The role of internal stresses on the plastic deformation of the Al–Mg–Si–Cu alloy AA6111

In this work, we have investigated the internal stress contribution to the flow stress for a commercial 6xxx aluminium alloy (AA6111). In contrast to stresses from forest and precipitation hardening, the internal stress cannot be assessed properly with a uniaxial tensile test. Instead, tension–compression tests have been used to measure the Bauschinger stress and produce a comprehensive study which examines its evolution with (i) the precipitation structure, and (ii) a wide range of applied strain. A large set of ageing conditions was investigated to explore the effect of the precipitation state on the development of internal stress within the material. It is shown that the Bauschinger stress generally increases with the applied strain and critically depends on the average radius of the precipitate and is thus linked to the shearable/non-shearable transition. Further work in the case of non-shearable particles shows that higher strain eventually leads to particle fracture and the Bauschinger stress then rapidly decreases. Following the seminal work of Brown et al. a physically based approach including plastic relaxation and particle fracture is developed to predict the evolution of the internal stress as a function of the applied strain. Knowing the main characteristics of the precipitation structure–such as the average precipitate radius, length and volume fraction–allows one to estimate accurately the internal stress contribution to the flow stress with this model.     

Modelling work hardening of aluminium alloys containing dispersoids

The influence of dispersoids on tensile deformation behaviour has been studied by comparison of aluminium alloys containing different dispersoid densities. It was found that a fine dispersion of non-shearable particles led to an increased work hardening at the initial plastic deformation, but the effect was opposite at higher strains. The reason has been attributed to the generation of geometrically necessary dislocations (GNDs). A new model has been proposed for the evolution of GNDs based on a balance of storage and dynamic recovery of GNDs. The model predicts a rapid saturation of GNDs and a reduced work hardening at small strains, consistent with the experimental results.     

Vacancy–solute interactions during ageing of a cold-worked Mg–RE-based alloy

The vacancy–solute interactions during artificial ageing at 250^○C of cold worked samples of a commercial magnesium alloy WE54 (Mg–RE based) were studied by coincidence Doppler broadening of positron annihilation radiation and positron annihilation lifetime spectroscopy. The results show that, in the as-cold-worked state, the vacancies are associated with dislocations that are generated by the cold work and that, after artificial ageing at 250^○C, the vacancies are associated with solute elements and help the formation of precipitate precursors. This mechanism accelerates the formation of hardening precipitates without any apparent changes in the precipitation sequence and in the products of the decomposition of the supersaturated solid solution. The present study demonstrates that the stronger hardening response achieved in the cold-worked samples originates from the presence of a higher concentration of vacancies that is introduced by the cold work and is retained in the first few minutes of ageing.     

Solute-diffusion-controlled dislocation creep in pure aluminium containing 0.026 at.% Fe

Pure aluminium containing about 200?at.ppm Fe in solution is shown to creep about 10⁶ times slower at 200°C than the same aluminium containing a negligible amount of iron in solution. The high creep resistance of the Al–200?at.ppm?Fe alloy is attributed to the presence of subgrain boundaries containing iron solute atoms. It is proposed that the opposing stress fields from subgrain boundaries and from the piled-up dislocations during creep are cyclically relaxed, by iron solute diffusion, to allow climb of the lead dislocation in the pile-up. The mechanism is a form of mechanical ratcheting. The model is applied to Al–Fe alloys and correctly predicts that the creep rate is controlled by the rate of iron solute diffusion and by a temperature dependence equal to the activation energy for iron diffusion, namely Q _c?=?221?kJ?mol^?1. Basic creep studies on solid-solution alloying with solute atoms that diffuse slowly in the lattice of aluminium (e.g. manganese, chromium, titanium and vanadium) appear worthy of study as a way of enhancing creep strength and of understanding creep mechanisms involving solute-atom-containing subgrain boundaries.     

