Microstructure and Mechanical Properties of MoSi2/Nb Composites Made by MA-Pulse Discharge Sintering

Abstract

MoSi 2 is one of the most promising candidates for high-temperature structural materials because of its high melting point and excellent oxidation resistance. However, the low room temperature fracture toughness and ductility, etc., still remain as major disadvantages to potential high temperature structural applications. MoSi 2 composites containing Nb have shown a great improvement in mechanical properties, especially in fracture toughness, over the monolithic MoSi 2 . However, the addition of refractory metals such as Nb or W has not been that acceptable because of the high reactivity of these additions with the MoSi 2 matrix, and the deleterious effect of reducing oxidation resistance and increasing density. Also, Nb reinforced MoSi 2 composites resulted in a penalty with decreasing strength, both at room and elevated temperatures. In the present study, MoSi 2 and MoSi 2 -Nb powders with a fine grain size were prepared by mechanical alloying. In order to retain the fine microstructure in the consolidated samples, pulse discharge sintering was used to consolidate these powders at a lower sintering temperature compared to that of the conventional hot pressing or HIP. The grain size of the consolidated MoSi 2 -10Nb composite was 0.18 to 1.2 μm, which was finer than that of monolithic MoSi 2 , 1.5 to 3.0 μm. The formation of the (MoNb) 5 Si 3 phase was pronounced in the Nb added composites, and this phase seemed to be formed during the sintering process. Both monolithic MoSi 2 and MoSi 2 -10Nb composites from the mechanically alloyed powders showed a significant amount of plastic deformation in a bending test at 1073 K. The yield stress of the Nb added alloy was higher than that of the monolithic MoSi 2 , which may be caused by the refinement of the microstructure.