Research On The Performance And Microstructure Of Cu-0.23Be-0.84Co Alloy During Heat-treatment Strengthening Process

Beryllium copper alloy is a kind of copper alloy containing 0.2%~2%(wt.) Be and a small amount of Ni and Co. Beryllium bronze alloy is a typical aging strengthening alloy. It has good comprehensive performance of high strength and hardness, high electrical conductivity and thermal conductors, excellent wear resistance and corrosion resistance, and outstanding high temperature anti-creep deformation. The alloy is extensively used in aerospace, national defence, power electronics, petroleum chemicals, and other fields. Presently, related researches at home and abroad mainly concentrate on beryllium bronze containing 1.6%~2%(wt.) Be with high strength(≥800MPa). However the researches on preparation process, heat treatment characteristics and strengthening mechanism of beryllium bronze including 0.2%~0.6%(wt.) Be with high conductive/thermal properties used in some special conditions such as casting roll and splice has rarely been reported.This paper is based on the self-made Cu-0.23Be-0.84 Co alloy. The effect of heat treatment process parameters like temperature, time, and cold deformation on the microstructure and properties of Cu-0.23Be-0.84 Co alloy was studied. The electric conductivity, hardness, tensile strength, and arc-erosion resistance of the alloy after different process were measured. The aging precipitation behavior was studied and the ageing strengthening mechanism was explored with optical microscope(OM), scanning electron microscope(SEM), and transmission electron microscope(TEM and HRTEM). According to the relationship between conductivity changes and transition rate of the precipitated phase, the phase transformation kinetics and electrical equations of the alloy after aging were established and on this base, the change mechanism of aging precipitation was explored.The results show that the change trend of electrical conductivity and hardness of Cu-0.23Be-0.84 Co alloy over time are consistent after 950℃×1h solution and followup aging at 460℃、480℃ and 500℃. The electrical conductivity and hardness of the alloy firstly increases sharply(0.5-8h) and increases steadily(1-4h) and decreases(4-8h). The electrical conductivity reaches to peak value at 4h, and the peak conductivity of 460℃、480℃ and 500℃ are 71.4%IACS, 71.6%IACS and 69.8%IACS. The hardness of the alloy reaches to peak value after aging 3-4h. The peak hardness of the alloy is 123 HB at 460℃ for 4h, and 117 HB at 480℃ for 3h, 112.3HB at 460℃ for 4h. With the aging time prolonging, the structure of precipitated phase has obviously change. At early aging the precipitates phase of the alloy(0.5-2h) is Be2 Cu phase, midterm(4h) Be Co phase, and later(8h) Be2 Cu again. This change leads to peak performance of the alloy appears in the middle of aging(4h) and decrease at 8h for the phenomenon of re-dissolution. According to the TEM of the Cu-0.23Be-0.84 Co alloy, the main age-hardening mechanism is the Orowan strengthening mechanism. The coarsening behavior of the precipitation phase in good agreement with the theoretical model of LSW shows the coarsening mechanism is diffusion controlled. This paper analyzed the relationship between electrical conductivity and the transition rate of precipitates. And then the Avrami equation of phase transformation kinetics and electrical conductivity equation are obtained. Adding certain amount cold deformation on alloy before aging can effectively improve the properties of the alloy. In the range of experiment, after the process of 950℃×1h+50% cold deformation+(460-480)℃×(3-4)h,the performance of Cu-0.23Be-0.84 Co alloy is better with conductivity of 71.6-72.8%IACS, and hardness of 167-176 HB. By comparison, the alloy after drawing and aging has better arc-resistance than that after casting and solid solution. The results of electrical contact test show that electrical erosion resistance of alloy after drawing and aging is better than that after casting and solid solution. This is because the flat contact surface, little change arcing energy and small amount of mass loss of the alloy after drawing and aging.