| Nickel based superalloy powders are the raw materials for preparing aeroengine turbine disk. The main defects in the powders include inclusions, heat induced hole and the original grain boundary. These defects significantly reduce the processing and mechanical properties of the powder metallurgy(PM) superalloys, which seriously hinder the development and application of PM superalloys parts working at high temperature environment. The defects in PM superalloy are directly related to the morphology, structure, gas content and impurities in the powders. Aiming at providing useful information for improving the performance of PM superalloys, this thesis work foucused on the structure and properties investigations of various types of superalloy powders, provided by Institute of XXX, including argon and nitrogen atomized nickel-based powders. The effects of powder particle size and atomization medium on the morphology, shrinkage defects, porosity and microstructure have been systematically investigated. In addition, the electric plating process for preparing Ni based superalloys powders embedded in Ni coating was also investigated in order to resolve the problem of TEM sample preparation. The main results and conclusions are as follow:1) SY02 powders prepared by argon atomization show good spherical shape with only a few planetary powders and coated powders. With decreasing particle size, the cooling rate increases. As a result, the contents and sizes of the shrinkage and pores decreases. The microstructure on the surface and in the interior mainly shows dendrite and cellular crystal structure. With the reduction of particle size, the dendrite structure transforms to cell structure. The matrix of powders is nickel-base gamma phase and blocky carbide phases precipitate in grain and grain boundaries. The lattice constant of the gamma phase increases and the micro-hardness increases with the decreasing powder particle size. 2) SY07 and SY08 powders prepared by nitrogen atomization show less spherical than SY02 powders. There are a lot of irregular shaped powders, including planetary adhesive powders, coated powders and crushed powders. Compared to argon atomized powders, the nitrogen atomized powders have less porosity, smaller shrinkage and higher density. The results show that the cooling rate of nitrogen atomized powders of SY07 is fastest, and the powders of argon atomized SY02 is slowest. With the decrease of particle size, lattice constant of matrix phase increases, the microhardness increases slightly. 3) The procsess of composite plating method for preparing Ni-based superalloy powders embedded in Ni coating has been investigated. The effects of current density, plating duration on the density and thickness of the coating were studied. The results show that using a current density of 1A/dm2 and electroplating time of 3 hours at room temperature, coating thickness can reach 37μm so that powders in less than 32μm can be embedded in nickel coating. It is also found that with the increase of electroplating time, coating thickness could still increase. When the plating time increased to 8 hours, the thickness of the coating reaches 75μm so that even the maximum powder particles in this experiment can be embedded in nickel coating. The obtained density coatings provide the precursors for future TEM sampl preparation.4) The superalloy ribbons were prepared by melt spinning. With the high at roller speed of 50m/s, the alloys still show ctystalline structure. Columnar crystal is found in the ribbon surface, which contacted the roller during melt spinning. Microhardness of melt spun of superalloy reach 654 HV, which is lower than those for gas atomized powders. |
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