Preparation And Characteristic Of Me-Mo-Si(Me=W, Nb) High-temperature Ternary Silicides

Transition metal silicides have been considered as promising candidate structural materials to be used at high temperatures due to their high melting points,high strength retention at high temperatures,reasonable densities and oxidation-resistant. However,its brittleness is the drawback hindering its industrial applications,which is mainly determined by the phase composition and microstructure of the products.The brittle behavior can be reduced effectively by two different means:by alloying,or by microstructure control including in-situ composites.Based on the two thought,Mo-W-Si MSi₂-type silicides were prepared by mechanical alloying(MA) and hot pressing(HP) method;Mo-Nb-Si M₅Si₃-type slicides were prepared by MA/HP method and arc-melting/annealing method.The phase composition,microstructure,spectroscopy and fracture appearance of those high-temperature silicides were investigated through XRD,OM,SEM and EPMA etc. The room temperature hardness,fracture toughness,compressive strength and densification of samples were measured.The study result indicated that the main phases of(Mo_1-x,Nb_x)Si₂(x=0,0.2,0.4,0.5) alloys are MoSi₂,WSi₂,W₅Si₃ and sparse Mo_4.8Si₃C_0.6.With the adding of W element, the structure of MoSi₂ alloy is finer,MoSi₂ composites have higher hardness and compressive strength.The relative density would increase,then decrease,and MoSi₂ materials reach 95.32%in relative density when Mo:W is 6:4.The fracture mode of alloys shows brittle fracture and the toughness has no obvious improvement.The C₈₈-type(Mo_1-x,Nb_x)₅Si₃(x=0,0.2,0.4,0.6,0.8,1) alloys were prepared by arc-melting/annealing method and MA/HP method.The results show thatβ-Nb₅Si₃ is stabilized by the Mo addition.The alloys prepared by arc-melting/annealing mothod consist of(Mo,Nb)₅Si₃ andβ-(Nb,Mo)₅Si₃ solid solution as the matrix with spare Mo₃Nb₂Si₃ ternary intermetallic detected by EDS.The solubility of Nb in Mo₅Si₃ is much more than that in MoSi₂.Compare with samples not annealed,the structure of samples annealed are finer.There are some tiny cracks in the microstructure of samples prepared by arc-melting/annealing method,which lowers mechanical property. With the increase of Nb content the hardness of the alloys changes parabolically at first increasing and then decreasing.The maximum hardness of alloys can reach 1616.7HV. The hardness of annealed alloys is higher than that not annealed.The indentation fracture toughness and compressive strength of the alloys has not been improved significantly.Moreover,with the adding of Nb element,samples prepared by MA/HP method show an upward trend.Compare with samples prepared by arc-melting/annealing method,samples prepared by MA/HP mothod have lower hardness except Mo₅Si₃.But The indentation fracture toughness and compressive strength of samples prepared by MA/HP method are better than samples prepared by arc-melting/annealing method.On the opposite of samples prepared by arc-melting/annealing method,they have no tiny crack,because the thermal stress release adequately in the process of MA/HP method.