碳对镍基单晶高温合金凝固缺陷影响的研究进展

Nickel-based single crystal superalloy has been widely used in the turbine blade of advanced aircraft engine, due to its characteristic of high temperature resistance and excellent mechanical properties. In order to further improve the high temperature performance, the trace element carbon is completely removed from the initial and then uses in limited quantities, meanwhile the total amount of refractory elements(Re, W, Mo and Ta) addition is gradually increased in modern nickel-based superalloys. The addition of carbon can reduce oxides, improve the purity and castability of alloys. Moreover, it is more important that the addition of carbon can control the solidification defects such as freckles, microporosities, stray grains and low angle grain boundaries, which are caused by the increase of refractory elements. At present, some progress has been made in the study of the effect of carbon about the solidification defects in the nickel-based single crystal superalloys. However, there are still some problems. During directional solidification, the fluid flow within the mushy zone of solid and liquid causes the stagnation of dendritic growth or dendritic melting, which results in the formation of freckle defects. The density difference caused by the segregation of solute elements results in the lower density liquid at the bottom of the interdendritic region flowing to the top of mushy zone. The rise of segregation coefficients of the refractory elements is followed by the fall with the increase of carbon content, which could offset the density difference and suppress the thermal solutal convection. The tendency of freckle formation is reduced. However, the evidences of the relationship between the time and quantities of carbides precipitation and thermosolutal convection are quite absent. So it is necessary for further research. The formation of microporosities is mainly due to the present of tiny molten pools in interdendritic areas, which is attributed to the difference in the shrinkage of solid and liquid phases in the course of directional solidification. The addition of minor carbon can form carbides and the appropriate amount of carbides can fill microporosities, which makes the volume fraction of microporosities reduce. At present, the research only stays at the stage of experimental phenomena and general qualitative explanation. Specifically, the relationship between the quantities of carbide addition and microporosity content is not completely clear, so it has attracted much attention. The formation of stray grains is related to the dendritic fragments, supercooled nucleation on mould wall andseed crystal remelting. The addition of minor carbon can suppress thermosolutal convection, reduce dendritic fragments and supercooled nucleation. By this ways, the formation of stray grains can be avoided. The low angle grain boundary is due to dendritic bending and torsional deformation, however it has nothing to do with thermosolutal convection. Although the addition of minor carbon can not prevent the kind of defects, it can play an important role in strengthening the grain boundary. Now the effects of carbon on the stray grains and low angle grain boundaries are very deficient in both experimental phenomena and mechanism analysis, and these need to be studied systematically. In this paper, it reviews the research progress of carbon addition on the solidification defects in nickel-based single crystal superalloys. The types of defects, the effects of carbon on solidification microstructure defects and the mechanism of carbon impact defects are emphasized. The existing problems and future research trends are also put forward. We have confidence that it could provide a theoretical support for improving the mechanical properties of superalloy and product yield.