On the stability of coherent HfRu- and ZrRu-B2 precipitates in Nb-based alloys

High temperature creep strengths of Nb-based alloys have been limited by the lack of coherent precipitates that exist at temperatures above 1200 . In this investigation, a series of BCC Nb-based alloys with coherent HfRu- and ZrRu-B2 precipitates were investigated to determine the dependence of phase stability, misfit, and solvus temperatures on composition. Sequential anneals from 1000-1500 were used to determine the B2 solvus temperature ( T s , B 2 ) of each alloy and solvus lines were constructed for each system. HfRu-B2 is found to be more thermally stable than ZrRu, with HfRu-containing alloys demonstrating higher T s , B 2 at equivalent Ru concentrations. For alloys with T s , B 2 above 1200 , additional anneals at 1000 and 1200 provide insight into B2 volume fraction variations with temperature. Additional Hf- and Zr-rich tertiary phases also formed on the grain boundaries of the selected compositions at intermediate to high temperatures. Through transmission electron microscopy, the lattice misfits for the B2 precipitates were found to be ≈ 0.5% at 1000 and the grain boundary phases were identified as C14 Laves, L1 0 , β -Hf, and topologically close-packed P phases. Implications for the design of Nb-based alloys strengthened by Ru-B2 precipitates, including strategies to mitigate deleterious phase formation, are discussed throughout. • Design guidelines for Nb-based alloys strengthened by coherent HfRu- and ZrRu-B2 precipitates are developed. • Increasing the Ru concentration increases both B2 solvus temperature and B2 volume fraction. • HfRu was found to be more thermally stable than ZrRu, demonstrating higher solvus temperatures at equivalent Ru additions. • Small additions of Hf to ZrRu-containing alloys significantly suppress the formation of deleterious phases. • Using the Orowan bowing model, estimated yield strength improvements from the B2 precipitates range from 57-500 MPa.