Fabrication, strength and oxidation of molybdenum-silicon-boron alloys from reaction synthesis

High-temperature materials;Tensile strength;Oxidation resistance;Reaction synthesis;Powder metallurgy;Mo-Si-B Alloys

Middlemas, Michael Robert ; Materials Science and Engineering

University:Georgia Institute of Technology

Department:Materials Science and Engineering

关键词: High-temperature materials;    Tensile strength;    Oxidation resistance;    Reaction synthesis;    Powder metallurgy;    Mo-Si-B Alloys;   

Others  :  https://smartech.gatech.edu/bitstream/1853/28253/1/middlemas_michael_r_200905_phd.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

Mo-Si-B alloys are a leading candidate for the next generation of jet turbine engine blades and have the potential to raise operating temperatures by 300-400°C.The alloys of interest are a three-phase mixture of the molybdenum solid solution (Moss) and two intermetallic phases, Mo3Si (A15) and Mo5SiB2 (T2).A novel powder metallurgical method was developed which uses the reaction of molybdenum, silicon nitride (Si3N4) and boron nitride (BN) powders to synthesize a fine dispersion of intermetallics in a Moss matrix.The covalent nitrides are stable in oxidizing environments up to 1000ºC, allowing for fine particle processing.The process developed uses standard powder processing techniques to create Mo-Si-B alloys in a less complex and expensive manner than previously demonstrated. This powder metallurgy approach yields a fine dispersion of intermetallics in the Moss matrix with average grain sizes of 2-4μm.Densities up to 95% of theoretical were attained from pressureless sintering at 1600°C and full theoretical density was achieved by hot-isostatic pressing (HIP). Sintering and HIPing at 1300°C reduced the grain sizes of all three phases by over a factor of two.Microstructure examination by electron back-scatter diffraction imaging was used to precisely define the location of the phases and to measure the volume fractions and grain size distributions.Microstructural quantification techniques including two-point correlation functions were used to quantify microstructural features and correlate the BN reactant powder size and morphology to the distribution of the intermetallic phases.High-temperature tensile tests were conducted and yield strengths of 580MPa at 1100°C and 480MPa at 1200°C were measured for the Mo-2Si-1Bwt.% alloy.The yield strength of the Mo-3Si-1Bwt.% alloy was 680MPa at 1100°C and 420MPa at 1300°C.A review of the pertinent literature reveals that these are among the highest yield strengths measured for these compositions.The oxidation resistance in air at 1000 and 1100°C was examined. The protective borosilicate surface layer formed quickly due to the close spacing of intermetallic particles and pre-oxidation treatment was developed to further limit the transient oxidation behavior.An oxidation model was developed which factors in the different stages of oxidation to predict compositions that minimize oxidation.