The Effect of Growth Rate on the Microstructure and Tensile Behaviour of Directionally Solidified Ti-44Al-9Nb-1Cr-0.2W-0.2Y Alloys

Abstract

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...

Review on Progress of Lamellar Orientation Control in Directionally Solidified TiAl Alloys

“…When the growth rate varies greatly, it is easy to undergo a change in solidification mode on most occasions. Jiang et al [ 45 ] found that for Ti-44Al-9Nb-1Cr-0.2W-0.2Y alloy, within the range of growth rate of 10~20 μm/s, as the growth rate increased, the angles between the columnar grain orientation and the ingot axial direction gradually decreased, and the preferred grain orientation gradually changed from the heat flow direction to the pulling direction. Moreover, the difference in growth rate caused changes in the primary solidification phase.…”

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