Study On Microstructure And Properties Of Cu-Fe-Ag And Cu-Fe-Ag-P Deformation Processed In Situ Composites

Cu-Fe in situ deformation processed composites are of particular interest because of the good strengthening effect of Fe filaments and the relatively low cost of iron. Due to the relatively high solution of Fe in Cu at high temperature, coupled with sluggish kinetics of iron precipitation at low temperature, a large amount of Fe dissolve in Cu matrix, which reduce the electrical conductivity. For that reason, at present many researchers focus much on how to promote the precipitation of Fe from the Cu matrix and optimizing the strength and electrical conductivity of Cu-Fe in situ composites. In our work, we use the multi-alloying method to inhibit the dissolution of Fe in Cu matrix and optimize the microstructure and properties of Cu-Fe alloys. The effect of alloying element on the composite is to be discussed.In this thesis, the technical routine is described as follows. Firstly, we calculate the phase equilibrium thermodynamic of Cu-Fe-Ag and Cu-Fe-Ag-P system using CALPHAD (Calculation of Phase Diagram) technique in order to guide the design of the composition and the heat treatment parameters based on possible phases. Secondly, the Cu-Fe-Ag and Cu-Fe-Ag-P alloys are prepared using the Melt-Cast method and cold drawing process. The effect of Ag and P on microstructure and properties of Cu-Fe in situ composites is carried out. Finally, the homogenization treatment and the aging treatment are carried out to study the microstructure and properties of the Cu-Fe-Ag and Cu-Fe-Ag-P in situ composites. In the work, we use X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), tensile test and resistivity measurement to study the effect of Ag and P element on the microstructure and properties of Cu-Fe in situ composites during solidification, cold drawing, homogenization treatment and aging treatment. The main work and conclusions are described as the following:The calculated Cu-Fe-Ag phase diagrams show that Ag can reduce the maximum solid solubility of Fe in Cu. With an increase of Ag content, the amount of dissolved Fe atoms in Cu decreases gradually.α-Fe phase initially occurs at 500℃and liquid phase occurs at 800℃in Cu-rich corner of Cu-Fe-Ag system. The miscibility gap in the Cu-Fe system produces with presence of Ag. With increasing Ag, the miscibility gap expands to higher temperature zone. The calculated Cu-Fe-Ag-P equilibrium phase diagrams show that when the P content is lower than 0.1at.%, the new phases such as FeP, Fe2P, Fe3P and Cu3P may exist in Cu-rich corner of Cu-Fe-Ag-P system.Ag can refine the primary Fe dendrites in as cast alloys. With increasing Ag content, the average diameter of primary Fe dendrites becomes small. When the content of Ag exceeds 1wt.%,the refinement efficiency is not obvious. The refinement mechanism of the dendrites is that Ag reduces the interfacial energy between Cu andγ-Fe and increase the nucleation ofγ-Fe. It is a fact that the Cu-Fe-Ag alloys facilitate the production of filamentary microstructure during deformation process. At the same deformation, with increase of Ag content, the thickness and spacing size of the filaments decrease.Investigation on the strengthening mechanism of Cu-Fe-Ag and Cu-Fe-Ag-P in situ composites reveals the strength of the composites consists of three parts. The first part is enhanced Hall-Petch strengthening of Fe filaments, which results from the narrowed spacing of deformed Fe filaments due to heavy deformation. The second one is the solution strengthening of Fe and Ag in Cu matrix. The last one is the precipitate strengthening effect. Therefore, the strength of the composite can be described as followed: matrixThe homogenized treatment refines the primary Fe dendrites and promotes the precipitation of secondary Fe particles and nano-scale particles. With an increase of Ag content, the primary Fe dendrites become smaller. At heavy deformation, the homogenized composites can obtain high strength, which results from the interface strengthening caused by finer filament of the composites. The electrical conductivity is increased 4~5%IACS, which is ascribed to the iron precipitate from Cu matrix during homogenization treatment.Investigation on the effect of intermediate aging treatment and final aging treatment on the microstructure and properties of Cu-Fe-Ag and Cu-Fe-Ag-P in situ composites is carried out. For Cu-Fe-Ag, many nano-scale Fe particles (20nm) precipitate during intermediate heat treatment, which leads to the increase of the electrical conductivity (~20%IACS). Ag can promote the nucleation and precipitation of Fe meanwhile the presence of Ag can reduce the thermal stability of Fe filaments, which make the filaments split. For Cu-Fe-Ag-P, many nano-scale Fe3P (50nm) andα-Fe particles (20nm) precipitate from Cu matrix during final heat treatment, which makes the electrical conductivity increased 18%IACS. The strength of the composites slightly reduces due to the occurrence of the recovery and recrystallization in matrix during aging process.The ASAXS( Anomalous small angle X-ray scattering) experiment is carried out to study the precipitation of Fe in Cu-0.6Fe and Cu-0.6Fe-0.5Ag at 500℃. The kinetics of Fe precipitates is obtained: R1=0.08t1/2 and R2=0.09t1/2 , which reveals that the presence of Ag can promote the nucleation of Fe during isothermal aging treatment.