Microstructure and microtexture in pure copper processed by high-pressure torsion

The evolution of microstructure and microtexture in high purity copper was examined after processing by high-pressure torsion (HPT). Copper disks were annealed for 1 h at 800 °C and later processed monotonously in HPT at ambient temperature for 1/4, 1/2, 1, and 5 turns under a pressure of 6.0 GPa. Electron backscattered diffraction (EBSD) measurements were taken for each disk at three positions: center, mid-radius, and near-edge. Results from EBSD for samples processed between 1/4 and 1 turn indicate the formation of Σ3 twin boundaries by recrystallization before complete microstructural refinement. The results show a gradual increase in the homogeneity of the microstructure with increasing numbers of turns, reaching a stabilized ultrafine-grained structure at 5 turns with a bimodal distribution of fine and coarse grains of 0.15 and 0.5 μm in diameter, respectively. The occurrence of recrystallization in the early straining stages was further supported by examining microtexture development with increasing numbers of turns, where this shows a gradual transition from a shear texture to a mixture of shear and recrystallization and later to a shear texture at high HPT strains. The promotion of recrystallization during HPT is probably related to the high purity of the copper.     

Structural and mechanical properties in AA 5083 processed by ECAE

Abstract

Equal channel angular extrusion has been used to analyse refining of grains in an industrial 5083 aluminium alloy during severe plastic deformation. The influence of the total strain as well as of the processing route were studied by tensile tests and TEM. The room temperature behaviour and the high temperature properties suggest that large strains increase the density of high angle boundaries in the material. The optimal processing route to achieve grain refinement appears to be route B for the present investigation.     

Microstructure development and texture evolution of ME20 sheets processed by accumulative roll bonding

Magnesium alloy ME20 sheets were produced by accumulative roll bonding (ARB) up to 8 cycles. The rolling microstructure was significantly refined during the first 2 cycles and remained homogeneous up to 6 cycles. After 8 cycles (ε ~ 6.4) the homogeneous equiaxed microstructure was replaced by a very fine shear band microstructure. With increasing ARB cycles, the texture intensity of basal poles decreased leading to a higher sheet tensile strength but with decreasing ductility.     

Quasi-static and dynamic mechanical properties of commercial-purity tungsten processed by ECAE at low temperatures

In this work, we have processed commercial purity tungsten (W) via different routes of equal-channel angular extrusion (ECAE) at temperatures as low as 600 °C. We have systematically evaluated the quasi-static and dynamic compressive behaviors of the processed W. Quasi-static compression tests were performed using an MTS hydro-servo system at room temperature. It is observed that samples ECAE processed at 800 °C show higher yield and flow stresses than those processed at other temperatures; no obvious strain hardening is observed in the quasi-static stress–strain curves. Quasi-static strain rate jump tests show that the strain rate sensitivity of ECAE W is in the range of 0.02 to 0.03, smaller than that of coarse-grained W. Uni-axial dynamic compressive tests were performed using the Kolsky bar (or split-Hopkinson pressure bar, SHPB) system. Post-loading SEM observations revealed that under dynamic compression, the competition between cracking at pre-existing extrinsic surface defects, grain boundaries, and uniform plastic deformation of the individual grains control the overall plastic deformation of the ECAE W. The existence of flow softening under dynamic loading has been established for all of the ECAE W specimens.     

微米级短碳纤维/铜基复合材料组织和摩擦性能研究

以电解铜粉和酚醛树脂包覆的毫米级短碳纤维为原料,通过球磨-冷压-加压烧结制备了微米级短碳纤维/铜复合材料,研究了短碳纤维长度和分散程度对材料力学和摩擦性能的影响.结果 表明:球磨工艺可有效缩短碳纤维长度,制备均匀分散且长度均匀(20 ～40 μm)的微米级碳纤维,进而保证了材料在摩擦过程中可连续产生均匀细小的碳颗粒以阻碍材料的黏着,改善材料的摩擦稳定性和耐磨性.球磨时间不足时,短碳纤维长度差异大且局部存在纤维缠结,摩擦过程中富铜区黏着加剧,易产生片状脱落,磨损较大(2.32×10-4mm3/(m· N));球磨时间过长时,短碳纤维损伤严重,产生大量碳颗粒与短碳纤维共存,过多的碳铜界面和缺陷促进了材料摩擦过程中疲劳损伤导致的大量剥层磨损,耐磨性降低;球磨3h制备的复合材料综合性能较好,弯曲强度和体积磨损率达到332.9 MPa和1.00×10-4mm3/(m·N),摩擦系数波动范围宽度约为0.0834.     

