Pure Fcc Metals Research Articles

New creep region in pure face-centered cubic metals at low temperatures

Creep behavior was investigated carefully for typical face-centered cubic (fcc) metals, i.e., high purity Al, Cu, and Pb, below 0.3Tm, where Tm is the melting temperature. All samples showed marked creep behavior at such low temperatures, with an apparent activation energy of 15–30 kJ mol−1, a stress exponent of 2–5 and a grain size exponent of 0. These results show a new creep region that has not appeared in deformation mechanism maps of pure fcc metals.

Strength Effects in Micropillars of a Dispersion Strengthened Superalloy

AbstractThe present paper investigates the uniaxial compression behavior of highly alloyed, focused ion beam (FIB) manufactured micropillars, ranging from 200 up to 4000 nm in diameter. The material used was the Ni‐based oxide‐dispersion strengthened (ODS) alloy Inconel MA6000. Stress–strain curves show a change in slip behavior comparable to those observed in pure fcc metals. Contrary to pure Ni pillar experiments, high critical resolved shear stress (CRSS) values were found independent of pillar diameter. This suggests that the deformation behavior is primarily controlled by the internal obstacle spacing, overwhelming any pillar‐size‐dependent mechanisms such as dislocation source action or starvation.

Evolution of dislocation patterns in fcc metals

The effect of strain on the microstructure and detailed internal structure of dislocation boundaries in pure polycrystalline fcc metals (aluminum, copper, nickel and gold) was systematically studied and compared as a function of strain following compression at room temperature. At low strains all metals form a cellular structure. The dislocations in the cell walls tend to rearrange themselves from tangles to ordered arrays of parallel dislocations as the strain is increased. However the rearrangement does not correlate well with the stacking fault energy. A good correlation is found with the cross-slip activation energy.

Diffusion in Metallic Elements and Intermetallics

Starting from some fundamentals of solid-state diffusion, we remind the reader to the major techniques for lattice diffusion measurements. Self-diffusion is the most basic diffusion phenomenon in any solid. The paper covers main features of self-diffusion in pure fcc and bcc metals and some important facts about diffusion of substitutional solutes in metals. Binary intermetallics are compounds of two metals or of a metal and a semimetal. Their structures are different from those of the constituents. Some intermetallics are interesting functional materials others have attracted attention as high-temperature structural materials. The paper reviews some results mainly from our laboratory on diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si and Co-Nb, which have been published in detail elsewhere. Some results for the ternary system Ni-Fe-Al are also mentioned.

A new model of dynamic recovery for Stage III of pure fcc metals without cross slip

Abstract Dynamic recovery in Stage III of strain hardening of pure fcc metals has been known for decades and has been associated with dislocation cell formation and refinement in scale with increasing plastic shear resistance when this exceeds a critical and temperature-dependent threshold level. Its rate controlling process had been attributed by Seeger 50 years ago to massive cross slip of screw dislocation groups circumventing Lomer-Cottrell type linear sessile dislocations blocking the primary slip system. Consideration of this cross slip-controlled mechanism has persisted in various modified forms in the literature up to the present, even though it is not consistent with the cellular dislocation structure of Stage III, and its evolution. Here a new mechanism is proposed in which dynamic recovery occurs through the systematic removal of short Lomer – Cottrell type sessile dislocation segments, i. e. LC locks, in the cell walls by statistical processes stimulated by the impingement of glide dislocations on cell walls when the applied stress exceeds a temperature-dependent threshold shear resistance τIII.

Twinning and dislocation activity in silver processed by severe plastic deformation

Severe plastic deformation (SPD) provides attractive procedures for producing ultrafine-grained (UFG) metals in bulk form [1, 2]. One of the most frequently used methods is equal-channel angular pressing (ECAP) where it is possible to produce relatively large bulk UFG metals having dimensions of several centimeters in all directions [3]. It is now well established that the high strength of metals processed by ECAP is due to their high dislocation densities and small grain sizes [4]. There have been extensive reports documenting the evolution of the microstructure during the processing by ECAP of pure face-centered cubic (fcc) metals having medium or high stacking fault energies (SFE). Thus, it was reported that in metals such as Al and Cu, the grain size reaches a minimum value and the dislocation density saturates after about 4 passes of ECAP [4]. Nevertheless, very little information is available at present describing microstructural evolution during ECAP in pure fcc metals where the SFE is very low. It was shown recently that in Ag, where the SFE is only *16 mJ m-2 [5], the dislocation density was *46 ± 5 9 1014 m-2 after 8 passes of ECAP [6, 7] where this density is exceptionally high by comparison with other fcc metals, such as Au or Cu [6, 8–10]. The high dislocation density in Ag is a consequence of the very low SFE because the annihilation of dislocations is hindered by their high degree of dissociation into partials. The present investigation was therefore initiated to provide a comprehensive report on the evolution of grain structure and dislocation and twin densities when Ag is processed to very high strains up to a maximum of 16 passes through the ECAP die. As will be demonstrated, the experimental results are consistent with, and provide the first direct support for, a theoretical model of deformation in fcc metals with low SFE developed earlier by Mu¨llner and Solenthaler [11].

