Ultrafine-grained Copper Research Articles

Ultrafine-Grained High Strength Cu-Ni-Si Alloys

Ultrafine-grained (UFG) pure copper has been in the focus of materials scientists over the last two decades, however ultrafine-grained high-strength copper alloys have scarcely been processed or characterized so far industrially.In this contribution, UFG copper alloys, especially Cu-Ni-Si alloys, being well known as ideal materials for electromechanical connectors, springs and leadframes, are presented. Precipitation hardened Cu-Ni-Si alloys are a well established and technologically important class of materials for a wide range of applications where high strength and good conductivity are required. Yield strength and fatigue properties of metallic alloys can be significantly enhanced by severe plastic deformation methods. In contrast to other strengthening methods such as solid solution hardening, severe plastic deformation leads to a weaker decrease of electrical conductivity and is therefore a means of enhancing strength while maintaining acceptable conductivity for current bearing parts and components. Characterization of these materials after severe plastic deformation by swaging, wire drawing and subsequent aging was carried out using conductivity-, hardness-and tensile tests as well as highly-resolved microstructural characterization methods.The results reveal that UFG low alloyed copper alloys exhibit impressive combinations of properties such as strength, conductivity, high ductility as well as acceptable thermal stability at low and medium temperatures. By a subsequent aging treatment the severely plastically deformed microstructure of Cu-Ni-Si alloys can be further enhanced and thermal stability can profit from grain-boundary pinning by precipitated nanoscale nickel silicides.

Deformation and Failure of Ultrafine-Grained Cu at Subambient Temperature

The ultrafine-grained copper was obtained by 12 passes of equal-channel angular pressing method. The uniaxial tensile tests at room temperature and the subambient temperature of 77 K show that the yield stress increases from the value of 128 MPa to the value of 138 MPa, respectively. In addition, the lowering the test temperature tends to the increase of the deformation before the failure. The fractographic analysis shows the transcrystalline ductile failure for all samples. Due to the high plasticity of nanostructured copper no influence of the nanoporosity on the failure process was observed.

Shear fracture mechanism in micro-tension of an ultrafine-grained pure copper using synchrotron radiation X-ray tomography

In order to investigate the early fracture of ultrafine-grained (UFG) pure copper with a partially recrystallized microstructure and simple shear texture, the evolution of surface strain was measured using in situ micro-tension with digital image correlation. The spatial distribution of voids in tensile specimens was revealed after testing using synchrotron radiation X-ray tomography. The results show that the shear fracture behavior is associated with void evolution in UFG copper and this is strongly affected by the simple shear texture produced by equal-channel angular pressing (ECAP). The results have important implications for use in micro-forming.

Structural instabilities during cyclic loading of ultrafine-grained copper studied with micro bending experiments

The cyclic mechanical properties and microstructural stability of severe plastically deformed copper were investigated by means of micro bending experiments. The ultrafine-grained structure of OFHC copper was synthesized utilizing the high pressure torsion (HPT) technique. Micron sized cantilevers were focused-ion-beam milled and subsequently tested within a scanning electron microscope in the low cycle fatigue regime at strain amplitudes in the range of 1.1 − 3.2 ∗ 10−3. It was found that HPT processed ultra-fine grained copper is prone to cyclic softening, which is a consequence of grain coarsening in the absence of shear banding in the micro samples. Novel insights into the grain coarsening mechanism were revealed by quasi in-situ EBSD scans, showing i) continuous migration of high angle grain boundaries, ii) preferential growth of larger grains at the expense of adjacent smaller ones, iii) a reduction of misorientation gradients within larger grains if the grain structure in the neighborhood is altered and iv) no evidence that a favorable crystallographic orientation drives grain growth during homogeneous coarsening at moderate accumulated strains, tested here.

