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Ti-44Al-9Nb-1Cr-0.2W-0.2Y alloys were directionally solidified (DS) at different growth rates varying from 10 to 20 µm/s using a modified liquid metal cooling (LMC) method. The results show that an increase in the growth rate leads to both a decrease in the size of the columnar grains in the directional solidification stable growth zone and a deterioration of the preferred orientation of the α 2 (Ti 3 Al)/γ(TiAl) lamellar structure in the columnar grains. The growth direction of the primary dendrite in the quenching zone gradually deflected along the axial direction as the growth rate increased. At the same time, the morphology changed from dendrite to a cystiform dendritic structure, with considerable B2 phase segregation in the dendritic core. Correspondingly, the tensile properties of the alloy decreased at 800 • C with a gradual increase in the cleavage fracture area. These findings show that the low growth rate is beneficial for the preferred orientation and the mechanical properties of the alloy. The content of the B2 phase and the change in the lamellar orientation are the main limiting factors for the tensile properties of the materials at high temperatures.Metals 2018, 8, 535 2 of 10 growing α-phase is required to be accurately incorporated into the parent phase during the freezing process. In contrast, it is easier to control the lamellar orientation because β phase precipitation is the only constituent phase of the initial solidification stage.The elemental composition is the only requirement for a complete β phase transition from the liquid [12]. There are many β-stabilizing elements such as Nb, W, Mo, Cr, Mn and V [13][14][15][16]. While ensuring that the alloy solidifies completely along the β-phase path, these stabilizing elements are very likely to cause a certain amount of the B2 phase to remain in the room-temperature tissue. This process is referred to as β-segregation because of the large amount of β-stabilizing elemental segregation in the B2 phase [17,18]. The extreme hardness of the B2 phase is responsible for the ambient temperature brittleness of the β phase TiAl alloy. Due to the extremely high cooling rate, β-segregation is prone to occur during directional solidification. The dendrite growth in the quenched zone shows the initial growth stage of the solidification and may reveal the solidification process of the material at different growth rates. During this period, the analysis of the β-segregation process is very convenient.Appropriate additions of β-stabilizing elements within a specific range of Al (<49%) can ensure that the material completely solidifies in the β path. However, some β-stabilizing elements, such as W, will greatly increase the nucleation rate during the solidification process, which may lead to difficulties in obtaining columnar crystal growth in the DS TiAl alloy. Therefore, an additional amount of such elements should be strictly limited. As a rare metal, Y may significantly increase the microstructure uniformity and grain boundary strength of the alloys [19]. T...

R ecently, research on lightweight materials has been an essential aspect in aircraft manufacturing and aerospace industry. The γ-TiAl based alloys with high content of Nb are promising candidates as a new high temperature material due to their low density, excellent specific strength, specific stiffness, and good high temperature oxidation and creep resistance [1,2] . Compared with nickel-based superalloy, which has been widely used for more than fifty years, TiAl alloy containing high Nb is lighter in weight. The high Nb TiAl alloy exhibits superior creep properties and high temperature oxidation resistance compared to normal TiAl based alloy over 973 K [1][2][3][4][5][6][7][8][9] .TiAl-based alloys have high mechanical strength both at room and high temperatures because of their ordered structure, but single γ phase TiAl-based alloys have very low room temperature ductility. The near γ phase TiAl based alloy shows better ductility with the presence of a small amount of the second α 2 phase. The microstructure of the near γ phase is of α 2 -γ layered structure which is called full lamellar structure. The Abstract: Three novel multi-microalloying TiAl-based alloys containing high Nb were designed and fabricated.Thermogravimetric method was applied to investigate the influence of Cr on the oxidation behavior of high NbTiAl alloy at 1,073 K for 200 h in laboratory air. The 2at.% and 4at.% Cr were added into the alloy, (respectively named 2Cr and 4Cr compared to the Cr-free ternary alloy, 0Cr alloy). The alloys' microstructure and composition as well as the composition distribution of the oxidation scale were analyzed by means of Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and X-Ray Diffractometry (XRD). The results show that the addition of Cr decreases the grain size of the Nb-TiAl alloy and leads to a transformation from a fully lamellar structure to a nearly fully lamellar structure. When oxidized at 1,073 K for 200 h, the oxidized mass gain of the alloy increases with an increase in Cr addition amount in the first 100 h and decreases in the last 100 h. With the increase of Cr content, the oxidation surface turns compact but uneven in morphology, which may affect the oxidation resistance of the alloy by increasing the peeling off risk of the oxidation layer at friction conditions. room temperature ductility of the near γ phase TiAl based alloy is closely related to the grain size and microstructure distribution. Small grain size and good grain boundary strength can significantly improve the toughness of the alloy. One of the most interesting approaches to decrease grain size is micro-multicomponent alloying, in which a variety of alloying elements are added in trace amounts. The near γ-phase TiAl-based alloys have very good solid solubility for many alloying elements, so micro-multi-component alloying is very effective for the microstructure optimization of the material. Alloying elements such as Cr, B, Si and W are usually added in the TiAl alloy. It has been found that the addit...

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