Coarse-grained Copper Research Articles

Effect of cryogenic impact deformation and subsequent annealing on microstructure and microhardness of pure copper

This article presents an application of cryogenic impact deformation in effective grain refinement in coarse-grained copper. Ultrafine-grained microstructure was processed by means of cryogenic impact deformation using an air hammer with an initial impact velocity of 7.35ms−1. For strain ε=2.21, TEM observation showed that the initial equiaxed coarse grains (18μm) evolved into a lamellar structure of submicron size narrow domains delineated by low-angle grain boundaries. The microhardness of the deformed sample increased from 74.2±1.2HV to 118.3±1.8HV. Subsequently annealed at 190°C for 60min led to a significant reduction of dislocation density. Elongated ultrafine grains (with a mean width of 175nm) embedded with remaining nanoscale deformation twins were observed in the annealed sample. This study highlights the potential of cryogenic impact deformation to produce bulk ultrafine-grained materials.

Deformation and fracture mechanisms and structural changes in coarse-grained copper under shock-wave loading

Results of experiments on shock-wave deformation of M2 copper under uniaxial loading are presented. Light, scanning, and transmission electron microscopy methods are used to reveal specific features of mechanisms of deformation and fracture of copper during the formation of a main spall crack. The parameters of spall strength, damage, and self-similarity of the spall crack contour are determined.

Near threshold fatigue crack growth in ultrafinegrained copper

The near threshold fatigue crack growth in ultrafine-grained (UFG) copper at room temperature was studied in comparison to conventional coarse-grained (CG) copper. The fatigue crack growth rates da/dN in UFG copper were enhanced at ΔK ≤ 7 MPa√m compared to the CG material. The crack closure shielding, as evaluated using the compliance variation technique, was shown to explain these differences. The effective stress intensity factor amplitude AKeff appears to be the same driving force in both materials. Tests performed in high vacuum on UFG copper demonstrate the existence of a huge effect of environment with growth rates higher of about two orders of magnitude in air compared to high vacuum. This environmental effect on the crack path and the related microstructure is discussed on the basis of fractography observations performed using scanning electron microscope and completed with field emission scanning electron microscope combined with the focused ion beam technique.

Activation parameters and deformation mechanisms of ultrafine-grained copper under tension at moderate temperatures

Mechanical properties, characteristic features of deformation behavior, the activation energy for plastic deformation, strain rate sensitivity and activation volume of ultrafine-grained (UFG) and conventional coarse-grained (CG) copper have been studied by tension in the temperature interval of 293–573K and in the strain rate interval of 1.3×10−2–3.0×10−5s−1. It is found that both the properties and the activation parameters differ significantly in UFG and CG copper suggesting the different deformation mechanisms. Plastic flow is shown to be controlled by grain boundary diffusion in the case of UFG copper. Considering the thermal activation analysis data and deformation relief appearing on the pre-polished surface of the test samples, the significant contribution of grain boundary sliding to the overall deformation during plastic flow of UFG copper at moderate temperatures is supposed.

Fatigue Crack Growth in Ultrafine-Grained Copper Obtained by ECAP

The propagation of long fatigue cracks in ultra fine-grained (UFG) copper obtained by equal channel angular pressing (ECAP) is investigated in the mid ΔK range and in the near threshold regime. The crack growth rates in UFG copper are substantially faster than in coarse-grained (CG) copper. A huge influence of environment is observed, with growth rates faster of more than two orders of magnitude in air compared to vacuum. The crack growth mechanisms are discussed on the basis of microfractographic observations and the deformation texture.

