 0Cr20Al6RE as the research object, the morphology, distribution, chemical composition and phase structure of RE precipitated phases were characterized by SEM, EPMA and TEM. It is found that with ultra-low oxygen (3 ppm) and RE/(O + S) = 37.1, the main existence form of rare-earth is RE-P-As in which the La-P phase is lanthanum monophosphide with a rock-salt structure, while the structure of complex inclusion is mainly RE-Al-O/RE-P-As/(Ti,Nb)(C,N). According to Gibbs free energy, precipitated phase distribution and solidification segregation calculation, it is confirmed that La-P-As is formed at the solidification front. This finding provides a technology idea for the composition design and process improvement of RE steels by avoiding the emergence of large-size RE-O-S inclusions.