Preparing Of Ultrafine-grain Copper By ECAP And Research On Its Microstructure And Properties

It is well known that the properties of metal materials are in contact with its grain size. According to the equation of Hall-Petch,mechanical strength and hardness will increase with the minishing of grain size.Therefore,grain refinement is always an effective method of improving comprehensive performance of metal material.However,it is too hard to refine some materials's grain size to submicrometer in the past,due to all kinds of restrictions about technology and process.With the development of material science and the improvement of material preparation and processing,people began to study the grain size of the material below submicrometer in the 1980s,so-called the ultra-fine grain material.ECAP is an effective method to process ultra-fine grain material by deforming similar to simple shearing.Through the tests and analysis with ECAP,at theΦ=120°,Ψ=60°through the route B_C and pressing through eight passes,the average grain sizes of the industry pure cupper attained 1.5,μ_m.Test with the route B_C for contrast,the corner of the dieΦ=105°,Ψ=37°is better thanΦ=120°,Ψ=60°.The tensile strength of pressing through eight passes attained 430MPa,Vickers hardness attained 167HV,the elongtation is 20.1%.Comparing the result ofΦ=120°,Ψ=60°route A andΦ=120°,Ψ=60°route B_C,the former is better than the latter, which the tensile strength of pressing through eight passes attained 425MPa,the elongation is 19.1%and Vickers hardness is 184HV.After the anneal heat treatment at temperature 200°for one hour,the resault show that the suitable heat treatment technological parameter can produce strongly-refined microstructures.For example,after the route B_C annealing one hour at the temperature 200°, the tensile strength reach 435Mpa and the elongtation is 23.01%and Vickers hardness is 153MPa.ECAP specimens were examined by TEM for determinations of the grain sizes and the nature of any dislocation interactions occurring within the grains.The elongated diffraction spot showed that the grain size minished.After heat treatment,the dislocation decreased and diffraction spot displayed elongated state. The microstructures was analysised after 1,4,8 passes(dieΦ=120°,Ψ=60°)by route B_C and then heat treatmented,it indicates that the fracture was plastic rupture,the amounts of the dimple increased and the void turned deepen.After heat treatment,the ductility is evaluated.