A novel formation mechanism: one-step synthesis of high-purity MAX phase bulk via enlarged reaction depth

The synthesis of high purity MAX phase bulk still faces a big challenge. In this study, we fabricated 98 vol.% MAX phase bulk doped with Al2O3 nanoparticles through spark plasma sintering by using larger size TixAl1-x powders (particle size ~100 μm) for the first time, in which 91 vol.% Ti2AlN was the dominant component. The phase transition mechanism for Ti2AlN was deeply revealed by both experiments and theoretical calculations. Ti proportion of TixAl1-x powders played a key role in the formation of Ti2AlN, and increasing Ti proportion leaded to higher content of Ti2AlN. Both Al migration from TixAl1-x and TiN nanotwins generated by N vacancies could enlarge the reaction depth, resulting in fully transformation of larger size reactants and thus high purity MAX phase as obtained. Further, the transition model for the synthesis of Ti2AlN was accordingly proposed, which contained three stages and was additionally verified by experimental observations.