| ABSTRACT:Cu-Be alloys with high strength, elasticity, hardness, wear resistance and good electrical conductivity are widely used for manufacturing of dies, die mosaic pieces and elastic components. However, the alloying element beryllium is toxic and it is harmful to humans during the alloy production. Furthermore, the strength and elasticity of Cu-Be alloys decrease drastically as performed at temperature above200?, and the stress relaxation rate is about40%, which cause materials failure. Therefore, substantial efforts have been made to develop a new Be-free conductive elastic copper alloy.In this paper, an environmental-friendly Cu-6.0Ni-1.0Si-0.5Al-0.15Mg-0.1Cr alloy with ultrahigh strength, good electrical conductivity and excellent anti-stress relaxation resistance was designed and prepared by a medium-frequency induction furnace. The composition design, thermo-mechanical heat treatment, process parameters, precipitation behavior, stress relaxation behavior and corrosion behavior were investigated in detail. Some conclusions were drawn as following.(1) Developed dendrites appeared in the initial casting ingot, including dendrite skeleton, non-equilibrium precipitates and transition zone. The dendrites were rich in Cu, while non-equilibrium precipitates were rich in Ni and Si. The precipitates were Ni2Si, Ni3Si (Ni74Si26) and Ni3Al particles. Elements of Al, Mg and Cr were disturbed in the alloy evenly.(2) The stress-strain curves of alloy deformed at evaluated temperature exhibit dynamic recrystallization feature, and the process could be divided into working hardening, dynamic recovery and dynamic recrystallization. Hot processing map was established on the basis of dynamic material model theories and Prasad instability criterion, indicating that appropriate hot processing temperature range for hot deformation was850-900?. After hot compression deformed at750?, Cube texture of{001}<100> was formed on the cross section. Gauss texture of{0115<100> and rotated cubic texture of{001}<110> were formed as deformed at800?; Copper texture of{112}<111>, S texture of{123}<634> and Gauss texture of{011}<100> were formed as deformed at850?and900?.(3) The appropriate thermo-mechanical heat treatment method for designed alloy is that:homogenization treatment at940?for4h?hot rolling by80% at900??solution treatment at980?for4h?cold rolling by50%?aging treatment at450?for1h. After above treatments, the alloy could achieve good properties:hardness was341HV, electrical conductivity was26.5%IACS, tensile strength was1090MPa, yield strength was940MPa, and elongation was3.5%. The hardness was330.1HV even as the alloy was aged at450?for20h, indicating that the alloy have good anti-over aging resistance. After loading at room temperature,100?and200?for100h, the stress relaxation rates of peak-aged specimens were4.05%,6.52%and9.74%, respectively.(4) The order of the phase transformation in supersaturated solution solid during ageing at450?was that:supersaturated solution solid?L12ordering (Ni3Si)??-Ni3Si??-Ni3Si+5-Ni2Si. Precipitation occurred as the supersaturated solid solution was aged at450?for5min, and dimension of precipitates was3-4nm. Dimension of precipitates grew up to4-5nm as aged for15min. Precipitates of (3-Ni3Si and ?-Ni2Si with6-8nm appeared as aged for60min. Precipitates of ?-Ni3Si,?-Ni2Si and ?'-Ni2Si with13-15nm appeared as aged for480min. Precipitates of ?-Ni3Si and ?-Ni2Si with100-200nm appeared as aged for24h.(5) L12ordering appeared in the supersaturated solid solution as aged at500?for5min, and discontinuous precipitation appeared as aged for10min. Discontinuous precipitation appeared as aged at550?for5min. Continuous precipitation appeared as aged at600?for10min, and two variants with perpendicular growth direction were observed. Rod shape ?-Ni2Si precipitate with size of100nm appeared as aged at650?for10min, and size of ones grew up to150nm and200nm as aged for30min and60min, respectively.?-Ni2Si precipitates with size of100nm appeared as aged at700?for10min. Rod-shape8-Ni2Si precipitate with size of150nm appeared as aged at750?for10min, while disk shape ones with size of250nm appeared as aged for30min.(6) The corrosion rates of designed alloy with treatment states of under-aged, peak-aged and over-aged aged were0.033226mm/a,0.022975mm/a and0.0019456mm/a, respectively. The order of anti-corrosion resistance among the three aged treatment states was that: over-aged> peak-aged> under-aged. XPS results showed that the corrosion product layer mainly contained compounds of CuO, CU2O, CuCl and CuCl2. With increasing immersion time, Cu was oxidized to CU2O, partial Cu2O were further oxidized to CuO, and some CU2O was transformed into Cu2(OH)3C1and CuCl2·3Cu(OH)2, which could protect the matrix from corrosion. |
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