"Nitride-Forming Surface Reactions in Refractory Multi-Principal Element Alloys for Tunable Property Control"
Thermo-structural materials with improved temperature capabilities and mechanical durability are critical to achieve the performance goals for a variety of applications. Simultaneously achieving the required bulk properties (e.g., strength, toughness) and surface durability (e.g., resistance to wear, erosion, and corrosion) is a longstanding challenge in metal alloy design. However, alloy designs employing multiple principal elements (MPEAs) provide attractive opportunities to simultaneously tune the alloy chemistry for controlled surface reactivity during processing or in service while retaining good bulk mechanical properties. To enable the design of new alloys, this work focuses on understanding the dynamics of nitrogen dissolution and nitride compound layer formation during nitriding-based surface hardening processes for refractory MPEAs. Building on understanding of the reaction behavior and individual refractory elements and the thermodynamic models of alloy thermochemistry the microstructure development during gas nitriding was studied for NbTaTiZr and MoNbTaTi alloys. Parallel studies were performed with Al additions to control the formation of a single BCC solid solution or dual-phase BCC and ordered B2 phase. Characterization of the nitrided zone by TEM diffraction and microchemical analysis showed a tendency for the metals to de-mix according to nitrogen affinity, rather than to form solid solution nitrides. The results will be discussed in the context of mechanical property enhancement and future opportunities for alloy and process design.