| Copper alloys with high strength and high electrical conductivity have been widely used in many fields,such as integrated circuit lead frames,railway contact wires,spot-welding electrodes and electromagnetic gun rails.To date,there are many problems in the fabrication process of the high strength and high electrical conductivity copper plates in China.One of the problems is that large-size copper plates fail to meet the practical requirement of both high strength and high electrical conductivity,limiting the development of our basic industry.For preparing the high-performance copper plates,a structure composed of nano-twins and nano-precipitates was designed to address the trade-off between strength and electrical conductivity.The processes suitable to prepare the large-size copper plates were developed via this strategy,and high-performance copper plates with yield strength lager than 640 MPa and electrical conductivity-80%IACS have been obtained successfully.What's more,this strategy can help to broaden the understanding of how to fabricate the high strength and high electrical conductivity copper plates.This study focused on two aspects:first,how to obtain the structure composed of nano-twins and nano-precipitates,and second,how this structure influences the properties of the copper alloys.A series of experiments and discussions were conducted following the two aspects.The previous studies have indicated that the nano-deformation twins have an obvious strengthening effect.Unfortunately,the intervals between twin bundles and the interiors within the twin bundles are "weak zones" because of the weak dislocation-dislocation interactions.The introduction of nano-precipitates into the copper matrix will strengthen these "weak zones",and thus set off the disadvantage of the nano-deformation twins.The alloying element which can reduce both the stacking fault energy and precipitation from the copper matrix was firstly selected,and then the selected alloying element dissolved into the copper matrix in form of solute atoms after the solution-treatment,Cryorolling.i.e.,rolling at liquid nitrogen temperature,was employed to restrict the cross-slipping and climbing,and thus promoted the formation of deformation twins.Finally,the solute atoms precipitated from the copper matrix via the aging treatment,obtaining a structure composed of nano-deformation twins and nano-precipitates.This route can both strengthen the copper alloys significantly and avoid degrading the electrical conductivity,resulting in the simultaneous improvement of strength and electrical conductivity.This study was divided into three stages as follows:Firstly,the above strategy,i.e.,the co-existence and co-strengthening of nano-twins and nano-precipitates,was verified via the cryorolling of a Cu-0.3Zr alloy.The microstructure and texture evolutions during the cryorolling of the Cu-0.3Zr alloy were investigated.The mechanism of producing deformation twins and factors influencing the strength and electrical conductivity were analyzed and discussed.The results showed that both the Zr element and lowering the rolling temperature can promote the formation of deformation twins.The cryorolling can promote the transition of shear bands from copper-type to brass-type.The deformation twins near the shear bands tend to bend and fragment,and thus evolve to many nano-crystallites.Furthermore,the cryorolling can also promote the transition of texture from copper-type to brass-type.The deformation twin bundles stimulate the initial formation of brass-type texture,and the brass-type shear bands promote further development to brass-type texture.During the aging treatment,the precipitates can pin the movement of the twin boundaries,and thus stabilize the deformation twins.After the aging treatment,the precipitates distribute homogeneously at the twin boundaries,sub-grain boundaries,within the twin bundles and the copper matrix,finally obtaining the structure composed of nano-twins and nano-precipitates.The Cu-0.3Zr alloy with this structure has good combination of tensile strength(601 MPa)and electrical conductivity(81.40%IACS).Compared with the Cu-0.3Zr alloys subjected to the room temperature rolling,its tensile strength was increased by 12.5%,and its electrical conductivity only decreased 3.33%IACS.Secondly,the methods to produce more deformation twins were developed and investigated from the two aspects,i.e.,lowering the stacking fault energy further and increasing the driving stress for twin nucleation.A precipitation strengthening Cu-Cr-Zr-Hf alloy with a lower stacking fault energy than the Cu-0.3Zr alloys was designed.A two-step rolling-aging process which can increase the driving stress for twin nucleation effectively was developed.A better comprehensive property was achieved in the Cu-Cr-Zr-Hf alloy subjected to the two-step rolling-aging process.The mechanisms of refining the deformation bands and producing the deformation twins were analyzed.The results showed that the Hf element can reduce the stacking fault energy effectively.Therefore,some deformation twins formed in the Cu-0.4Cr-0.2Zr-0.2Hf alloys subjected to the room temperature rolling.The two-step rolling-aging process can increase the driving stress for twin nucleation effectively,and thus produce a large number of deformation twins in the Cu-0.4C r-0.2Zr-0.2Hf alloys subjected to the two-step room temperature rolling-aging process.A series of experiments were conducted to optimize the two-step rolling-aging processes,and the best one was cryorolling 60%,intermediate aging at 400? for 120 min,cryorolling 30%.and final aging treatment at 450?for 300 min.Subjected to this process,the Cu-0.4Cr-0.2Zr-0.2Hf alloy achieved an excellent combination of yield strength(644 MPa),tensile strength(684 MPa)and electrical conductivity(79.85%IACS),meeting the requirement of yield strength(600 MPa)and electrical conductivity(?80%IACS).The numerous deformation twins and the refined deformation bands contributed to the increase in strength.The nano-precipitates formed during the intermediate aging treatment will pin the movement of dislocations and promote the dislocation density during the following rolling process,leading to the increase in local flow stress.The increased flow stress will refine the deformation bands and stimulate more deformation twins.If the driving stress for twin nucleation is large enough,the deformation twins will form within the deformation bands.Finally,on the basis of those above,the friction and wear performances were optimized further.The effects of Cr contents on the mechanical properties,electrical conductivity and friction and wear behaviors of Cu-Cr-Zr-Hf alloys were investigated.High strength,high electrical conductivity and good friction and wear performance were finally achieved simultaneously.The results showed that appropriate Cr contents can improve the friction and wear performance significantly.When the loads were 15 N and 30 N or the sliding speeds were 60 mm/s and 120 mm/s,the Cr contents had little influences on the friction and wear behaviors.In this case,all the Cu-Cr-Zr-Hf alloys performed well in the friction and wear test,and the volume wear losses were between 0.09 and 0.26 mm3.When the load was 45 N or the sliding speed was 180 mm/s,the Cr contents influenced the friction and wear behaviors significantly.In this case,the Cu-Cr-Zr-Hf alloy with 1%Cr performed best in the friction and wear test;Once the Cr content exceeded 1%,the friction and wear behaviors degraded significantly,which can be ascribed to the shedding of the large-size Cr particles.The shedding of Cr particles aggravated the abrasive and fatigue wear.The submicron Cr particles can improve the friction and wear performance effectively,and the nano-precipitates Cr can improve the tensile properties effectively.With the co-existence of submicron Cr particles,nano-precipitates Cr and deformation twins,the Cu-1Cr-0.2Zr-0.2Hf alloys achieved a combination of high strength(yield strength 655 MPa,tensile strength 705 MPa),high electrical conductivity(79.00%IACS)and good wear-resistance. |
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