Ultrafine-grained Copper Research Articles

Ultrafine-grained copper produced by machining and its unusual electrochemical corrosion resistance in acidic chloride pickling solutions

Ultrafine-grained (UFG) copper was prepared by facile machining procedure. High resolution transmission electron microscopy images revealed that, in UFG Cu, minimum grain size of 80nm could be formed when a small machining rake angle was applied. The electrochemical corrosion behavior of UFG Cu in 0.5M HCl was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. Comparing with coarse-grained Cu, UFG Cu exhibited notably declined corrosion current density. Particularly, when the size of Cu grains were reduced from 500μm to 80nm, the charge transfer resistance of anodic dissolution step dramatically increased from 200 to 621Ωcm2.

Compressive behavior and rate-controlling mechanisms of ultrafine grained copper over wide temperature and strain rate ranges

In the present work, we systematically investigated the mechanical behaviors of ultrafine grained copper under quasi-static and dynamic loading conditions at temperatures ranging from 77 to 573K. Based on experimental results, we explore the coupling effect of strain rate and temperature on the plastic deformation of ultrafine grained copper. The strain rate, temperature sensitivity and the apparent activation volume have been determined. The results indicate that the flow stress of ultrafine-grained copper shows enhanced sensitivity to both testing temperature and strain rate comparing with its coarse grained counterpart. Meanwhile, the strain rate sensitivity also increases with rising temperature while the temperature sensitivity seems to decrease with increasing strain rate. The estimated thermal activation volume of ultrafine-grained copper is about on the order of ∼10b3, which is of the same order with that for grain boundary diffusion-controlled processes. However, it is concluded that such processes should be ruled out as the dominant mechanisms under our experimental temperature and strain rate conditions. Instead, thermal activated dislocation interaction appears to be the dominant rate-controlling mechanism.

Fatigue-induced grain coarsening and crack growth behavior in ultrafine-grained copper under different loading histories

The crack growth direction in ultrafine-grained copper under constant stressing depends on the applied stress amplitude. The growth direction is perpendicular to the loading axis at low stresses and inclines at 45° to the loading axis at high stresses. To clarify physical causes of such different behaviors at high and low stress amplitudes, two-step fatigue stress tests are conducted. The formation mechanisms of crack paths at high and low stress amplitudes and effect of pre-stressing on subsequent crack growth direction are discussed considering the morphological features of surface-damage formed under different cyclic-stress histories.

A micromechanical model of the evolution of stress and strain fields in ultrafine-grained metal structures under tension

In this study, the authors present a micromechanical finite element modelling approach to the analysis of a set of ultrafine-grained polycrystalline copper structures. The models take into account both realistic grain distribution and the number of refinement passes of the microstructure. The grain size effect is successfully predicted, with non-uniform deformation and shear localization reproduced during global plastic flow in agreement with experimental observations. The models provide a useful tool with which to interpret damage and fracture mechanisms occurring in ultrafine-grained structures.

Grain Size Variation during Low Temperature Creep and Tensile Deformation of Ultrafine-Grained Copper

Grain Size Variation during Low Temperature Creep and Tensile Deformation of Ultrafine-Grained Copper

Thermal stability of highly nanotwinned copper: The role of grain boundaries and texture

Abstract

Formation mechanism and organizational controlling of ultra-fine-grain copper processed by asymmetrical accumulative rolling-bond and annealing

The initial copper with large grain sizes of 60–100 μm was processed by six passes asymmetrical accumulative rolling-bond (AARB) and annealing, the ultra-fine-grained (UFG) copper with grain size of 200 nm was obtained, and the microstructures and properties were studied. The results show that there are large sub-structures and also texture component C for the UFG copper obtained by six passes AARB, possessing high strength and microhardness in company with poor elongation and conductivity. Thereafter, the UFG copper was annealed at 220 °C for 35 min, in which the sub-structures disappear, the grain boundaries are composed of big angle grain boundaries, and the textures are composed of a variety of texture components and parts of twins. Compared with the UFG copper obtained by six passes AARB, the tensile strength and yield strength for the UFG copper obtained by six passes AARB and annealing at 220 °C for 35 min are decreased slightly, the elongation and conductivity are improved obviously.

