Dynamic Behavior Of Liquid Phase Separation And Segregation In Al-Bi-(Sn) Immiscible Alloys

Immiscible alloys are well suited as structural and functional materials, such as self-lubrication materials, superconducts, electrical contacts and solder, etc. As there is a liquid phase separation process in solidifying immiscible alloys, this kind of alloy easily forms seriously segregated or even layered structure of little use for industry applications. In order to solve this issue, the liquid phase separation mechanism and segregation evolution in aerodynamically levitated Al–Bi immiscible alloys was discussed. Full views of segregation evolution in Al–10 wt.% Bi immiscible alloys were also obtained by means of synchrotron radiation X-ray imaging technique, and the dynamic behavior of minor droplets and bubble-induced segregation were analyzed. Minor droplet nucleation as well as its structure was investigated through synchrotron radiation small angle X-ray scattering(SAXS) technique. Based on the above research, the Al-Bi-Sn core-shell structure and crescent structure have been revealed. The main results are as follows:1. When the Al-Bi alloy melt is levitated stably under aerodynamic condition, it can rotate around its vertical axis, and thus the centrifugal force will drive the minor droplets to migrate along radial direction horizontally. The coarsening degree of minor droplets can be described by the dimensionless average droplet radius:, where and are the average droplet radius at t = 0 and time t, respectively; N0 the initial droplet number at t = 0; Y is the collision volume. The liquid phase separation process in solidifying immiscible alloys is controlled by Marangoni, Stokes and centrifugal motions at different stages. Irrespective of compositions, if the solidification time is permitted, the Al/Bi core–shell structure can be formed finally.2. The synchrotron radiation X-ray imaging results directly confirm that surface segregation occurs before the liquid decomposition. Due to Soret effect, the Bi atoms settle down to the bottom part of the alloy melt. The Marangoni motion dominates at the initial stage of the liquid phase separation while the Stokes motion plays a key role in the later stage when the droplets have grown into the lar_ge sized. The distribution of Bi droplets agrees with the Gaussian type under the higher cooling rate while it changes from the Lifshitz-Slyozov-Wagner(LSW) diffusion-controlled to a superimposed congregation type under the lower cooling rate. And a logistic model can well describe the growth dynamic behavior of Bi droplets.3. The synchrotron radiation SAXS results show that the liquid phase separation results from a nucleation and growth process rather than spinodal decomposition mechanism. The liquid decomposition process has fractal characteristic, and the nanometer-sized Bi-rich droplets in Al matrix melt present mass fractal structure, its fractal dimension values are located in the range of 2.39~2.56.4. The dynamic behavior of segregation driven by bubble evolution in solidifying Al-10 wt.% Bi alloy was caught by synchrotron radiation X-ray imaging technique. Thermodynamic calculating indicated that, in order to decrease the total free ener_gy, both the Bi atoms from the Al-rich liquid and the already existing Bi droplets can approach the outer surface of bubbles. However, under the present conditions, there is no movement of the Bi droplets toward to the bubble because the temperature gradient between the melt around bubble and other surrounding areas is so small that it is negligible. So, the Marangoni motion of Bi droplets would be insignificant. The trapped Bi atoms do not change the stochastic growth of bubbles, but do change the scaled size distribution of bubbles into a type neither of stochastic nature with Gaussian distribution nor of LSW-diffusion dominated mechanism.5. The synchrotron radiation X-ray imaging results that, positive segregation and Marangoni motion can cause the Bi solute appear in the upper part of the sample after the solidification of the Al-10 wt.% Bi alloy in this thesis.6. Surface segregation, Soret effect, thermal Marangoni motion, solutal Marangoni motion, and Stokes motion play a key role in the formation of Al/Bi–Sn core-shell structure. If the alloy melts solidify on the condition that the radius(r) of the Bi-rich minor droplets is smaller than a critical value(r_g), it will form Al/Bi–Sn core-shell structure, otherwise, the crescent structure will be formed.