Study On Liquid Phase Separation Behavior Of Cu-Fe Alloy And Cu/Fe Bimetal Composite

Cu-Fe alloy belongs to metastable immiscible alloy,and there is a metastable immiscible gap under the liquidus of binary equilibrium phase diagram.In the solidification process of the alloy,the liquid phase separation occurs when the melt is subcooled to the metastable immiscible gap.Foreign countries have studied the liquid-phase separation of Cu-Fe alloys by rapid solidification.However,few studies have reported the effects of alloying elements on the liquid phase separation of Cu-Fe alloys.Earlier studies in this paper found that the addition of element C will cause liquid phase separation of Cu-Fe alloys.Since the occurrence of liquid phase separation has a significant impact on the microstructure and properties of the alloy,it is necessary to study the influence of C on the behavior of liquid phase separation of Cu Fe alloy,which has a great significance to the actual production of Cu-Fe alloy and the development of new materials.Based on the previous research,the Cu-14Fe-xC alloy with different C content(x=0,0.05,0.2,0.7 wt.%)has been prepared by vacuum melting casting.The microstructure and properties of Cu-14Fe with C addition are researched.Combined with the phase diagrams of Cu-Fe binary alloy and Cu-Fe-C ternary alloy,the solidification behaviors of Cu-14Fe alloy and Cu-14Fe-C alloy are analyzed in depth.The mechanism of microstructure evolution of the alloy after the addition of C element is further revealed,and the relationship between the amount of C element added and the microstructure and properties of the alloy is clarified.Finally,the Cu-Fe-C bimetal composite was prepared by centrifugal casting technology,and its microstructure and microhardness were analyzed.The main conclusions are as follows:1.The solidification mode of Cu-14Fe alloy is normal liquid-solid transformation.In the solidification process,the primary Fe phase first precipitates from the alloy melt and grows in the form of dendrite.The final solidification structure of the alloy is mainly composed of Cu-rich matrix and dendrite Fe-rich second phase,among which the dendrite with the size between 2-7 ?m accounts for the most,and the average size is 3.86?m.The immiscible region of Cu-Fe alloy changes from metastable state to steady state after adding trace C element,which makes it stable under the liquidus.Therefore,in the process of preparing Cu-14Fe-C alloy by conventional vacuum melting and casting method,when the alloy melt is cooled to the stable immiscible gap area,liquid phase separation will occur,and the homogeneous melt will be decomposed into two different components of Cu-rich phase and Fe-rich phase.The separated Fe-rich phase is in the form of small spherical droplets,which collide and coalesce under the action of Brownian motion and become larger spherical droplets.Large scale Fe rich globular droplets continue to collide and coalesce under the action of Marangoni migration and Stokes motion,and become larger globular droplets.The final solidification structure of the alloy is mainly composed of Cu rich matrix and spherical granular Fe rich second phase.The size of the second Fe rich phase is mainly determined by the time of collision and coalescence of the Fe rich phase droplets after the liquid separation of the melt.The more C element is added,the larger the temperature range of the immiscible gap in the Cu-Fe-C alloy is,the longer the collision and coalescence time of the Fe-rich droplets after the liquid phase separation occurs in the alloy melt,and the larger the size of the spherical Fe-rich second phase in the final Cu matrix.2.The addition of C element can reduce the maximum solid solubility of Fe in Cu of Cu-Fe alloy.The more C element is added,the smaller the maximum solid solubility of Fe in Cu is.However,with the further increase of C element,the decreasing trend of solid solubility of Fe in Cu becomes slower.With the decrease of the maximum solid solubility of Fe in Cu,the volume fraction of Fe-rich second phase in the matrix increases,and the strengthening effect of the second phase is strengthened.The microhardness of Cu-14Fe-C alloy is obviously improved.When the content of C element continues to increase,the Fe-rich second phase grows and aggregates excessively,which leads to the weakening of the strengthening effect of the second phase and the decrease of the hardness of the alloy.The conductivity of Cu-Fe alloy is closely related to the scattering resistance of impurities in Cu matrix.The addition of C element reduces the maximum solid solubility of Fe in Cu,decreases the content of Fe atom in Cu matrix,reduces the scattering effect of impurities,and greatly improves the conductivity of the alloy.When the addition of C element continues to increase,the electric conductivity of the alloy will decrease because the size of the second phase is too large,which seriously hinders the movement of electrons in the matrix.3.During the process of centrifugal casting,the Cu-rich phase of Cu-Fe-C alloy melt will segregate toward the outer layer of the metal cavity under the action of centrifugal force,and the Fe-rich phase will move toward the inner layer of the metal cavity.Segregated to form a microstructure with Cu-rich phase in the outer layer and Fe-rich phase in the inner layer.However,at the initial stage of pouring,the alloy melt that has just been poured first contacts lower temperature outer wall of the cavity.The Fe-rich phase can not move relative to each other under the effect of chilling,and it first solidifies and precipitates on the outer wall.An Fe-rich phase structure is formed near the outer wall.Therefore,the centrifugal casting Cu-Fe-C alloy composite mainly consists of the outermost Fe-rich phase,the middle Cu-rich phase,and the inner Fe-rich phase.The hardness test found that the hardness of the outermost Fe-rich phase and the inner-rich Fe-rich phase was between 750-800 HV,the hardness of the intermediate Cu-rich phase was between 80-100 HV.