Study On Synthesis And Properties Of Mo-Si-B Materials

For civilian and military applications, there is an urgent and increasing need for materials which can be used at high temperature to improve energy efficiency; a considerable amount of research has concentrated on developing new materials. Multiphase Mo-Si-B alloys containing different combinations of equilibrium ternary phases are under examination for high temperature structural application due to their high temperature stability and attractive material properties. To meet the need of developing useful structural molybdenum-based alloys, two distinct multiphase Mo-Si-B alloy systems have been proposed, based on the phases:(1)Mo₃Si,Mo₅SiB₂(T₂),and Mo₅Si₃(T₁),by Akinc and co-workers; and (2)α-Mo,Mo₃Si,and Mo₅SiB₂(T₂),by Berczik. With regard to fracture resistance, the latter alloys hold more promise since they contain the relatively ductileα-Mo phase, with Si and B in solid solution, in addition to the brittle intermatallic phases Mo₃Si and Mo₅SiB₂.Alloys comprised of Mo(solid solution(ss)), T₂(Mo₅SiB₂) and Mo₃Si phases appear together favorable combinations of mechanical property and oxidation resistance due to the formation of a adhesive borosilicate coating during high temperature oxidation and sufficient creep resistance and high temperature strength .A balance of toughness, oxidation resistance, and creep resistance is sought from the metal and silicide phases. The.assumption for the alloys design is that a continuous bcc matrix will provide for improved toughness, and that the presence of the silicide phases will aid in the formation of a protective scale in addition to providing good high temperature strength. To date, people mainly use arc melting method for synthesis Mo-Si-B alloys, sometime also use powder metallurgic method(PM).But only a few researchers use PM, because it will get more internal defects, especially pore. So in this work, we conducted the research on the fabrication of Mo-8.9Si-7.7B alloy with a not so severe process. Also, the microstructure (by XRD and SEM), the physical and electrical properties, as well as oxidation behavior of the obtained materials have been investigated, respectively.The high purity bulk tri-phaseα-Mo-Mo₃Si-Mo₅SiB₂ alloy has been sintered by hot pressing with the start atom ratio Mo-8.9Si-7.7B and (Mo-8.9Si-7.7B)-3Sn at 1600℃with a pressure of 30MPa in high vacuum. The addition Sn was uniform distributed in the matrix with the form of solid solution.. When rising the temperature under 1600℃,the contents of Mo₃Si and Mo₅SiB₂ phases all increased. When the temperature higher than 1600℃,the composition of the alloys changed, containing a lot of MoSi₂ phase, and the contents of Mo₃Si and Mo₅SiB₂ phases decreased. When extending the soaking time, the composition of the alloys didn't change much.Density, physical and electrical properties of MSB and MSBS bulk materials has been also carefully investigated. Alloy with 3at.%Sn hold the highest density. The measured flexural strength, fracture toughness, Vickers hardness and electrical conductivity of MSB alloy were 129.564 MPa, 2.361 MPa·m^1/2,704.14 Gpa and 34.481μΩ.cm. The measured flexural strength, fracture toughness, Vickers hardness and electrical conductivity of MSB alloy were 462.712 MPa,6.550MPa·m^1/2,1121.28 Gpa and 61.914μΩ.cm.The solution of Sn improved each physical property, mainly because the solution strength of Sn, and the densify of the alloy by adding Sn..The oxidation behavior of MSB and MSBS alloys were also investigated by means of TGA, XRD, SEM in the work. The cyclic oxidation behavior of MSB and MSBS at 800 and 1000℃for 30 cycles in air followed a parabolic rate law. The pore in the samples made the materials lost more during oxidation. It had been demonstrated that MSB and MSBS alloys had some oxidation resistance due to the dense continuous oxide scale consisted of B-SiO₂ forming during oxidation in air. Generally, the oxide scale was grown by the inward diffusion of O and the outward diffusion of Si and B. O₂ meet with Mo and form MoO₃, MoO₃ volatilized and form a lot of pores between oxide scale and matrix. Once formed the layer of pores can be, though not always, an effective barrier to further diffusion Si and/or B outward, but does not hinder the continued inward diffusion of oxygen. MSB sample obtained littler pores before oxidation, but MSBS sample had fewer pore because of the adding of Sn, so the oxidation resistance of MSBS was much better than MSB.The corrosion rates of MSBS material in H₂SO₄ solution with different concentration were better than that of MSB material.