多道次ECAE动态成型Al-Mg-Si合金导线组织与性能

借助S4800扫描电子显微镜、Philips DM420透射电子显微镜、SANS CMT5105电子万能材料试验机和QJ48双臂直流电桥,研究了多道次ECAE动态成型Al-Mg-Si合金导线的组织与性能。结果表明:4道次ECAE动态成型可制备平均尺寸在10μm左右甚至更小的Al-Mg-Si合金导线晶粒。随着Mg、Si含量的增加,合金导线的抗拉强度增大,伸长率与等效导电率降低。经160~170℃/7h时效处理后,Al-0.59%Mg-0.59%Si合金导线的抗拉强度、伸长率和等效导电率分别为305.71~309.63 MPa,4.7%~5.4%和55.18%IACS~56.33%IACS,与目前国产Al-Mg-Si合金导线(295 MPa,52.5%IACS)相比,导电性能显著提高。     

Investigation of texture and mechanical properties of copper processed by new route of equal channel angular pressing

The evolution of crystallographic texture and the mechanical properties of copper subjected to severe plastic deformation (SPD) using equal channel angular pressing (ECAP) were investigated. Samples were subjected to ECAP under two different processing routes: B₆₀ and B_C. As the cross sections of the samples were circular, a new route with a rotation angle of 60° in the same direction between consecutive passes was introduced. The material exhibited texture development similar to the simple shear texture in both routes and the most significant changes in texture strength in both processing routes took place after the second pass. Microstructure of ECAP processed samples were investigated using electron backscatter diffraction (EBSD) analysis. Comparison of the EBSD data with optical micrograph of the initial sample confirmed that ECAP process has led to a significant decrease in grain size. Significant increases in hardness and tensile strength were observed after the first pass of ECAP. Variations of tensile strength as a function of the number of passes were related to the dislocation densities and the average boundary spacing.     

室温ECAP和冷轧复合变形工业纯钛的组织和性能

采用ECAP技术和常规冷轧复合变形工艺制备了高强度工业纯钛,研究了复合变形后试样的力学性能与显微组织的关系.结果表明,工业纯钛经室温单道次ECAP和冷轧复合变形后,晶粒被严重拉长,形成了明显的纤维状组织,试样的抗拉强度高达805MPa;随着冷轧变形量的增大,变形组织的细化程度和均匀性提高,使试样的强度和塑性进一步提高.位错滑移和孪生是工业纯钛室温ECAP和冷轧复合变形的主要变形机制.     

Microstructure evolution of diamond with molybdenum coating and thermal conductivity of diamond/copper composites fabricated by spark plasma sintering

Journal of Materials Science: Materials in Electronics - Molybdenum (Mo) coating was deposited on the diamond surface by vacuum micro-vapor deposition. Effects of deposition parameters on the...     

Microstructure and tensile behavior of Al and Al-matrix carbon nanotube composites processed by high pressure torsion of the powders

Carbon nanotubes (CNTs) are expected to be ideal reinforcements of composite materials used in aircraft and sports industries due to their high modulus and low density. In the present paper, severe plastic deformation by high pressure torsion (HPT) of powders at elevated temperature (473 K) was employed to achieve both powder consolidation and grain refinement of aluminum-matrix nanocomposites reinforced by 5 vol% CNTs. Before the HPT, the powders were ball milled using planetary ball mill in order to achieve molecular level mixing. Aluminum was treated by the same process for a reference. The HPT processed disk were composed of considerably equilibrium grain boundaries with high misorientation angles. The CNT-reinforced ultrafine grained microstructural features resulted in high strength and good ductility.     