Irradiation damage of single crystal, coarse-grained, and nanograined copper under helium bombardment at 450 °C

Abstract

Wear resistance and electroconductivity in copper processed by severe plastic deformation

The wear properties and electroconductivity of three ultra-fine grained (UFG) commercially pure copper materials, subjected to combinations of high-pressure torsion (HPT) and equal-channel angular pressing (ECAP), were studied and compared with conventional coarse-grained (CG) copper. The results are discussed as a function of microstructure and microtexture. The UFG specimens demonstrate no significant decrease in electroconductivity by comparison with CG copper. The conductivity of an ECAP+HPT specimen showed a value of 99.3% of annealed copper and the same sample showed the lowest wear rate among the UFG specimens. However, all UFG specimens gave higher wear rates than CG copper and there was no evidence for the enhanced wear resistance reported for nanocrystalline copper obtained by surface mechanical attrition treatment (SMAT) and electrodeposition. This result is discussed in the context of two competing processes: shear by sliding and normal compression by impact load.

Fatigue cracking and fracture behaviors of coarse-grained copper under cyclic tension–compression and torsion loadings

In this study, fatigue damage and fracture behaviors of coarse-grained copper are investigated under cyclic tension–compression and torsion loadings. The cyclic stress response and dislocation patterns of coarse-grained copper under the two fatigue tests are compared. Fatigue crack initiation, propagation and fracture surfaces of the specimens are found to rely on the loading mode. In general, the initiation and early propagation of fatigue cracks are mostly controlled by the direction of the maximum shear stress. The effect of normal stress can be used to explain the common tendency for fatigue cracks to propagate from stage I to stage II. Combining the formation mechanism of fatigue striations with the analysis of stress state, it is found that fatigue cracks are much easier to transform from stage I to stage II under tension–compression fatigue than under torsion fatigue. But under torsion fatigue with higher strain amplitude, because the driving force is much higher, stage II fatigue crack propagation obviously exists. Additionally, by analyzing the growth condition of longitudinal and circumferential cracks, the fracture process of torsion fatigue with different strain amplitudes is discussed.

Study on the effect of temperature rise on grain refining during fabrication of nanocrystalline copper under explosive loading

Nanocrystalline (NC) copper was fabricated by severe plastic deformation of coarse-grained copper at a high strain rate under explosive loading. The feasibility of grain refinement under different explosive loading and the influence of overall temperature rise on grain refinement under impact compression were studied in this paper. The calculation model for the macroscopic temperature rise was established according to the adiabatic shock compression theory. The calculation model for coarse-grained copper was established by the Voronoi method and the microscopic temperature rise resulted from severe plastic deformation of grains was calculated by ANSYS/ls-dyna finite element software. The results show that it is feasible to fabricate NC copper by explosively dynamic deformation of coarse-grained copper and the average grain size of the NC copper can be controlled between 200∼400 nm. The whole temperature rise would increase with the increasing explosive thickness. Ammonium nitrate fuel oil explosive was adopted and five different thicknesses of the explosive, which are 20 mm, 25 mm, 30 mm, 35 mm, 45 mm, respectively, with the same diameter using 20 mm to the fly plate were adopted. The maximum macro and micro temperature rise is up to 532.4 K, 143.4 K, respectively, which has no great effect on grain refinement due to the whole temperature rise that is lower than grain growth temperature according to the high pressure melting theory.

Molecular dynamics investigation of shock front in nanocrystalline copper

The elasto-plastic deformation behavior, yield strength and strain rate of material under shock compression can be represented by shock front, and the shock front is also related to the variation of strength after shock compression. In this paper, we study the dynamic plastic deformation processe of nanocrystalline copper under shock compression through molecular dynamics simulations. We also explore the dependences of the shock front and the mechanism of elasto-plastic deformation on grain boundary, and make a comparison with the case of the shock response of nanocrystalline aluminum. This investigation shows that the contribution of grain boundary to the shock-front width of nanocrystalline copper are smaller than that of nanocrystalline aluminum. The plastic mechanism of nanocrystalline copper is dominated by the emission and propagation of partial dislocations, and the full dislocation and deformation twin are rarely found in the samples. From the simulations are also found that the shock-front width decreases with the increase of loaded shock stress. A quantitative inverse relationship between the shock wave front width and the shock intensity is obtained. This quantitative inverse relationship is close to other simulation result of nanocrystalline copper and quite different from results of coarse-grained copper compression experiments.

