Microstructure And Mechanical Properties Of TiBw/TA15(Mo,Si) Composites

Recently,titanium alloys have become the focus of aerospace research because of their light weight and high strength.They have better forming ability and weldability than TiAl intermetallic compounds,lower density and better corrosion resistance than traditional nickel-based superalloys,making them the preferred material for structural weight reduction of aerospace components.Titanium matrix composites with uniformly distributed reinforcing phases have a contradictory relationship between strength and plasticity.Titanium matrix composites with network reinforcement not only solved the brittleness problem of powder metallurgy,but also has excellent comprehensive properties.A series of TiBw/TA15（Mo,Si）composites with different composition were fabricated by combining low energy ball milling and vacuum hot pressing sintering technology.Adding Si element can improve the high temperature properties of the composites,but the plasticity is significantly reduced by increasing the addition mass fraction,however,adding Mo element can improve the plasticity and machinability of the composites.So using MoSi₂ powder as addition in order to add Si and Mo elements simultaneously.The properties of the composites were tested at room temperature and high temperature.The structure of sintered composites and composites after heat-treatment,the fracture surfaces of tensile specimens were observed and analyzed by optical electron microscopy（OM）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and X-ray diffraction（XRD）.The change of microstructure was determined.The effect of structure and heat treatment process on the mechanical properties and the fracture mechanism were studied.The strengthening mechanism of the composites were analyzed.The sintered structure of series TiBw/TA15（Mo,Si）composites showed that adding MoSi₂ powder alone to TA15 alloy could not obtain the network structure.The high volume fraction of alpha phase and continuous large-scale silicides on grain boundary had adverse effects on the properties of the composites.TiBw whiskers formed a first-order network around TA15 alloy particles by in-situ reaction,which limited the growth of beta grain.The addition of Mo element can increase the volume fraction of beta-Ti in the matrix and refine the layer of alpha-Ti.The addition of Si element can increase the size of the layer of alpha-Ti and disperse silicide in the matrix.The silicides and Mo elements were dissolved in the matrix at high temperature.The silicides first nucleated and grew at the network interface during furnace cooling,precipitated into the matrix as the alpha/beta interface began to disperse,finally formed a secondary network structure.Meanwhile,solid solution of Mo element in beta phase produces solution strengthening.The size of lamellar and silicide and the distribution of silicide can be controlled by changing the amount of MoSi₂ poewder addition.The results of room temperature property test,high temperature tensile test and fracture morphology observation of sintered composites showed that adding MoSi₂ alone can improve the strength but reduce the plasticity.TiBw/TA15+1.6MoSi₂ combined the high strength and high plasticity because of the addition of Si and Mo elements.The tensile strength and elongation of TiBw/TA15+1.6MoSi₂ at room temperature were 1113MPa and 4.3%,respectively.Increasing the addition of MoSi₂ can improve the strength of the material,the quantity and size of silicide,but decreased the plasticity obviously.The change trend of compression at room temperature and tension at elevated temperature was same.The high temperature tensile strength of composites at 650℃is similar to that of TA15 alloy at 500℃which indicated that the composites increases the usage temperature of the alloy by nearly 100℃.The TiBw network structure made contribution to the improvement of strength of composites by refining the microstructure.The addition of Mo element improved strength by solution strengthening.Furthermore,the silicide network structure increased the deformation compatibility of composites.The size,distribution of silicide and the microstructure of matrix can be controlled by changing the heat treatment process.The fine silicide in sintered materials can be dissolved by solution treatment at 1200℃,and then dispersed by aging treatment.The increasement of aging temperature can promote the matrix martensite decomposition and precipitatation of more fine silicide phases.Martensite completely decomposed when the aging temperature was increased to 650℃.The fine silicides were distributed at the alpha/beta interface,and the size of silicides increased by prolonging the time of aging treatment,and the stability of precipitated phase was obvious improved.