相场方法研究Al-Ag合金γ相周围溶质析出过程

PHASE-FIELD SIMULATION OF SOLUTE PRECIPITATIONS AROUND THE γ PHASE IN Al-Ag ALLOY

针对Al-Ag合金溶质的Spinodal亚稳互溶隙曲线的特点,考虑溶质场与析出相的相互作用,通过构造时效温度和溶质浓度相关联的局域相互作用自由能密度函数,建立了低Ag溶质的Al-Ag合金析出过程的相场模型.模拟了Ag溶质浓度分别为4.2%和22%的体系中溶质场的Spinodal分解和析出Guinier-Preston区(GPZ),以及在析出相附近形成一种无沉淀区(PFZ)的演化过程.模拟结果表明,在析出相周围形成了椭圆形的PFZ,其宽度大约是棒宽度的2倍,而在远离PFZ的区域出现了由于Spinodal分解而形成的Ag溶质分布图案.经过足够长的时效时间,溶质场析出GPZ.当Ag溶质浓度较高时,浓度场会在PFZ周围形成多条液滴状的平行流线环绕着的析出相.模拟结果与实验观察结果很好地吻合.

Interactions between different precipitation products during phase decomposition for different alloys have been observed.Spinodal decomposition and intragranular precipitation are the two well-know mechanisms for explaining phase decomposition,both of which interaction mechanisms have been investigated experimentally in recently.A local free energy density function depending on aging temperature and composition has been proposed to describe the interaction between the Ag solute field andγprecipitates in phase-field simulation of spinodal decomposition in Al-Ag alloy.The evolution of spinodal decomposition in Al-Ag alloy with 4.2%Ag and 22%Ag has been simulated by the phase-field method using this function to represent numerically the precipitated Guinier-Preston zones（GPZ） around aγphase.The simulated results show that PFZ around a precipitated phase is an elliptical and its width is about two times the width ofγphase.In the region far from PFZ,a pattern of Ag solute field appears due to spinodal decomposition.When Ag-depleted zones are relatively far apart with each other,spinodal decomposition is strongly affected by them.The formation of PFZ resulting from spinodal decomposition has been initiated in the central region of the supersaturatedαmatrix before the modulation effect of Ag solute field at edge reachs here.It is found that two or three Ag-rich bands appear around the PFZ.During aging,Ag diffuses from not only theαmatrix but also the edge of Ag-depleted zones,where an accumulation of Ag occurs.After long time aging,many droplet-like Ag solute bands are formed near PFZ aroundγphase.These simulated results are in better agreement with the experimental results.