Liquid Phase Separation Mechanism And Microstructure Control Of The Immiscible Cu-Pb、Cu-Fe Alloy

Owing to the frequent occurrence of liquid phase separation during the cooling process,immiscible alloys,in its solidified state,are easily subjected to severe reduction in value,which greatly limits the application and development of such alloys.Based on the different characteristics of the liquid phase separation of the alloy melt,the immiscible alloys fall into two types: one is the steady-state immiscible alloy represented by the monotectic alloy,and the other is the metastable immiscible alloy represented by partially peritectic alloy.Among them,Cu-Pb monotectic alloy,as a kind of steady-state immiscible alloy has received extensive attention in various fields due to its excellent self-lubricating properties;Cu-Fe alloy,a typical metastable immiscible alloy,has attracted widespread attention due to their excellent electrical conductivity and low cost.In this paper,Cu-40 wt.%Pb,Cu-15 wt.%Fe,and Cu-35 wt.%Fe are selected as the research object.A combination of glass fluxing technique and cyclic superheating method is used to study the liquid phase separation process and the evolution mechanism of the solidification structure.At the same time,the control of the solidification structure of the immiscible alloy is achieved by adding a third component and applying a pulse current.The research results obtained are as follows:The Cu-40 wt.%Pb hypermonotectic alloy specimens were produced by a rapid quenching method using copper molds.Combined with energy spectrum analysis and related theories,it was concluded that in the initial stage of liquid phase separation,L₁ Cu-rich phase was nucleated from the alloy melt as the primary phase.In the initial nucleation stage of the liquid phase separation of immiscible alloys,the nucleation of L₁ and L2 phases is a rapid succession process.That is,the nucleation of one phase will inevitably trigger that of another phase.For Cu-15 wt.%Fe peritectic alloys,L2 Fe-rich phase was nucleated from the alloy melt as the primary phase.The nucleation mechanism is completely different from that of Cu-Pb monotectic alloys.The samples of Cu-40 wt.%Pb,Cu-15 wt.%Fe,and Cu-35 wt.%Fe alloys with different degrees of undercooling were produced respectively by a combination of glass fluxing technique and cyclic superheating method.The results show that degrees of undercooling exert great influence on the solidification structure of the immiscible alloy,and at the same time,the change of the alloy composition will largely affect the solidification microstructure.Combined with theoretical analysis,it is concluded that Ostwal ripening is the main mechanism leading to the coarsening of crystals of a few phases in the initial stage of liquid phase separation,that Brownian movement and Marangoni migration are the main mechanisms leading to the coarsening of crystals of a few phases in the middle stage of liquid phase separation,and that at the later stage of liquid phase separation,Stokes movement serves as the dominant factor in the coarsening,segregation and even delamination of crystals of a few phases.Ostwal ripening,Brownian movement,Marangoni migration and Stokes movement all go through the whole process of liquid phase separation,only the roles they play at different phases being different.Through the addition of the third component Ni to the Cu-40 wt.%Pb monotectic alloy and the Cu-15 wt.%Fe peritectic alloy,as well as the introduction of pulse current to the Cu-40 wt.%Pb monotectic alloy during solidification process,their effects on the solidification structure of an immiscible alloy are investigated.The results show that the addition of the third component Ni greatly changes the solidification microstructure of the immiscible alloy.For the Cu-40 wt.%Pb monotectic alloys,the addition of the third component Ni,causes the second phase grains in the alloy melt to become significantly finer and diffusely distributed in the matrix,and at the same time,with the increase in the amount of the third component Ni,the range of the homogenization degree of undercooling of Cu-40 wt.%Pb alloy also expands significantly.The refining mechanism mainly includes: the influence of the third component Ni on the interfacial energy,the second phase volume fraction and the lattice structure of the alloy melt.For the Cu-15 wt.%Fe peritectic alloy,the addition of the third component Ni results in a complete change in the solidification microstructure of the alloy melt,and with the increase of the degree of undercooling,the solidification structure becomes growingly finer.Combined with XRD analysis,the addition of Ni results in the precipitation of α-Cu phase and Fe3Ni2 phase in the alloy melt,and lattices of the two phases are the same,all belonging to fcc structure,thus enhancing the combination of the two phases.By an analysis of the refining mechanism,it is believed that the granulation of the first granular crystals in the melt is mainly due to the dendrite melting mechanism,while the mechanism that leads to the formation of the second type of granular crystals is mainly the dendrite breakingrecrystallization mechanism.As for the effect of pulse current on the solidification process of Cu-40 wt.%Pb monotectic alloy,the result is clearer.The pulse current has a significant refining effect on the solidification structure of the immiscible hypermonotectic alloy Cu-40 wt.%Pb.With the increase of pulse current,the effect becomes more obvious.The reason is mainly due to the change of the liquid structure of the alloy melt with the introduction of the pulse current.The specific results are: the activity of the internal components of the melt is reduced,and the medium and small sizes of the solvent clusters increase,eventually resulting in the refinement of the solidification structure of the Cu-40 wt.%Pb hypermonotectic alloy.From another point of view,the introduction of the pulse current increases the degree of undercooling of the Cu-40 wt.%Pb alloy melt.The greater the pulse current is,the greater the degree of undercooling of the alloy melt is.In essence,the introduction of the pulse current has changed the liquidphase separation property of the Cu-40 wt.%Pb hypermonotectic alloy.Like the conventional alloy,its solidification structure is significantly refined as the degree of undercooling increases.A test of the hardness and wear resistance of Cu-40 wt.%Pb hypermonotectic alloy sample subjected to pulse current treatment shows that the introduction of pulse current can refine the microstructure of Cu-40 wt.% Pb monotectic alloy and optimize the self-lubricating properties of the material.It is of guiding significance for the study of the practical production of the stable-state immiscible Cu-Pb monotectic alloy.