Study On High Strength And High Conductivity Deformation Processed Cu-Fe-Ag In Situ Composites

Deformation processed Cu-Fe in situ composites have attracted much interest because of the good strengthening effect of Fe filaments and the lower cost of Fe. Due to the relatively higher solubility of Fe in the Cu matrix and the particularly harmful effect of the dissolved Fe on the conductivity of Cu, namely 9.2μ?·cm/wt%Fe, the Cu-Fe in situ composites usually have a conductivity less than 40%IACS, which can be partly improved through long-time heat treatments, while dramatic loss of the composites strength occurrs at the same time. Therefore, in many previous researches much focus had been put on optimizing the strength and conductivity of Cu-Fe in situ composites.In this thesis, the general train of thoughts lies in two aspects, one is to reduce the maximum solubility of Fe in the Cu matrix at high temperature and the other is to accelerate the precipitation kinetics of dissolved Fe from the Cu matrix during the intermediate heat treatment. Ag was selected as the third alloying element based on theoretical analysis of interactions between elements, first principle calculation and many previous researches on binary Cu-Fe in situ composites. Three Cu-Fe-Ag in situ composites, Cu-14Fe-1Ag, Cu-14Fe-3Ag and Cu-11Fe-6Ag were prepared using"melt and drawing"process, and binary Cu-12Fe was also prepared along with Cu-Fe-Ag for comparison purpose. Effect of Ag on the microstructure and properties of Cu-Fe in situ composite during the solidification, cold drawing and intermediate heat treatment were studied through X-Ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile test, Vickers hardness and resistivity measurement. The main results of the research are listed below:Theoretical calculation base on first principle theory revealed that in a system composed of 32 Cu atoms, energy increment of the system is 760eV and 320eV respectively after one Cu atom was replaced by one Fe or Ag atom, which indicates that competitive dissolution exists in the Cu matrix between Fe atoms and Ag atoms, and Ag atoms are in the favorable situation.Presence of Ag can reduce the maximum solubility of Fe in the Cu matrix at high temperature during the solidification, accelerate nucleation ofγ-Fe from molten Cu and refine the Fe dendrites as well. Fe content in the matrix decreases with the increasing content of Ag and in the matrix of Cu-11Fe-6Ag it decreased to 2.5%, which is 45% lower than that in Cu-12Fe.Investigation on the strengthening mechanism of Cu-Fe-Ag reveals that strengthening effect of Ag lies in two aspects, one is the solution, precipitation and filament strengthening of Ag itself, and the other is enhanced Hall-Petch strengthening of Fe filaments, which results from the narrowed spacing of defromed Fe filaments due to the refinement effect of Ag on the as-cast Fe dendrites. Atη=6.1 the increment in theHall-Petch strength resulting from Ag is about 200MPa. Based on the relevant researches on deformed Cu-Nb in situ composite and microstructure and property investigation in the experiment, a physical model for strengthening mechanisim of heavily drawn Cu-Fe-Ag in situ composite has been developed. In the model, strength of Cu-Fe-Ag is considered as combination of the Hall-Petch strengthening effect of deformed Fe filamentsσH?P, strengthening effect of Fe ?σFe?Min the form of dissolution and precipitation and strengthening effect of Ag ?σAg, that isσ=σH ? P+?σFe?M+?σAg, WhereσH ? P= 1319×λ?1/2,λis the average spacing between Fe filaments; ?σFe ?M=57×CF1e/2, CFe is the Fe content in the matrix excluding that in the form of filaments; and as the Ag content CAg 6%, deformed Ag filaments contribute much to the total strength, and ?σAg reaches 340MPa and 670MPa at draw ratioη=6.1 and 7.5 respectively in Cu-11Fe-6Ag.Investigation on the effect of annealing temperature on the microstructure and properties of the matrix of Cu-Fe-Ag composites revealed that Ag can accelerate the precipitation kinetics of dissolved Fe from the Cu matrix and lower the thermal stability of Fe filaments. The mechanism behind which lies in that the higher interface energy between Ag precipitates and the matrix is favourable for the nucleation ofγ-Fe, and the increased diffusion coefficient of Fe in the Cu matrix due to Ag addition.Effect of annealing temperature, time and draw ratio on the mechanical and electrical properties of Cu-Fe-Ag in situ composite was investigated and a new thermo-mechanical treatment process was put forward. The remarkable merit of the process is that stepped annealing treatment and cold deformation are properly combined, which permits relatively thorough precipitation of Fe on one hand and retains the strength resulting from the cold deformation on the other hand, leading to a good combination of strength and conductivity. A new type of high strength and high conductivity Cu-11Fe-6Ag in situ composite was successfully prepared using the above intermediate heat treatment and the strength/conductivity of Cu-11Fe-6Ag obtained 1020MPa/70.5%IACS, 1140MPa/68.8%IACS, 1260MPa/63.8%IACS and 1500MPa/53%IACS atη=6.9, 7.6, 8.6 and 9.2 respectively. The combination property of Cu-11Fe-6Ag is much better than that of Cu-Fe composites reported and near that of Cu-24Ag, whose materials costs is 3 times higher than Cu-11Fe-6Ag investigated.