Mechanical properties of C/C composites processed by wet impregnation and P-CVI methods

Wet impregnation with phenolic resin and P-CVI methods were used to manufacture C/C composites. The influence of impregnation process of porous 2D carbon fibre substrate with resin and pyrocarbon deposited by CVD technique on mechanical properties of formed composites was studied. The results indicate that using P-CVI method large pores remain in the matrix resulting in lower mechanical strength. This fraction does not undergo any changes during thermal treatment. The flexural modulus of C/C composites depends mainly on the type of reinforcing fibres. The values of moduli measured in composites obtained by both methods do not differ significantly. Comparison of two methods of fabrication of C/C composite show that much better strengths can be achieved by forming the carbon matrix in solid state.     

Microstructure of low temperature processed CNFs/glass nanocomposites

Carbon nanofibers/glass (CNF/G) nanocomposites were obtained from a glass powder of low melting point and pristine CNFs. Green bodies containing from 0 to 22 % (v/v) of CNFs were sintered under nitrogen atmosphere in the 550–700 °C temperature range with different holding times. A fully microstructure characterization, by means of Hg porosimetry and N₂ adsorption, was carried out for understanding the CNFs/G composites behavior during the sintering process. This understanding is required to optimize the microstructural design of CNFs/glass nanocomposite materials. During sintering two different and simultaneous phenomena occur the matrix crystallization and the pore formation. The glass matrix crystallization temperature decreases from 650 to 550 °C, when CNFs concentration increases to 22 % (v/v). The glass matrix produces the CNFs degradation and generates gaseous species which lead to homogeneous or foamy materials. This depends on the CNFs concentration and thermal treatment conditions. Foamy nanocomposites present pore size distributions with pores <0.1 and close to 20 μm. The glass matrix wets the CNFs and produce their degradation been of 1 % of carbon loss in all nanocomposites.     

Microstructural evolution of silicon carbide/aluminum oxide composites processed by melt oxidation

The nucleation and growth mechanisms during high temperature oxidation of liquid Al-3 wt% Mg and Al-3 wt% Mg-7 wt% Si alloys were studied to provide a better understanding of the composite fabrication process, especially in the presence of SiC reinforcement. Al2O3-matrix composites with and without SiC particulates have been produced by directed oxidation of aluminum alloys. The microstructure consists of three interpenetrating phases: the SiC preform, a continuous -Al2MO3 matrix, and a network of unoxidized metal. The volume fraction of metal within the oxidation product decreases with increasing processing temperature. The preform does not show any evidence of degradation by the molten alloy, but the growth front tends to climb up the particles, increasing the oxidation area and therefore enhancing the rate of composite growth. The amount of porosity was found to increase with the Mg content in the alloy, from 2.0 vol% for 0.5 wt% Mg to 5.8 vol% for 3 wt% Mg. The role of magnesium and silicon in the growth process are discussed.     

Microstructure and properties of BN/Ni-Cu composites fabricated by powder technology

Microsize Powders of Ni and Cu were prepared by water atomization technique to fabricate metal matrix composites containing various percentages of nanosized boron nitride particles (1, 2, 3, 4, 5 wt. % of BN in a matrix containing (20 wt. %Ni and 80 wt. %Cu). The prepared mixtures were cold compacted under 400 MPa, and sintered for 2 h at 1000 °C in a controlled atmosphere of 3:2 N₂/H₂ gas mixtures. The microstructure and the chemical composition of the prepared powders as well as the consolidated composites were investigated by X-ray diffraction as well as field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS). The produced Cu and Ni powders have spheroid shape of size less than 100 microns, but the investigated BN has an equiaxed particle shape and particle size of ～ 500 nm. It has been also observed that BN and Ni particles were homogeneously distributed in the Cu matrix of the present BN/Ni-Cu composites. The density, electrical resistivity, saturation magnetization and hardness of the composites were measured. It was observed that, by increasing BN content, the relative density was decreased, while the saturation magnetization, electrical resistivity and hardness were increased.     

