Molecular Dynamics Simulation Of The Influence Of Y,Cr-Mg-Ti On The Microstructure Of Al-8Fe Melts And Al₃Fe Nucleation

The mechanical properties of Al-Fe alloys are reduced due to the cracking of the coarse needle-like,flake-like,and elongated Al₃Fe phases within the matrix.The addition of rare earth or alloying elements to Al-Fe alloys is one of the most common methods used to refine the primary Fe-rich phase.However,the effects of rare earth or alloying element additions on the microstructure of Al-Fe alloy melts and the nucleation process have been rarely studied.In this paper,the melt structure and nucleation process of four alloys,namely Al-8Fe,Al-8Fe-1.5Y,Al-8Fe-2Cr-0.4Mg-0.3Ti,and Al-8Fe-2.5Cr-0.6Mg-0.5Ti,were simulated using classical molecular dynamics with the LAMMPS simulation package.The effects of the addition of rare earth element Y and alloying elements Cr,Mg,and Ti on the microstructure of the Al-8Fe alloy melt and the nucleation process were studied using radial distribution functions,coordination numbers,HA bond pairs,Ackland-Jones-defined clusters,and Voronoi indices.This was done to explain their mechanisms in shaping the morphology of the primary Al₃Fe phase.Simulating the structure of Al-8Fe alloy melt and the nucleation process of Al₃Fe phase,it can be observed that there is a strong interaction between Al and Fe atoms in the Al-8Fe alloy melt.The Al-8Fe alloy melt is composed of a large number of"other"type clusters,a certain amount of hcp clusters and bcc clusters,and a small amount of fcc clusters and ideal icosahedra.Prior to nucleation,a large amount of hcp structure precipitates and reaches the critical value,leading to the transition to the fcc structure and the initiation of nucleation within the melt.During the process of cooling down to the beginning of nucleation,the Al-8Fe alloy melt experiences volume expansion.The increase in fcc structure represents the initiation of growth of the primary Al₃Fe phase.Comparing the structure of Al-8Fe-1.5Y alloy melt and the nucleation process of Al₃Fe phase,it can be observed that the addition of 1.5wt.%Y weakens the Al-Fe bonds in the melt.Y atoms have a certain probability of occupying the first coordination layer of Al atoms.Simultaneously,the size effect forces the first peak of the Al-Fe coordination number to shift to the left,increasing the system’s coordination number and promoting the formation of defect icosahedral clusters within the melt,thus reducing the content of hcp clusters.During the nucleation process,the addition of 1.5wt.%Y increases the nucleation temperature,lowers the critical value for hcp structure transformation,and increases the number of primary Al₃Fe phase nuclei within the melt prior to nucleation.In the nucleation process,the addition of 1.5wt.%Y promotes the generation and growth of Al₃Fe phase nuclei.Furthermore,the addition of 1.5wt.%Y increases the content of post-nucleation icosahedral clusters,inhibiting the growth of the primary Al₃Fe phase and refining its morphology.Comparing the structure of the Al-8Fe-2Cr-0.4Mg-0.3Ti and Al-8Fe-2.5Cr-0.6Mg-0.5Ti alloy melt and the nucleation process of Al₃Fe phase,it can be observed that the addition of Cr-Mg-Ti enhances the ordering degree within the melt and increases the tendency for the formation of icosahedral clusters.In the Al-8Fe-2Cr-0.4Mg-0.3Ti melt,the interaction between Al and Fe is strengthened,resulting in a decrease in the average coordination number and an increase in the content of hcp clusters.Conversely,in Al-8Fe-2.5Cr-0.6Mg-0.5Ti,the content of fcc clusters increases,and the melt structure is similar to that of Al-8Fe alloy melt.During the cooling and nucleation process,Cr-Mg-Ti enhance the interaction between Al and Fe,increase the tendency for the formation of Al-Fe clusters,and promote the formation of primary Al₃Fe phase nuclei.The Al-8Fe-2Cr-0.4Mg-0.3Ti alloy melt has a smaller critical value for hcp structure,resulting in a higher number of primary Al₃Fe phase nuclei within the melt.After nucleation,the Al-8Fe-2Cr-0.4Mg-0.3Ti alloy melt contains more icosahedral clusters,suppressing the growth of the primary Al₃Fe phase and refining its morphology.In contrast,the number of icosahedral clusters decreases in Al-8Fe-2.5Cr-0.6Mg-0.5Ti,and the refining effect of primary Al₃Fe phase is weakened.