Study On The Synthesis, Microstructure And Properties Of MoSi₂ Alloyed With Nb,Al And Composited With SiC

The alloys with the nominal composition of(Mo0.97Nb0.03)(Si0.97Al0.03)2,(Mo0.94Nb0.06)(Si0.97Al0.03)2,(Mo0.91Nb0.09)(Si0.97Al0.03)2 and(Mo0.88Nb0.12)-(Si0.97Al0.03)2 were prepared by combustion synthesis method,and Mo, Nb, Si, Al powders were used as the starting materials. The combustion mode, combustion temperature, velocity of the flame-front propagation and combustion products were investigated. The results indicate that the combustion mode depends on the content of Nb. With the increase of the content of Nb, the combustion mode gradually transits from a spinning combustion to a stable combustion. The X-Ray diffraction patterns show that the combustion products of(Mo0.97Nb0.03)(Si0.97Al0.03)2 and(Mo0.94Nb0.06)-(Si0.97Al0.03)2 specimens are mainly Mo Si2, and a small amount of Mo5Si3 are also detected. The Nb Si2 phase appears in(Mo0.91Nb0.09)(Si0.97Al0.03)2 specimen. The SEM photographs show that the combustion products are porous structure materials.Research on effect of Nb and Al alloying elements on mechanical properties of Mo Si2 by vacuum hot pressing showed that with the increase of Nb content, the density, fracture toughness and vickers hardness followed a trend of decrease before increase. The relative density and bending strength followed a trend of increase before decrease. Therefore, the optimum content of Nb and Al alloying elements were found to be 6 atom% and 3 atom%, respectively.By combustion synthesis product of(Mo0.94Nb0.06)(Si0.97Al0.03)2 and commercial Si C nanopowder as raw materials, dense(Mo0.94Nb0.06)(Si0.97Al0.03)2/x vol% Si C composite were prepared by hot pressing. Systematic research on mechanical properties of hot-pressed materials at room temperature was carried out, the results showed that with the increase of Si C content, the The relative density followed a trend of decrease before increase, the bending strength showed continuous increase, the fracture toughness and vickers hardness followed a trend of increase before decrease.(Mo0.94Nb0.06)(Si0.97Al0.03)2/15 vol% Si C has the highest bending strength(472 MPa) and maximum fracture toughness(3.51 MPa.m1/2), respectively, which is 39% and 80% than that of Mo Si2(338 MPa) and(1.95 MPa.m1/2), respectively.The oxidation behavior of dense Mo Si2 and(Mo1-xNbx)(Si0.97Al0.03)2 monoliths were studied at 773 K and 1023 K in air for 216 h.(Mo0.97Nb0.03)(Si0.97Al0.03)2 shows a parabolic oxidation kinetics curves and a dense scale forms on the surface at 773 K.(Mo1-xNbx)(Si0.97Al0.03)2(x>0.09) show poorer oxidation resistance than Mo Si2, and the highest mass gain is 33.3494 g/m2, which is much higher than that of Mo Si2, 11.8509 g/m2. The oxidation mass gain and scale thickness are decreased at 1023 K. Mo Si2 shows good oxidation resistance at relative high temperature of 1023 K than that at relative low temperature of 773 K. With the increase of Nb content,the oxidation of(Mo1-xNbx)(Si0.97Al0.03)2 alloys showed be accelerated. Results from this work indicated the potential application in preventing the pest oxidation behavior by micro-alloying of Nb and Al of Mo Si2, such as(Mo0.97Nb0.03)(Si0.97Al0.03)2.