Microstructures And Properties Study Of Mo-Si-B-X(X=W,Cr,Ti) Alloys

Mo-Si-B multi-phase alloys become attractive materials in the field of high temperaure structural application for their high melting temperature?above 2000 ??, high temperature stability, excellent strength, good creep resistance. However, the poor fracture toughness and the high-temperature oxidation resistance of Mo-Si-B alloy have yet to be improved.For improving the mechanical properties of the alloy, alloying elements were introduced in Mo-Si-B alloy. In addition, the improvement methods of high-temperature oxidation resistance of Mo-Si-B alloy include the alloying and siliconizing. In this paper,the Mo-12Si-8.5B alloy containing ?-Mo, Mo3 Si and Mo₅SiB₂ was selected to study.Mo-Si-B-W?Cr,Ti? alloys were sintered by spark plasma sintering?SPS?. Effects of sintering process on microstructure and mechanical properties of alloys were investigated, and optimal process for preparing the alloys was formulated. Different W-added, Cr-added or Ti-added Mo-Si-B alloys were prepared by SPS in optimal process. Effects of alloy compositions on phase composition,microstructure and mechanical properties of Mo-Si-B alloys added alloying elements were tested,and mechanism was discussed. The coating was prapared on Mo-Si-B-W?Cr,Ti? alloys through siliconizing. The high-temperature oxidation resistance of coating were tested,and the formation mechanism were discussed.The Mo-12Si-8.5B composites prepared by SPS contains ?-Mo, Mo3 Si and Mo₅SiB₂.The alloys added W still contain elemental tungsten. The element Cr in Mo-Si-B-Cr was unequally solubled in three phases of alloys. The Ti in alloys was mainly dissolved into intermediate phase, and was rarely solubled in Mo solid solution.The optimal process for preparing the alloys was formulated:sintering temperature was 1700 ?,holding time at maximum temperature was 6 min and the external pressure is 30 MPa.The vickers hardness of alloys was improved by adding Ti, and the hardness of alloy is highest, HV1123, with 8at.% Ti adding. When the Cr content of alloys was increased to8 at.%, the hardness of alloys could be improved. But the hardness of alloys were decreased with adding W. The bending strength of alloys trended to improve with the increment of W content or Cr content, but they were lowered by adding higher content Ti. The fracture toughness of Mo-12Si-8.5B is 9.24MPa·m1/2. The fracture toughness of alloys can't beimproved by adding W, Cr or Ti, and were lowered by adding higher content alloy element.The oxidation resistance of the alloy was slightly improved by adding Cr. The coating of alloys were prepared by siliconizing. The coatings on different composites contain MoSi2 main layer and Mo5Si3 transition layer. The results of high-temperature cyclic oxidation at800 ?,1000 ? and 1200 ? show that composites with no coating appear poor oxidation resistance. The existence of the coating greatly improves the oxidation resistance of the composites. The oxidation kinetic curves of the coating on different composites follow a parabolic law. Oxidation product of the coating contain SiO2, and which of the coating on W-added or Ti-added composites also found the existence of WO3 and Ti3O5. The thickness of transition layer was increased with the oxidation temperature improving.The thickness of transition layer can be increased to 18?m.