Research On Microstructure And Mechanical Properties Of High Temperature （TiB+La₂O₃）/Ti Composites

Titanium matrix composites （TMCs）, reinforced with ceramic particles, haveconsiderable potential for improving properties and service temperature and can beextensively applied in areas such as aerospace, advanced weapon systems, because oftheir high specific strength, good specific modulus and resistance to elevated temperatures.With the development of titanium matrix composites, the researches on improving alloy’schemical composition, reinforcement choice and prepared of titanium matrix compositesare much more than heat treatment about titanium matrix composites. However, it iswell-known that heat treatments can improve effectively Ti alloys’ properties. Therefore,improving the comprehensive properties of TMCs by heat treatment becomes much moreimportant. A new heat treatment method, TRIPLEX heat treatment, is adopted and theeffects of it on microstructure and properties of TMCs are investigated in my paper.In situ synthesized （TiB+La₂O₃）/IMI834composites were prepared in a consumablevacuum arc-remelting furnace as reaction between Ti and LaB6. The theoretical volumefraction of TiB and La₂O₃is1.82%and0.58%respectively. The ingots （Φ580mm） werehot-forged into rods with a diameter of Φ70mm. The majority of the TiB whiskerreinforcements were aligned along the direction of forging. The size of La₂O₃paticals arein the nano-scale. The reinforcements were uniformly distributed in the TMCs.TRIPLEX heat treatment （β₃HT） is β solution （WQ, OQ, AC）, α+β solution, andthen aging. TRIPLEX heat treatment, β heat treatment （βHT） and α+β heat treatment （α+βHT） were adopted in my paper. The microstructure of specimens after β₃HT heattreatment is laminar structure, that of βHT is widmanst tten, and α+βHT is bimodalstructure. The faster the cooling speeds are in β phase district, the thinner α laminar is.The width of α laminar after β₃（WQ, OQ）HT is thinner than that of βHT. The width of αcolony is more; the low angle grain boundaries are more. After heat treatment, theinterface between reinforcement and matrix is clear, so both the TiB whiskers and La₂O₃particles are stable without interfacial reaction. Reinforcements prevent grain boundarymigration during grain growth, β grain decrease. The different of matrix orientationincreases around of reinforcements.Tensile properties of TMCs after α+βHT, β₃（WQ） HT and β₃（OQ） HT are best, andβ₃（AC） HT the better, and β（AC） the worst one. The elongation of specimens after β₃（OQ,WQ） HT and α+βHT is increase as twice as that of β（AC）. With temperature increasing,ultimate strength of specimens decreases, but elongation increases significantly. Theelongation of specimens after β₃（WQ） HT is best, β₃（OQ）HT and α+βHTi is better, thenβ（AC）HT is worst. At700℃, elongation of specimens after β₃（WQ） HT improves no lessthan200%compared with that of β（AC）HT. After thermal exposure, only a slight changeof ultimate strength has been found, however the ductility of specimens is sharply reducedafter thermal exposure. The ductility of all specimens is the worst at650℃thermalexposure. Compared with specimens treated by thermal exposure at600℃and650℃, theductility of specimens thermal exposured at700℃shows an abnormal increase. Afterthermal exposure, the ductility of specimens treated by β₃HT and α+βHT heat treatmentis better than that of β heat treatment. With temperature and stress increasing, the steadystate creep rates increase. The steady state creep rates of specimens treated by α+βHT isfastest, that of β₃（OQ） HT increases slightly compared with β HT. The fracture toughnessof α+Î'ht, β₃（OQ）HT, β₃（WQ）HT and β₃（AC）HT increases22%,34%,50.5%and34%respectively compared with β HT.The fracture mechanism of TMCs is that if a short fiber with AR（AR: length-to-diameter ratio） is higher than critical aspect ratio （ARc）, TiB whiskers bear tensile stress inthe processing of tensile. Most TiB whiskers of the TMCs are higher than ARc. Withtemperature increasing, ARcof TiB whiskers increase, so few debonding TiB whiskers areobserved. Both the TiB whiskers and La₂O₃particles are stable without interfacial reactionafter hermal exposure at600℃,650℃and700℃for100h, the stable reinforcements can ensure the stability of TMCs. After creep fractured, large numbers of interface debondingsbetween TiB whiskers and matrix are observed. This is attributed to both the interfacestrength and grain boundary strength decrease at high temperature for long time. La canreduce oxygen concentration, depress precipitate of Ti3Al. The reinforcement of La₂O₃canimprove the strength, thermal stability and creep properties of TMCs.