Effect Of Delta Phase On Hot Deformation And Recrystallization Behavior Of Alloy GH4169

The evolution ofδphase in alloy GH4169 during static state dissolution were studied, and the dissolution process and its mechanisms were analyzed. The constitution equations describing the hot deformation behavior of the alloy at annealed state and the delta-processed state were established by thermal simulation compression tests, respectively. The effect ofδphase on the hot working performance of the alloy was discussed. Microstructure evolution of the alloy at the annealed state and delta-processed state under different deformation conditions were investigated by optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) technique and transmission electron microscopy (TEM). The mechanism of dynamic recrystallization for the alloy was analyzed, and the effect ofδphase on the hot deformation behavior of the alloy was also discussed. According to the dynamic materials model, processing maps of the alloy at the annealed state and delta-processed state were obtained, respectively. The stable and instable regions for hot working of the alloy at the different states were determined, and the suggested processing parameters were presented.The results show that the dissolution amount ofδphase reaches to stable values at temperatures of 980℃and 1000℃, and theδphase are continuously dissolved at temperatures of 1015, 1025 and 1035℃. The morphology change of needle-shapedδphase during dissolution process can be divided into two stages: from a long needle shape into a short needle and globular ones at the early stage, and the decrease in the dimension of short needle-shaped and globularδphase at the late stage. The dissolution of the long-needle-shapedδphase is mainly controlled by long range diffusion of Ni or Nb, which can be described by one-dimensional atomic diffusion dynamic model. The dissolution of the globularδphase is mainly controlled by interfacial reaction, which can be described by three-dimensional interfacial reaction dynamics model.The results of the thermal simulation compression tests show that the hyperbolic sine-type function is suitable for describing the relationship between the flow stress and the deformation condition for the alloy at both the annealed state and the delta-processed state. The activation energies for hot compression are 443 and 467 kJ/mol, respectively. After delta processing, the stable stress and the peak strain for hot compression of the alloy are decreased, and the flow softening degree after peak stress is enhanced. The deformation mechanism of the alloy at annealed state is mainly the dynamic recrystallization accompanied dislocation climbing. After delta processing, the mechanism for hot deformation of the alloy is different, and the apparent activation energy and activation volume are increased due to the pre-precipitatedδphase. Meanwhile, the elongation of the alloy is increased due to the pre-precipitatedδphase, which is beneficial to the processability.The results of the microstructure analysis for hot compressed samples at annealed state show that both the size of dynamic recrystallization grains and the frequency of high angle boundary can be quantitatively described by the Zener-Hollomon parameter. The nucleation mechanisms of dynamic recrystallization for this alloy are closely related to the value of Zener-Hollomon parameter. Under low Z conditions, the main nucleation mechanism is the bulging of original grain boundaries accompanied by twinning. Under high Z conditions, the main nucleation mechanism nearby original grain boundaries is the bulging of original grain boundary accompanied by subgrain rotation. The nucleations inside the original grains focus on the deformation bands, and the continuous dynamic recrystallization can also occur in the local region for the annealed alloy.The results of the microstructure analysis for hot compressed samples at delta-processed state show that the dissolution rate ofδphase under hot deformation conditions is much faster than that under static state conditions. After delta processing, the size of dynamic recrystallization grain decreases to some extend. The nucleation mechanisms of dynamic recrystallization for the alloy are different because of the pre-precipitatedδphases. The main nucleation mechanisms of dynamic recrystallization for delta-processed alloy include theδphase stimulated nucleation and the bulging of original grain boundaries.The results of the processing maps for the alloy at annealed state and delta-processed state show that three classic zones of dynamic recrystallization appear in the region with moderate and low strain rates. The starting forging for the alloy at annealed state are suggested to be at strain rates between 10^-2.7 and 10^-1.5s^-1 and temperatures between 1087.5 and 1100℃, and the final forging are suggested to be at strain rates between 10^-2.5 and 10^-1.5s^-1 and temperatures between 1000 and 1065℃. The flow instability occurring at the strain rates between 10^-0.25 and 1 s^-1 and the temperatures between 950 and 1100℃at the annealed alloy is related to the cracking induced by the local plastic flow. The hot working parameters for the alloy at delta-processed state are suggested to be at strain rate between 10-2.5 and 10^-1.5 s^-1 and temperature between 950 to 1015℃. The higher dissipation efficiency occurring at the lower temperature region (T=950℃) is closely related to the occurrence ofδphase stimulated nucleation of dynamic recrystallization. The higher value of dissipation efficiency occurring at the region with higher temperatures and lower strain rates for this alloy is associated with the acceleration of dynamic recrystallization due to the dissolution ofδphase. The flow instability occurring at the strain rates between 10^-0.35 and 1 s^-1 and temperature between 950 to 1100℃for the alloy at delta-processed state is related to the local shear bandings.