The Effect Of In-situ Formed Phase On The Liquid-liquid Separation In Al-Bi Immiscible Alloy

Immiscible alloys have been investigated with special metallurgy characteristic for decades.Many kinds of them have great potential applications in aerospace,automobile,electronic industry,etc.It is very difficult to obtain homogeneous microstructure by conventional casting method due to the large density difference between the minority phase and matrix.The mechanical properties of immiscible alloys are dependent on both the alloy composition and the distribution of dispersive droplets in matrix.Recently,new studies focus on the liquid-liquid phase separation and solidification behavior in the multi-component immiscible alloy.It takes into account the concurrent actions of the nucleation,diffusional growth,Ostwald coarsening,Stokes motion,Marangoni migration and collision-and-coagulation of minority droplets.The formation mechanism of microstructure in the multi-component immiscible alloys receives more and more attentions.This study valuable in both theoretical and practical significance for the development and utilization of these alloys.In this dissertation,we studied the inhibition mechanism on liquid phase separation and properties by adding the third elements into Al-Bi immiscible alloy.During the solidification process,the third elements are added into Al-Bi immiscible alloys to generate the in-situ formed phase,which eliminate the separation of Bi-rich droplets.The addition of elements lead to the refinement of microstructure and improve the mechanical properties.The different morphology of in-situ phase due to alloy composition,and its interaction with Bi-rich droplets has been analyzed.The influence mechanism of the in-situ phase of nucleation,growth,motion and distribution of the Bi-rich droplets has been studied theoretically.Moreover,the influence of the addition proportion and the solidification rate on the solidification microstructure have been analyzed.Also,the mechanical properties of Al-Bi-X immiscible alloys were studied.The inhibition mechanism of liquid phase separation and properties of Al-Bi-Ti,Al-Bi-B,Al-Bi-RE and Al-Bi-RE-Cu alloys were discussed in detail.The main research results are shown as following:(1)Adding the third elements result in the in-situ formation of Al3Ti,Al2B,Bi2RE particles,which have the effect to inhibit the sedimentation of Bi-rich droplets.The size of Bi-rich droplets were refined with a dispersive distribution.Because of the different mechanism of interaction with Bi-rich droplets,Al-Bi-X immiscible alloys with the different Bi-rich droplets sizes have been fabricated.(2)The model of interaction between in-situ particles and Bi-rich droplets in Al-Bi-X immiscible alloys have been established.The in-situ formed Al3Ti particles were dispersively distributed in the matrix,which have been used to impede the sedimentation of Bi-rich droplets.The Al3B particles have been in-situ formed and dispersively distributed in the matrix,which is used to accelerate the nucleation of Bi-rich droplets.The Bi2RE particles have been wetted by Bi-rich droplets and acted as heterogeneous nucleation sites to improve the nucleation rate.According to the wettability of in-situ formed particles,the order of refinement effect of in-situ formed particles is Bi2RE,Al2B,Al3Ti particles.The numerical calculations of Al-Bi-RE alloys have been performed to indicate the development of the distribution of Bi-rich droplets in different time.(3)We have analyzed the effect of in-situ formed particles on the microstructure and the wear properties of Al-Bi immiscible alloys.The particles were in-situ formed by adding the third elements,and these particles improve the uniform distribution of Bi-rich droplets and reduced the coefficient of friction of Al-Bi alloys.Meanwhile,they have been main carriers in the matrix and reduced the wear rate of Al-Bi alloys.With the Addition of Cu into Al-Bi-RE alloys,the microstructure and the wear resistance of Al-Bi-RE-Cu alloys have been investigated,which achieved the desired properties for developing the high-performance self-lubrication bearing materials.