Influence of Al3Ni crystallisation origin particles on hot deformation behaviour of aluminium based alloys

Abstract

Binary Al–Ni, Al–Mg and ternary Al–Mg–Ni alloys containing various dispersions and volume fraction of second-phase particles of crystallisation origin were compressed in a temperature range of 200–500 °C and at strain rates of 0.1, 1, 10, 30 s^?1 using the Gleeble 3800 thermomechanical simulator. Verification of axisymmetric compression tests was made by finite-element modelling. Constitutive models of hot deformation were constructed and effective activation energy of hot deformation was determined. It was found that the flow stress is lowered by decreasing the Al₃Ni particle size in case of a low 0.03 volume fraction of particles in binary Al–Ni alloys. Intensive softening at large strains was achieved in the alloy with a 0.1 volume fraction of fine Al₃Ni particles. Microstructure investigations confirmed that softening is a result of the dynamic restoration processes which were accelerated by fine particles. In contrast, the size of the particles had no influence on the flow stress of ternary Al–Mg–Ni alloy due to significant work hardening of the aluminium solid solution. Atoms of Mg in the aluminium solid solution significantly affect the deformation process and lead to the growth of the effective activation energy from 130–150 kJ/mol in the binary Al–Ni alloys to 170–190 kJ/mol in the ternary Al–Mg–Ni alloy.     

Precipitation in solution-treated aluminium–4 wt% copper under cyclic strain

Solution-treated Al–4 wt% Cu was strain-cycled at ambient temperature and above, and the precipitation and deformation behaviours investigated by TEM. Anomalously rapid growth of precipitates appears to have been facilitated by a vacancy super-saturation generated by cyclic strain and the presence of continually refreshed dislocation density to provide heterogeneous nucleation sites. Crystallographic texture appears to be responsible for latent hardening in specimens tested at room temperature. Increasing temperatures lead to a gradual hardening throughout life due to precipitation. Specimens machined at 45° from the rolling direction, which exhibit rapid precipitation hardening, show greater texture hardening due to increased axial stress required to cut precipitates in specimens. In the temperature range 100–200°C, precipitation of Θ″ is suppressed by cyclic strain, and precipitation of Θ′ promoted. The rapid growth of precipitates generated by cyclic strain operates with diminishing effect at higher temperatures due to faster recovery of non-equilibrium vacancy concentrations. Θ′ precipitates generated under cyclic strain are smaller and more finely dispersed than those produced via quench-ageing due to heterogeneous nucleation on dislocations and possess a low aspect ratio and rounded edges of the broad faces caused by the introduction of ledges into the growing precipitates by dislocation cutting. Frequency effects indicate that dislocation action is responsible for the observed reduction in aspect ratio. Accelerated formation of grain-boundary precipitates appears partially responsible for rapid inter-granular fatigue failure at elevated temperatures, resulting in coexistent fatigue striations and ductile dimples on the fracture surface.     

Shear modulus of shock-compressed LY12 aluminium up to melting point

Asymmetric plate impact experiments are conducted on LY12 aluminium alloy in a pressure range of 85--131\,GPa. The longitudinal sound speeds are obtained from the time-resolved particle speed profiles of the specimen measured with Velocity Interferometer System for Any Reflector (VISAR) technique, and they are shown to be good agreement with our previously reported data of this alloy in a pressure range of 20--70\,GPa, and also with those of 2024 aluminium reported by McQueen. Using all of the longitudinal speeds and the corresponding bulk speeds calculated from the Gruneisen equation of state (EOS), shear moduli of LY12 aluminium alloy are obtained. A comparison of the shear moduli in the solid phase region with those estimated from the Steinberg model demonstrate that the latter are systematically lower than the measurements. By re-analysing the pressure effect on the shear modulus, a modified equation is proposed, in which the pressure term of $P/\eta^{1/3}$ in the Steinberg model is replaced by a linear term. Good agreement between experiments and the modified equation is obtained, which implies that the shear modulus of LY12 aluminium varies linearly both with pressure and with temperature throughout the whole solid phase region. On the other hand, shear modulus of aluminium in a solid-liquid mixed phrase region decreases gradually and smoothly, a feature that is very different from the drastic dropping at the melting point under static conditions.     