Comparison of dislocation patterns in cyclically deformed fcc metals

Cyclic deformation behaviors of face-centered cubic (fcc) metals, especially Cu and Ag single crystals, were compared. It was found that the dislocation patterns of fatigued Cu and Ag single crystals display similar features with ladder-like persistent slip bands (PSBs). The current finding confirms that, after Cu and Ni, Ag is the third pure fcc metal to form the ladder-like PSB structure. Based on the two-phase model of the saturation shear stress, a new criterion for PSB formation has been established.

Mechanical behavior and microstructural evolution upon annealing of the accumulative roll-bonding (ARB) processed Al alloy 1100

The results of mechanical testing on ultra-fine grained aluminum processed by accumulative roll-bonding (ARB) were analyzed with TEM observation and in accordance of the microstructural evolution upon annealing. It was found that rapid grain growth, with the corresponding decrease in strength, did not occur until the annealing temperature of 200 °C or higher. The oxide rolled into the material near the bonding interfaces was seen to act as an obstruction for grain boundary migration across said interfaces. More interestingly, the strain near the interface due to the surface preparation technique used during ARB was found to form discontinuous segregates consisting of smaller grains formed during annealing or even ARB processing of higher number of cycles. Such phenomenon is attributed to recovery or polygonization due to the strain incurred. This study has also demonstrated that yield point phenomenon may be observed in a commercially pure fcc metal when the grain size is within a certain range.

Modelling Deformation Structures in Metals Based on an Entropic Analysis

Non-equilibrium thermodynamics theory is applied to the description of plastic deformation in pure FCC metals at the steady state. The saturation flow stress is predicted as a function of temperature and strain rate for Al, Cu, Ni and Ag. The implications on the cell/subgrain size and dislocation density are explored.

Plastic anisotropy in aluminum and copper pre-strained by equal channel angular extrusion

The mechanical response of as-processed equal channel angular extrusion materials is anisotropic, depending on both direction and sense of straining. The stress–strain curves exhibit hardening characteristics different from the usual work hardening responses, e.g., Stages I–IV, expected in annealed fcc metals under monotonic loading. In this work, the anisotropic flow responses of two pure fcc metals, Al and Cu, processed by route Bc are evaluated and compared based on pre-strain level (number of passes), direction of reloading, sense of straining (i.e., compression versus tension), and their propensity to generate subgrain microstructures and to rearrange, should the slip activity change. In most cases, either macroscopic work softening or strain intervals with little to no work hardening are observed. Application of a crystallographically based single-crystal hardening law for strain-path changes [Beyerlein and Tome, Int. J. Plasticity (2007)] incorporated into a visco-plastic self-consistent (VPSC) model supports the hypothesis that suppression of work hardening is due to reversal or cross effects operating at the grain level.

An engineering-oriented embedded-atom-method potential fitting procedure for pure fcc and bcc metals

A numerical fitting procedure for developing embedded-atom-method (EAM) potentials for pure fcc and bcc metals is presented in this paper. The focus is on developing an approximate EAM potential that suffices for engineering applications to pure metals. The EAM inter-atomic potential consists of two parts: the pair-potential and the embedding function. By assuming a parameterized form for the pair-potential, the embedding function is numerically fit into the hydrostatic linear-elastic stress equations of the metal at hand. Following this, the single crystal anisotropic Young's modulus and Poisson's ratio of the metal are calculated through a uniaxial molecular dynamics (MD) simulation, and are compared to experimental values. The parameter of the pair-potential is then changed and the embedding function recalculated until the Young's modulus and Poisson's ratio are satisfactorily predicted. Following this, a parameterized relation between temperature and kinetic energy is fit into the thermal expansion data of the metal, and a temperature dependent volume factor for calculating Young's modulus at 0 and 100 K accurately is numerically fit. Finally, the potential is adjusted without changing the slopes (i.e. forces) to fit the cohesive energy of the metal. The resulting EAM potential for pure copper is tested to see how accurately the thermal and elastic properties of single crystal copper are predicted. The thermal and elastic properties are found to be predicted accurately. This shows that the EAM potential fitting procedure thus developed is suitable for atomic-level engineering applications.

