Ab Initio Molecular Dynamics Study On Structure,Kinetics And Thermodynamics Of Liquid Ti-Al Alloys

The solidification process of an alloy has an important influence on its macroscopic properties.As the initial phase of the process,the structural study of liquid alloys is of great importance in relation to alloy processing and alloy design.The existence of short-range ordered structures and medium-range ordered structures in liquid alloys is essential for the study of nucleation in solidification processes.In this paper,the liquid structure,self-diffusion coefficient and mixing enthalpy(formation enthalpy)of Ti-Al alloy system are calculated and analyzed in two modes of isothermal and isothermal degrees using ab initio molecular dynamics method,and the liquid structure of practical Ti alloy Ti6A14V is also studied to explore the liquid-solid structure correlation.It was found that the Ti-rich and Al-rich dilute alloy sides in both computational modes have stronger medium-range ordered structures,and in the other composition ranges,the shortrange structures are more ordered.The analysis of the partial correlation functions,coordination numbers,and the HA bonded pairs shows that the interaction between dissimilar atoms is stronger than that between similar atoms.In the whole composition range of Ti-Al alloys,the icosahedral short-range ordered structure dominates,and the short-range ordered structures decrease with the increasing of Al content,while the disordered structure increases.In Ti-rich dilute alloys,in addition to icosahedron,the content of clusters with bcc-like structure is higher,we believe that the presence of such clusters reduces the solid-liquid interface energy,which influences the nucleation and morphology selection during solidification.The magnitude and asymmetry of the calculated mixing enthalpy are in good agreement with the experimental values,and the minimum value lies around 55 at.%Al.Based on the subnormal solution model,it can be concluded that the heterogeneous interatomic interactions on the Al-rich side are about 50%stronger than those on the Ti-rich side.It was found that the variation of self-diffusion coefficient with composition in Ti-Al liquid alloys is closely related to the evolution of short-range ordered structures.Before 50 at.%Al,the self-diffusion coefficients of Ti and Al atoms coupled with each other,and the two kinds of atoms move together in a similar way;with the increase of Al content,the self-diffusion coefficients of the two kinds of atoms begin to decouple.However,the results of isothermal and isothermal superheat calculations begin to differ after 50 at.%Al.The self-diffusion coefficients of Al calculated at a constant temperature increase sharply while DTi increases only slightly,in the meanwhile,the short-range order in the liquid alloy decreases while the disorder increases.DAl calculated under the condition of isothermal superheat increases gradually with the abundant of Al content,whereas begins to decline when the Al content rise over 95 at.%.At this time,the short-range ordered structures also change correspondingly:decreases gradually and then increases after 95 at.%Al.That’s to say,in the liquid Ti-Al alloys,the diffusion of atoms is limited by the ordered structure and driven by the disordered structure.The viscosity of the liquid Ti-Al alloy estimated from the self-diffusion coefficient of the atoms at constant temperature is in good agreement with the values reported in the literatures.By observing the partial correlation functions of titanium alloys Ti92Al8(the corresponding commercial titanium alloy grade is TA7),Ti86V14(the corresponding commercial titanium alloy grade is TB6)and Ti86Al10V4(the corresponding commercial titanium alloy grade is TC4),we found that in the Ti92Al8 liquid alloy,the interaction between dissimilar atoms is stronger than that between similar atoms,and in the Ti86V14 liquid alloy,the interactions between dissimilar and similar atoms are close,while in Ti86Al10V4,the Al-Al and V-V interactions are weaker,which lead to a conclusion that solid solution-like short-range ordered structures with abundant Al or V atoms may exist.The difference between the coordination number ratio and the nominal ratio shows that Al and V atoms in Ti86Al10V4 alloy are more inclined to coordinate with Ti atoms respectively,and the chemical order is weaker and easy to form solid solution.Among the short-range ordered atomic clusters in the ternary alloys,the bcc-like structure is high in content,second only to the icosahedra which can be commonly found in the liquid structure,and we speculate that the presence of this structure reduces the solid-liquid interfacial energy,making the bcc structure of β-Ti the dominant crystal structure in high-temperature solids.