Phase-field Simulation Of ?' Morphology Evolution For Ni-based Alloys Considering Elastic And Plastic Fields

Nickel-based superalloys are widely used as hot sections for advanced aero-engines and industrial gas turbines because of their excellent high temperature mechanical properties.They are key materials for the development of high performance engines.It is well known that the macroscopic properties of the material are closely related to its microstructure.Therefore,it is necessary to deeply understand the mechanism of the microstructure evolution of the alloys and the effect of microstructure evolution on the properties,which is of great significance to for alloy composition design and process optimization.With the rapid development of modern scientific theory and the quick improvement of computer performance,numerical simulation has become an equally important research method as experimental research.The phase-field method has gradually become one of the powerful tools for studying the process and mechanism of microstructure evolution.Some research work on phase-field simulation of microstructure evolution in nickel-based superalloys has been carried out.However,studies on the effects of internal factors,such as stress,elastic-plastic deformation and system energy,on microstructure evolution are still very limited.In addition,the phase-field model of the microstructure evolution under creep conditions needs to be improved.In this paper,the Ni-Al binary nickel-based alloy is taken as the research object.The phase field method numerical simulation combined with experimental verification is used to deeply analyze the elastic and plastic strain field evolution and system energy variation.Furthermore,the evolution processes of the growth and coarsening of ?' phase,?' rafting,?' shearing and instability of?' raftsduring aging and creep are investigated,which is aim to reveal the evolution mechanism of ?' phase and its influence on properties.These studies provide a new phase-field model and design basis for the development of nickel-based superalloys.The main results are as follows:(1)The influences of the antiphase domains and elastic energy on the ?' evelotions during the isothermal aging process have been studied through the phase-field model coupling with elastic field.In the early stage of aging,the “merging-splitting” phenomenon can occur between the neighboring ?' phases with different antiphase domains.The driving force for the splitting is the release of antiphase boundary energy.The antiphase domain impedes the coalescence of neighboring ?? phases while the elastic energy originated from??/? lattice misfit accelerates the growth and coarsening of ?? phases.The evolution process of ?? size with aging time can be divided into three stages,i.e.the growth stage,transition stage and coarsening stage,and the corresponded growth mechanisms are short-range diffusion,coalescence + Ostwald ripening and Ostwald ripening,respectively.(2)The ?'/? lattice misfit in the phase-field model which can simulate the microstructure evolution during continuous two-step aging is modified to be a function of temperature.The effects of cooling rate after the first aging on the ?' evolution during the continuous two-step aging have been studied.The higher the cooling rate after the first aging,the larger the number and the larger the size of the secondary ?' phases precipitated during the secondary aging.(3)The ?' rafting phenomena under four different stress states,i.e.tension,compression,shear and monoclinic,are systematically studied through the phase-field model coupling with elastic and plastic fields.The simulation results are qualitatively consistent with the experimental observations.Emphasis has been put on the 45°-type rafting mechanism under shear conditions.Under shear conditions,the high-and low-equivalent-stress areas in the ? channels are distributed along the [011] and [ 1]directions.This non-uniform distributed equivalent stress,on the one hand,causes the chemical potential difference between the elements in these two areas;on the other hand,it promotes the formations of preferential diffusion channels through adjustment of lattice distances.Under the influence of chemical potential difference,the ?'-forming elements diffuse directionally from the high-to low-equivalent-stress areas along the preferential diffusion channels,thus forming the 45°-type rafts.(4)A phase-field model suitable for the simulations of microstructures and properties in nickel-based alloys under creep conditions has been established.Through this model,the synchronous simulation of creep microstructure and property is realized firstly.The influences of elastic and plastic deformations,as well as creep damage,on the microstructure evelutions are considered in this creep phase-field mode.This model can completely simulate the microstructure evelutions and creep deformation during the whole three stages of the creep,which lays a foundation for quantitative prediction of creep microstructure and properties.(5)The effects of creep stress and ?' volume fraction on the microstructure evolutions and creep properties are studied,respectively,through the creep phase-field model.The results show that the creep life decreases with the increased creep stress.As the ?' volume fraction increases,the creep life first increases and then decreases.When the ?' volume fraction is 65%,the steady state creep rate is the lowest and the creep life is the longest,due to the fact that the internal stress,elastic strian and plastic strain in ? channels are all in the lowest states.(6)The validity and accuracy of creep phase-field model are verified through the creep experiments of Ni-18.5 at.%Al single crystal alloy.Good agreement between the experimental and simulated results has been shown under the tensile creeps of 80,100 and120 MPa at 1223 K.