| In order to satisfy the requirements of supersonic aircrafts, future gas turbine engines and thermal protection system for high-strength light alloy sheets applied in the range of 800～1000℃, it is of crucial importance for manufacture of TiAl alloy sheets. Based on this, two novel methods to fabricate high Nb-TiAl alloy sheet have been developed including foil metallurgy and direct hot-pack rolling of cast ingot. This thesis focuses on the research from the following aspects:reaction behaviors occurring in Ti/Al foil, hot deformation behaviors of high Nb-TiAl alloy ingot, microstructure and mechanical properties for wrought alloy. Meanwhile, the superplastic behavior was systematically investigated for the hot-rolled high Nb-TiAl alloy with near γ (NG) microstructure. The main results and innovations are listed as follow:(1) The reaction behaviors occurring in the Ti/Al multilayers were detailedly studied using hot pressed sintering. The results showed that through the reaction between molten Al and Ti, TiAl3/Ti multilayers can be achieved with complete consumption of AI; during subsequent high-temperature heat treatment, TiAl3/Ti multilayers will eventually turn into Ti3Al/TiAl multilayers accompanying with simultaneous formation and successive disappearance of intermediate phases, such as TiAl2 and Ti2Al5. Under the guidance of the above result, the Ti-46-7.5Nb alloy sheet with fully lamellar microstructure was obtained by heat treatment of Ti/Al/Nb multilayers. Ti, Al and Nb distribute homogeneously. The procedure of fabricating high Nb-TiAl alloy sheets is 680℃/2h+1200℃/36h+1400℃/24h.(2) By means of thermal simulation, the hot-deformation behaviors of Ti-45Al-8.5Nb-(W, B, Y) alloy were studied; and the microstructure evolution and effect of strain, deformation temperature and strain rate on plastic flow stress were revealed. The influence of the strain on the material constants in the constitutive equation was described by 5th fitting, and then the constitutive equation of hot deformation for high Nb-TiAl alloy was established, which has a good accuracy in predicting the flow stress. Moreover, based on the dynamic materials model (DMM), the hot processing map of the high Nb-TiAl alloy was established and then the range of rolling parameter was determined according to the processing map.(3) A novel technological process was established by utilizing the direct hot pack rolling of ingot without the HIP and isothermal canned forging/extrusion in conventional rolling mill, which is more cost-effective and more suitable for industrialized production. Using this technology, the large scale Ti-45Al-8.5Nb-(W, B, Y) alloy sheet was successfully fabricated, and its size reaches 1000mm×70mm×2mm. By control of rolling temperature, three kinds of typical microstructure were obtained, such as near gamma (NG), duplex (DP) and near fully lamellar (NFL). Of these microstructures, the NFL microstructure is very fine, and the average size of lamellar colonies is 38μm, which is minimum size for the wrought high Nb-TiAl alloy. Room-temperature tensile yield strength and ultimate strength of NFL microstructure reach 1116MPa and 1133MPa respectively, and the elongation is 0.46%. Meanwhile, the NFL microstructure shows excellent high-temperature strength retention, that its tensile strength maintains above 1000MPa from room temperature to 800℃ and even can stay at 680MPa for 900℃.(4) The hot rolled Ti-45Al-8.5Nb-(W, B, Y) alloys with NG microstructure show excellent superplasticity. A maximum superplastic elongation of 824% was obtained at the optimal deformation temperature of 1050℃ and an initial strain rate of 2×10-4 s-1. Especially, it is worth noting that high Nb-TiAl alloy with deformed microstructure exihibits good low-temperature superplasticity. At 950℃ and 2×10-4 s-1, the elongation reaches 410%, which is of great significance for the industrial application of superplastic forming technology for high Nb-TiAl alloy.(5) For superplastic deformation of high Nb-TiAl alloy within the range of 950～1100℃, the m value ranges from 0.41 to 0.62, and the apparent activation energy equals to 238kJ/mol. It is suggested that the superplastic deformation behavior belongs to grain boundary sliding mechanism controlled by grain boundary diffusion with coordinater of dynamic recrystallization, and β phase also contributes greatly to the superplastic deformation. During superplastic deformation, the high Nb-TiAl alloy shows good resistance to void nucleation and propagation, and the failure mechanism of superplastic deformation is intergranular fracture mode. |
PDF Full Text Request