Synthesis of Bulk Nanocrystalline Materials by Repeated Cold Rolling

AbstractThe article gives an overview on the preparation and characterization of bulk nanocrystalline or bulk amorphous metallic materials by repeated cold rolling with intermediate folding. The results that are highlighted for pure fcc and hcp metals and one binary glass‐forming alloy indicate that truly nanocrystalline materials with grain sizes below 10 nm can be obtained by this process that does not apply an additional hydrostatic pressure and that is compatible with continuous processing routes.

A first-principles measure for the twinnability of FCC metals

Twinnability is the property describing the ease with which a metal plastically deforms by twinning relative to deforming by dislocation-mediated slip. In this paper a theoretical measure for twinnability in face-centered-cubic (fcc) metals is obtained through homogenization of a recently introduced criterion for deformation twinning (DT) at a crack tip in a single crystal. The DT criterion quantifies the competition between slip and twinning at the crack tip as a function of crack orientation and applied loading. The twinnability of bulk material is obtained by constructing a representative volume element of the material as a polycrystal containing a distribution of microcracks and integrating the DT criterion over all possible grain and microcrack orientations. The resulting integral expression depends weakly on Poisson's ratio and significantly on three interfacial energies: the stacking-fault energy, the unstable-stacking energy and the unstable-twinning energy. All these four quantities can be computed from first principles. The weak dependence on Poisson's ratio is exploited to derive a simple and accurate closed-form approximation for twinnability which clarifies its dependence on the remaining material parameters. To validate the new measure, the twinnability of eight pure fcc metals is computed using parameters obtained from quantum-mechanical tight-binding calculations. The ranking of these materials according to their theoretical twinnability agrees with the available experimental evidence, including the low incidence of DT in Al, and predicts that Pd should twin as easily as Cu.

Formation of stacking-fault tetrahedra in aluminum irradiated with high-energy particles at low-temperatures

Stacking-fault tetrahedra (SFT) are typical vacancy clusters in fcc metals. SFT have been, however, considered to be unstable in aluminum because of its high stacking-fault energy, until the recent finding of SFT in thin aluminum foils subjected to a tensile fracture. This study confirmed that SFT form in aluminum following irradiation with high-energy particles. In electron irradiation, SFT with an average size of 2 nm formed below 203 K, while a larger irradiation intensity at lower temperature induced SFT with a larger number density. Irradiation with 60-keV Al + ions induced SFT below room temperature, although the defect yield (the ratio of the number of defect clusters to the number of incident ions) was about 10 3 , considerably smaller than that in the other fcc pure metals. With neutron irradiation below 15 K to a fluence of 2 × 10 2 1 neutrons m - 2 , SFT were not observed at room temperature. Instead, dislocation loops were observed to form and to disappear during observation with 120-kV electrons that do not cause atomic displacements. Tensile fracture of aluminum thin foil induced SFT at up to 400 K, which is close to the temperature at which SFT become unstable in isochronal annealing experiments. These results suggest that a high concentration of vacancies at lower temperature tends to cause SFT to form rather than vacancy-type dislocation loops in aluminum.

Textural Features in Warm Rolled and Subsequently Annealed Silver Sheet

In order to obtain a sharply textured silver sheet with a single orientation as a substrate, which is favorable for high temperature superconductor (HTS) film having high Jc (critical current density), the sheet metallurgical processing of pre-heated rolling combined with two step annealing treatment has been performed. We used two kinds of starting material, which are silver ingots of commercial purity obtained by casting in air and vacuum, to examine the effect of oxygen on texture development. The main feature of warm rolling texture obtained in this study was a strong Brass {011}‹211› component with minor S {123}‹412› component, and in some cases, cube {001}‹100› component or Copper {112}‹111› component appears also depending on the warm rolling procedures. Upon crystallization, it has been found that cube {001}‹100›, {124}‹4, 12, 7› and {13, 6, 15}‹365› orientations were formed as the dominant components in silver sheets and the relative amount of their orientation components depended on the concrete annealing conditions applied after the warm rolling and on the oxygen content. A very sharp single-crystal like cube texture has been successfully obtained in the specimen, which was cast “in vacuum”, warm rolled at 95 percent and subsequently annealed 10 minutes at 150°C and 30 minutes at 500°C in nitrogen. Finally, we discussed the technological basis on sharp cube texture formation in fcc pure metals with low stacking fault energy.