Nanostructured pure copper fabricated by simple shear extrusion (SSE): A correlation between microstructure and tensile properties

In the present paper the variation of microstructural parameters and tensile properties of ultrafine-grained copper processed by simple shear extrusion (SSE) via namely route C in 1, 2, 4, 6, 8 and 12 passes is described. TEM analysis showed that the microstructure evolves from lamellar boundaries and elongated cells towards a more equiaxed homogeneous microstructure. After 12 passes, the grain fragmentation occurred in all the directions without any significant elongation in the grains. The minimum cell size is achieved after eight passes. Evaluation of dislocation density using scanning transmission electron microscopy observations shows a gradual increase of dislocation from one to eight passes following a reduction afterward. Yield stress and ultimate tensile stress reach a maximum after eight passes. The uniform elongation attains its minimum after eight passes. Reduction in dislocation density, grain growth, formation of Moiré fringes and twinning after twelve passes of SSE are some of the evidences for the softening. The critical grain size for the formation of nano twins (the onset of grain growth) is predicted. Prosperous prediction of yield stress using a strength–structure relationship helps in the understanding of the effect of dislocation density and microstructural observations.

Plastic deformation mechanisms of ultrafine-grained copper in the temperature range of 4.2–300 K

Main microstructural features of ultrafine-grained (UFG) polycrystalline oxygen-free copper (Cu–OF) obtained by direct and equal-channel angular hydrostatic extrusion were studied by EBSD and XRD methods. The effect of microstructure on the temperature dependences of the yield stress and strain rate sensitivity of the deforming stress was investigated using tensile and stress relaxation tests in the insufficiently studied temperature range of 4.2–300 K. Using thermal activation analysis it was established that in the range 77–200 K the rate of plastic deformation is controlled by the thermally activated mechanism of crossing the forest dislocations and its empirical parameters were obtained. The experimental anomalies below 77 K unaccountable by the forest crossing mechanism were explained by the inertial properties of dislocations revealed under conditions of high effective stress and low dynamic friction. The inversion of the temperature dependences of the activation volume observed above 200 K was attributed to the thermally activated detachment of dislocations from local pinning centers within the grain boundaries.

In-situ X-ray diffraction during tensile deformation of ultrafine-grained copper using synchrotron radiation

At the synchrotron facility, Super Photon Ring – 8 GeV, in-situ X-ray diffraction during tensile deformation was conducted on ultrafine-grained Cu with a grain size of about 300 nm fabricated by equal-channel angular pressing. The diffraction profile was observed with the time resolution of about 1 s using multiple MYTHEN detectors, and the diffraction angle and the full-width at half-maximum of some Bragg peaks were determined using the pseudo-Voigt function. From the analysis of Bragg peaks, it was found out that there are microscopically three regions; elastic, plastic and transition regions. The 0.2% proof stress obtained from the stress–strain curve locates within the microscopic transition region. Microstrain was evaluated using the Williamson–Hall method and the dislocation density was also obtained from the microstrain. The dislocation density starts increasing before 0.2% proof stress, which is associated with dislocation bow-out and emission from grain boundaries. The Taylor relationship seems to be still satisfied after 0.2% proof stress.

Microstructure and Mechanical Properties of Ultrafine-Grained Copper Produced Using Intermittent Ultrasonic-Assisted Equal-Channel Angular Pressing

We proposed intermittent ultrasonic-assisted equal-channel angular pressing (IU-ECAP) and used it to produce ultrafine-grained copper. The main aim of this work was to investigate the microstructure and mechanical properties of copper processed by IU-ECAP. We performed experiments with two groups of specimens: group 1 used conventional ECAP, and group 2 combined ECAP with intermittent ultrasonic vibration. The extrusion forces, microstructure, mechanical properties, and thermal stability of the two groups were compared. It was revealed that more homogeneous microstructure with smaller grains could be obtained by IU-ECAP compared with copper obtained using the traditional ECAP method. Mechanical testing showed that IU-ECAP significantly reduced the extrusion force and increased both the hardness and ultimate tensile stress owing to the higher dislocation density and smaller grains. IU-ECAP promotes conversion from low-angle grain boundaries to high-angle grain boundaries, and it increases the fractions of subgrains and dynamic recrystallized grains. Group 2 statically recrystallized at a higher temperature or longer duration than group 1, showing that group 2 had better thermal stability.

