Study On The Structure,thermodynamics And Kinetics Of Liquid Al-(Cu,Ni)Alloys

The interaction between atoms is the chemical root of forming clusters;moreover,the enthalpy of mixing explains the interatomic forces from a thermodynamic point of view.During solidification,the diffusion behavior plays an essential role in the formation of clusters.Therefore,the study of the liquid structure,diffusion coefficient and mixing enthalpy plays an important role in understanding the formation mechanism of microstructure,analyzing the physical properties of alloys,and providing insight into the solid-liquid phase transition.This paper used ab initio molecular dynamics to investigate the self-diffusion coefficients of 22 pure metals in the liquid state.The structure,diffusion coefficients and mixing enthalpies were also studied for liquid Al-(Cu,Ni)alloys.The available experimental self-diffusion coefficients of twenty-two liquid metals near their melting temperatures are compared with the calculated data by the first-principal dynamics simulation.The holistic evaluation shows that the calculated self-diffusion coefficients are in reasonable agreement with the experimental data.For liquid metals with loose-packed structures,the self-diffusion coefficient decreases in general with the increase of packing fraction.For liquid metals with a close-packed structure in which the coordination number is larger than 11.5,the self-diffusion coefficient fluctuates around 2.5×10-9 m2s-1,except for liquid Al,Ti and Li of which the self-diffusion coefficients are anomalously large.The packing fraction can only partially account for the variation of self-diffusion coefficient of liquid metals.For liquid Al-Cu alloys,two different behaviors of chemical short-range order vs the concentration of Cu(cCu)for Al and Cu centered clusters respectively were observed.For Al-centered clusters,Al tends to coordinate with more Cu atoms than random distribution only when cCuu>50 at.%;while for Cu-centered clusters Cu prefers hetero atom coordination in the entire concentration range.Icosahedral atomic clusters are dominant in liquid Al100-xCux alloys.The self-diffusion coefficients of Al and Cu atoms are quite different at the Al rich end,and tend to be consistent with the increase of Cu concentration.There is a convex upwards segment between cCu≈20～65 at.%in the mixing enthalpy curve,which is anomalous and can not be excluded by alternative calculations using LDA functional and other temperatures.The mechanism of this anomalous mixing enthalpy needs further study.For liquid Al-Ni alloys,there is a strong chemical interaction between Al and Ni.The Ni-Al short-range order is easier to formed at the Al-rich end,while at the Ni-rich end there exists both Al-Ni clusters and Ni-Al clusters.The icosahedral structure is dominant in liquid Al-Ni alloy.The local structure formed at 50 at.%Ni is relatively stable and is retained with the content of Ni increasing.With the increase of Ni content,the self-diffusion coefficients of Al and Ni atoms decrease dramatically.The self-diffusion coefficients of the two types of atoms differ greatly at the Al-rich end and gradually converge with the content of Ni increasing.The chemical short-range ordered structure and icosahedral structure influence the diffusion coefficients to a certain extent.The mixing enthalpy curve of the liquid Al-Ni alloy is a regular parabola,and the interaction between Al and Ni atoms shows symmetry.