| Precipitation hardening is the main strengthening mechanism of Nickel based superalloy and ?' phase is one of the most important reinforcement phase.According to the theory of precipitation hardening,microstructure including size,number density,distribution of second phase particles are significant factors that influence the strengthening.The study of the microstructure of ?' phase in Ni-Al-Cr ternary alloy and precipitation kinetics under aging process will help with better understanding of strengthening mechanism in more complex commercial superalloy and provide guidance to the heat treatment and increasing the performance of superalloy.In this work,a ternary phase field theory linked with CALPHAD method and coupled with classical nucleation theory was built up to model the aging process of the alloy Ni-5.2Al-14.2Cr at.%at the temperature of 873 K.Additionally,by incorporating elastic strain energy into the phase field model,Ni-Al-Cr ternary alloy systems with different elastic strain energy were also studied.Further,by changing the composition,Ni-Al-Cr ternary alloy systems with different equilibrium phase fraction were simulated.This work studied the evolution of the microstructure of precipitation in Ni-5.2Al-14.2Cr at.%alloy during aging process at the temperature of 873 K and the precipitation kinetics is quantitatively characterized.It is shown that ?' particles stay as the shape of sphere and are distributed randomly on the matrix during the aging process and only a few necks due to coalescence of particles were observed.Compared with simulation results of alloy systems with only one variant of precipitation,simulation results of alloy system with four variants of precipitation indicate that the existence of anti-phase boundary will retard the coalescence between particles in the early stage of aging.Aging process of 60 h can be divided into 4 stages dominated by different mechanisms:nucleation stage,transient stage,quasi coarsening stage and pure coarsening stage.After comparing the simulation results and ones obtained from experiments,it is found that phase field simulation reproduced the aging process including nucleation,growth and coarsening.The precipitation kinetics is also observed to agree with experiments.By incorporating elastic strain energy into the model,the influence of elastic strain energy on the microstructure is studied.The simulation results show that the influence is remarkable:due to elastic strain energy,round precipitation particles will transform to square shape,align along two elastic-soft direction and finally form the separated raft pattern observed in experiments.By changing the elastic strain energy,the influence of elastic energy on the precipitation kinetics of Ni-AI-Cr alloy is investigated.Simulation results show that elastic strain energy will lead to the division of the whole aging process.Precipitation kinetics in the first stage agree with the cubic law of Ostwald coarsening theory,while it deviates from the cubic law in the second stage and the coarsening is retarded in this stage.The effect of volume fraction on coarsening is studied by changing composition of Ni-AI-Cr alloy system.Simulation results show:precipitation kinetics of alloy systems with different volume fractions agrees with the cubic law of Ostwald coarsening theory;coarsening rate constant increases with the volume fraction. |
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