The Dynamics Of Liquid Ce-based Alloys

Based on the quasi-elastic neutron scattering,sliding cell and multi-slice sliding cell techniques,this paper studies the diffusion of liquid Ce-based alloys systematically.And it mainly involves the influence of alloying effect on diffusion of melts,the relationship between diffusion and viscosity,the correlation of self-diffusion and inter-diffusion,the effect of dynamical cross correlation factor,the analysis method for inter-diffusion of multicomponent melts and so on.In Ce_70-xAl₁₀Cu₂₀Co_x?x=0⁵?melts,the results measured by quasi-elastic neutron scat-tering show that the addition of Co has significantly influence on the dynamics of melt.On one hand,the temperature dependence of diffusion changed from Arrhenius type to VFT relationship with Co microalloying.On the other hand,the trace addition of Co makes the diffusion of the atoms slower.Especially near the melting temperature,the mobility is significantly reduced and the diffu-sion activation energy highly increased.In the Ce-Al-Cu-Co system,the diffusion activation energy is strongly correlated with the reduced glass transition temperature,which indicated the connection between the melt dynamics and the glass forming ability.Furthermore,there is a correlation between the pre-exponential factor and diffusion activation energy used to describe the diffusion-temperature dependence,which is similar to the crystal and metallic glass.However,the relevant parameters are huge different with the outcomes in crystal and metallic glass.These results imply the commonness and characteristics of atomic diffusion in Ce-based melts.In liquid Ce₇₀Al₁₀Ni₂₀alloys,the relationship between self-diffusion of Ni and viscosity can be well described by S-E function at high temperature.However,there is a clear“decoupling”between self-diffusion and viscosity near the liquidus temperature,and the relationship transform from S-E into FSE function.This transform is attributed to the strong dynamical heterogeneities.In Ce₇₀Al₁₀Ni₂₀melts,viscous flow are dominated by the slowest regions,which are mainly consist of Ce atoms.And the mobility of Ni is faster than the slowest regions which led to the failure of S-E relationship.The inter-diffusion constants measured by sliding cell method are significantly smaller than the prediction by Darken formula in Ce₈₀Ni₂₀melts.These results indicate that the dynamical correlation plays an important role to the inter-diffusion in this melts.And different with the thermodynamic fac-tors,the dynamical cross correlation factor,whose value falls inside the range of 0.6-0.8,suppressed the inter-diffusion between the atoms.Macroscopically,dynamical cross correlation factor represents the deviation of diffusion from the ideal diffusion system.And it is determined by the atomic interac-tions on a micro level.In Ce₈₀Ni₂₀melts,there is an attractive chemical interaction between Ce and Ni,at the same time,a repulsive one for Ni-Ni.The dramatic differences of the chemical interaction give rise to the obviously disparities of the movement tendency among Ce-Ce,Ce-Ni and Ni-Ni,and eventually led to the failure of Darken formula.For the sake of solving the problem of multicomponent inter-diffusion,we construct the objec-tive function which contains the information of diffusion process in melts.And the inter-diffusion coefficient matrix can be obtained by optimizing the objective function.In order to guarantee the globality of the optimal solution,we propose a global optimization algorithm,which was based on the genetic algorithm,simulated annealing algorithm and L-M algorithm.The solving process of this algorithm is only based on one experimental measurement,hence greatly reduces the workload.At the same time,it is not required to smooth the experimental data and determined the gradient and integral.Therefore,the calculated accuracy is improved.In addition,this method also has high ver-satility.And based on high precision experimental composition distribution,it is easy extended into four-component melt system and above to determine the inter-diffusion coefficients.In this paper,it was used to calculated the diffusion coefficients of Ce₇₀Al₁₀Cu₂₀from the measurements of multi-slice sliding cell method.The huge different results from solid alloys are that the inter-diffusion is domi-nated by the main diffusion coefficients,and the cross diffusion coefficients are very small which can be negligible.Furthermore,Darken formula is not suitable for this melts,which means that the dynam-ical cross correlation factor may have a significant impact on the inter-diffusion of the multicomponent melts,and the link between self-diffusion and inter-diffusion must consider this impact.