| A series of new alloys of Cu-10Ni-20Zn-xAl (x=1.2,1.6,2.0, and 2.4 wt.%) with high strength, conductivity, and elasticity were designed and prepared in this study. By means of physical property measurements of materials, such as hardness and electric conductivity measurements, room temperature tensile tests, X-ray diffraction (XRD), optical microscopy (OM), and scanning and transmission electron microscopy analysis (SEM, TEM), and energy spectrum analysis, and electrochemical property measurements (i.e., electrochemical impedance spectroscopy), the relationship of the structure and performance of the alloys with the Al content was studied, mechanical properties, electrochemical performance, microstructure and it's evolution of the alloys were investigated. At the same time, the influence of Al content on the microstructure and properties in the studied alloys was also investigated. And then the strengthening mechanism and the form and effect of the Al element in the alloys were presented. Several important conclusions can be summarized as follows:1. The Cu-Ni-Zn-Al alloys prepared by adding the Al element into the relevant ternary Cu-Zn-Zn alloys is a new type of hardened alloys by ageing precipitation. The Al content plays an important role in determining the structure and performance of the Cu-Ni-Zn-Al alloys. Both the hardness of the Cu-Ni-Zn-Al alloys and the temperature for the homogenization and solid-solution treatment are increased with the increase of the Al content.2. The Cu-Ni-Zn-Al alloys has a strong hardening effect by ageing precipitation and the suitable ageing after the solid-solution treatment was found to be at 500℃for 8-16h. Under this ageing condition, the peak hardness and the conductivity increase of the Cu-Ni-Zn-Al alloys can reach 128-135Hv and 3-7%IACS, respectively. During the ageing process, both the hardness and the conductivity of the Cu-Ni-Zn-Al will increase obviously with the increase of the Al content.3. The structure and composition of precipitates in the Cu-Ni-Zn-Al alloys are related to the Al content. When the Al content is low (1.2 wt. %), the precipitate is the Ni3Al with a small quantity of Cu and Zn called theγ' phase that resembles the Cu3Au structure with an Ll2 ordered cubic, plays a major role in the hardening of the alloys, and has a definite orientation relationship of [011]n//[011]m and (100)n//(200)m with the a matrix. When the Al content is high (1.6-2.4 wt.%), theγ' phase will precipitate at the early ageing, but the so-calledβphase will also precipitate with the ageing proceeding. Theβphase is the NiAl with a small quantity of Cu and Zn and may be written as a formula of (NixZn1-x)(AlyCu1-y). The incubation for theβphase will be shortened and the volume fraction of theβphase will be markedly increased when the Al content is gradually increased.4. The precipitation process of precipitates in the Cu-Ni-Zn-Al alloys at the early ageing was analyzed by the XRD method, and the strengthening mechanism of the precipitates in the alloys was presented. The coherent precipitation of the precipitates induces a severe distortion in the solid-solution matrix, and the extent of the distortion increases with the prolonging of the ageing time. Under the cooperative influences of the coherent precipitates and the matrix distortion, the hardness of the alloys increases obviously, and then a strong hardening effect by ageing precipitation is observed.5. The distribution feature of precipitates in the Cu-Ni-Zn-Al alloys during the solid solution-ageing process is related to the Al content. When the Al content is 1.2 wt.%, the precipitates have a typical feature of continuous precipitation structure. As the Al content increases, the discontinuous precipitation structure will be observed, and moreover its occurrence time will be shortened gradually and its volume fraction will be increased gradually.6. The 80% cold rolling before the ageing can markedly increase the hardness of the alloys and accelerate the processes to increase the hardness and conductivity of the alloys. The more the Al content, the more remarkable the above effects. After the 80% cold rolling, the proper temperature for ageing the Cu-Ni-Zn-Al alloys is 450℃, and the ageing time is 0.5-8h. Under the above treatment conditions, the mechanical performance and electrical conductivity of the alloys can be increased greatly, and the tensile strengthσb, the yield strengthσ0.2, the elongationδ, the electrical conductivity EC, and the elastic modulus E can reach 901-1155MPa,844-1125MPa,1.1-4.0%,9-15%IACS, and 130-134GPa, respectively. Hence, the mechanical performance of the Cu-Ni-Zn-Al alloys can be comparable to or even superior to those of the Cu-Ti, Cu-Ni-Sn, and Cu-Be alloys under similar treatments, and the Cu-Ni-Zn-Al alloys is believed to be a new potential type of Cu-based alloys of high strength, conductivity, and elasticity.7. There exists an evident interaction between the ageing precipitation and the recrystallization in the Cu-Ni-Zn-Al alloys during the cold rolling-ageing process. The precipitation of the precipitates severely blocks the recrystallization of the deformed structure in the alloys, and greatly increases the recrystallization temperature of the alloys, thus restraining the discontinuous precipitation behavior of grain boundaries.8. The ageing precipitation dynamics in the Cu-Ni-Zn-Al alloys was analyzed with the electrical resistance method, and the ageing dynamical equation was obtained. The theoretical curve predicated by the ageing dynamical equation was found to be in agreement with that obtained by the experimental method, which clearly interprets the ageing precipitation dynamical process during the solid solution-ageing and the solid solution-cold rolling-ageing processes in the investigated alloys.9. The Cu-Ni-Zn-Al alloys have a better capability to resist corruption than the corresponding ternary Cu-Ni-Zn alloys without Al addition. After the immersion in 3.5 wt.% NaCl corruptive solution for a long time, the corruption products formed on the surfaces of the alloys were found to be composed of the outer layer of (Cu, Ni, Zn)2Cl(OH)3 and the inner layer enriched in A12O3 and Cu2O. The metal oxide layer is mainly responsible for the corruption resistance of the alloys.
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