Papers by Keyword: Cu-Al Alloy

Abstract: Recently, it was reported that Cu-Al alloys of low stacking fault energy (SFE) processed by severe plastic deformation show excellent tensile properties due to TWIP (Twinning induced Plasticity) phenomenon. In this study, Cu-15at% Al sheets were heavily processed by conventional multi-pass cold rolling up to 90% in reduction in thickness without annealing. In order to reveal the change in mechanical properties and the microstructure evolution, tensile test, hardness test, optical microscopy (OM) and electron backscattering diffraction (EBSD) analysis were performed. Deformation twinning due to low SFE is observed even in the case of low reduction in thickness. As the reduction increases, grains are refined by intersections of shear bands. It is found that the balance of strength and elongation of the processed sheets is comparable to those by severe plastic deformation followed by annealing in literature.

Abstract: Internal oxidation Cu-Al alloy sheet processing has been used to prepare dispersion-strengthened Al₂O₃/Cu composites. A new technique has been developed that successfully avoids mixed Cu-Al powder preparation, H₂-reduction, and sintering processes. The microstructure and phase were investigated by metallurgical microscope, SEM, and TEM. Holding-time, internal oxidation temperature and thickness of Cu-Al alloy sheet were the three key factors with regard to preparing Al₂O₃/Cu composites by internal oxidation in Cu-Al alloy sheet. The mechanical characteristics and electric properties of Al₂O₃/Cu composites sheet and hot-extruded rod were examined. The results show hot extrusion process enhances microhardness and ultimate tensile strength to a higher level with and exerts a little influence on high electrical conductivity.

Abstract: The transformation temperature and time of α+γ₂ to ß in a Cu-Al alloy after cryogenic treatment during heating were measured by DSC, and the transformation activation energy of α+γ₂ to ß was also calculated. It is indicated that the Cu-Al alloy with heating rate of 10°C/min, the phase transformation onset and ending temperature is 561.75°C and 582.88°C, respectively, and the phase transformation time is126.6S. The phase transformation activation energy decreases with the increasing volume fraction of their phase transformation.

Abstract: After heat treatment at 800°C for 15min, a Cu-Al alloy was cryogenic treated at -196°C for 30min, the micro-mechanical properties of the Cu-Al alloy before and after cryogenic treatment were measured by nanometer mechanical testing system and the microstructure of the Cu-Al alloy were also analyzed by optical microscope, SEM/EDS and XRD. On the basis of that, the effects of cryogenic treatment on micro-mechanical properties of the Cu-Al alloy were investigated. The results show that the cryogenic treatment can refine the grains, and increase the hardness, elastic modulus, elastic recovery rate and ratio of hardness to modulus of the Cu-Al alloy. As a result, cryogenic treatment can improve the ability to resist applied load and anti-indentation creep effectively.

Abstract: The purpose of this study is to highlight the influence of the strain hardening on internal friction behaviour of a copper single crystal alloy Cu-Al (9% at.) using Isothermal Mechanical Spectroscopy (IMS) technique. To do this, the sample was cold worked about 1% by torsion and tested at different stabilized levels of temperatures. Thus the specimen have been progressively heated to 1179 K and then cooled down to room temperature. The advantage of IMS experiments is to allow to compare the isothermal internal friction spectra obtained during heating (in this case, the annealing temperature (T_A) is equal to the temperature of measurement (T_M) with the measurements performed at various temperatures during cooling after annealing at 1179 K (T_M < T_A in this case). The results obtained in increasing temperature step by step (T_A = T_M) show the existence of two (02) independent relaxation peaks from 914 K to 1179 K. The first one, called here P₁ (at about 0.7 T_Melt at 1Hz), decreased with increasing the annealing temperature, while a new relaxation peak P₂ (at about 0.9 T_Melt at 1Hz) progressively developed. However for high temperature annealing, IMS tests (T_A > T_M) does reveal only one peak P₂. It appears, therefore, clearly that the strain hardening is responsible for the P₁ existence. To monitor the evolution of P₁ and P₂ with the temperature, a method of peaks decomposition already published was applied. The relaxation parameters, deduced from the Arrhenius plots, were thus accurately determined. The mechanism responsible of the P₁ and P₂ peaks appearance has been attributed to the movement of dislocation segments inside the sample microstructure.

Abstract: The microstructural evolution and grain refinement of Cu-Al alloys with different stacking fault energies (SFEs) processed by equal-channel angular pressing (ECAP) were investigated. The grain refinement mechanism was gradually transformed from dislocation subdivision to twin fragmentation with tailoring the SFE of Cu-Al alloys. Concurrent with the transition of grain refinement mechanism, the grain size can be refined into from ultrafine region (1 m~100 nm) to the nanoscale (<100 nm) and then it is found that the minimum equilibrium grain size decreases in a roughly linear way with lowering the SFE. Moreover, in combination with the previous results, it is proposed that the formation of a uniform ultrafine microstructure can be formed more readily in the materials with high SFE due to their high recovery rate of dislocations and in the materials with low SFE due to the easy formation of a homogeneously-twinned microstructure.

Abstract: In the present investigation, a bimodal structured alloy with ultrafine-grained (UFG) eutectoid matrix embedded with micrometer-grained pre-eutectoid phase was introduced into the hypo-eutectoid Cu-10.8wt.%Al and Cu-11.3wt.%Al alloys by means of pre-pressing heat-treatment, equal-channel-angular pressing (ECAP) and subsequent annealing. Different size of micrometer grained pre-eutectoid phase was obtained by controlling the cooling rate during pre-pressing heat-treatment of the hypo-eutectoid alloy. The tensile deformation behavior of the developed microstructures is characterized by a maximum tensile yield strength up to 800MPa, which is three times higher than that of the un-treated alloy. It is found that the size of the micrometer grained pre-eutectoid phase is critical to the improvement of the bimodal structured alloy. With larger micrometer grained pre-eutectoid phase, no obvious improvement in plastic elongation was observed with the increase of volume fraction of the pre-eutectoid phase from 20% to 40%, but a decrease in the yield tensile strength was observed. An optimal combination of strength and ductility was obtained particularly in those samples embedded with small-sized micrometer-grained pre-eutectoid phase, which provide extra strain gradient hardening effect.

Abstract: In order to make clear the micro-yielding mechanisms of polycrystalline metals including twins, the movement of dislocations in the surface grains of Cu-6.8at%Al alloy and pure Mg polycrystals during the early stages of deformation was directly observed by using etch pit technique. The fresh dislocations multiply from the Frank-Read sources within the grains, and pile up against the twin and grain boundaries of two kinds of specimens. The pile-up dislocations on the primary and/or secondary slip planes are also confirmed in Cu-6.8at%Al alloys. Especially during the compressive loading for pure Mg, the occurrence of deformation twins is remarkable with an increase of strain rate, whereas the distribution of fresh dislocations tends to decrease in the surface grains. The present results suggest that the effect of twin boundaries on micro-yielding is almost equivalent for that of grain boundaries, which act as barriers to moving dislocations even in the pre-yield deformation.