Shear modulus of shock-compressed LY12 aluminium up to melting point

Asymmetric plate impact experiments are conducted on LY12 aluminium alloy in a pressure range of 85-131 GPa. The longitudinal sound speeds axe obtained from the time-resolved particle speed profiles of the specimen measured with Velocity Interferometer System for Any Reflector （VISAR） technique, and they are shown to be good agreement with our previously reported data of this alloy in a pressure range of 20-70 GPa, and also with those of 2024 aluminium reported by McQueen. Using all of the longitudinal speeds and the corresponding bulk speeds calculated from the Gruneisen equation of state （EOS）, shear moduli of LY12 aluminium alloy are obtained. A comparison of the shear moduli in the solid phase region with those estimated from the Steinberg model demonstrate that the latter are systematically lower than the measurements. By re-analysing the pressure effect on the shear modulus, a modified equation is proposed, in which the pressure term of P/η^1/3 in the Steinberg model is replaced by a linear term. Good agreement between experiments and the modified equation is obtained, which implies that the shear modulus of LY12 aluminium varies linearly both with pressure and with temperature throughout the whole solid phase region. On the other hand, shear modulus of aluminium in a solid-liquid mixed phrase region decreases gradually and smoothly, a feature that is very different from the drastic dropping at the melting point under static conditions.     

Texture development during cold rolling of Fe–Cr–Ni alloy-experiments and simulations

Abstract

In the present work, evolution of microstructure and crystallographic texture during cold rolling of two phase Fe–Cr–Ni alloy was investigated. Fe–Cr–Ni alloy (in initially solution annealed condition) was uni-directionally cold rolled in a laboratory rolling mill to different thickness reductions. Scanning electron microscopy was used to observe the changes in microstructure, while X-ray diffraction was used to investigate changes in crystallographic texture of austenite and ferrite (through changes in orientation distribution function). Crystallographic texture was also simulated using different crystal plasticity models (Full constraint Taylor, relaxed constraint Taylor (lath and pancake) and co-deformation Visco Plastic Self Consistent (VPSC)). With the increase in plastic deformation, there were morphological as well as crystallographic changes in the microstructure. Strong α-fibre (RD//〈1?1?0〉) texture was developed in ferrite, while brass ({1?1?0}〈1?1?2〉) and Goss ({1?1?0}〈0?0?1〉) was dominant in austenite after 80% cold rolling. The formation of brass type texture after deformation has been attributed to the formation of shear bands and presence of strong crystallographic texture in the initial solution annealed material. Both Taylor as well as VPSC models could not capture the changes in texture with deformation accurately. For ferrite: γ-fibre (ND//〈1?1?1〉) and for austenite: Cu ({1?1?2}〈1?1?1〉) component was always present in the simulated textures. Possible reason for this could be the pining effect of interface boundaries and non-incorporation of non-crystallographic shear banding in the Taylor and VPSC models.     

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al–Cu–Mg alloy

The hardening precipitation of an Al–Cu–Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier–Preston–Bagaryatskii (GPB) zones type or to the so-called S-Al₂CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces.     

Spatially nonlocal model of surface erosion by ion bombardment

A model of solid surface erosion under ion bombardment is proposed. This model takes into account the dependence of the local sputtering yield on the nanoscale relief at a given point and the point of incidence of a primary ion. Such an approach leads to spatially nonlocal erosion model, which predicts the formation of a wavelike relief in the presence of nanoscale inhomogeneities on the surface.     

Influence of precipitate-assisted nucleation on the microstructure and mechanical properties of Al-Mg-Si-Cu-Zn alloys

The influence of precipitation on the recrystallization nucleation and mechanical properties of Al-Mg-Si-Cu-Zn alloys was investigated by means of tensile tests, SEM, TEM, XRD and EBSD. The results reveal that there are distinct contributions from the various precipitates that form during annealing and that these critically influence the evolution of microstructure and its associated texture, as well as mechanical behaviour. In contrast to alloy sheets A, B, and C annealed at a lower temperature or for a shorter time, the T4P alloy sheet D with an annealing at 450°C for 3?h not only possesses almost identical strength and elongation, but also a higher average r (0.659) and n (0.313) values, and also a lower Δr (0.091) value. After solution treatment, the four alloy sheets are comprised of equiaxed grains with somewhat different grain sizes and different textures, but texture volume fraction and grain size in alloy sheet D both are decreased due to the effect of precipitate-assisted nucleation during solution treatment. The corresponding nucleation and growth mechanisms of recrystallization grains were established and the relationship between textures and r value in the four alloy sheets was also analyzed on the basis of a Visco-plastic self-consistent (VPSC) model.     

