Distribution Of Metallic Impurities In Gadolinium During Directional Solidification Process

Directional solidification technology has been widely studied and applied in the purification of polycrystalline silicon,aluminum and copper with high purification efficiency.In this paper,aiming at the problems of limited purification effect and low purification efficiency of typical metal impurities such as Ti,Fe,A1 and Cu in gadolinium,the technology of purifying metal gadolinium by directional solidification is studied.The main conclusions are as follows:The theoretical calculation and analysis shows that the equilibrium distribution coefficients of Ti,Al,Fe,Cu,Mg and Mn in gadolinium are 0.018,0.273,0.117,0.123,0.405 and 0.014,respectively,all less than 1,so it is feasible to purify gadolinium by directional solidification.Through the study of the different directional solidification rate and the heating temperature,the rules of the migration and occurrence of metal impurities in the metal gadolinium are obtained as follows:At 3 ?m/s,the influence of heating temperature of directional solidification at 1350?,1400? and 1450 ? shows that the higher the heating temperature is,the stronger the convection in the melt is,the better the removal efficiency is.At 1450?,the effective distribution coefficients of metal impurities Ti,Al,Fe and Cu are 0.021,0.297,0.355 and 0.275 respectively.Ti and A1 are close to the Scheil distribution,indicating that they are perfect mixed in the melt;but Fe and Cu are higher than Scheil distribution,which are enriched at the interface forming forming a solute boundary layer with a thickness greater than 5mm.Impurities of Mn and Mg are also affected by evaporation effect.The higher the heating temperature,the higher the removal rate.At 1450?,the influence of different solidification rate of 3?m/s,6?m/s,12?m/s,24?m/s and 50?m/s shows that the slower the solidification rate of directional solidification,the smaller the effective distribution coefficient,the better removal effect of metal impurities.At the solidification rate of 3?m/s,the crystal is preferred to grow into a coarse columnar crystal,and the removal efficiency is the best.When the solidification rate reaches to 24?m/s,the component undercooling increases and the impurity removal efficiency weakens.When the solidification rate reaches 50?m/s,the crystal is equiaxed and the impurity does not remove the effect.The transmission electron microscopy and energy spectrum scanning results of directional solidification gadolinium show that Al and Cu impurities are distributed in the matrix metal in the form of solid solution,and the impurities of Fe and Ti are segregated at the grain boundary with TiFe phase or ?-Ti phase.Under the conditions of 1450? and 3 ?m/s,the purification effect of metal gadolinium shows that the total amount of metal impurities in once directional solidified gadolinium is reduced from 1858 ppmw to less than 356 ppmw,and the removal rate is above 82.7%;the total amount of metal impurities in the twice directionally solidified gadolinium is reduced to below 173ppmw,the removal rate is higher than 93%,and the purity of the metal is above 99.98wt.%.