Core-shell bimetallic particles, prepared by sequential impregnations

Bimetallic particles (10-20 nm), with a Cu or Ag core and a Pd shell were elaborated by sequential impregnations on various oxides. Their shapes are limited by (100) and (111) faces which correspond to the planes of lowest surface energy for pure fcc metal particles of the same size. The Pd shells are epitaxially oriented on the core and locally accommodated to the metal lattice. In the case of Pd/Cu particles, annealing at 400 °C induces the structural transition to the ordered structure with shapes limited by (111), (001) and (110) faces. The adhesion energy of the annealed particles does not seem to depend on the substrate.

Patterning in crystal plasticity of FCC metals

In this overview of the following four contributions to the present Viewpoint Set we present a factual assessment of the present understanding of the phenomenon of cellular dislocation patterning occurring in pure FCC metals in monotonic deformation in Stage III and discuss the most promising mechanistic developments accounting for the driving forces, emerging forms of patterning and sources of dynamic recovery.

FEM-simulation of the hardening behavior of FCC single crystals

Although the deformation behaviour of single crystals has been investigated experimentally and theoretically for several decades, a generally accepted theory for the underlying hardening processes has not been found yet. In the last years, computer simulations offered additional investigation possibilities, whereby especially the use of constitutive equations in combination with the finite element method has delivered important results. The publication reports on an FEM crystal plasticity model-which was previously used for multicrystals-and its application to single crystals. The model is designed for the low temperature behaviour of pure fcc metal crystals. The rate dependent equations include kinematic and isotropic hardening, with formulations founded on the responsible slip system processes. The obtained simulation results coincide very well with typical single crystal experiments like tensile or latent hardening tests, which confirms the chosen mathematical approaches. It is shown that both kinematic and isotropic processes determine hardening, whereby the underlying slip system interactions play an important role. Moreover, experimentally not visible processes can be studied in detail and are discussed concerning the metal physics theories, which finally contributes to a better understanding of metal deformation behaviour.

Microstructure of both as-irradiated and deformed 304L stainless steel irradiated with 800 MeV protons

A 304L stainless steel water degrader was irradiated with 800 MeV protons at Los Alamos Neutron Science Centre (LANSCE) up to 8.5 dpa at temperatures up to 250°C. Tensile tests showed that hardening and embrittlement were induced in the material. In order to understand the irradiation hardening and embrittlement mechanism, the microstructure in both as-irradiated and deformed material has been studied. The results of TEM investigations show that in the as-irradiated material the main features are: (a) very dense small defect clusters, part of them can be resolved as stacking fault tetrahedra (SFT), with a mean size of about 1.6 nm independent of irradiation dose; (b) large Frank loops, whose size increases with dose but whose density varies little with dose; (c) amorphization of precipitates; and (d) no observable helium bubbles or cavities. The main feature in the deformed material is the formation of twin lamellae and bundles of twin lamellae. In all of the four samples (0, 0.7, 3.4 and 6.8 dpa) studied, dense twin lamellae have been observed. The twin planes are {1 1 1}. Similar to channels observed in irradiated and deformed fcc pure metals, the original microstructures inside the twin lamellae, namely small clusters and Frank loops, have been removed. The width of the twin lamellae and their bundles varies from a few nanometers to more than 100 nm. The structure outside the twin lamellae is little changed.

Modelling strain hardening and steady state deformation of Al–Mg alloys

A new approach to the modelling of work hardening during plastic deformation of pure fcc metals has recently been proposed by the present authors. The model is based on a statistical approach to the problem of athermal storage of dislocations. By combining the solution for the dislocation storage problem with models for dynamic recovery of network dislocations and sub-boundary structures, a general internal state variable description is obtained. In the present work this model is further developed by including the effects resulting from grain boundaries, elements in solid solution, and the presence of non-deformable precipitate particles. The result is a work hardening model and associated computer code, capable of providing the stress–strain behaviour for a given solid solution alloy under any combination of constant strain rate and temperature. The model has been applied to the problems of work hardening and flow stress saturation in Al–Mg alloys. It is demonstrated that the model predictions, in terms of microstructure evolution and associated strengthening, are in good agreement with experimental observations.