Simultaneous Grain Growth and Grain Refinement in Bulk Ultrafine-Grained Copper under Tensile Deformation at Room Temperature

Grain growth and grain refinement behavior during deformation determine the strength and ductility of ultrafine-grained materials. We used asymmetric cryorolling to fabricate ultrafine-grained copper sheets with an average grain width of 230 nm and having a laminate structure. The sheets show a high-true failure strain of 1.5. Observation of the microstructure at the fracture surface reveals that ultrafine laminate-structured grains were simultaneously transformed into both equiaxed nanograins and coarse grains under tensile deformation at room temperature.

Investigation on hot deformation behavior of nanoscale TiC-strengthened Cu alloys fabricated by mechanical milling

In this work, ultrafine-grained (UFG) copper alloys dispersed with nanoscale TiC particles were produced by combination of mechanical milling and spark plasma sintering (SPS). Microstructure evolution during uniaxial compression to height reduction of 20–60% in the temperature range of 400–700°C was investigated systematically. The stress-strain curves in all conditions were characterized by a fast increase in stress at initial stage and a subsequent yield drop. No obvious microstructure change was observed with increasing reduction ratio. There exhibited a tendency in grain size refinement with increasing strain rate even the change in grain size is slight. A slight grain growth was found with the compression temperature high up to 700°C. The high temperature tensile tests results suggested that the yield stress showed a relatively weak dependence on strain rate with a strain rate sensitivity parameter of around 0.02, and a low degree of strain hardening was observed. In addition, the recrystallization behavior was found to be inhibited by the ultrafine grains and nanoscale TiC particles in starting microstructure, because the grain boundaries or junctions in initial structure act as the severe pinning points, limiting bowing, and the nanoscale TiC particles prevent the nuclei formation of recrystallization. The fracture morphologies occurred during the tensile test were closely related to the change in particles distribution from random to aligned distribution during the hot press deformation.

Strengthening Mechanisms and Electrochemical Behavior of Ultrafine-Grained Commercial Pure Copper Fabricated by Accumulative Roll Bonding

In this study, the four-cycle accumulative roll bonding (ARB) process at room temperature was successfully used for grain refining in commercial pure copper. Atomic force microscopy (AFM) images revealed that the average grain size reduced from about 26 µm in the unprocessed material to about 180 nm after four cycles of ARB. Also, transmission electron microscopy image indicated that the average grain size reached to 200 nm after four cycles. The yield strength of the ultrafine-grained pure copper after fourth cycle (360 MPa) was about 400 pct higher than that of the annealed unprocessed sample (70 MPa). The contribution of dislocations in strengthening of the pure copper decreased from ~30 to ~3 pct whit increasing the number of ARB cycles from 1 to 4. Scanning electron microscopy micrographs of fractured surfaces of the tensile test specimens revealed that ductile fracture of annealed sample with deep equiaxed dimples replaced by shear ductile rupture with shallow and small elongated dimples in ARB-processed samples. Moreover, electrochemical impedance spectroscopy and Mott–Schottky analysis showed that the electrochemical behavior improved by increasing the number of ARB cycle.

An investigation on rolling texture transition in copper preprocessed by equal channel angular pressing

The effects of equal channel angular pressing (ECAP) and subsequent rolling path on the evolutions of rolling texture and flow stress anisotropy in the finally cold-rolled copper sheet was investigated. Copper billets processed by 1, 2, 4, and 8 passes of ECAP were subjected to cold rolling via three different paths: unidirectional rolling along extrusion or transverse direction of ECAP, or cross rolling. The microstructure, texture, and flow stress were characterized by EBSD, TEM, XRD, and tensile testing, respectively. The rolling texture was found transformed from copper-type to brass-type as the initial prerolling microstructure was refined from coarse grained to ultrafine grained (UFG) by multipass ECAP; Cross rolling, which is conventionally considered effective in reducing texture strength and thus mechanical anisotropy in coarse-grained materials, has proven to be ineffective in UFG copper. The flow stress anisotropy in the rolled copper sheet was found mainly controlled by the microstructural-dependent anisotropy of critical resolved shear stress rather than texture strength, and this leads to decrease of flow stress anisotropy in the cold-rolled copper sheet upon the increase in the number of preprocessing ECAP passes.