Investigation of Specimen- and Grain-Size Dependence of Yield Stress in Electrodeposited Nanocrystalline Copper through Micropillar Compression

ABSTRACTThe specimen-size dependence of yield stress of nanocrystalline copper with average grain size (d) of 360 nm has been investigated through uniaxial compression tests of micrometer-size pillars fabricated via the focused ion beam method. The yield stress decreases with the decrease in the micropillar size while the yield stress is almost constant for larger micropillars. The critical specimen size (t) is approximately 12.5 μm, correspoinding to the critical (t/d) value, (t/d)*, of 35, which is much larger than that for coarse-grained copper polycrystals.

Size effects in micro cup drawing

Experimental studies into the effect of blank thickness on the deep drawing response of the coarse-grained and ultrafine-grained copper demonstrated the occurrence of a size effect: the dependence of the maximum load and the limit drawing ratio on the blank thickness in sub-millimetre range. A dislocation based constitutive model taking into account the thickness effects was used for numerical simulations of the process. It was demonstrated that the occurrence of the blank thickness effect is governed by the ratio of the blank thickness t to the grain size D of the material. Critical values of the t / D ratio below which the size effect comes to bearing were determined. The obtained results can be seen as a demonstration of more general suitability of the model developed for predicting microforming operations with full account of the specimen or work-piece dimensions.

Effects of grain refinement due to severe plastic deformation on the growth behavior of small cracks in copper

To clarify the effects of grain refinement on the growth behavior of small surface cracks, fatigue tests on round bar specimens were conducted on coarse-grained (CG) copper and ultrafine-grained (UFG) copper processed with equal channel angular pressing. The growth behavior of a small crack was monitored using a plastic replication technique. The difference in the growth behavior tendency between the CG and UFG copper occurred at an extremely low fatigue crack growth rate (FCGR) of dl/dN<10−6mm/cycle. For the UFG copper, the FCGR temporarily dropped at around dl/dN=3×10−7mm/cycle and then gradually recovered with subsequent cycling. After the FCGR exceeded dl/dN=10−6mm/cycle, it was nearly proportional to the crack length. On the other hand, the FCGR of the CG copper showed no temporary drop but rather increased steadily with cycling. To understand the reason for the temporary decrease of the FCGR of UFG copper, a crack growth model based on the reversible plastic zone size at a crack tip and the related microstructural factors were developed. In addition, FCGR evaluation was discussed by applying σanl (σa, nominal stress amplitude; l, crack length; n, material constant) to both materials.

Formation of thick nanocrystalline surface layer on copper during oscillating sliding

Severe plastic deformation on the subsurface layer of metals by means of sliding under loading could lead to the formation of nanocrystalline surface tribolayer. In this work, a nanocrystalline surface layer with a thickness of 120μm is formed on a coarse-grained pure copper sheet after oscillating sliding at room temperature in argon. In the as-formed top surface layer, the average transverse and longitudinal grain size is about 20nm and 37nm, respectively. This work provides experimental evidence that dry sliding friction can be developed as a surface nanocrystallization technology to produce nanocrystalline surface layer with a thickness exceeding 100μm.

Influence of grain size and temperature on micro upsetting of copper

Grain size and temperature significantly influence flow behaviour in metal-forming processes. This influence is even more pronounced in micro metal forming, because so-called ‘size effects’ cause inhomogeneity within the material and inaccuracy in the shape of the items produced. In this study, a specialized metal forming system was developed to investigate the effects of grain size and temperature on the micro upsetting of copper. To refine the grains of copper, the study used equal channel angular extrusion (ECAE) and annealing techniques, after which the treated copper was carefully machined to prepare billets. The results clearly indicate how the flow stress of fine-grained copper exhibited a greater degree of sensitivity to forming temperature than coarse-grained copper. With the refinement to the grain size and increased forming temperature, it was possible to reduce inhomogeneous flow, leading to improvements in the edge quality of the deformed workpieces in micro upsetting.

