Directional Solidification Of High Nb Containing TiAl Alloy And Its Influencing Factors

Comparing with normal TiAl alloy, high Nb containing TiAl alloy can be used at higher temperature, it has better oxidation resistance, tensile strength and creep strength at elevated temperature. So the high Nb TiAl alloy has been an important development direction. But the brittle of TiAl alloy limit its industrial application. Directional solidification technique can control the grain orientation, and improve the plastic of TiAl alloy.This method can improve the comprehensive performance of high Nb containing TiAl alloy.First, using arc melting to prepare high Nb containing TiAl alloy ingot, getting TiAl alloy bar through the induction smelting and suction casting. Second, the bars were placed into alumina crucibles moulded with Y2O3, which can reduce the reaction between melting alloy and crucible. Third, designing appropriate induction heating coil for the directional solidification. Finally, we use a W-Re5/26 thermocouple to test the heat preservation temperature.The macroscropic structure of directional solidified Ti-45Al-8Nb bar shows four regionsxasting region, directional anneal region, transition region and directional solidified region. Directional solidified region are composed by columnar crystal.The lamellar orientation is shown in the columnar crystal. There are certain angle between most of lamellar orientation and the pulling direction, a few of lamellar orientation parallel or vertical to the pulling direction. Compositions of the various microstructural of the directional solidified bars were determined by the energy dispersive analysis(EDS). There are three major component defects, one is the Y2O3 cast from the coating of corundum crucible, the other is the composition microsegregation during directional solidification, especially for the P-segregation.The effects of pulling rate of directional solidification and trace elements B were investigateded in the Ti-45Al-8Nb alloy. Directional solidified Ti-45Al-8Nb alloy bar can be obtained at the heat preservation temperature 1550℃and with the pulling velocity less than 20μm/s. At the pull velocity 25μm/s, transverse grain boundaries are observed, but the morphologies are still columnar grain. As the pulling velocity increasing, the size of columnar grain and the lamellar spacing decrease. At the same heat preservation temperature and the same pulling velocity, the size of columnar grain decrease with the increasing elements B, but the lamellar spacing does not change. With the increasing elements B, dendrite arm can be observed. When the mole fraction of elements B reach to 0.5at%, a lot of β-segregation are presented in the directional solidification morphology.