| Nano-scaled Cu-rich phase will be precipitated from Cu-containing steel during irradiation or aging treatment,which improve the yield strength,tensile strength,hardness,and other mechanical properties of steel.However,in the application of reactor pressure vessel(RPV),the irradiation-induced phase will lead to a high ductile-brittle transition temperature and embrittlement phenomenon,limiting the application in the reactor.To prepare high-strength alloy steel and avoid irradiation embrittlement in service,it is significant to master and control the precipitation behavior of the Cu-rich phase for improving the comprehensive properties of steel.In the solid phase transformation,the precipitation transformation path and microstructure morphology are mainly influenced by the elastic strain energy and interaction of the alloying elements.It is significant to explore the regulation mechanisms of elasticity and alloying elements on the transformation structure.Thus,a multi-component phase-field model of FeCu-Mn-Ni-Al coupling elastic field is established by using the thermodynamic subregular solution model and microelastic theory.The precipitation process of quinary Fe-Cu-Mn-Ni-Al alloys during isothermal aging process is simulaited to clarify the regulation mechanisms of elasticity and alloying elements on phase transformation structure and growth kinetics.According to the thermodynamic analysis of the Fe-Cu-Mn-Ni-Al system,we get the binodal line and spinodal line,and the two-phase region can be divided into two parts by them: the spinodal region and the metastable region.The precipitation processes of alloy systems with different Cu compositions in these two regions are simulated to reveal the regulation mechanisms of internal elastic distortion and external strain on the microstructure and morphology.It is found that Cu/Mn/Ni/Al atoms are segregated and formed stable Mn Ni Al@Cu microstructure in both unstable and metastable regions.The core-shell structure has gone through the following stages:(Cu/Ni/Al/Mn)segregation state,in which Mn/Ni/Al are trace;(Mn Ni Al(Cu)Mn Ni Al)state,single-shell structure;(Mn Ni Al(Mn(Cu)Mn)Mn Ni Al)state,double-shell structure;Mn(Ni Al(Mn(Cu)Mn)Ni Al)Mn state,triple-shell structure.Mn Ni Al ring-shaped shell phase adheres to the outside of the ?-(BCC-Cu)or ?-(FCC-Cu)phase by a non-classical nucleation.Meanwhile,Ni-Al-Mn shell layer will inhibit the coarsening and structural transformation of precipitates.Our simulation results are the same as existing experimental studies,which further prove the accuracy of this model.In the spinodal region,the period of forming individual particles from the component wave is shortened under the effect of the internal elastic distortion energy,and the component waves change from random arrangement to regular vertically crossed arrangement.Under the effect of elastic anisotropy,the particles are oriented along the elastic "soft" direction <11> and merge into regular ellipsoidal or rod-shaped morphology.In the metastable region,the internal elastic effect inhibit the precipitation process,and make the particles possess the tendency of <11> orientation arrangement,but this phenomenon is not obvious.In the effect of external strain,the particles grow up along the specific direction,which is decided to the comparison of elastic anisotropy of two-phase.Because the precipitates are softer than the precipitates,the particles will grow along the vertical direction of the external strain direction;And there is a corresponding relationship between the growth rate and the strain value,the growth rate at big strain value(0.4%-1.0%) is about twice as much as that at small strain condition(0% ? 0.4%).According to the study of the effect of temperature and elasticity on structural transformation,we find that the nucleation driving force at low temperature is greater than that at high temperature.With the rise of temperature,the nucleation process is prolonged,the growth,coarsening and structural transformation are inhibited.Therefore,the particles' morphology is effectively controlled by applying external strain.The effects of Mn and Ni/Al on the phase separation process are discussed by threedimensional phase-field simulation.The results show that Mn can accelerate the phase separation process and promote later growth and coarsening.According to the analysis of free energy and volume fraction,it is found that the volume fraction increases with the rise of Mn content.However,Ni/Al will impede the decomposition of component waves and delay the phase separation process.With the increase of Ni/Al compositions,the inhibition effect is more obvious.Through the analysis of the later coarsening process and the comparison with the standard LSW theory,we find that higher Mn content accelerate the growth and coarsening rate of particles,and the time exponents after fitting are 0.41,0.42,and 0.37 respectively,which are all greater than the theoretical value of 0.33.This is mainly due to the influence of mixed mechanisms of Ostwald ripening and coalescence coarsening.With the increase of Ni/Al composition,the time exponents are 0.29,0.26,and 0.32 respectively,which are less than the theoretical value.It is mainly caused by the fact that the aggregation degree of Ni/Al and the thickness of Ni-Al-Mn shell are increased with the rise of Ni/Al contents,inhibiting the diffusion of Cu atoms and the growth of Cu rich phase.Furthermore,the addition of Mn/Ni/Al elements obstruct the phase separation,growth and coarsening of the Cu-rich phase,thus leading to particle refinement and the clustering of Ni-Al-Mn atoms. |
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