High Temperature B2 Precipitation in Ru-Containing Refractory Multi-principal Element Alloys

Ru-based B2 phases present an opportunity to design two-phase BCC + B2 refractory multi-principal element alloys (RMPEAs) with higher temperature stability compared to B2 phases observed in RMPEAs. In this investigation, seven equiatomic Ru-containing RMPEAs were characterized in the as-cast and annealed conditions. Of the two Hf-free alloys, Mo _25 Nb _25 Ta _25 Ru _25 was determined to be a single-phase B2 alloy and Mo _20 Nb _20 Ta _20 W _20 Ru _20 was single-phase BCC. Within all five Hf-containing alloys, phases formed during solidification included HfRu–B2, disordered BCC, and HfO _2 phases. The Hf-containing alloys also precipitated B2 nanoparticles within the BCC phases after further cooling in the solid. All phases were still present after annealing at 1500 ^∘ C to 1600 ^∘ C. The HfRu–B2 nanoparticles in as-cast Hf _20 Mo _20 Nb _20 Ta _20 Ru _20 were characterized by transmission electron microscopy (TEM), and a lattice misfit of < 1 pct between the BCC phase and B2 nanoparticles was calculated. Room-temperature micropillar compression tests were performed on BCC + B2 nanoparticle regions in annealed Hf _20 Mo _20 Nb _20 Ta _20 Ru _20 . Post-mortem TEM analysis revealed precipitate shearing by dislocations, resulting in paired dislocations, along with bowing of dislocations around precipitates. Utilizing the insights from this investigation, compositions for RMPEAs with solutionable B2 precipitates stable above 1200 ^∘ C are suggested.