Microstructural evolution and micro/meso-deformation behavior in pure copper processed by equal-channel angular pressing

Pure copper was processed by equal channel angular pressing (ECAP) using route Bc with a die channel angle of 110°. Electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to analyze the microstructure and texture during ECAP processing and after combining ECAP with micro-compression. Ultrafine-grained pure copper with an average grain size of ~0.5µm was achieved after ECAP processing through 12 passes. The anisotropic behavior during micro-compression is caused by the micro-texture of pure copper introduced by ECAP processing. Three kinds of strain softening behaviors are found during micro-compression tests in pure copper processed by ECAP. The strain softening of micro/meso deformation in UFG pure copper is caused by the accelerated dislocations annihilation due to the high dislocation recoveries at HAGBs, which is consistent with the deformation mechanism of grain boundary sliding and grain rotation.

Investigation on Grain Size Effect of Rolling Texture in Copper

Cold rolling (CR) was conducted on coarse grained (CG) and ultrafine-grained (UFG) coppers, obtained by 1 and 8 passes in the equal channel angel pressing (ECAP), to investigate the effect of grain size on rolling texture. The microstructure was refined to UFG (~420 nm) with the ECAP pass increased to 8, while only band-like CG microstructure was observed in the 1 pass processed copper. The influence of the texture before CR could be excluded as the crystallographic texture kept similar for different ECAP pass. Pole figures (PFs) showed that the shear texture introduced by ECAP was replaced by rolling texture after CR. Furthermore, the rolling texture was a kind of classical copper-type for the CG copper, while a brass-type rolling texture was observed in the UFG copper. TEM results confirmed that the deformation nanotwins were only observed in the UFG copper, while the microstructure of CG copper was further compressed and subdivided. It indicated that the observed differences in rolling texture component and density might be contributed to the grain size effect which resulted in different deformation mechanism and grain subdivision behavior.

Twin Boundaries merely as Intrinsically Kinematic Barriers for Screw Dislocation Motion in FCC Metals.

Metals with nanoscale twins have shown ultrahigh strength and excellent ductility, attributed to the role of twin boundaries (TBs) as strong barriers for the motion of lattice dislocations. Though observed in both experiments and simulations, the barrier effect of TBs is rarely studied quantitatively. Here, with atomistic simulations and continuum based anisotropic bicrystal models, we find that the long-range interaction force between coherent TBs and screw dislocations is negligible. Further simulations of the pileup behavior of screw dislocations in front of TBs suggest that screw dislocations can be blocked kinematically by TBs due to the change of slip plane, leading to the pileup of subsequent dislocations with the elastic repulsion actually from the pinned dislocation in front of the TB. Our results well explain the experimental observations that the variation of yield strength with twin thickness for ultrafine-grained copper follows the Hall-Petch relationship.

Modified strain rate regime in ultrafine grained copper with silver micro-alloying

Ultrafine grained (UFG) copper with an average grain size of 115nm was micro-alloyed with silver (UFG CuAg) to analyze its effect on strain rate sensitivity. The materials were prepared by sintering Ag microalloyed-Cu ultrafine powder, using the spark plasma sintering (SPS) technique and after H2 pre-annealing for oxide reduction. Mechanical tests show that UFG CuAg follows the law of behavior established for UFG Cu: m≈B(1−σ0/σ), relating the strain rate sensitivity m and the applied stress, σ. In this relation, σ0 is a threshold grain size dependent stress and B is characterizing the grain boundaries diffusion properties. Main difference compared to pure UFG copper is a large value of the parameter B for UFG CuAg, indicating a dominant role of the grain boundaries structure and chemistry on the rheology of UFG metals.

