Structural Evolution And Cluster Behavior In The Solidification Process Of Binary Alloy's Melts

In the present paper, the structural evolution, cluster behavior and viscosity with temperature in the solidification process of Cu-Ag, In-Sn and Cu-Sn binary alloys were investigated by means of X-ray diffraction and viscosity measurement. The different cluster behaviors in the process from cluster to crystalline nucleus have been discussed. The correlations between liquid structure and viscosity have been revealed. The concept of structural fragility of liquids has been proposed, and its correlation with the glass-forming ability (GFA) has also been discussed.The results of X-ray diffraction show that the structure factors of Cu-Ag alloys have the symmetrical first peaks under all the investigated temperatures, and the liquid structures of them are micro-homogeneous. For the structure factor of In-Sn alloys, there is a shoulder on the right of the first peak due to the influence of Sn, and it is more obvious with increasing Sn content and decreasing temperature. The structure factors of Cu-Sn alloys have the symmetrical first peaks with Sn content at 10% and 20%, but they show obvious asymmetry when Sn content increase to 30% and 40%, especially under 900℃. This is caused by the formation of Cu₃Sn structures in the liquid, which makes the liquid structure micro-inhomogeneous, and both the X-ray diffraction and DSC curves show abnormal changes near 900℃.All the mean nearest neighbor distance r₁ of Cu-Ag, In-Sn and Cu-Sn alloys show increasing with decreasing temperature, which indicates the micro-thermal contraction phenomenon of atom clusters exist in the binary alloys. The results of Gaussian peaks decomposition of the radial distribution (RDF) of Cu-Ag alloys reveal that the Cu-Cu correlations are enhanced near the liquidus in copper enriched Cu₈₀Ag₂₀ and Cu₆₀Ag₄₀ alloys, and the Ag-Ag correlations are enhanced near the liquidus in silver enriched Cu₂₀Ag₈₀ alloys.The correlation radius r_c and the correlation length D of Cu-Ag and In-Sn alloys were analyzed, and the results show two different cluster behaviors. In the Cu-Ag alloys, r_c keeps stable near the liquidus, and D shows a near linear change from high temperature to liquidus. In the In-Sn alloys, however, r_c shows an obvious change near the liquidus, and D shows an obvious non-linear but a near exponential change with temperature, in addition, an obvious change near the liquidus like r_c is also found in D. As the cluster plays a crucial role in the nucleation process, we consider them as two different nucleation mode, i.e., the stabilization mode and the saltation mode. The nature of the stabilization mode is the similar structures of liquid and solid, the liquid structure keeps stable with temperature variation, the cluster in the liquid can serve as the embryo of the crystalline nucleus directly. The nature of the saltation mode is due to quite different structures of liquid and solid, the liquid structure change obviously with temperature, the cluster in the liquid needs an abrupt structure change before it acts as the embryo of the crystalline nucleus.All the r_c of Cu-Sn alloys show obvious increase near 900℃. This is related to the formation of Cu₃Sn structure. Furthermore, the changes of r_c introduced by the nucleation appear at about 30～40℃below the liquidus. This is mostly owing to the existence of the Gu₃Sn structure, which restrains the atom diffusion and structure reconfiguration, consequently, stabilizes the liquid structure, thus the nucleation needs some supercooled driving force. The temperature dependence of D of Cu-Sn alloys shows a relation between linear and exponential.The viscosities of Cu-Ag, In₃₀Sn₇₀ and Cu-Sn alloys were measured. The results show that the viscosities accord with the Arrhenius equation at high temperatures, but all show a departure from this equation when temperatures are near their liquidus. Thereinto, Cu-Ag and Cu-Sn alloys depart to the increscent side, and the departure degree of Cu-Ag alloys are more than Cu-Sn alloys, while In₃₀Sn₇₀ alloy departs to a little decreasing side. According to the liquid structure model of Frenkel, the temperature dependence of the mean nearest neighbor distance r₁ near the liquidus can qualitatively reflect the above viscosity departure behaviors. Based on the results of Cu-Ag, In₃₀Sn₇₀ and Cu-Sn alloys, we found a simple quantitative relation among the correlation length D, the viscosityηand temperature T, i.e.,η= aD + bT + c, where a, b, c are constants.A new concept, the structural fragility of liquids, was proposed based on different structural changing behaviors with temperature. Different from the fragility concepts of supercooled liquids and superheated melts, we directly use the liquid structural parameters (the correlation length D) instead of viscosity, consequently, to study the fragility of liquids from a more essential property. Synchronously, to quantify the structural fragility, a structural fragility parameter, F_s, is defined as,where T^* is the liquid-solid transition temperature and D^* is the correlation length at T^* The correlation between the new concept and the GFA of some binary alloys was discussed. The results show that the structural fragility is closely related to the GFA of liquids, a smaller F_s corresponds to a higher GFA.