Abstract:

Molybdenum (Mo) alloys with different La₂O₃ particle additions (0.6, 0.9, 1.5 wt.%) were prepared by powder metallurgy to investigate the effect of La₂O₃ particles on microstructural evolution and creep behavior of the alloy. Pure Mo, annealed at 1500 °C for 1 h, presented a fully recrystallized microstructure characterized by equiaxed grains. The alloys doped with La₂O₃ particles (Mo-La₂O₃ alloys), on the other hand, exhibited fibrous grains elongated in the rolling direction of the plate. In contrast to the shape of the grains, the average grain size of the alloys was found to be insensitive to the addition of La₂O₃ particles. Nanosized La₂O₃ particles with diameters ranging from 65 to 75 nm were distributed within the grain interior. Tensile creep tests showed that dislocation creep was the predominant deformation mode at intermediate creep rate (10⁻⁷ s⁻¹-10⁻⁴ s⁻¹) in the present alloys. The creep stress exponent and activation energy were found to decrease with increasing temperature, particularly within the low creep rate regime (< 10⁻⁷ s⁻¹). The Mo-La₂O₃ alloys exhibited remarkably greater apparent stress exponent and activation energy than pure Mo. A creep constitutive model based on the interaction between particles and dislocations was utilized to rationalize the nanoparticle-improved creep behavior. It was demonstrated that low relaxed efficiency of dislocation line energy, which is responsible for an enhanced climb resistance of dislocations, is the major creep strengthening mechanism in the Mo-La₂O₃ alloys. In addition, the area reduction and creep fracture mode of the Mo-La₂O₃ alloys were found to be a function of the creep rate and temperature, which can be explained by the effect of the two parameters on the creep and fracture mechanisms.

Key words: Mo alloys, Nanosized particles, Creep strength, Deformation and fracture