Study On The Melt Structure And Solidification Process Of Al-In(Bi)-Sn(Sb) Monotectic Alloys

The alloy melt structure has significant effect on the microstructure and properties of the solidified alloy.The resistivity,as one of the sensitive physical properties of the metal melt structure,plays a very important role in the study of the melt structure,which reflects the transport properties of electrons.The change in the internal structure of the alloy melt(composition fluctuation,concentration fluctuation,energy fluctuation)can cause the change of its physical properties,such as resistivity.Concentration fluctuation takes place in monotectic alloy melt during liquid phase separation.With the deepening research of the liquid metal,the liquid phase separation of the monotectic alloys is gradually revealed by measuring the sensitive physical parameters of the melt,from which the liquid separation temperature,the monotectic reaction temperature and the shape of the immiscible area can be obtained.For ternary and above monotectic alloys,the mechanisms of liquid separation are more complex,and the lack of accurate phase diagrams makes it more difficult to study.By measuring the change of physical properties of alloy melt with changing temperature can characterise the liquid phase separation process of ternary monotectic alloy which provides a new effective method for the study of liquid-liquid phase transition of monotectic alloys,and has a positive guidence effect on the preparation of monotectic alloys.In this paper,the resistivity temperature characteristics of Al-In-Sn monotectic alloys and Al-Bi-Sb monotectic alloys were investigated by DC four-electrode method.The non-uniformity of the monotectic alloy melt and phase separation phenomenon were explored,DSC analysis was used to characterise the thermodynamic properties of each component alloy,which can further verify the resistivity temperature characteristics.Based on resistivity experiment results,800?,850?,900?(Al-In-Sn system)and 900?,1000?(Al-Bi-Sb system)above the liquid separation temperature were selected as the melting and casting temperature in the solidification experiment.The effects of melt temperature on the liquid phase separation process and solidification morphology of Al-In-Sn monotectic alloys and Al-Bi-Sb monotectic alloys were investigated.The effects of alloy compositions on solidification morphology of two kinds of ternary monotectic alloys were detailedly explored at a certain temperature.For Al-In-Sn system,the effects of the cooling rate on the solidification process and the solidified morphology were investigated by casting into different molds.The mechanisms of the solidification process were analyzed detailedly by drawing schematic diagrams of the solidification process of the two systems.The macrostructure,microstructure,phase composition and components of the alloy were analyzed by SEM,EDS and XRD methods.The results are as follows:The occurrence of nonlinear changes above monotectic temperature on ?-T curves of ternary Al-In-Sn monotectic alloys consistented with the transition points on the DSC curves in a certain error range which verified the occurrence of liquid phase separation.The two obvious abnormal changes(an upward sharp peak and a convex shape)on ?-T curves of ternary Al-Bi-Sb monotectic alloys at the heating process corresponded to the peaks with unique and different height on DSC curves,which indicates the occurrence of monotectic reaction and liquid phase separation.The changes on ?-T curves and DSC curves of two kinds of monotectic alloys caused by liquid phase separation are attributed to the concentration fluctuation inside the melt,and the addition of Sn and Sb reduced the monotectic reaction temperature of Al-In and Al-Bi systems.Liquid phase separation took place during solidification of Al-In-Sn monotectic alloys located in the immiscible area,core-shell structure could be obtained by casting into a copper mold.The alloy composition affected the solidified morphology by changing the volume fraction of two kinds of enrichment liquid phase in Al-In-Sn monotectic alloys,and the smaller volume fraction enrichment phase is generally formed core.The increase of melt temperature increased the interface energy of the system,increased the temperature gradient,accelerated the Marangoni convection rate of the second phase droplets,the movement time was shortened greatly,which were beneficial to the formation of the regular core-shell structure of Al-In-Sn monotectic alloys.The rapider cooling rate was also beneficial to the formation of core-shell morphology in the solidified structure of Al-In-Sn monotectic alloys.The core-shell structure morphologies with black Al-rich phase wrapped by offwhite Bi-rich phase were obtained through the way of casting the Al-Bi-Sb monotectic alloy melt into a copper mold.The core-shell morphologies were not changed regularly with the composition like Al-In-Sn monotectic alloys that was due to the significant Stokes effect in the Al-Bi-Sb system.What is more,the Stokes effect was different in different compositions which were also relatated to the width of the immiscible area of different Al-Bi-Sb monotectic alloys.The increase of the melt temperature increased the Marangoni convection and the Stokes motion rate of the Al-rich phase in the Al-Bi-Sb monotectic alloy,which was beneficial for Al-rich liquid phase to move to the center of the sample.