Hardening Of Copper Research Articles

Creep strength contribution due to precipitation hardening in copper\u2013cobalt alloys

In spite of its huge technical significance, there does not seem to be consensus about how to model the precipitation contribution to the creep strength. Most contributions in the literature are based on a constant internal stress (also called back stress or threshold stress) from the precipitation. It is well-known and it will also be demonstrated in the paper that this assumption is at variance with observations except for some ODS alloys. There is, however, one model developed by Eliasson et al. (Key Eng Mater 171–174:277–284, 2000) that seems to be able to represent experimental data without the use of any adjustable parameters. It has successfully been applied to describe the creep strength of austenitic stainless steels. Due to the fact that various mechanisms contribute to the creep strength in these steels, the model has not been fully verified. The purpose of this paper is to apply the model to published creep data for Cu–Co alloys, where the precipitation totally dominates the strength contribution to validate the model. In the paper, it is demonstrated that the model can indeed describe the influence of applied stress, alloy composition and heat treatment for the three analysed Cu–Co alloys.

Strengthening effects in precipitation and dispersion hardened powder metallurgy copper alloys

The hardening of copper and copper alloy matrix using powder metallurgy (PM) techniques and different ways for dispersoids formation, as well as analysis of their single and combined effects on the strength of obtained material at room and elevated temperatures, have been presented and discussed. Gas atomized Cu–3.8wt.%Ti and Cu–0.6wt.%Ti–2.5wt.%TiB2 (Cu–Ti–TiB2) powders and mechanically alloyed powder Cu–4wt.%TiB2 were used as starting materials. The powders were consolidated by hot isostatic pressing (HIP) and hot pressing (HP). Optical, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX), as well as transmission electron microscope (TEM) were used for microstructure characterization of the compacts. High strengthening of the Cu–Ti compacts was achieved by thermal treatment (aging) as a consequence of the development of modular structure and precipitation of metastable Cu4Ti(m). Hardening in the Cu–Ti–TiB2 compacts is due to simultaneous influence of the following factors: the development of modular structure, precipitation of metastable Cu4Ti(m), and the presence of TiB2 dispersoid nanoparticles. In case of Cu–TiB2 compacts, high starting values of hardness and hardness on the elevated temperatures result from the presence of finely distributed TiB2 particles in copper matrix obtained by mechanical alloying. Cu–Ti–TiB2 composite yields much higher hardness values compared with the binary Cu–Ti alloys, owing to primary TiB2 dispersions formed during atomization. Separation of metastable Cu4Ti precipitate and the presence of significantly finer TiB2 particles in the copper matrix are the reason for higher hardness values at peak temperatures (400–500°C) in multiple-hardened copper alloy compared to the dispersion-hardened.

Sliding friction wear of hydrogenated pure copper

The effect of cathodically charged hydrogen on the wear characteristics of pure copper has been investigated using pin on disc wear tests. Sliding distance and applied normal load were taken as wear test variables, while sliding velocity was kept constant. The tests revealed that cathodic hydrogen charging resulted in a reduction of the coefficient of sliding friction of copper. The coefficient of sliding friction was found to decrease with increasing charging time. Most of the wear debris of the hydrogen charged copper had a particle like shape and presented features of brittle fracture, whereas the wear debris of the uncharged copper was indicative of mechanical shearing and plastic deformation. The amount of copper wear debris was found to be a uniform function of charging time. A long time of charging produced more wear debris. The wear track of the hydrogen charged copper showed discontinuous smearing of copper and oxide particles, whereas the uncharged copper showed very smooth and uniform smearing of copper and oxide particles. Microcracks were observed in the wear tracks of the charged copper. Cathodic hydrogen charging was found to cause hardening in copper and the severity of hardening increased with increasing time.

Anomalous hardening in copper due to the growth of deformation-induced micro-twins after annealing

We report the annealing-induced anomalous hardening in rolled single crystal and polycrystalline copper caused by the appearance of a large amount of annealing twins. The origin of those twins is traced to the vast amount of micro-twins with width of 2–3 nm formed in the process of rolling.

Latent Hardening in Copper and Copper Alloys

A new phenomenological latent hardening model is developed for rate-dependent single crystal plasticity. The model quantitatively predicts the latent hardening evolution and latent hardening material dependence for f.c.c. single crystals. Increased overshoot, typically observed in copper alloys as opposed to copper, is rationalized based on the history dependence of latent hardening.

