Study On Microstructure Evolution Of Ternary TiAl-5Nb Alloy During Directional Solidification

The engine vanes with monoaxial stress condition are one of the important developmental directions. In order to fully display the advantage of TiAl-based alloys using in engine vanes, we hope that the solidification mode is the full beta solidification during the directional solidification of TiAl-based alloys. Consequently, we obtain the lamellar microstructure inclined at angle of 0°or 45°to the growth direction. Nb additions not only can enhance the high temperature properties and the temperature ductility of TiAl alloys, but also Nb additions play very important role for obtained TiAl-based alloys of the full beta solidification. In summary, Nb additions offer the possibility in order to design the full beta solidified high Al content TiAl alloys of high properties and controllable lamellar orientation.Firstly, Ti-Al-Nb ternary phase diagrams were calculated by Thermo-Calc software. By the analysis of the vertical sections of Ti-Al-Nb with lower Nb content than 15at.%, the following are the results. First, when Nb content is 5at.% and Al content is lower than 52.3at.%, the primary phase isβphase during solidification. Second, when Nb content is 10at.% and Al content is lower than 55.8at.%, the primary phase isβphase. Third, when Nb content is 15at.% and Al content is lower than 56.9at.%, the primary phase isβphase. The vertical sections of Ti-Al-Nb ternary system are further simplified into pseudo-Ti-Al binary phase diagram. According to the pseudo-Ti-Al binary phase diagram, the expression of the aluminum equivalent is obtained in TiAl-Nb, which provides the theoretical foundation for obtained TiAl-Nb alloys of the beta solidification.In order to further confirm the beta solidification of TiAl-Nb alloys and verify the full beta solidification critical values in the pseudo-Ti-Al binary phase diagram, the microstructure evolution of TiAl-Nb button ingots is investigated by non-consumable tungsten electrode arc melting equipment. For TiAl-5Nb button ingots, when Al content is lower than 48at.%, the primary phase isβphase. If Al content is higher than 48at.%, the primary phase isαphase. We synthetically analyzed the pseudo-Ti-Al binary phase diagram and the experimental results of non-consumable tungsten electrode arc melting, and considered that the directional solidification process is the quasi-equilibirium state. Simultaneously, in order to retain low density of TiAl-based alloys and obtain good comprehensive mechanical properties, the composition is confirmed to use in the directional solidification experiment. These alloys are Ti-45Al-5Nb, Ti-47Al-5Nb, and Ti-50Al-5Nb alloy. The directionally solidified microstructures of Ti-45Al-5Nb, Ti-47Al-5Nb, and Ti-50Al-5Nb alloys were investigated under the temperature gradient of 6.9K/mm and 11.4K/mm. The results show that when the temperature gradient is 6.9K/mm the macrostructures of TiAl-5Nb alloys have the poor directional effect. Whereas, when the temperature gradient is 6.9K/mm we can find that the primary phase isβphase under the directional solidification of TiAl-5Nb alloys according to make an angle of the lamellar orientation with the growth direction. When temperature gradient is 11.4K/mm, the macrostructures of Ti-50Al-5Nb alloys have the good directional effect. Experimental studies show that the primary phase isβphase during the directional solidification when the temperature gradient is 11.4K/mm and the growth rate is lower than 20μm/s. Therein the lamellar orientations of Ti-50Al-5Nb alloys mainly have paralleled to the growth direction when the growth rate is lower than 15μm/s. It is realized to control the lamellar orientation in high Al containing TiAl-5Nb alloys. In conclusion, the experimental results show that the beta solidification can be obtained in TiAl-5Nb alloys under the experimental condition. Moreover, the microstructures of Ti-50Al-5Nb alloys have the good lamellar orientations under the temperature gradient is11.4K/mm and the growth rate is lower than 15μm/s. In TiAl-5Nb alloy containing high Al content, we obtain expecting directionally solidified microstructure. Based on the criterion of nucleation and constitutional undercooling (NCU model), the phase and microstructure selection of Ti-(44-53)Al-5Nb alloys was constructed during the directional solidification, which is in good agreement with the experimental results.