| Superalloy has played an important role in the aviation field because of its excellent resistance,creep property and corrosion resistance under high temperatures.Some components of aviation turbine engines,such as turbine blades,guide blades and turbine discs,are made of superalloys.Nickel-based superalloy was the most widely used superalloy because it can form a solid solution with various elements such as Al,Ti,W and Re while maintaining good organizational stability.Due to the non-equilibrium crystallization of nickel-based single crystal superalloy castings,severe component segregation exist between the dendrite stems and the interdendritic regions,and the lower melting point eutectic are generated in the interdendritic regions,that are harmful to the performance of the alloy.Therefore,the solution treatment is necessary to homogenize the components and eliminate the eutectic.A new type of nickel-based superalloy（DD9-X）developed by Jiangsu University is used in the experiments of this article.The single crystal specimens are made by high rate solidification method（HRS process）.In this work,the genesis and change of the homogenized micro-pore,the cause of the incipient melting structure and the increase of the maximum homogenized temperature of the nickel-based single crystal superalloy DD9-X are researched,and the complete solution treatment that is suitable for the complete single crystal bar is also gotten.A model of diffusion of Rhenium during homogenization is established on the basis of the data,and the law of change of the diffusion coefficient of Rhenium is obtained by this model.The main conclusions of this paper are as follows.The casting structure of DD9-X nickel-based single crystal superalloy is a branched crystal,there are some layeredγ/γ′eutectic in the interdendritic regions.The segregation is exist both in radial direction and lengthwise of the casting structure.The radial segregation is more serious than the longitudinal segregation.There are some irregular shrinkage pores on the edge of the interdendritic regions,the porosity is 0.067%.After 1300℃/4h+1330℃/4h+1340℃/6h treatment,some regular circular pores are generated on the edge of the interdendritic regions,the porosity is increased to 0.169%.The cause of the homogenized pores is the aggregation of vacancy that is caused by different diffusion rate in different elements（unbalanced diffusion）.With the increase of temperature and hold time,the porosity is decreased,and is unchanged after decrease to 0.061%.The micro-pores cannot be completely eliminated by solution treatment.The maximum temperature of solution treatment is determined by the melting point of the pore regions,is increased with the diffusion of high melting point elements Re and W.The melting point of the pore regions is small because of the enrichment of Al and Ni,and the initial melting structures are generated by the melted pores.After 1300℃/4h+1330℃/4h+1340℃/6h+1345℃/22h,the maximum temperature of solution treatment is increased to more than 1350℃.The law of change of diffusion coefficient of Rhenium during 1300℃1340℃does not conform the Arrhenius equation（D（28）D0exp（-Q/（RT）））,because the activation energy in theγ′phase is higher than it in theγphase.The activation energy is the highest in pure Ni,second in theγ′phase（single crystal）,third in theγphase（single crystal）,and the smallest in the Ni-1wt%Re and Ni-4.8wt%Re diffusion couple.Based on the model built in this experiment,the diffusion coefficient of Re is6.67×10-3μm2/s at 1300℃,7.99×10-3μm2/s at 1330℃and 1.86×10-2μm2/s at 1340℃.The complete solution treatment of DD9-X nickel-based single crystal superalloy is room temperature（5℃/min）→1300℃/4h（5℃/min）→1330℃/4h（5℃/min）→1340℃/6h（1℃/min）→1345℃/22h（1℃/h）→1350℃/5h. |
