Investigation Of Fabrication And Properties Of Alloying And Nana-particle Compositing (Mo, W) Si₂-SiC

MoSi₂ has been presently considered as the most promising high-temperature structural material due to its excellent properties. But some problems such as room temperature brittle and high-temperature creep, limit its practical application. In this thesis, alloying of WSi₂ and compositing with nano SiC particles were used to fabricate (Mo,W)Si2-SiC nanocomposites, and further improved the mechanical property of MoSi₂ matrix composites. The main work involved in the present study includes: the analysis of valence electron structure for MoSi₂, WSi₂ and (Mo,W)Si₂; calculation of reaction thermodynamics of (Mo,W)Si2 composites in the thermal explosion mode of SHS; the research on the characteristic of pyrolysis of polycarbosilane; study on optimization of hot-pressing sintering technique using artificial neural network and genetic algorithm; discussion of strengthening and toughening mechanisms based on room temperature and high temperature mechanical properties; and finally oxidation resistance property of this nanocomposite in static atmosphere and the high-velocity flame of a burner rig.According to analysis of the valence electron structure of MoSi₂, WSi₂ and (Mo,W)Si₂ based on the empirical electron theory of solid and molecule, Mo in MoSi₂ and W in WSi₂ were of c 3th and c 7th hybrid level, respective, whereas Si was of 1st and 3th hybrid levels. The strongest bonds in the unit cells of MoSi₂ and WSi₂ were Mo-Si and W-Si bond along <331> direction, respective, whose bond energy were 26.420kJ/mol and 33.397kJ/mol, valence electron numbers were 0.25888 and 0.30697, respectively. In (Mo_1-x,W_x)Si₂ solid solution, the valence electron numbers and the strongest bond energy increased with the increase of x, showing that hardness and melting point of solid solution increased with the increase of molar fraction of WSi₂. And the percentage of valence electron numbers of solid solution increased with the increase of x, showing that the strength of solid solution increased with the increase of molar faction of WSi₂.It proved theoretically by calculating the gibbs free energy of possibly occurred reaction in Mo-W-Si system that the element powder with appropriate proportioning could be used to fabricate (Mo_1-x,W_x)Si₂ composite powder and materials. (Mo_1-x,W_x)Si2 composite powders with different molar ratios were prepared by the thermal explosion mode of SHS using Mo, W and Si powder. Mo₅Si₃ and W₅Si₃ in the product were eliminated by adding excessive Si. And pure (Mo_1-x,W_x)Si2 composite powders were gained by eliminating the excessive Si in the product by insulation above Si melting point.The flexural strength at room temperature and 1200℃gradually increased with the increase of WSi₂ content The flexural strength at room temperature and 1200℃of (Mo_0.5,W_0.5)Si₂ composite were increased by 40.7% and 112.4% compared with MoSi₂, respectively. The hardness of (Mo_0.5,W_0.5)Si₂ composite was increased by 12.1% compared with MoSi₂, and increased with the increase of WSi₂ content. All this was in agreement with the result of the valence electron theory analysis.Using precursor conversion method, SiC-(Mo,W)Si₂ composite powder coated homogeneously with nano SiC particles was fabricated by pyrolysis of PCS on the surface of (Mo,W)Si₂ composite powder. Fabricating process was fitted, forecasted and optimized by artificial neural network and genetic algorithm. The process conditions corresponding to maximal flexural strength of nanocomposite were WSi₂/(MoSi₂+ WSi₂)=50%, SiC(vol%)=15%, hot-pressing temperature being 1695℃and holding time being 65min, and the corresponding maximum target value was 714MPa.The mechanical property of the material was significantly improved by alloying of WSi₂ and compositing with nano SiC particles. (Mo_0.5,W_0.5)Si₂-15%SiC nanocomposite had the best comprehensive mechanical properties, with the room temperature flexural strength, fracture toughness and hardness being 712MPa, 7.03MPa·m^1/2 and 12.4GPa, respectively, increased by 176%, 134% and 50% compared with MoSi₂, respectively. Its flexural strength at 1200℃and 1300℃were 786MPa and 640MPa, respectively, and the yield strength at 1400℃was 491MPa. WSi₂ and nano SiC particles strengthened the matrix cooperatively and simultaneously. The residual stress is the main cause for dislocation, and dislocation multiplication is formed when the shear stress components exceed the critical shear stress needed by (Mo,W)Si₂ slip system to slip and (Mo,W)Si2 matrix around nano SiC particles begins plastic deformation. Lots of dislocation, dislocation networks, jog on dislocation and dislocation pile-up were observed in (Mo,W)Si₂-SiC nanocomposite by TEM.The main causes for strengthening and toughening (Mo,W)Si₂-SiC nanocomposite are: a. The addition of nano SiC particles helpful to the grain refinement and to prevent the abnormal grain growth; b. With the addition of nano SiC particles, the dominant fracture mode of the composite change from the intergranular fracture into transgranular fracture; c. The residual stress causing dislocation, and crack deflection and shielding in the matrix; d. Solid solution strengthening.The high-temperature strengthening mechanisms of (Mo,W)Si₂-SiC nanocomposite are mainly composed of: a. creep resistance and high temperature properties of materials are significantly improved because of the hindering effects of nano particles on both dislocation movement and the slippage of the grain boundary of matrix grain, and restraining the generation of the internal crack and cavity in materials as well. b. the high temperature strength of the matrix of nanocomposite is significantly improved after alloying of MoSi₂ with WSi₂.The addition of WSi₂ and nano SiC particles lowered the oxidation activation energy of the material, but dense and smooth protective coat could be formed on (Mo,W)Si2-SiC nanocomposite in high-temperature and static atmosphere. (Mo_0.5,W_0.5)Si₂-15%SiC nanocomposite passed application evaluation successfully with no crack and spall occurring after being oxidized for 10h in the 1823K flame of a burner rig.