Ductilizing Mo-La₂O₃ alloys with ZrB₂ addition

Mo-0.6wt%La₂O₃-xZrB₂ (x=0, 0.5, 1.0, 1.5, 2.0wt%) alloys were prepared by using the solid-solid mixing/doping method. Microstructure, tensile strength, and ductility were experimentally examined to investigate the effect of ZrB₂ addition. Microstructural observations showed that the addition of 0.5-1.5wt% ZrB₂ effectively reduces the grain size of the alloys and remarkably varies the second phase particles distributed at grain boundaries (GBs). The size of intergranular La₂O₃ particles was decreased with the increasing ZrB₂ addition up to 1.5wt%. ZrO₂ particles were produced through reaction between ZrB₂ and oxygen, which were predominantly distributed at GBs, increasing the volume fraction of intergranular second phase particles while decreasing the oxygen concentration at GBs. Tensile testing results displayed that the yield strength of alloys was relatively insensitive to the ZrB₂ addition, which was quantitatively understood with respect to the strengthening mechanisms. The ductility, however, was strongly dependent on the additions. In particular, the 1.5wt% ZrB₂-doped alloy exhibited a striking ductility increased by over 40% when compared with the ZrB₂-free Mo-La₂O₃ alloy, although the two alloys have the same level in yield strength. The ductilization is mainly related to the GB purifying effect derived from the reaction addition, which overwhelms the weakening effect of intergranular particles on ductility. The findings in the current work provide a possible approach to modify the GBs and hence to improve the ductility of BCC alloys such as in present Mo alloys.