Feedstock stoichiometry steers Ta4AlC3 polymorph formation

The MAX phases constitute a family of atomically layered ternary carbides/nitrides with hexagonal structure (space group P6 3 /mmc ) applauded for their compositional versatility and properties bridging the gap between metals and ceramics. Ta-Al-C is a unique system for MAX phase synthesis because of the existence of two Ta 4 AlC 3 polymorphs with distinct stacking order. This work allowed correlating Ta 2 AlC and β-Ta 4 AlC 3 formation to competing peritectoid reactions between φ-Ta 22 Al 20 and cubic TaC monocarbide or trigonal Ta 2 C hemicarbide, respectively. This work suggests that α-Ta 4 AlC 3 formation relies on the intermediate Ta 2 AlC MAX phase with narrow processing window. Conversely, it suggests that β-Ta 4 AlC 3 requires no intermediate and can form rapidly under broadly varying thermal/compositional conditions. Accordingly, this work reports on bulk, high-purity MAX phase synthesis by spark plasma sintering feedstocks based on Ta 2 H, Al, and graphite powders in the 1150-1650 °C range. Delicate variation of feedstock stoichiometry successfully steers antecedent phase formation towards high polymorph specificity.