On dislocation interaction with radiation-induced defect clusters and plastic flow localization in fcc metals

Abstract Plastic instability associated with formation of narrow flow channels results from dislocation pinning–unpinning by defect clusters. We investigate the dynamics of dislocation interaction with radiation-induced defect clusters, and specifically with, firstly, sessile self-interstitial atom clusters in dislocation decorations and, secondly, stacking-fault tetrahedra (SFTs) in the matrix. It is shown that the critical stress to free trapped dislocations from pinning atmospheres can be a factor of two smaller than values obtained on the basis of rigid dislocation interactions. The unpinning mechanism is a consequence of the growth of morphological instabilities on the dislocation line. Dislocation sources are activated in spatial regions of low SFT density, where their destruction by glide dislocations leads to subsequent growth of localized plasticity in dislocation channels. We show that removal of SFTs is associated with simultaneous dislocation glide and climb. Jogs of atomic dimensions are formed when a fraction of SFT vacancies are absorbed by pipe diffusion. The width of a flow channel is explained in terms of two length scales: the size of an individual SFT, and the dislocation source-to-boundary distance (d of the order of micrometres). While dislocation segments climb by a few atomic planes with each SFT destruction event, d determines the total number of such events. Numerically computed channel widths (about 70–150nm), and the magnitude of radiation hardening in copper are consistent with experimental observations.

Cyclic hardening in copper described in terms of combined monotonic and cyclic stress-strain curves

Cyclic hardening in copper described in terms of combined monotonic and cyclic stress-strain curves

Cyclic hardening in copper described in terms of combined monotonic and cyclic stress-strain curves

Hardening of polycrystalline copper subjected to tension-compression loading cycles in the plastic region is discussed with reference to changes in flow stress determined from equations describing dislocation glide. It is suggested that hardening is as a result of the accumulation of strain on a monotonic stress-strain curve. On initial loading, the behaviour is monotonic. On stress reversal, a characteristic cyclic stress-strain curve is followed until the stress reaches a value in reverse loading corresponding to the maximum attained during the preceding half cycle. Thereafter, the monotonic path is followed until strain reversal occurs at completion of the half cycle. Repetition of the process results in cyclic hardening. Steady state cyclic behaviour is reached when a stress associated with the monotonic stress-strain curve is reached which is equal to the stress associated with the cyclic stress-strain curve corresponding to the imposed strain amplitude.

A numerical simulation of cyclic hardening in copper.

The constitutive equation for analyzing the cyclic deformation was developed by introducing the concept of the internal stress and the dislocation multiplication into the Johnston-Gilman's equation modified by the authors. Then, the numerical simulation of the cyclic stress-strain hysteresis loop accompanied by the cyclic straining was done for the annealed copper. The stress and plastic strain ranges estimated were slightly different from the test results in the transition hardening stage under the total strain range-controlled condition. Meanwhile, the former was in good agreement with the latter in the saturation hardening stage, where the cyclic mechanical properties of materials were commonly represented in the practical use. The analyzed hysteresis loop tends to be fatter in shape than the experimental one at the larger strain range controlled in the test. This problem, however, is solved by adjusting one of the parameters included in the proposed constitutive equation, which are determined by the static tensile and stress-relaxation tests.

Dislocation structure and work hardening in polycrystalline ofhc copper rods deformed by torsion and tension

The dislocation structure during work hardening in copper deformed by torsion and tension is investigated by X-ray line broadening and TEM. In the case of torsion the equivalent tensile flow stress and the rate of work hardening is lower than that obtained in tensile experiments. At the same time the dislocation density at the same equivalent strain is considerably larger in the torsionally deformed material and the TEM microstructure indicates a double-slip deformation mechanism. In the tensile deformed samples asymmetric X-ray line broadening indicates long-range internal stresses with relatively lower dislocation densities. In this mode of deformation multiple slip takes place. The lower equivalent flow stress and the smaller rate of work hardening in the torsionally deformed material is correlated with the restricted number of slip systems and the absence of strong long-range internal stresses, which leads to the relative ease of generating high dislocation densities.

Features of the hardening of copper and copper-aluminum alloys in microdeformation

Features of the hardening of copper and copper-aluminum alloys in microdeformation

The role of texture in work hardening of copper

The texture of previously-studied copper of grain sizes 3.4, 15 and 150 µ m has been measured, and compared to copper either mechanically processed to “randomize” texture, or cast in an equiaxed condition. Work-hardening behavior was also determined for the latter two types of material. The 3.4 µ m material was found to be very weakly textured, yet its stress-strain curve differed considerably from the Taylor-predicted curve based on a single crystal. More strongly fiber-textured material, such as the 150 µ m material, and also the weakly-textured cast material of 405 µ m grain size, agreed much more closely with the Taylor prediction. It was concluded that any texture effects on work hardening in copper are overwhelmed by grain size effects.

