Liquid Phase Separation and Regulation of Solidification Structures of Aluminum-based Immiscible Alloys

Author:Jia Peng

Supervisor:hu xun geng hao ran


Degree Year:2019





In this paper,the resistivity and DSC method was used to investigate the nature,the characteristics of thermodynamics and kinetics of liquid-liquid structure transition of Sn–Bi alloys.In addition,the effect of liquid-liquid structure transition on solidification process and solidfied structures of Sn–Bi alloy was studied.The resistivity method was used to investigate the composition dependence of clusters in Cu–Sn alloys.The La element was added into Al–25Bi immiscible alloy to suppress its macrosegregation.The liquid-liquid phase separation and solidification process of Al75Bi9Sn16 immiscible alloys were studied.The effect of melt superheating treatment on its solidification structure was investigated.The size-dependence of the morphology of Sn–Bi-rich minority droplets was also studied.The effect of composition on the solidification structure of Al–Bi–Sn immiscible alloys was studied.The effects of composition,copper roller speed and melt superheating treatment on the thickness,width,phase composition and microstructures of Al–Bi–Sn alloy ribbons were studied.The detailed contents are as follows:(1)The thermodynamic and kinetic characteristics of liquid phase transition(LPT)were investigated using resistivity method and DSC method in Sn50Bi50 alloy.The results showed that the first-order liquid phase transition was completed in Sn50Bi50 melt and controlled by temperature and the concentration of the substrate.The nature of transition was the rearrangement of residual Sn\\Sn covalent bonds and the release of free electrons.The Kissinger method,Ozawa method and Doyle method are suitable for the investigation of LPT.In addition,a facile strategy was established for investigating the thermodynamic and kinetic characteristics of the LPT.(2)The liquid–liquid structure transition(L-LST)of Sn75Bi25 alloy were studied via two direct methods such as a high temperature X-ray diffractometer and a modified high temperature and hall effect instrument as well as several in direct methods including resistivity,density,viscosity and thermal analysis methods to present the essence,thermodynamics and dynamics of liquid-liquid structural transition(L-LST).The results indicated that the reversible first-order liquid–liquid structure transition did occur in the range from 990 to 1068 K during heating and occur in the range from 865 to 945 K during cooling.From the atomic and molecular scale,the nature of the L-LST was the rearrangement of covalent atom clusters during heating.The transformation of liquid-liquid structure reflected the qualitative change of melt structure.Although the structure rearrangement of covalent clusters was remained after liquid-liquid structure transition,which was only a quantitative process of melt structure.From the view of the electronic structure,the nature of the L-LST was the releasement of free electrons and the localized-extended transition of electron.Besides,a novel strategy combining atomic structure and electronic structure was developed to reveal the nature of the L-LST.The influences of the L-LST on the solidification behaviors and morphologies of Sn75Bi25 alloy also were investigated.The results showed that the undercooling of the eutectic phase increased after the L-LST,which was responsible for the drastic decrease in the spacing of the eutectic phase.(3)The liquid structure of CuxSn100-x(x=0,10,20,33,40,50,60,75,80 and 100)alloys with atom percentage were investigated with resistivity and viscosity methods.It can be found from the resistivity data that the liquid Cu75Sn25 and Cu80Sn20 alloys had a negative temperature coefficient of resistivity(TCR),and liquid Cu75Sn255 alloy had a minimum value of-9.24μΩcm K-1.While the rest of liquid Cu–Sn alloys had a positive TCR.The results indicated that the Cu75Sn25 atomic clusters existed in Cu–Sn alloys.In addition,the method of calculating the percentage of Cu75Sn25 atomic clusters was established on the basis of resistivity theory and the law of conservat ion of mass.The Cu75Sn25 alloy had a maximum volume of the atomic clusters and a highest activation energy.The results further prove d the existence of Cu75Sn25 atomic clusters.Furthermore,the correlation between the liquid structure and the resistivity was established.These results provide a useful reference for the investigation of liquid structure via the sensitive physical properties to the liquid structure.