| GH99 alloy is a typical γ′-hardened nickel-based superalloy. In the alloy, W, Mo, Co are used as solution strengthening elements, and B, Ce, Mg are used as grain boundary strengthening elements. The alloy has been widely used in aerospace engines due to its excellent high temperature strength, creep resistance and corrosion resistance. The alloy is a kind of superalloy with high service temperature, which can reach about 1000℃. The hot deformation behavior of the alloy is very sensitive to the temperature and strain rate, and the deformation resistance is very large. Hence, the forming of the alloy is very hard. Moreover, the microstructure evolution of the alloy is very complex at high temperatures due to its high alloying degree. The processing parameters have an important effect on the microstructure evolution of the alloy, which is very difficult to control. On the other hand, there are a large number of annealing twins in the alloy, and it is necessary to explore the evolution characteristics of the annealing twins during the process of heat treatment and hot deformation.In this paper, the microstructure evolution characteristics of GH99 alloy were studied. By means of isothermal compression tests, the flow behavior of the alloy was investigated. Meanwhile, the evolution characteristics of dynamic recrystallization(DRX) and annealing twins during hot deformation process were also explored.The evolution characteristics of Σ3 boundaries and γ′ phase in the alloy were obtained through heat treatment experiment. The volume fraction of Σ3 boundaries increased firstly and then decreased with the increasing temperature and holding time during solution treatment process. During aging treatment process, the structure of Σ3 boundaries was very stable, and the variation of volume fraction of Σ3 boundaries was very small, which was maintained in the range of 0.553~0.633. In addition, the coarsening of γ′ phase was obviously observed with the increasing aging temperature and holding time. The growth activation energy of γ′ phase was about 248.8k J/mol. With the growth of γ′ phase, the shape of some γ′ phase transferred from spherical to square.The constitutive models were established to describe the flow stresses during the work hardening-dynamic recovery period and dynamic recrystallisation period by researching the flow behavior of the alloy during hot deformation. Meanwhile, the accuracy of the constitutive equations was also verified. In addition, the thermal deformation activation energy of the alloy was calculated by Arrhenius equation, Q=427.626 k J/mol. Based on DMM model, the power dissipation maps of the alloy at different strains were established. Through microstructure observation, the optimal processing parameters of the alloy were also obtained, which were located in the temperature range of 1090~1160°C and strain rate range of 0.01~0.3s-1. On the basis of four different instability criteria(Prasad, Murty, gegel malas), the processing maps of the alloy were established with the true strain of 0.65. Through microstructure observation, the flow instability is predicted to occur at two domains. The first domain was located in the temperature range of 1010~1080℃ and strain rate range of 0.16~1s-1, and the other one was located in the temperature range of 1080~1160℃ and strain rate range of 0.32~1s-1. The microstructure observation revealed that the main mechanisms of instability were flow localization and adiabatic shear band during hot deformation of GH99 alloy.The DRX evolution of GH99 alloy during hot compression was studied by using EBSD technology. The results revealed that the DRX grain size and the volume fraction of DRX grains increased with the increase of deformation temperature and the decrease of strain rate. The DRX kinetics model and the prediction model of DRX grain size in the alloy were established according to the evolution characteristics of DRX with processing parameters. In addition, discontinuous DRX(DDRX) and continuous DRX(CDRX) were the main nucleation mechanisms of DRX in GH99 alloy. Although CDRX and DDRX occurred simultaneously in the alloy during hot deformation, CDRX was only an assistant nucleation mechanism of DRX. There were two processes during DDRX, i.e. grain nucleation and growth process. Under high temperature and low strain rate, growth process dominated DDRX, while grain nucleation dominated DDRX at low temperature and high strain rate. Though CDRX was only an assistant nucleation mechanism of DRX, the effect of CDRX was strengthened in the initial stage of hot deformation. With the increasing strain, the effect of CDRX was weakened again. On the other hand, the effect of CDRX was weakened with the increase of deformation temperature, while it was strengthened with the increase of strain rate.The annealing twin evolution of GH99 alloy during hot compression was studied by using EBSD technology. The results revealed that lots of Σ3n(n=1,2,3) boundaries lost their characteristics during the initial stage of hot deformation, leading to the decreasing volume fraction of Σ3n(n=1,2,3) boundaries and density of Σ3 boundaries. With the increasing strain, lots of Σ3 boundaries appeared in form of coherent Σ3 boundaries in the DRX microstructure, leading to the increasing volume fraction of Σ3n(n=1,2,3) boundaries and density of Σ3 boundaries. In addition, the formation of new Σ3 boundaries was mainly through growth accidents for the alloy, and the formation of Σ9 and Σ27 boundaries was mainly through regeneration mechanism of grain boundary. With the increasing deformation temperature and decreasing strain rate, the volume fraction of Σ3n(n=1,2,3) boundaries and density of Σ3 boundaries increased firstly and then decreased, while the proportion of coherent Σ3 boundaries in the Σ3 boundaries increased continually. |
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