| TiAl-base alloys have enormous potential in the high-temperature applicationssuch as the aerospace engine application due to their excellent mechanical propertiesand oxidation resistance at elevated temperature. However, the inherent poor ductility oftitanium aluminides at ambient temperatures has limited their applications. Thus, thedevelopment to improve their low-temperature ductility and high temperaturedeformability has become the major challenge for this alloy system. High Nb containingTiAl alloys is attractive.In this paper, Ti-45Al-5Nb alloy with elemental powders as raw material has beenfabricated and ameliorated by the process routes of mechanical milling+hot pressingsintering+heat treatment and isothermal forging. The sintering temperature were set at1250℃,1300℃and1350℃. Two kinds of heat treatment process were established, oneof which is1325℃/2h for the compacts sintered at1250℃and1300℃and the other is1300℃/1h for the compacts sintered at1350℃. The isothermal forging temperature was1250℃. The evolutions of the microstructure and property with different processconditions have been analyzed. The correlation between preparation, microstructure andhigh temperature properties was investigated systematically.The element powders with the composition of Ti-45Al-5Nb were prepared bymechanical milling. the original powders have been sufficiently refined and evenlydistributed. The composite powders by ball milling of8h show the best properties,which have an average particle size of20.8μm. Analysis shows that there was noalloying reaction occured in the composite powders.The fine near-gamma microstructures are formed in the compacts sintered at1250℃and1300℃. The grain size is10-20μm,which is slightly larger sintered at1300℃. Thenearly-lamellar microstructure is formed in the compacts sintered at1350℃with thesize of25-35μm. B2phase particles are discovered in all of them. In addition, someenrichment of Nb taken place in the three compacts, which contributes to the formationof large lamellar microstructure in the compacts sintered at1350℃. The roomtemperature brittleness is significant in the three sintered compacts. The maximumcompressive strength is1844MPa. While the strength and elongation of the compact sintered at1350℃are significantly better than the others.The near-gamma microstructures of the compacts sintered at1250℃and1300℃are transformed into fully lamellar microstructure after the heat treatment. The coarselamellar structure of the compact sintered at1350℃was finer and more homogeneousafter the heat treatment, and the gamma grains disappeared. In addition, extremely fineB2phase precipitates were observed in the lamellar structure. The strength of thecompacts sintered at1250℃and1300℃increased up to350MPa after heat treatment,and the elongations were improved by almost150%. The strength of the compactssintered at1350℃increased to426MPa after heat treatment. The study shows that thedeformation temperature and strain rate have a great influence on the flow stress.The microstructure of the compact sintered at1350℃was refined after isothermalforging. While some coarse lamellas with enrichment of Nb exist by solid solutionstrengthening and all were stretched along the deformation direction by rotating andbending. The directional stretched state of coarse lamellas has disappeared by the heattreatment for900℃/20h, and the fine recrystallized structure has been grown. Theforged alloys after heat treatment have a great increase on yield strength by30%andelongation by100%. |
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