Clusters Type In Metal Melts And Their Roles During Solidification Process

This present work uses first principle molecular dynamics combined with X-ray diffraction (XRD) methods, discussing the influences of clusters formed by like or unlike atoms on the structures of metal melts; the rapid solidification (RS) technique is further applied to reveal the roles of clusters types in metal melts during solidification process and final structures, exploring the solidification essence from the evolution of atomic clusters.Firstly, atomic clusters in pure metal melts are investigated, differences between the pair correlation functions (PCFs) of pure Ga and In melts are explained by the structure of atomic clusters in the melt. The first peak of PCF for In melt is higher and wider than that for Ga melt, indicating the short-range order degree of In melt is higher than that of Ga melt and the structure of In melt is losser than Ga melt. There exists a second peak at105°of angle distribution function in both two melts, Some clusters resembled with body-centred tetragonal configurations reserve in In melt; the second peak in In melt is higher than that in Ga melt, leading to the structure of In melt more ordered than that of Ga. By Voronoi polyhedra (VP) analysis, VP configurations formed by high sides in Ga melt make its structure denser and the principle peak in the PCF steeper; VP configurations formed by low sides in In melt make its structure more open and the principle peak in the PCF wider.Secondly, three different Fe-B, Ga-In and In-Sn systems are chosen to investigate the clusters types in their melts by ab initio molecular dynamic (AIMD) method. In Fe-B alloy melts, the existence of the shoulder in the second peak of the PCFs of Fe80B20alloy melt is confirmed. Tri-caped trigonal prism and its derivatives of B-centered clusters are the dominant clusters in FegoB2o alloy melt. The shoulder is caused by the unequal opportunity of connection modes of B-centered clusters. For Ga-In alloy melts, the positions of the first peaks in gGaGa(r)、gInIn(r) are close to the pure Ga and In and unchanged with the compositions, the position of first peak in gGaIn(r) is larger than the average value of pure Ga and In, indicating that the unlike atoms are prone to repulse and GaGa, InIn clusters formed by like atoms are easy to emerge in the system. The most popular1311pair bonds existed in liquid pure Ga and In are dominant in liquid alloys, proving that GaGa, InIn clusters coexist in Ga-In alloy melts. By the Voronoi analysis, with increasing In, in the Ga100-xInx(30≤x≤50at.%) region, the highest content coordination number around Ga atoms abruptly decrease from12to10, proving that GaGa and InIn clusters are prone to separate. For In-Sn alloy melts, the first peak position of gInSn(r) is smaller than the average value of pure In melt (3.24A) and pure Sn melt (3.10A), indicating that the binding bonds between In and Sn atoms are not ignored. Partial coordination numbers NInIn and NInnsn show a linear relationship with alloy compositions, the atomic arrangement around In atoms is consistent with the random close-packed (RCP) model. The results prove that the existences of Sn-Sn covalent clusters cause the atomic distribution around Sn atoms open. Two main peaks in the angle distribution functions gInSnIn(θ) and gsnInSn(θ) locate at56°and105°, indicating that In atoms are easy to form tetrahedral clusters with Sn atoms in the alloy melts. The In-Sn alloy melts can be considered as mixture of Sn-Sn covalent clusters and dense random close-packed clusters.Finally, the roles of atomic clusters in melts during solidification process are studied. The structures of Ga100-xInx(0<x≤21at.%) alloys and pure Ga at373K and298K have been investigated by X-ray diffraction (XRD). The structure factor results reveal that the structures of Ga-In melts deviate from RCP model at373K, when the temperature decreases to298K, the melts get close to the RCP model. By AIMD simulations, the atomic contents on the coordination shell of Ga atoms are close to the nominal alloy compositions and they are slightly affected by the changing temperature; the In atomic concentration around In atoms is higher than the nominal alloy composition, clusters formed by like atoms segregate and the lowing temperature makes the segregation degree decrease. The evolution of InIn clusters in the cooling process determines the structural changes of alloy melts. The melts can be divided into two types by AIMD simulations:first, melt mainly contains clusters formed by the like atoms, such as Al95In5melt, unlike atoms distribute in topological short-range order; second, the bonding force between unlike atoms is strong, such as Al90Cu10andAl84Ni16melts. The RS ribbons of Al95In5, Al90Cu10, Al84Ni]6, Cu6Sn5and Ga2oSn8o alloys have been prepared using single roller melt-spinning device. The morphologies and compositions of these ribbons are investigated by field emission scanning electron microscope (FESEM) combined with energy dispersive X-ray (EDX) instruments. From these results, when the the bonding force between unlike atoms in the melt is strong, such as Al90Cu10, Al84Ni16and Cu6Sn5melts, intermetallic compounds generate in the RS ribbons, some crystalline-like Al2Cu clusters are found in the Al90Cu10melt by calculations and these clusters transform Al2Cu intermetallic compound during RS process; when the clusters formed by the same atoms are easy to appear in the melt, such as Al95In5and Ga2oSn80melts, the elements distribute in micro-heterogeneous way in the RS ribbons.