Competing regimes of rate dependent plastic flow in ultrafine grained metals

Low temperature (77K, 243K and 255K) mechanical tests were carried out at strain rates between 7.10−4s−1 and 5.10−2s−1 on pure ultrafine grained (UFG) copper (grain size 100nm) to mimick room temperature high strain rates. Variation of the activation volume measured as a function of the stress is consistent with the analytical model proposed in [C. Duhamel, Y. Bréchet, Y. Champion Int. J. Plast. 26 (2010) 747–757] to explain the rheology of UFG metals. This model is based on dislocations interactions at grain boundaries and grain boundaries sliding. The experiments confirm that the strain rate sensitivity (which should be a criterion for plasticity in absence of macroscopic work hardening) increases with the strain rate (or stress) at high strain rate, as also observed for nanotwinned copper. From the model, a strain rate sensitivity criterion is derived, function of a characteristic UFG strength parameter and grain boundary properties. The analytical description of deformation of UFG including microstructural properties should help for a quantitative control of the macroscopic properties of UFG.

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.

Effect of pre-annealing treatment on the compressive deformation and damage behavior of ultrafine-grained copper

Abstract The effect of short-term pre-annealing treatment on the compressive deformation and damage behavior of ultrafine-grained (UFG) copper produced by equal channel angular pressing (ECAP) has been studied by mechanical tests and microstructural observations. It is found that a controlled 10-min pre-annealing treatment on the UFG copper below the recrystallization temperature can result obviously in a higher strength (higher flow stress level) and a higher flow stability (lower stress softening rate) under uniaxial compression, as compared to the case of as-ECAPed sample. Observations of surface and interior deformation damage features indicate that, compared to the as-ECAPed copper sample, the pre-annealed ECAPed copper at less than recrystallization temperature may exhibit a better compressive plastic deformation capacity, namely, the destructive deformation morphology of large-scale shear bands normally observed in compressed as-produced UFG copper sample would not appear in compressed pre-annealed sample, except for a relatively smooth surface morphology featuring some ‘step’ reliefs with a few discontinuous shear bands and non-propagation voids. Finally, the deformation mechanism of different kinds of severe plastic deformation (SPD) processed UFG materials as well as their pre-annealing effects are further discussed, and a general rule is summarized as follows: an appropriate selection of pre-annealing temperature and annealing time is needed for improving both strength and ductility of SPDed materials with a microstructure comprising equiaxed grains and well-defined grain boundaries.

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.

Microstructure and mechanical behavior of UFG copper processed by ECAP following different processing regimes

Commercially pure (99.9%) copper was severe plastically deformed by equal-channel angular pressing (ECAP) following route-Bc in three different processing regimes in order to obtain ultrafine-grained (UFG) microstructures leading to improved mechanical properties. In regime-1, the billets were processed at room temperature up to eight passes. The billets were processed at 200°C up to eight passes in regime-2. Regime-3 is a hybrid regime by which the billets were deformed at 200°C up to four passes first, and these billets were then processed at room temperature for one, two and four passes. In all regimes, the ECAP processing results in a refinement of the conventionally grained (CG) initial microstructure of copper down to sub-micron level leading to a great improvement in the strength as compared to CG copper. Among the regimes applied, regime-3 was found to be the best regime for improvement in strength along with adequate ductility. The samples showed more than eight times increases in yield strength after processing in regime-3 for 4 + 4 passes, from 47 MPa for CG copper to about 408 MPa for the UFG sample. Such improvement in strength was accompanied by a 16.9% total elongation and 6% uniform elongation. The processing in regime-2 resulted in the best elongation to failure of about 22% after eight passes, but it gave the lowest strength values among others.

A comprehensive study on the damage tolerance of ultrafine-grained copper

In this study the fracture behavior of ultrafine-grained copper was assessed by means of elasto-plastic fracture mechanics. For the synthesis of the material high pressure torsion was used. The fracture toughness was quantitatively measured by JIC as a global measure by recording the crack growth resistance curve. Additionally, the initiation toughness in terms of the crack opening displacement (CODi ) was evaluated as a local fracture parameter. The results presented here exhibit a low fracture initiation toughness but simultaneously a remarkably high fracture toughness in terms of JIC . The origin of the large difference between these two parameters, peculiarities of the fracture surface and the fracture mechanical performance compared to coarse grained copper will be discussed.

