The effect of Zr, Ti, and Hf on the microstructure and mechanical properties of the Nb_1.6Mo_0.3C_0.04 refractory alloy

Nb-based alloys combine low density with good room-temperature ductility, but commercial alloys such as Nb-1Zr and C-103 lack sufficient strength above 1200℃. In this study, an Nb_1.6Mo_0.3C_0.04 refractory alloy was modified by adding small amounts of Zr, Ti and Hf to form MC-type carbides (M = metal) in the matrix and enhance its high-temperature performance. Four alloys, Zr_0.04Nb_1.6Mo_0.3C_0.04, Ti_0.04Nb_1.6Mo_0.3C_0.04, Hf_0.04Nb_1.6Mo_0.3C_0.04 and Ti_0.024Zr_0.016Nb_1.6Mo_0.3C_0.04, were fabricated, and their microstructures and compressive properties were characterized. All alloys consist of a body-centred cubic (bcc) Nb-Mo solid-solution matrix in which plate-like MC carbides are uniformly dispersed and coherent with the matrix. Carbides containing carbon vacancies deform by dislocation slip and stacking faults, which relax interfacial stresses and delay crack initiation. Differences in melting point and lattice misfit among Zr-, Ti- and Hf-containing carbides control their morphology and stability, thereby governing the balance between high-temperature strengthening and softening. In the Zr-containing alloy, boundary carbides and intragranular (Zr,Nb)C needles provide the most stable high-temperature strengthening. In the Ti-containing alloy, intragranular needle-like (Ti,Nb)C precipitates increase yield strength but reduce resistance to high-temperature softening. In the Hf-containing alloy, grain-boundary (Hf,Nb)C carbides raise yield strength but promote high-temperature softening because of stress concentrations. All four alloys show densities below 8.9 g/cm³, compressive strains above 50 % at room temperature and specific yield strengths at 1450℃ in the range of 27.1–30.4 MPa·cm³/g, comparable to those of NbMoTaW refractory high-entropy alloys but with better room-temperature workability, lower density and lower cost.