EFFECT OF MICRO-ALLOYING ELEMENT Bi ON SOLIDIFICATION AND MICROSTRUCTURE OF Al-Pb ALLOY
Qian SUN1,2,Hongxiang JIANG1,2,Jiuzhou ZHAO1,2
1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Cite this article:
Qian SUN,Hongxiang JIANG,Jiuzhou ZHAO. EFFECT OF MICRO-ALLOYING ELEMENT Bi ON SOLIDIFICATION AND MICROSTRUCTURE OF Al-Pb ALLOY. Acta Metall Sin, 2016, 52(4): 497-504.
Monotectic alloys are characterized by a miscibility gap in the liquid state. Many of them have great potentials to be used in industry. For example, alloys based on Cu-Pb and Al-Pb are good candidates to be used as advanced bearing materials if the soft Pb phase is dispersed in the Al or Cu matrix. Cu-Cr alloy is a high-strength, high conductivity material and Cu-Co alloy is an excellent magneto-resistive material, etc.. However, when a homogeneous monotectic alloy melt is cooled into the miscibility gap, it will transform into two liquids. The liquid-liquid decomposition generally causes the formation of a phase segregated microstructure. In recent years, considerable efforts have been made to investigate the solidification behavior of monotectic alloys. A lot of experiments have been carried out under microgravity conditions in space as well as under the gravitational conditions on the earth. The solidification behaviors of monotectic alloys under the conventional and rapid solidification conditions as well as the effect of external fields, such as electric current, magnetic field etc., are investigated. Models describing the solidification process have been built and the microstructure formations under different conditions have been calculated. It has been demonstrated that the microstructure evolution during cooling an alloy in the miscibility gap is a result of the concurrent actions of the nucleation, growth, Ostwald ripening and motions of the dispersed phase droplets. The nucleation of the dispersed phase droplets has a dominant influence on the solidification microstructure of monotectic alloys. In this work, solidification experiments were carried out to investigate the effect of micro-alloying element Bi on the solidification of Al-Pb alloys. The experimental results demonstrate that micro-alloying element Bi can cause an obvious refinement of the Pb-rich particles. The refining effect increases with the increase of the Pb content of Al-Pb alloys. The affecting mechanism of micro-alloying element Bi on the solidification process of Al-Pb alloys was analyzed. The microstructure formation process was calculated. The numerical results indicate that the addition of micro-alloying element Bi causes a reduction in the interfacial energy between the two liquid phases and, thus, enhances the nucleation rate of the Pb-rich droplets and promotes the formation of Al-Pb alloys with a well-dispersed microstructure.
Fig.1 SEM images of Al-7%Pb-xBi alloys with x=0 (a), x=0.05% (b) and x=0.10% (c) continuously solidified at the solidification rate of 10 mm/s
Fig.2 2D size distributions of Pb-rich particles in Al-7%Pb-xBi alloys with x=0 (a), x=0.05% (b) and x=0.10% (c) continuously solidified at the solidification rate of 10 mm/s
Fig.3 SEM images of Al-5%Pb-xBi (a, b) and Al-9%Pb-xBi (c, d) alloys with x=0 (a, c) and x=0.10% (b, d) directionally solidified at the solidification rate of 10 mm/s
Fig.4 Average 2D diameters of the primary Pb-rich particles in Al-Pb alloys with an addition of 0.10%Bi vs Pb content
Fig.5 Schematic of the variations of the interfacial energy σ between the two liquids (dash line) and the composition difference Cβ-Cm between the two liquids (solid line) with temperature (Cβis the molar concentration of the solute in the dispersed phase droplets, Cm is the molar concentration of the solute in the matrix liquid, T is thermodynamic temperature and Tc is the critical temperature of the system)
Fig.6 σ between the two liquid phases for Al-Pb and Al-Bi [34] alloys
Fig.7 Schematics of surface zones formed by the gathering of micro-alloying element Bi (a) and the distribution of element Bi in the melt (b) (C represents the concentration of micro-alloying element Bi and CBiinteris the concentration of micro-alloying element Bi at the interface between the two liquids; A/B are the boundaries between the surface area and the Al matrix/Pb-rich phase; S is the interface between the two liquids)
Fig.8 Temperature profile in front of the solidification interface along the central z axis of the sample solidified at the solidification rate of 10 mm/s
Fig.9 Nucleation rate I (solid lines), the number density N (dash lines) and the 2D average radius <R> (dot dash lines) of the dispersed phase droplets as a function of position in front of the solidification interface for Al-5%Pb alloy (black lines), and Al-5%Pb-0.10%Bi alloy (red lines)
Fig.10 Under-cooling of the matrix liquid ΔT (dot dash lines) and I of the Pb-rich droplets (solid lines) as a function of position in front of the solidification interface for Al-5%Pb alloy (black lines) and Al-5%Pb-0.10%Bi alloy (red lines)
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