超微細結晶粒銅における転位の熱活性化運動過程

超微細結晶粒銅における転位の熱活性化運動過程

Retardation of Softening of Ultrafine-Grained Copper during Low Temperature Annealing under Uniaxial Tensile Stress

It is known that pure metals having ultrafine grains (UFGs) exhibit softening and grain coarsening at temperatures about one third of the melting temperatures. When such low-temperature annealing (LTA) of UFG copper is conducted under uniaxial tensile stress, the retardation of the softening is found to occur compared with that without stress. Observations of the microstructure and texture analysis indicate that the softening and the grain coarsening are attributed to recrystallization, and the presence of uniaxial tensile stress during LTA retards recrystallization. High voltage transmission electron microscopy revealed that the dislocation density of UFG copper annealed under uniaxial tensile stress is lower than that after LTA without stress. The retardation of recrystallization is associated with the reduction of the dislocation density of unrecrystallized UFG copper induced by dynamic recovery. [doi:10.2320/matertrans.MD201117]

Fatigue Crack Growth and Related Microstructure Evolution in Ultrafine Grain Copper Processed by ECAP

Fatigue crack growth tests were carried out using centre-cracked tensile (CCT) specimens of ultrafine grained (UFG) copper, aiming at clarifying microstructure evolution around the fatigue crack tip and at better understanding of fatigue crack propagation mechanisms. The fatigue crack growth tests revealed that UFG copper had the lowest threshold stress intensity factor range ΔKth. On the other hand, the crack growth rates were almost identical among the UFG and conventional specimens tested at higher ΔKI regime. From the microstructure observation after the fatigue crack growth test, significant grain growth was detected in the intimate vicinity to the fracture surface in UFG copper. The size of the grown grains in the UFG copper increased with increasing stress intensity factor range and with the size of the cyclic plastic zone.

Vacancies, twins, and the thermal stability of ultrafine-grained copper

Ultrafine-grained metals have impressive strength but lack the thermal stability necessary for most applications. Nano-scale, deformation twinned copper microstructures exhibit a rare combination of strength and stability. While storing less energy in their interfaces than other nanostructured metals, they also exhibit lower vacancy supersaturations, reducing the driving force and mobility for microstructure evolution. From a thermal stability perspective, the nano-twinned microstructure may thus be preferred over the more commonly produced nano-scale equiaxed microstructures.

Formation behavior of different topographies of crack growth path under high and low cyclic stresses in ultrafine-grained copper

Fatigue tests of ultrafine-grained copper specimens were conducted. The crack growth direction under constant stress depended on the magnitude of the stress amplitude. It occurred perpendicular to the loading axis at low stresses and was inclined at 45° to the loading axis at high stress. To clarify the different growth mechanisms between the high- and low-stress amplitudes, two-step fatigue stress tests were conducted. The crack path formation under high- and low-stress amplitude is discussed from the viewpoint of the evolution of damaged areas around the crack.

Deformation, structural changes and damage evolution in nanotwinned copper under repeated frictional contact sliding

Nanotwinned metals have the potential for use as structural materials by virtue of having a combination of high strength as well as reasonable ductility and damage tolerance. In the current study, the tribological response of nanotwinned copper has been characterized under conditions of repeated frictional sliding contact with a conical tip diamond indenter. Pure ultrafine-grained copper specimens of fixed grain size (∼450nm), but with three different structural conditions involving relatively high, medium and negligible concentrations of nanotwins, were studied. The effects of twin density and number of repetitions of sliding cycles on the evolution of friction and material pile-up around the diamond indenter were studied quantitatively by depth-sensing instrumented frictional sliding. Cross-sectional focused ion beam and scanning electron microscopy observations were used to systematically monitor deformation-induced structural changes as a function of the number of passes of repeated frictional sliding. Nanoindentation tests at the base of the sliding tracks coupled with large-deformation finite-element modeling simulations were used to assess local gradients in mechanical properties and deformation around the indenter track. The results indicate that friction evolution as well as local mechanical response is more strongly influenced by local structure evolution during repeated sliding than by the initial structure. An increase in twin density is found to result in smaller pile-up height and friction coefficient. Compared to the low-density nanotwinned metal, high-density nanotwinned copper showed significantly higher resistance to surface damage and structural changes, after the initial scratch. However with an increase in the number of sliding passes, the friction coefficient and rate of increase of pile up for all specimens acquire a steady value which does not change significantly in subsequent scratch passes. The frictional sliding experiments also lead to the striking result that copper specimens with both a high and low density of nanotwins eventually converge to a similar microstructure underneath the indenter after repeated tribological deformation. This trend strongly mirrors the well-known steady-state response of microcrystalline copper subjected to uniaxial cyclic loading. General perspectives on contact fatigue response of nanotwinned copper are developed on the basis of these new findings.

Effect of counterpressure during equal-channel angular pressing on nanoporosity formation in ultrafine-grained copper

Results of small-angle X-ray scattering and high-precision density measurements showed that the application of counterpressure during the equal-channel angular pressing (ECAP) of ultrafine-grained copper leads to a decrease in nanoporosity and an increase in mechanical properties of the ECAP-processed metal.