Modification Of Alloying And Composition Based On Mo₅Si₃ Refractory Metal Silicides

The Mo-Si system metal silicides (Mo₅Si₃) are attractive materials in the field of high temperature structural applications for their high melting point, low density, excellent strength and corrosion resistance. However, the high brittleness at room temperature, insufficient strength at high temperature and oxidation resistance shortcomings hindered the actual applications. Alloying and compounding is common method to improve the performance of metal silicides. Based on reciprocity replacement alloying principle of Mo₅Si₃ and Nb₅Si₃, (Mo_1-xNb_x)₅Si₃ silicide alloy was produced by arc-melting and hot-pressing sintering process, respectively. And the microstructure and mechanical properties and corrosion resistant properties have been studied. Based on the in-situ compounding principle, the paper studies mechanical alloying process, products and synthesis mechanism of MoO₃-Si-Al, MoO₃-Mo-Si-Al and MoO₃-CuO-Mo-Si-Al systematic. Combining the hot-pressing sintering technology, bulk composite of Cu-Al₂O₃/Mo₅Si₃ in situ toughening by Cu and Al₂O₃ was prepared. And the mechanical properties, microstructure, wear resistance, oxidation resistance and toughen mechanism of these composite materials have been investigated in detail.Comparing with the alloys prepared by hot-pressing sintering method, (Mo_1-xNb_x)₅Si₃ alloys prepared by arc-melting have higher relative density and micro-hardness, low fracture toughness and compressive strength. Micro-hardness, fracture toughness and compressive strength of Mo₃Nb2Si3 alloy were 1616.7 HV, 3.2 MPa·m1/2, 1017MPa and 1334.6 HV, 4.7MPa·m1/2, 1637MPa, respectively. Lamellar organization of Mo₅Si₃/ MoSi2 formed in alloy organization of (Mo_1-xNb_x)₅Si₃ alloys. The formation mechanism of lamellar organization and the effect of lamellar on mechanical properties were studied. With the reducing of distance between layers, micro-hardness, fracture toughness and compressive strength increased. Through the method of section line, layer distances and layer densities were calculated. The results showed that with the annealing time prolong, at first the layer distances reduced, and then increased. The layer distances of (Mo_1-xNb_x)₅Si₃(X=0,0.2,0.4) alloys were 57.3μm,48.2μm and 24.9μm after annealing for 50h.(Mo_1-xNb_x)₅Si₃ alloy has very excellent corrosion resistance in HCl and H2SO4 solution. And its corrosion weightlessness is 100 times lower than 0Cr18Ni9 stainless steel. In low voltage, Mo in alloy surface forms MoO₂ passivation coating. While in high voltage, MoO₂ passivation coating forms H4MoO4 and dissolves, and the same time, the Si in the surface forms a stable SiO₂ passivation coating, thus making alloy show second passivation phenomenon. Metal silicide alloys of (Mo_1-xNb_x)₅Si₃ have high covalent bond and chemical stability, are able to effectively resist solution corrosion and improve the corrosion resistance of alloy. In NaOH solution, SiO₂ preservative reacts with NaOH in the solution and produces Na2SiO4, damaging the integrity of SiO₂ preservative, and so reduce the corrosion resistance of (Mo_1-xNb_x)₅Si₃ metal silicides alloy.Al₂O₃-Mo₃Si/Mo₅Si₃ composite powders were synthesized by mechanical milling for 5h using MoO₃-Si-Al powders as raw materials. The reaction is similar with explosion mode of self-propagation high temperature synthesis. The first reaction was thermite reaction of MoO₃ and Al in the milling process, Then a series of reactions between Mo-Si have been triggered. Using MoO₃-Mo-Si-Al as raw material can prepare Al₂O₃/Mo₅Si₃ composite powders with different Al₂O₃ content (mass ratio) by means of controlling the contents of MoO₃ and Al. 20%Al₂O₃/Mo₅Si₃ composite powder has the best response characteristics. Composite powder particles are small and uniform with particle size less than 5μm after ball milling for 30h. The grain size of Al₂O₃ and Mo₅Si₃ are 29.9nm and 42.7nm, respectively. Composite powders no phase transformation take place after annealing for 1h at 1000℃and composite powders has good thermal stability.Cu-20%Al₂O₃/Mo₅Si₃ composite materials are in-situ synthesized by using MoO₃-CuO-Mo-Si-Al as raw material, combining mechanical chemical reduction method with hot pressing sintering technique. The structure of composite is uniform and compact. With the increase of Cu content, the microstructure becomes fine and sintering density becomes higher. Fine Al₂O₃ particles exist in grain boundary and within the crystal of composite materials, forming intragranular particles, strengthening the matrix. Elemental Cu with good plastic toughness disperses distribution in matrix materials, improving the interface combination among matrix, preventing crack propagation, increasing plastic deformation ability. Composite material 15%Cu-20%Al₂O₃/Mo₅Si₃ have the best comprehensive mechanical performance, its micro-hardness, compressive and flexural strength and fracture toughness are 1090HV, 2160MPa, 512MPa and 7.33MPa?m1/2, respectively.The introduction of Al₂O₃ and Cu improves the antioxidative properties of Mo₅Si₃ in middle and low-temperature and matain good oxidation resistance in high temperature. Composites materials have good circulation oxidation characteristics, and its oxidation dynamic curves approximately show a parabola rule. CuO in the surface of Cu-20%Al₂O₃/Mo₅Si₃ composites reacts with MoO₃ and produces Cu3Mo2O9 and CuMoO4 with better liquidity, thus improving the compactness of the oxide film. SiO₂ and Al₂O₃ in surface forms rigid and compact Al6Si2O13 mullite protective cover because of Cu3Mo2O9 and CuMoO4 fusion decomposition and fluxing action in higher temperature. Oxidation failure appears in experimental materials after cycle oxidation for a long time. Mo₅Si₃ oxidation failure is because oxidation film surface crazes in bigger thermal stress, 20%Al₂O₃/Mo₅Si₃ shows local oxide layer flaking. While Cu-20%Al₂O₃/Mo₅Si₃ is mainly because stress concentration in the corners caused by sample shape, causing crackle in the corners and accelerating material oxidation.Composite material with good resistance to frictional wear, composite materials has low coefficient of friction and wear rate using Si₃N₄ ceramics ball for friction pair. Adding a few content of Cu to composite materials can improve the interface bond and plastic toughness, enhancing the friction properties of materials. Using GCr15 bearing steel ball as friction pair, coefficient of friction of composite materials has increased nearly 10 times, and wear rate reduces. The wear form of 20%Al₂O₃/Mo₅Si₃ composite gives priority to particle attrition, at the same time with fatigue wear and oxidative wear; after join in Cu, the wear form of composite material are primarily adhesion wear and fatigue wear.