Studies On The Healing Mechanism And Model Of Creep Cavities In K465alloy

Nickel-based superalloys are widely used to manufacture hot section components of aero-engines,and generally operate under severe service conditions,i.e.high temperature,high pressure,complex stresses and gas corrosion.During the long-term service,nickel-based superalloys inevitably suffer microstructural degradation and even generate creep cavities,reducing mechanical properties and service life of materials.Hot isostatic pressing(HIP)rejuvenation technology has been demonstrated to heal service damages of nickel-based superalloys through recovering the degenerated microstructures and closing the creep cavities.However,determination of the optimal HIP processing parameters generally depends on abundant experimental data and engineering experience.Investigations on healing mechanism and kinetics model of creep cavities under HIP conditions can provide guidance for the selection of HIP parameters.In the present study,the microstructures of the samples of K465 superalloy in the original as-cast condition,creep damaged condition and the HIP processed condition were quantitatively analyzed by using the optical microscope and the scanning electron microscope.Microstructural evolution behavior of K465 alloy was studied and the stress field around creep cavities in K465 alloy under HIP condition was analyzed by the finite element method(FEM),in order to investigate the healing behavior of creep cavities of nickel-based superalloys.The results show that the concentrically-oriented γ’ rafting structure occurs in the vicinity of creep cavities when HIP temperature is not high enough to dissolve the primary γ’ particles completely but the HIP condition can induce effective healing.The application of isotatic pressure changes the stress fields of the γ matrix and the γ’ particles around creep cavities,which exhibiting obvious stress anisotropy.It can result in the directional diffusion of solute atoms and form the concentrically-oriented γ’rafting structure.The healing kinetics model of creep cavities under the application of HIP was proposed,taking both grain-boundary diffusion and creep of matrix into consideration.Numerical analysis for model parameters was conducted,and effects of HIP temperature and isostatic pressure on the healing time for creep cavities were quantitatively analyzed,constructing the HIP healing map for creep cavities.The results show that healing time decreases drastically with increase of temperature and pressure.As compared to HIPpressure,healing time is more sensitive to HIP temperature.For large-size cavities at grain boundaries or even the complete separation of grain boundary,the required healing time is much longer than that of the small sized cavities.When further increasing the isostatic pressure above 150 MPa,there is little improvement for healing effects.