Deformation Processing, Microstructure And Properties Of Cu-based In-situ Composites

With good combination of mechanical properties and electrical conductivity, Cu-based composites are suitable for the applications of high field resistive and pulse magnets, trolley wires for power car and the materials for lead frame. However, our research and development on these materials have far fallen behind developed countries. In order to meet the demand of native modern industry, competitive and low cost Cu-based composites with both high strength and high conductivity should be researched deeply. In the present investigation, Cu-16Fe, Cu-14.5Fe-1.5Cr, Cu-12Cr and Cu-16Cr composites have been prepared by in-situ synthesis technique and heavy deformation processing combined with heat treatments. The microstructure and mechanical properties as well as electrical conductivity have been studied by using Optical Microscopy (OM), Scanning electron microscopy (SEM), Electronic District Analysis (EDAX) and other test facilities.After heavily cold working, the dendrites of Fe or Cr phase in the as-cast billets were elongated into filaments. Higher strains could be obtained by cold drawing instead of rolling. With the increase of drawing strains, filaments became longer and thinner,and the spacing between them was more shortened. The average size of Fe or Cr filaments was around 200nm. Work hardening produced by deformation process caused significant improvement of tensile strength, while resulting in the reduction of electrical conductivity and plasticity. The correlation between ultimate tensile strength and spacing of filaments could be described by Hall-Petch equation. The increase of drawing strains and second-phase content induced the improvement of strength and decrease of electrical conductivity and plasticity.A little amount of Cr addition increased the strength of Cu-based composites at higher strains and also improved the electrical conductivity. However, because of brittleness and high hardness of Cr, high Cr content in Cu-16Cr composite caused the deterioration of processing properties.With several intermediate annealing treatments at 450℃for 2 hours , the work hardening of Cu-Fe based composites could be eliminated to enable the deformation process better and to reduce the disadvantage for electrical conductivity. At the same time, annealing accelerated Fe or Cr precipitates and then improved electrical conductivity.The results of heat treatments after deformation process showed that recovery and recrystallization occurred in Cu matrix, which resulted in the decrease of strength to some extent and remarkable improvement of plasticity. The increase of annealing temperature accelerated recrystallization and increased amplitude of fluctuation in mechanical properties. Small Fe or Cr particles precipitated from Cu matrix and distributed uniformly, which significantly raised electrical conductivity of Cu-Fe or Cu-Cr composites. Appropriate choice of heat treatment processes had a very important effect on electrical conductivity. By annealing above 500℃forseveral hours, conductivity of Cu-16Fe was found to decrease. After annealing at 550℃and 500℃for 7 hours, conductivity of Cu-14.5Fe-1.5Cr and Cu-12Cr reached peak value respectively. For Cu-16Cr, its conductivity obtained maximal value of 76.9%IACS by annealing at 550℃for 1.5 hours.