Giant Young’S Modulus Variations in Ultrafine-Grained Copper Caused by Texture Changes at Post-Spd Heat Treatment / Gigantyczne Zmiany Modułu Younga W Ultra Drobnoziarnistej Miedzi Spowodowane Przez Zmiany Tekstury W Trakcie Obróbki Cieplnej Po SPD

The effect of annealing on dynamic Young’s modulus, E, of ultrafine-grained (UFG) copper obtained by combined severe plastic deformation (SPD) is investigated. It is established that Young’s modulus in the SPD-prepared samples exceeds that in the coarse-grained fully annealed (CGFA) samples by 10 to 20 %. Isothermal annealing at elevated temperatures between 90 and 630°С leads to a sharp decrease of Young’s modulus for annealing temperatures above 210°С. After annealing at 410°С, the value of E reaches its minimal value that is 35 % lower than E in CGFA samples (total change in E is about 47 % of the initial value). Further annealing at higher temperatures leads to an increase in Young’s modulus. It is shown, that the unusual behavior of Young’s modulus is caused by formation of the <111> axial texture in the SPD-treated samples which then is replaced by the <001> texture during the post-SPD heat treatment.

Microstructural Evolution and Micro‐Compression in High‐Purity Copper Processed by High‐Pressure Torsion

Pure copper is subjected to severe plastic deformation at room temperature using quasi‐constrained high‐pressure torsion through 1/4 to 10 turns. The evolution of the microstructure is monitored using X‐ray diffraction and electron backscatter diffraction. Specimens are analyzed both before and after micro‐compression of ≈50%. The results show that after 10 turns there is a homogeneous ultrafine‐grained (UFG) microstructure with an average grain size of ≈250 nm and with a simple shear texture. The micro‐compression testing shows that UFG specimens exhibit a better surface quality after compression thereby demonstrating an excellent potential for using UFG copper in micro‐forming.

Passivation Behavior of Ultrafine-Grained Pure Copper Fabricated by Accumulative Roll Bonding (ARB) Process

In this study, passivation behavior of ultrafine-grained (UFG) pure copper fabricated by ARB process in 0.01 M borax solution has been investigated. Before any electrochemical measurements, evaluation of microstructure was obtained by transmission electron microscopy (TEM). TEM observations revealed that with increasing the number of ARB passes, the grain size of specimens decrease. Also, TEM images showed that UFGs with average size of below 100 nm appeared after 7 passes of ARB. To investigate the passivation behavior of the specimens, electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis was carried out. For this purpose, three potentials within the passive region were chosen for potentiostatic passive film growth. EIS results showed that both passive film and charge-transfer resistance increases with increasing the number of ARB passes. Moreover, Mott-Schottky analysis revealed that with increasing the number of ARB passes, the acceptor density of the passive films decreased. In conclusion, increasing the number of ARB passes offers better conditions for forming the passive films with higher protection behavior, due to the growth of a much thicker and less defective films.

The effects of reducing specimen thickness on mechanical behavior of cryo-rolled ultrafine-grained copper

Tensile tests on ultrafine-grained (UFG) Cu prepared by equal channel angular pressing (ECAP) and cryo-rolling at liquid nitrogen temperature were performed at various strain rates in order to investigate size effect on their tensile plastic deformation behaviors. It was found that the tensile strength decreases with reducing specimen thickness, which is called as the first-order size effect due to the increased surface softening. A surface layer model is developed to evaluate the relationship between the degree of surface softening and the number of grains across thickness direction. Compared to coarse-grained Cu subjected to rolling at room temperature, both tensile strength and ductility of the UFG Cu after cryo-rolling are enhanced, which is ascribed to the grain refinement and the increased fraction of high angle grain boundaries. The elevated strain rate sensitivity and decreased activation volume of UFG Cu suggest that its plastic deformation mechanism is dominated by dislocation–boundary interactions.