| Cu-Fe alloys possess a metastable miscibility gap below the liquidus temperature and belong to metastable immiscible alloys.Cu-Fe alloys inherit both characteristics of its constituent elements copper and iron,have many advantages,e.g.high electrical conductivity and thermal conductivity,good ductility and electromagnetic compatibility(EMC),low cost,etc.Therefore,Cu-Fe alloys have a wide industrial application foreground.However,the alloys show a strong tendency to form spatial phase separation and large-scale composition segregation under conventional solidification process.The present preparation of Cu-Fe alloys with high Fe content(≥15wt.%)rely on mechanical alloying and vacuum arc-melting.Ascribed to the limited as-prepared sample size and complicated process,their industrial application is greatly restricted.Until now,the available investigations of Cu-Fe alloys principally focus on the solidification behaviour and liquid-liquid phase separation under microgravity or highly undercooling condition.However,such deep understanding and effective controlling methods of segregation behavior and corresponding formation reason during conventional solidification process are still in lack.In addition,the current available investigations on magnetic properties of Cu-Fe alloys were only carried out for the samples prepared by mechanical alloying.There are few studies on magnetic properties of Cu-Fe alloys prepared by casting method.To address the above issues,the formation mechanism and reason of liquid-liquid phase separation and strong tendency of large-scale composition segregation in Cu-Fe alloys from both perspective of thermodynamics and melt structure factors has been uncovered and elucidated.The microstructure evolution of Cu-Fe alloys during melting process was also clarified,and a thermodynamic model has been put forward to uncover the surface energy driven melting process.In addition,the common used graphite mold and copper mold in the field of copper alloy industrial preparation was applied to investigate the influence of cooling rate on solidification behavior and microstructure evolution of Cu-Fe alloys under conventional solidification process by means of induction melting,and the corresponding mechanisms were also intensively analyzed and uncovered.The correlation between the microstructure and magnetic properties of immiscible alloys was also investigated and analyzed.The main results of present paper are as follows:(1)Whether from the calculation of thermodynamic perspective including mixing enthalpy and excess Gibbs free energy or from the perspective of alloy melt structure factors including the calculation of Pair Correlation Function(PCF),Coordination Numbers(CN)and Bhatia-Thornton(B-T)structure factors,the Cu-Fe alloys demonstrate a strong and apparent demixing tendency.As a result,Cu-Fe alloys show a strong tendency to form spatial phase separation and large-scale composition segregation under conventional solidifications process.The microstructure evolution of Cu-Fe alloys during melting process was clarified by means of vacuum arc-melting with different melting times.More importantly,results indicated that to obtain the homogeneous liquid melt was of vital importance and prerequisite for the preparation of uniform Cu-Fe alloys.Herein a thermodynamic model was constructed to discuss and uncover the surface energy driven melting process based on experimental results.Eventually,a uniform Cu-Fe alloy,which possesses a maximum diameter of 35mm and Fe content as high as 40wt.%,without the occurance of liquid-liquid phase separation was successfully prepared based on the above constructed thermodynamic dissolution model.(2)There is a significant difference in solidification microstructure of Cu-Fe alloy under different solidification conditions.In the present study,for alloy solidified in graphite mold with cooling rates of 40±5K·s-1 and 95±10K·s-1,not only liquid-solid transformation primarily takes place,but also liquid-liquid phase separation including primary and second liquid-liquid phase separation occurs.The occurrence of liquid-liquid phase separation can be ascribed to the coaction of constitutional undercooling and unique melt structure of Cu-Fe alloy.While for alloy solidified in copper plate mold with cooling rate of 330～580K·s-1,only liquid-solid transformation occurs.In addition,the morphology of Fe-rich dendrite changes from developed shape into cellular shape with the increase of cooling rate,accompanied by the increasing of its number and decreasing of its size.While remarkable "solute trapping"can also be found in Fe-rich dendrites at relative rapid solidification process.(3)The cooling rate can significantly influence primary liquid-liquid phase separation and second liquid-liquid phase separation which occurs in the phase-separated Fe-rich spherulites.It was demonstrated that as the increase of cooling rate from 40±5K·s-1 to 95±10K·s-1,the average size decrease from 14.8μm to 5.2μm,and the size distribution of bimodal characteristic peaks of primary phase-separated Fe-rich spherulites decreases from 7.5μm and 21.3μm to 3.3μm and 9.5μm;while the Cu concentration of phase-separated Fe-rich spherulites decreases from 40～50wt.%to 30～40wt.%.In addition,the interior morphological pattern of minority Cu-rich phase in primary phase-separated Fe-rich spherulites can be greatly influenced by cooling rate due to the dynamic coupling between thermodynamic and kinetic effects.Such a clear experimental observation of dynamic microstructure evolution for minority Cu-rich phase provides a strong and visualized evidence for the asynchronous crystallization behavior of primary phase-separated Fe-rich spherulites during solidification process.(4)The self-driven second liquid-liquid phase separation was investigated and disclosed by means of experimental observation and phase field simulation.Various morphological patterns of minority Cu-rich phase in primary phase-separated Fe-rich spherulites were observed,which discloses the dynamic evolution process during self-driven second liquid-liquid phase separation.The Marangoni migration is believed to be the dominant motion that makes the minority Cu-rich phase move radial inward to center of primary phase-separated Fe-rich spherulites.Moreover,multiple mechanisms such as diffusion-controlled growth,coalescence and coagulation,and Ostwald ripening,etc.are the dynamic mechanisms mainly responsible for various morphological patterns of minority Cu-rich phase after phase separation.In addition,phase field simulation was also performed to reveal the dynamic evolution of minority Cu-rich phase during liquid-liquid phase separation,which is beneficial for the deep understanding of liquid-liquid phase separation process of immiscible alloys.(5)Experimental result indicated that both the cooling rate during solidification process and the composition of metastable immiscible Cu-Fe alloys greatly influenced its resultant magnetic properties.When the cooling rate is relative slow,the Cu-Fe alloy is more likely to precipitate α-Fe particles,which results in obvious improvement in saturated magnetization.It was found that the interior morphological pattern of minority Cu-rich phase in primary phase-separated Fe-rich spherulites could be greatly influenced by cooling rate.Therefore,the coercivity of the Cu-Fe alloys corresponding changes.In addition,with the increasing Fe content from 10wt.%to 40wt.%,the saturated magnetization of Cu-Fe alloy monotonously but not linearly increases from 15.1emu·g-1 to 74.8emu·g-1,while the coercivity slightly and non-monotonically changes.(6)After annealing treatment(950℃,4h),a considerable amount of dual-scale nano-sized Cu-rich phases precipitate from Fe-rich dendrite(9R twinning structure Cu-rich phases with size of 15.3±0.8nm and FCC structure Cu phase with size of 90±5nm).These precipitated Cu-rich phase certainly would impose a strong pinning on domain walls and tend to restrict their motion during magnetic reversal process,thus consequently leading to the increase of coercivity after annealing treatment.Meanwhile,many Fe-rich nano-particles(α-Fe)are also observed in Cu matrix,which is believed to be the sole reason for the enhancement of saturated magnetization.In general,it was demonstrated that the soft magnetic property of Cu-Fe alloys could be enhanced via annealing treatment with improving the saturated magnetization and slightly deteriorating coercivity. |
PDF Full Text Request