Microstructure Evolution And Strengthening Mechanism In Cu-6%Ag Filamentary Composites

Filamentary strengthening Cu-Ag nanocomposites prepared by cold deformation and heat treatment possess high strength and excellent electrical conductivity.Many studies were focused on the effects of Ag content,heat treatment and deformation on the properties.The strengthening components in the nanocomposites were analyzed and some models were proposed to predict the strength.In order to further study the microstructure evolution and strengthening mechanism during drawing strain,Cu-6%Ag nanocomposites were prepared by melting,casting,solution treatment,aging and then cold drawing.The microstructure at various preparation processes was observed by optical microscopy and electron microscopy.The tensile strength of the specimens at various drawing strains was tested by an electric universal test machine.The change of strength was related to the microstructure evolution.The relationship between the microstructure and properties of Cu-6%Ag nanocomposites was established.Plenty of Ag nano-rods are homogenously distributed in the Cu matrix in the aged Cu-6%Ag.The Ag precipitates have a habit plane of {111} and a growth direction of <110>. There is a cube-on-cube orientation relationship,<011>_Ag//<011>_Cu and {111}_Ag//{111}_Cu, between the Ag precipitates and Cu matrix.The Cu/Ag interface is planar and has misfit dislocations with an average interval of 9(111)_Cu.Cu and Ag phases gradually rotate to the drawing direction and evolve into filamentary structure at heavy drawing strain.The diameter and interval of the filaments are reduced with the increase in the drawing strain.At initial deformation,the dislocation density increases with the drawing strain and dislocation cells form.When the drawing strain reaches 6.0,the dislocation cells lose stability and some cell walls transform into subgrain boundaries.The dislocation density decreases and the distribution of dislocation becomes inhomogeneous, while a coarse Cu grain divides into several small grains with different orientation. Deformation twins appear at heavy drawing strain and the amount of deformation twins reaches the maximum at aboutη=5.0.Deformation twins are produced by emission of Shockley partials from the interface and always penetrate several filaments then terminate at the Cu/Ag interface.The Cu/Ag interface fails to keep parallel with {111} but tends to parallel with {422} during cold drawing.The planar interface changes to zigzag morphology and the interface matching decreases.The misfit dislocation interval departs from the theory value as the draw ratio increases.Most of the interface contains no misfit dislocation once the Ag filament diameter is below 2 nm and the interface changes from semi-coherent interface into coherent interface.The relationship between interface energy and filament scale is responsible for the change of the interface.The coherent interface has lower interface energy when the Ag filament diameter is below 2 nm and the semi-coherent interface has lower interface energy when the Ag filament diameter is over 2 nm.The strength of Cu-6%Ag increases with the increase in the drawing strain.The work hardening atη＜3.0 benefits from the dislocation pinning by dislocation cell.The significant work hardening at 3.0＜η＜7.0 benefits from the dislocation pinning by Cu/Ag interface and twin boundary.The neglectable work hardening atη＞7.0 is due to the refinement of filamentary structure which only contains single dislocation.The plenty of interface is responsible for the notable work hardening even at high drawing strain.Cu and Ag phases keep a dynamic synchronization deformation during cold drawing.The real drawing strains of both phases keep the same as a whole but have different in local deformation.The same crystal type and orientation,and similar work hardening behavior lead to the co-deformation of both phases.The different lattice parameter and phase scale result in a mis-matching of deformation in local.The mean free path of dislocation strengthening model well explains the strengthening behavior of Cu-6%Ag during cold drawing.The predicted strength by the model is also consistent with the experimental results of other Cu-Ag alloys.Therefore,the strengthening mechanism in most of the Cu-Ag binary alloys could be explained by that model.Compared with the crystal size,dislocation density and interface,the mean free path of dislocation is the essential strengthening factor.The mean path of dislocation could be dislocation cell size, filamentary interval or layer thickness in practical microstructure.