(4)The inhibition of gravity segregation has been a long-standing challenge in fabrication and applications of homogeneous immiscible alloys.Therefore,the effect of rare-earth La on the gravity segregation of Al–Bi immiscible alloys was investigated.The results showed that the addition of La can completely suppress the gravity segregation to form the homogeneous immiscible alloys.The formation mechanism of homogeneous immiscible alloys was heterogeneous nucleation of Bi-rich droplets on LaBi2 phase precipitated in situ,forming needle-like LaBi2@Bi composite droplets,which significantly reduced the probability of Bi-rich droplets coalescence and effectively inhibited the growth of Bi-rich droplets(heterogeneous nucleation effect).In the process of settling,needle-like LaBi2@Bi composite phase droplets encountered larger resistance and smaller settling rate(shape effect).In this work,it was pointed out that the shape of composite droplets had an important influence on the formation of homogeneous and immiscible alloys,which was the main reason for the formation of homogeneous and immiscible alloys.It was also pointed out that the volume fraction of LaBi2 in LaBi2@Bi composite droplets significantly affected the macro-segregation of alloys,which provided a new idea for the preparation of homogeneous and immiscible alloys.(5)The liquid-liquid phase separation(L-LPS)and solidification process of the Al75Bi9Sn16 immiscible alloy were studied with calorimetric and resistivity methods to ascertain the melt superheated treatment process.The impact of melt superheating treatment(MST)on the phase constitution and solidification microstructures were investigated using X-ray diffraction(XRD)and field emission scanning electron microscope(FESEM)to determine the structural sensitivity to the melt superheated degree,and find a new strategy for improving the forming ability of the core-shell structure of the Al75Bi9Sn16 alloy.The results show that:resistivity method is a more sensitive,intuitive,and effective method to investigate the liquid-liquid phase separation,and the liquid-liquid phase separation and precipitation of primary(Sn)phase occur in 1039 K–880 K and 460 K–403 K respectively.In addition,the core-shell structure with Sn–Bi-rich core and Al-rich shell can be formed under conventional casting conditions;the melt superheating treatment(MST)can promote the formation of core-shell structure by increasing solidification ti me t0 and decreasing the average solidification r ate v and viscosityη.(6)In this work,the Al100-x(Bi45Sn55)x(x=5,15,25 and 35)alloys with varied elemental compositions were prepared to understand the correlations of the elemental composition with the solidification structures of the alloys.The results showed that the Sfh,Smc,S2c-s(Sn–Bi-rich,Al-rich)and S3c-s(Sn–Bi-rich,Al-rich,Sn–Bi-rich)structures were formed successively with the decreasing aluminum content,due to the increased aggregation degree of droplets.The increased aggregation resulted from(1)the increased width of the miscibility gap(from 0 to 146 K);(2)the increased volume tricfraction of the minority phase particles(from 8.91 to 49.98%);(3)the increased time of the L–LPS(from 0 to 0.46 s);(4)the decreased solidification rate(from 10.2 to 4.6 mm s-1).The exchange of core and shell occurred in as-cast Al–Bi–Sn immiscible alloys.The quantitative relationship between the composition and the collision probability of drop lets was established to reveal the inner reasons for structural evolution.In addition,the hot-spot effect of the lower melting point droplets was responsible for the coarser monotectic structure around the lower melting point particles.The results from this work are a useful reference for regulating structural configuration of immiscible alloys via manipulating the composition.(7)Al85Bi6Sn9,Al75Bi9Sn16,Al60Bi14.4Sn25.6 and Al45Bi19.8Sn35.2 immiscible alloys ribbons were prepared using a single-roller melt-spinning technique.The influences of the composition,the rotating speed of copper roller and the melt superheating temperature were investigated in terms of the thickness,width,phase constitution and microstructures of Al–Bi–Sn ribbons.The results showed that the(Al),(Bi)and(Sn)solid solution phases existed in all the investigated Al–Bi–Sn ribbons.When the rotating speed was larger than2000 revolutions per minute(rpm),the amorphous phase also was formed.The thickness of ribbons increased and the width decreased with the increase in the aluminum content or with the descent in the melt superheating temperature.The grain sizes of Sn–Bi-rich phase and Al-rich phase decreased gradually with the increases in the rotating speed,the aluminum content or the distance to the free surface.Although there was no obvious macro-segregation under the rapid solidification,the micro-segregation still occurred under the rapid solidification.In addition,the grain size of Al75Bi9Sn16 ribbons had a minimum value after the melt superheating treatment at 1183 K.The impact of melt thermal treatment on the microstructure was analyzed from the views of thermodynamics and kinetics.The changes in the grain size and morphology were attributable to the differences in migration rate,solidification rate and the time of liquid-liquid phase separation(L-LPS).The results are conducive for understanding the effects of composition and melt treatment on the microstructures of immiscible alloys.