Effect Of Silicon Content On Microstructure, Mechanical Properties And Oxidation Resistance Properties Of Mo-Si-B-La₂O₃ Alloys

Mechanical Alloying （MA） and Spark Plasma Sintering （SPS） were used in this paper to fabricate Mo-8Si-8.5B alloy, Mo-12Si-8.5B alloy and Mo-16Si-8.5B alloy with 0.9.wt%lanthanum oxide （La₂O₃） doped. Microstructure, mechanical properties in room temperature and oxidation resistance properties in ultra-high temperature of Mo-Si-B-La₂O₃ alloys with different Si content were tested. The effect of Si content on microstructure and properties of Mo-Si-B-La₂O₃ alloys were analysised. Mechanical behaviours in room temperature and mechanism of oxidation resistance in ulta-high temperature were discussed.It had been showed that the MA could not only reduce the partical sizes of powders, but gain mixture microstructure with nano-grain and amorphous. The grain sizes of Mo-Si-B alloy after sintering are fine which are all about 200nm. Most La₂O₃ particles with nano-scale are distributed on the alloy, however, few coarse La₂O₃ particles which caused by agglomeration during sintering are also existed in alloy. With the increasing of Si content, volume fractions of intermetallics are increased. Besides, the microstrutures of alloys are also changed.Intermetallics are distributed on the continuous α-Mo matrix in Mo-8Si-8.5B and Mo- 12Si-8.5B,however,α-Mo phase are distributed on continuous intermetallics matrix in Mo-16Si-8.5B.The results of mechanical properties tests at room temperature of Mo-Si-B-La₂O₃ alloys showed that hardness values of alloy had been increased with the increasing of Si content,whereas, the compression strength value, the flexure strength value and the fracture toughness value are all decreased. The fracture morphology showed that intergranular was the major fracture mode of alloy, there also existe transgranular somewhere.There were two stages of oxidation processings at high temperature of Mo-Si-B-La₂O₃ alloys with different Si content which called rapid oxidation stage and steady oxidation stage.With the increasing of Si contents, the mass loss after 30h at high temperature would decrease,and the boronsilicate would cover better on the surface of alloys. In these cases, oxidation resistance properties of Mo-Si-B alloy would increase.