Preparation And Aging Of The Supersaturated Cu-cr Alloy Powder

Copper based alloys with high strength and conductivity properties have been became an important field in materials research and development, for it applied widely in electrical contact materials, IC frame materials, packaging, electric railway wire, conductor of magnetic field, the inner of continuous casting crystallizer, and so on. A Copper-Chromium (Cu-Cr) alloy is one of the interesting alloy systems in R&D of copper based alloys with high strength and conductivity properties. The supper saturated CuCr alloys with finer grain can be prepared by rapid by rapid solidification, and the synthetical properties of alloys with excellent mechanical properties and electric properties can be obtained by the proper heat treatment.CuCr alloyed powders were prepared by gas atomization under the vacuum atmosphere with the cooling rate of 10⁵-10⁷ K/S, the chemical and morphological analysis of the powders were carried out then. The composition of Chromium in the powders was determined in 0．34%, 0．76%, and 1．56% (in mass percent)respectively, and shape of the powders indicated to be slippery surface and well proportioned and spherical. XRD and EDS analysis from the powder compacts shows, that the Chromium dissolved into Copper matrix completely, and distributed in the matrix uniformly, the same results were also confirmed by the SEM analysis both in second electron (SE) images and back scatter electron (BSE) images. Compact ability and parameters of the powder were studied to determine the powder compacting condition, the proper compacting condition of the powders was 493．50MPa under the temperature of 150℃.After encapsulated in quartz tube with vacuum sphere, the compacts of CuCr alloyed powder were ageing under the temperature of 200℃, 400℃, 500℃and 600℃, respectively, and the electric conductivity and microhardness of the aged compacts under the different temperature were measured, the optimal ageing condition of the compact then obtained from the isothermal ageing characteristic curves of the compacts, that is the compacts aged in 45min at 500℃. The aged compacts under the different temperature were studied by high resolution transmission electron microscope (HRTEM). It is indicated from the morphology of aged compacts that solute element Cr were concentrated in some small zones (<10nm) in the initial stages of ageing, and so named modulated phaseI then formed. With the ageing progressed, the periods and stoichiometry of the modulated phaseâ… were adjusted to fit the changes of concentration of solute element Cr, then the modulated phaseâ… were substituted by the modulated phaseâ…¡after ageing 30min. After ageing 45min for the compacts, the simple substance Chromium was precipitated from the Copper matrix, it presented the shape of spherical or strip, with the diameter of 20nm for spherical, the length of 25nm and width of 5nm for strip. The precipitation of simple substance Chromium was kept in the nanometer level in size till to the end of ageing (8h), here, the precipitate occurred in crystalline, spherical shaped and the diameter was less than 50nm, and the thickness was about 20nm along the grain boundary precipitation. The period and structure of modulated phaseâ… andâ…¡were determined by HRTEM and SAED pattern analysis, it is identified that the precipitation sequence of super saturated CuCr alloys was as follows:fcc Cu solid solution→modulated structureâ… evolved from fcc→modulated structureâ…¡(DO₃ super structure)→bcc precipitate of simple substance Chromium.It is a first time to find the modulated structureâ… and its structure, and the precipitation sequence of super saturated CuCr alloys were got complementarity and consummately, and it is also a first time to express the precipitation sequence of super saturated CuCr alloys in details.Further analysis makes sure that the modulated structureâ… was three-dimensional modulated structure, which formed based on the lattice of copper and by the modulated slight. The modulated period of modulated structureâ… was about in the 7-9, and the orientation was three sorts of crystal orientation [111] , the modulated vector was about 0．120a*-0．132a*. The modulated structureâ…¡was a DO super structure, which was constituted from eight bcc lattices superposed in two layers. The crystallographic relationship between precipitate and matrix were: The crystallographic relationship between precipitate and matrix stood to the well known N-W relation. The modulated structureâ…¡maybe a parent phase for the precipitation of simple substance Chromium in the later stage of ageing. The precipitation of Chromium with fcc lattice mentioned in some literatures and the single substance Chromium incoherent with matrix were not found in present works.Two kinds of modulated structure accorded with the ageing characteristics curves vary well, in which some parts hard understood. By the studied in strengthen mechanism and electric conduction principle, the ,modulated structure have a contribution both in mechanical properties and electric properties for the CuCr alloys.