Hot deformation behaviour, recrystallization and evolution of GH761 alloy were investigated via hot compression test, aiming at providing groundwork for isothermal forging process which is an advanced technology to improve the microstructure and properties of wrought superalloy. In addition, heat treatment research was studied to obtain microstructure which matches with optimal mechanical properties.
The stress-strain behaviour of GH761 alloy was studied under the condition of 990°C-1100°C and 0.03s-1-0.001s-1. The peak stress σp and corresponding strain εp, steady-state stress σs and εs corresponding to the point the steady state flow begins decrease with decreasing strain rate at constant deformed temperature. At constant strain rate, σp, σs and εs drop with rising temperature, but εp does not change obviously. The hot deformation constitutive equation was calculated.
The map of efficiency of power dissipation was set up. Efficiency of power dissipation (η) increases with rising strain rate. The value of η decreases with increasing strain, and it is relatively higher at 1050°C at high strain rate while decreases slightly with rising temperature at medium and low strain rate. Dynamic recovery (DRV) and dynamic recrystallization (DRX) correlate with the value of η. DRX is fullest under deformation condition which corresponds to highest η. In this research, plasticity failure does not occur, which is in good agreement with the criterion of plasticity instability.
Microstructure and recrystallization behavior at different deformed conditions were analyzed. Nucleation during DRX is controlled by density of defect. DRV, which is reinforced with increasing temperature and decreasing strain rate, reduces dislocation density, and thus reduces the nucleation rate and promotes the grain growth. Thus, dropping temperature and enhancing strain rate is necessary to refine the grain microstructure. The microstructure is most homogenous and finest when the DRX is just finished. Further deformation would promote the grain growth due to low nucleation rate and high growth rate. Increasing temperature coarsens the grain size, favors defect annihilation and hinders nucleation, which offsets the effect of accelerating DRX, so εp is constant with increasing temperature. Reducing the original grain size refines the DRX microstructure of alloy. Residual energy is released more completely and grain grows larger when the deformed alloy is heat treated at higher temperature.
It was determined that η phase precipitates fastest around 990°C. The grain size was largely reduced by the precipitation of η phase which hinders the DRX and grain growth.
The effect of heating rate and solution time on microstructure and 650°C tensile property of GH761 are not evident. Grain size solution treated at 1120°C is larger than that at 990°C, and grain boundary precipitation aging at 830°C is less than that at 850°C. As a result, tensile plasticity is reduced by 1120°C solution and 830°C aging. It was determined that the 1090°C solution and 850°C aging is a proper heat treatment for GH761 alloy to obtain larger tensile plasticity and satisfactory combined mechanical properties.
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