Modifying Microstructure and Mechanical Properties of Mo-Nb Single Crystals via Thermal Annealing

Molybdenum-Niobium (Mo-Nb) single crystals possess excellent mechanical properties, such as high strength and low creep rate at high temperatures, which leads to Mo-Nb single crystals’ potential application in the irradiated and aerospace environment. Mechanical properties at high temperatures are closely related to structural defects, including the density of dislocation and low-angle grain boundaries in a single-crystal Mo alloy. It is well known that the density of defects is mainly contributed to by processing and annealing. To clarify the microstructural evolution of Mo-Nb single-crystal alloys, thermal annealing tests with temperatures varying from 1100 °C to 1700 °C were conducted. Two Nb contents (3 at.% and 6 at.%) were chosen to investigate the effect of Nb content on the thermal stability (≤1700 °C) of single-crystal Mo-Nb alloys. Samples with high Nb content (6 at.%) soften after annealing, while ductility at room temperature obviously enhances In the low Nb content (3 at.%) sample, however, hardening and softening occur after low (1100 °C) and high temperature annealing, respectively. The evolution of mechanical properties could be mainly attributed to the change of density in the low-angle grain boundary. Furthermore, the changing Nb element and dislocation density during annealing are still important to the strength and ductility of Mo-Nb single crystals.