Selective,low-temperature synthesis of niobium carbide and a mixed (niobium/tungsten) carbide from metal oxide-polyacrylonitrile composites by carbothermal reduction

Composites of polyacrylonitrile (PAN) with the layered oxides (C₆H₁₃NH₃)Nb₃O₈, (C₈H₁₇NH₃)Nb₃O₈ and -(C₆H₁₃NH₃)NbWO₆ undergo carbothermal reduction in an argon atmosphere at 1000 °C to give the cubic carbides NbC_x and (Nb, W)C_x, respectively. Reduction of the Nb₃O₈/PAN composites to NbC_x proceeds via the formation of tetragonal NbO₂, with no other intermediates being detected. Formation of NbC_x begins at 800 °C but is not complete until 1000 °C. The resultant carbide appears in a highly porous form in admixture with approximately 50% wt/wt amorphous carbon. The carbide content, x, of cubic NbC_x increases with heating time (at 1000 °C) as expected. Values of x ranging from 0.69–0.95 have been observed. The cubic mixed carbide, (Nb, W) C_x, is formed similarly from the -NbWO₆ system via an alkylammonium form in the presence of PAN, although progressive separation into cubic NbC_x and hexagonal WC_x occurs at temperatures above 1000 °C. The -NbWO₆ system does not form a well-defined alkylammonium salt; instead a mixture of -HNbWO₆ with PAN gives rise to a very poorly crystalline (Nb, W) carbide on reduction. In all cases, both a layered oxide and PAN are necessary to form the pure carbides at 1000 °C. The oxide/PAN composites appear to be intimate physical mixtures rather than ordered layered nanocomposites.     

Recent progress on the study of the microstructure and mechanical properties of ECAE copper

Results on the microstructure and the tensile properties of equal channel angular extruded (ECAE) copper processed for one to 16 passes are presented and compared with the available literature data. With increasing number of passes (N), the microstructure changes from a strongly elongated shear band structure after N = 1 and 2, towards a more equiaxed subgrain and grain structure. This is accompanied by a decrease in the cell wall or subgrain-boundary widths and an increase in recovered or even recrystallised grain structures with low dislocation densities. Electron backscatter diffraction measurements have indicated that for lower N, the location of Σ3 boundaries is restricted to shear bands, while at greater N, Σ3 boundaries were found to be more widely distributed. Texture measurements indicate close similarity with simple shear texture components and a spread of the orientation components with greater N. Upon comparing the tensile behaviour of as-ECAE Cu with the surveyed literature, broad agreement on the strength of the material is achieved. However, a strong variation in the percentage elongation to failure is also noted. Strain hardening and deformation kinetic analysis via strain rate jump tests indicate an evolution from stage III to V hardening during post-ECAE compression and a saturation in the strain rate sensitivity after N = 4 resulting in maximum values of ∼0.02. Our results suggest that rather than a change in deformation mechanism, the increase in ductility with increasing N is associated with an increase in the mean free path of dislocations—with the grain boundaries remaining actively involved as the transmitter of plastic strain and their interaction with dislocations being the rate controlling deformation mechanism.

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Microstructure and mechanical properties of spark plasma sintered titanium‐added copper/reduced graphene oxide composites

Copper matrix composites were fabricated through mixing fixed amount of reduced graphene oxide and the different amounts of titanium. The dried copper/titanium/reduced graphene oxide mixture powders were firstly obtained by the wet‐mixing process, and then the spark plasma sintering process realized their faster densification. In the as‐sintered bulk composites, the layered reduced graphene oxide network, uniform titanium particles and copper‐titanium solid solution are observed in copper matrix. Investigations on mechanical properties show that the as‐prepared bulk composites exhibit the hardness and compressive yield strength compared with single reduced graphene oxide added composites. Increased titanium addition resulted into higher hardness and strength. The relevant formation and failure mechanisms of the composites and their influence on mechanical properties were discussed.     

Wetting characteristics of copper on niobium

Sessile drop experiments have been performed in a temperature range between 1090 and 1300° C, aiming to study the wetting of niobium by liquid copper and the influence of different atmospheres (argon, hydrogen, vacuum), crystallographic orientation, roughness, and oxygen doping of niobium on the wetting angle. At the peritectic temperature in the Cu-Nb system (1090° C) the contact angle is high,=67°, denoting poor wetting. With increasing temperature, decreases steadily for all samples. The wetting is at its lowest for the oxygen-doped samples and at its best for samples annealed under a hydrogen atmosphere. A new mechanism is suggested and discussed for oxygen degassing of niobium through liquid copper.