Comparisons of dry sliding tribological behaviors between coarse-grained and nanocrystalline copper

Abstract Dry sliding tribological behaviors of nanocrystalline (NC) and coarse grained (CG) Cu were studied by using a ball-on-plate tribometer with a counterface ball of cemented tungsten carbide. The results showed that prior to oxidation and delamination, the steady-state friction coefficients (FCs) of NC and CG Cu are comparable (∼0.35). As oxidation with delamination of wear surface occur, the FC for either CG or NC Cu increases gradually, approaching a steady-state FC (∼0.63). The wear resistance of the NC Cu was enhanced by at least one order of magnitude under the measured loads ranging from 5 N to 25 N in comparison with the CG counterpart, which is mainly attributed to the higher hardness of the NC layer.

Microstructure and Properties of Surface Nanostructured Copper

Surface nanocrystallization (SNC) is a novel method for improving materials properties. Nanostructured surface layers of about 20 μm thickness were produced in copper plate samples by means of surface mechanical attrition treatment (SMAT). The behaviors of the SMAT samples were investigated by using transmission electron microscopy (TEM), Vickers hardness testing and potentiodynamic anodic polarization tests. The experimental results showed that the longer the peening time was performed on the copper pate samples, the thicker the deformation layers formed. The microhardness results for the top surface layer of the copper plate sample are 1.723 GPa and 1.752 GPa for 45 and 60 min, respectively, which are about two times higher than that of the matrix. The primary passivate potential of nanocrystalline copper was more negative than that of coarse-grain copper.

Method of research on micro- and mesoscale high-rate deformation of metals

The paper describes a new method of research on micro- and mesoscale high-rate (∼105-107 s−1) deformation of metals. The method consists in studying the collapse of cylindrical holes of initial diameter Do = 0.5–2 mm by shock waves of known intensity and duration. The method was tested on annealed coarse-grained Ml copper.

Fabrication of nanocrystalline copper by explosive loading and its dynamic mechanics properties

Nanocrystalline (NC) copper is fabricated by the method of severe plastic deformation of coarse-grained copper under explosive dynamic loading at high strain rates. The dynamic mechanical properties of NC copper are studied by the split Hopkinson pressure bar method. The results show that it is feasible to fabricate nanocrystalline copper by explosive dynamic plastic deformation of coarse-grained copper and the grain size of NC copper can be smaller than 100 nm. Twinning and formation of dislocations are the main mechanisms of grain refining. The dynamic yield strength of NC copper increases with decreasing average grain size and increasing strain rate.

Investigation on Corrosion Behaviors of Ultra-Fine Grain Copper in 3.5% NaCl Solution

The corrosion behavior of ultra-fine grain (UFG) copper bulk prepared by equal channel angular pressing (ECAP) was studied in 3.5% NaCl solution. The effect of ECAP deformation on the copper corrosion is controversial in the literature, and worth to verify by means of various experimental techniques. Corrosion performances of UFG copper were investigated in comparison with that in recrystallized coarse grain (CG) copper by polarization curves, Tafel extrapolation method, electrochemical impedance spectroscopy(EIS). The shape of polarization curves and type of corrosive attack remains the same in the UFG and the coarse-grain state. UFG copper exhibited a lower corrosion current and high self-corrosion potential in comparison with CG copper. Electrochemical experimental results showed that UFG copper increased in resistance to corrosion compared with CG copper. This decrease in corrosion resistance was mainly attributed to the more compact corrosion film of UFG copper. The compact passive film led to decrease of the diffusion capability of ions within the corrosion film and corrosion rate.