Molecular Dynamics Simulation On Rapid Solidification Of TiAl Alloys

Rapid solidification is an effective way to improve the properties of TiAl alloy.In the process of“liquid-supercooled liquid-amorphous”transition,the thermophysical properties,structure,and dynamic properties of the system will change continuously.In this paper,the thermophysical properties,structure,and dynamic properties of TiAl Alloy during rapid solidification are studied by molecular dynamics simulation,which provides a systematic theoretical basis for the design and development of TiAl amorphous alloy.First,Molecular dynamics simulations(MDS)employing an embedded-atom-method(EAM)are applied to calculate the liquidus temperature T_Las well as the density and specific heat of liquid TiAl alloys at temperatures above and below the melting temperature in a wide composition range.Both the excess volume and specific heat-composition dependences of TiAl alloys are investigated.TiAl alloys show highly non-ideal behaviors,indicating that Neumann–Kopp's rule does not apply.The excess volume reaches the minimum values at Ti₂₅Al₇₅composition,corresponding to the most pronounced volume effect,while the specific heat reaches its maximum value at Ti₅₀Al₅₀composition.The microstructure evolution of TiAl alloy during rapid solidification was studied by various structural characterization methods.The Volume-Temperature curve shows that the amorphous structure of Ti_100-xAl_x(x=10,25,50,75,90)is formed at 300K.The composition of the alloy has a significant effect on the Glass Forming Ability(GFA)and Ti₂₅al₇₅has the highest GFA.The split occurring in the second peak of the pair distribution function(PDF)curves proves the formation of metallic glass.The analysis of the HA bond pair index and Voronoi index shows that the number of pairs and bonds reflecting the development of the icosahedral short-range order increased with increased Al concentration and reaches the maximum at Ti₂₅Al₇₅,which has the highest fraction of Al-centered full icosahedral clusters.The dynamic behaviors of the undercooled liquid of Ti₂₅Al₇₅alloy were analyzed.With the decrease of temperature,the?relaxation time of undercooled liquid deviates from the Arrhenius equation at T_A,and the system gradually deviates from the Stokes-Einstein relation(SER)and begins to obey the fractional SER at T_S.Meanwhile,the dynamic heterogeneity of the system increases significantly at T_S.There is no decoupling of element dynamics in the system,so the dynamic heterogeneity increases significantly at T_Scannot be explained by the difference of dynamic behaviors between Ti and Al.The five-fold symmetry W of the system increased abnormally at T_S,which establishes the correlation between the structure and the dynamic behavior of Ti₂₅Al₇₅alloy.