Effect Of NB Content On The Microstructure And Property Of TiAl Based Alloy

ABSTRACT:TiAl based alloy has been widely used in the aerospace and automotive engine fields as structural materials because of its excellent high temperature strength, high temperature oxidation resistance, creep resistance, specific strength, specific stiffness and low density characteristics. In recent years, y-TiAl alloy has been the research focus in material science and engineering as its good deformation performance. Nb element is a kind of (3-stable element, TiAl alloy with a certain content of β phase can improve its high temperature deformation behavior, but brittle sequential B2phase would form if added too many Nb elements into alloy. However, research on the effect of Nb content on the microstructure and property of TiAl based alloy is still limited. The effect of Nb content on the microstructure and tensile property as well as high temperature oxidation resistance of as cast Ti-47Al-2Cr-xNb-0.2Mo(at.%)(x=1,3,5,7) alloy were systemically studied by means of OM, SEM, EDS, XRD and TEM, then researched the optimization of as cast and as forged structure of alloy through heat treatment. Main conclusions are listed in the following:(1) The microstructure of as cast Ti-47Al-2Cr-xNb-0.2Mo(at.%) alloy is lamellars made up of y phase, a2phase and few β phase; there are two kinds of segregations in lamellar structures, namely networking β phase and y segregation. With Nb content increasing, the average size of lamellar colonies and the content of β phase increased. The tensile strength of different Nb containing TiAl alloy under room temperature(RT) were lower than that under high temperature(HT), and ductility under RT were also lower than that under HT, therefore, RT plasticity was not as sensitive as HT plasticity, further validated the poor ductility of TiAl based alloy under room temperature, and suitable to plastic processing. With Nb content increasing, plastic deformation was difficult to occur in coarse lamellar colonies during high temperature tensile process.(2) The adding of Nb reduced the oxidation weight gain rate; with increasing the Nb content, the particle of oxidation became dense and tiny, percentage of Al2O3in the oxidation scale increased gradually. In the researched Nb content range, the average size of oxidation scale particle and Nb content fit a good linear relationship; with the increase of Nb content, the structure of oxidation scale became uniform gradually, and more adherent to the matrix. Nb addition suppress the mutual transmission between the matrix metal element and the external oxygen atom, improved the oxidation resistance remarkably.(3)α phase in the grains and grain boundries were eliminated completely in low Nb containing TiAl alloys, while β phase were only decreased in high Nb containing TiAl alloys. After canned forged, the microstructures of different Nb containing TiAl alloys were mainly consisted of tiny recrystallized equiaxed grains. With the deformation increased, dynamic recrystallization degree increased, the size of grains became fine and homogeneous. β phases in the as cast alloy were not removed completely. After heat treated for a certain time in single a phase or α&γ phases region,β phase in the canned forged alloys with different deformation were diffused homogeneously, different microstructures were obtained.