Latent hardening in copper and aluminium single crystals

Latent hardening tests are performed in tension on copper and aluminium single crystals. The latent hardening ratios (LHRs) are measured for different values of primary shear strain in Stage 1, and for various latent systems. These LHRs have an increasing variation in the earlier stage of primary slip before decreasing towards values higher than unity. Latent systems are classified into three groups, according to their LHR intensity, for both copper and aluminium, but the LHRs are higher in copper than in aluminium. These experimental results are discussed in terms of short range interactions between primary and latent dislocations. If the anisotropy of dislocation densities is taken into account, one obtains a synthetical description of latent hardening behaviour which is in agreement with all the reported experimental results.

Ternary solution-hardening of copper single crystals

Abstract The hardening of copper by two solute additions, germanium and silicon, has been measured in the plateau range. The results fit the predictions of a theory of Labusoh, and perhaps a180 one of Fleischer. They do not agree as well with a much-used computer interpolation formula.

Work hardening of copper, nickel, and G31 alloy by compression and explosion

Work hardening of copper, nickel, and G31 alloy by compression and explosion

Comparison of strain hardening in polycrystals and <111> single crystals: The example of copper

Tests on copper single crystals oriented about 2 deg from were in agreement with earlier work on similar crystals. Comparison to polycrystals using the Taylor factor showed agreement only for large-grained material. Literature data for single crystals in plane strain compression also agreed only with large-grained copper strain hardening curves. These results suggest that grain size strengthening occurs which is not accounted for in the Taylor analysis. It is therefore concluded that polycrystal strain hardening in copper is not simply a Taylor-averaged single crystal process.

The Bauschinger effect and cyclic hardening in copper

The Bauschinger Effect and cyclic hardening were studied during the first few cycles in copper single crystals of an “easy glide” orientation. Dislocation etch pitting was used to augment the mechanical measurements. It was found that significant rearrangements of the dislocation distribution occurred during stress reversal. Sharply defined kink bands which formed during forward straining broadened, and the overall dislocation distribution became more uniform during subsequent cyclic deformation. For cycling at constant strain amplitude, it was found that hardening above the prestress level occurred almost as rapidly as during unidirectional deformation. However, the absence of hardening during increments of Bauschinger strain yielded anomalously low hardening in terms of the cumulative strain. A cyclic hardening model is proposed in which the Bauschinger strains below the prestress level result from relaxation of internal stresses whereas “hardening strains” occur only above the prestress level. The non-symmetry, which causes higher Compressive stresses than tensile stresses during cycling at constant strain amplitude, is shown to result from larger Bauschinger strains below the prestress levelduring tensile half-cycles. Cell sizes and bundle spacings reported in the literature for cyclic tests have been correlated with the corresponding flow stresses and are consistent with predictions of the Long Range Stress Theory.

Deformation hardening of copper at low temperatures

1. A decrease in critical shear stress at 1.4°K was detected for all three fundamentally oriented copper single crystals. A similar temperature dependence for the elastic limit in copper polycrystals was not detected. 2. The studied hardening curve irregularities were either caused by avalanche, slip effects or twinning, depending on the single crystal strain axis. 3. The calculated activation parameters confirmed that the process controlling the plastic deformation in copper was the thermally activated intersection of gliding dislocations with forest dislocations. 4. An anomalous increase in relaxation depth at 1.4°K in comparison to the values at higher temperatures was detected.

Latent hardening in copper-aluminium alloys

The latent hardening in copper aluminium alloys has been investigated as a function of composition and prestrain. It is compared with the latent hardening in copper, the similarities between the pure metal and the alloys are emphasized, and it is suggested that the results are consistent with forest multiplication being inhibited more strongly in the early stages of deformation of alloys than in pure copper. A model based on the hypothesis that the high yield stress in alloys inhibits forest multiplication is proposed. This model may also explain some of the characteristics of Stages I in alloys. It is pointed out that latent hardening in f.c.c. metals and the alkali halides is very similar.

A study of quench hardening in copper

AbstractQuenching studies of copper, under clean conditions, have revealed a two stage hardening mechanism. The first stage corresponds to the formation of small vacancy clusters and occurs around room temperature, while the second stage is centered around 250°C. Measurements of the dislocation loop density in quenched, aged specimens in the second stage of hardening, show that Frank vacancy loops are the major obstacle to dislocation motion. The temperature dependence of the critical resolved shear stress, in quenched specimens, was also measured throughout the aging process and compared with various expressions for the interaction of mobile dislocations with different obstacles. These results indicate quite clearly that Seeger's theory of obstacle shearing is favored over Fleisher's theory of asymmetrical distortion in this case. An experimental value of the cutting process is about 5 eV. In agreement with this mechanism, electron microscope observations show that Frank dislocation loops, within a slip plane, are removed during the first few percent of plastic deformation. Finally, the quench hardening is only removed at very high temperatures, in the same temperature range where the Frank loops are removed.