| As one of the candidate materials for advanced aerospace technology, high Nb-TiAl alloy is light-weight with high strength, good creep resistance, corrosion resistance and oxidation resistance. To optimize the microstructure and mechanical performance, the addition of alloying element and adjustment of process parameters in directional solidification need to be used to control the lamellar microstructure of TiAl alloy. The effects of W on lamellar microstructure and properties are studied by changing alloying element as well as process parameters in directional solidification. Induction melting method and drop-casting are used to produce alloy bars of Ti-45Al-6Nb, Ti-45Al-6Nb-0.4W and Ti-45Al-6Nb-0.8W, respectively. Bridgeman method under heating temperature of 1550℃ is performed on the above alloy bars with the growth rate of 5 μm/s,10 μm/s and 20 μm/s, respectively. Pure directional solidified Ti-45Al-6Nb-0.4W bars were made by Optical floating zone method with the growth rate of 5 mm/h,7 mm/h,10 mm/h,15mm/h and 20 mm/h, respectively. Tensile tests are performed using standard tensile specimens intercepted from the stable area of the Optical floating zone bars. The high temperature oxidation experiments is tested in 900℃ with time of 200 hours.The results show that the lamellar orientation and the lamellar thickness is changed by changing growth rate and W content。The lamellar spacing of α2/γ phase firstly decreased and then increased with the increase of W content and the lamellar thickness decline with the increase of growth rate. The orientation of the lamellae can be influenced by the content of W and the change of growth rate. Process parameters and heating type in directional solidification have a great impact on the growth of TiAl alloy and the the final organization. Higher temperature gradient will not only shorter the competitive area, but also make the obtain of the lamellae which are parallel to the growth direction at a higher growth rate possible. The different heating types lead to different microstructure, due to the electromagnetic stirring, there are no B2 phase by induction heating while the optical beam heating lead to the presence of B2 phase.Tensile properties analysis shows that the number of grains, lamellar orientation and the presence of B2 phase have a significant influence to the mechanical properties of the alloy. When the orientation of the alloy is same, the mechanical properties of single crystal alloy is superior to the columnar crystal alloy; for single crystal alloy, when the lamellae are parallel to the tensile axis, the best mechanical properties can be gained. The alloy mechanical properties gradual decline with the increase of the angle between the lamellae and the tensile axis and the internal segregation phase (B2) significantly reduces the mechanical properties.The high temperature oxidation test results shows that after adding a small amount of W element, the alloy surface becomes more compact, and the amount of surface pores reduced significantly, the oxide particles becomes more uniform and small. Oxidation profile analysis shows that after a long time of exposure to hot air, a transition layer of discontinuous y phase forms between the oxide layer and the lamellae. The oxide layer thickness decreases with a slight increase of W, the γ phase becomes more continuous in the transition layer and the transition layer thickness increases. Nitride layer is formed between the oxide layer and the matrix. W element distributed uniformly in the oxide layer, Ti, Al elements distributed conversely in the oxide layer. When the lamellae is perpendicular to the outer surface, the γ phase in the transition area is discontinuous, the oxide layer drop off, while the lamellae is parallel to the outer surface, γ phase in the transition area is more continuous, the oxide layer abscission did not occur. |
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