Deformation behaviour of ultrafine and nanosize-grained Mg alloy synthesized via mechanical alloying

Bulk Mg–5Al alloys were consolidated from powders that had been mechanically alloyed over different milling durations. The microstructural evolution, and physical and mechanical properties of the alloys were investigated. Mechanical measurements revealed a change in deformation behaviour after certain milling durations. At short milling duration, high yield strength was obtained through dislocation strengthening mechanisms predominantly by grain refinement and to a lesser extent by solid solution strengthening and particle dispersion strengthening. However, at longer milling durations, low yield strength was observed and the strengthening mechanisms at work in short milling durations appeared to be no longer effective. Enhanced ductility with no work hardening behaviour was observed in samples with a mean grain size of 45?nm. It appeared that the significantly large increase in the grain boundary regions played an important role in the room temperature deformation of the alloys. The possibility of a diminishing effect of the dislocation strengthening mechanisms and the onset of grain boundary deformation modes for the softening phenomenon and the absence of work hardening at some nanoscale grain sizes are discussed.     

fcc solid solution alloy films formed in immiscible Fe-Ag system and their mechanical behaviors

Fe-Ag alloy films were deposited by magnetron sputtering. Fe K edge X-ray absorption near-edge structure (XANES) was performed by synchrotron radiation to evidence the structure of the films. Annealing experiments were carried out to study their stability. The hardness and elastic modulus were measured by nanoindentation. The experimental and calculated XANES spectra both reveal that Fe atoms replace part of Ag atoms and supersaturated fcc Ag (Fe) solid solution alloy films are formed up to 38 at.% Fe. The solid solutions are stable and begin to precipitate at 400 °C The elastic modulus increases with the increase of Fe concentration and satisfies the rule of mixtures. The hardness of the as-deposited alloy film is larger than that calculated based on the rule of mixtures. The mechanism responsible for the enhancement of the hardness is discussed in terms of Labusch model of solid solution hardening.     

Strain behavior of the hydrogenated submicrocrystalline Ti–6Al–4V alloy

Comparative studies of the influence of 0.002–0.12 mass % hydrogenation on the structure and phase composition of the submicrocrystalline and coarse-grained Ti–6Al–4 V alloys are performed. The evolution of the strain processes in the hydrogenated alloy is studied for both alloys upon tension at a temperature of 293 K depending on the hydrogen content and alloy state. It is established that the presence of hydrogen in the nanostructured Ti–6Al–4 V alloy in the solid solution leads to a decrease of its yield stress and an increase of its tensile strength and total strain before failure. The possible reasons for the increased duration of the uniform strain stage and the effect of strain hardening of the alloy in the presence of hydrogen in the solid solution are discussed.     

High-pressure torsion applied to nickel single crystals

Nickel single crystals with different crystallographic orientations were deformed by high-pressure torsion. Special attention is devoted to examining the evolution of the micro-texture and microstructure. The initial crystal orientation was found to have a significant effect on the mechanical hardening and evolution of micro-texture at low and medium equivalent strains, whereas at very high strains no effect of the initial orientation was observed and the behaviour was very similar to a polycrystal. The evolution of micro-texture is in good qualitative agreement with the full constrained Taylor model. At very high equivalent strains the initial crystal orientation has no influence on micro-texture. At such strains, the hardening, the refinement of the structure and the texture reaches a saturation. The final micro-texture is explained by the change from one preferred crystallographic orientation to another.     

Multi-scale homogenization-based modeling of semi-crystalline polymers

This paper presents a finite-strain, multi-scale constitutive model for semi-crystalline polymers, accounting explicitly for the current state and evolution of the underlying crystallographic, lamellar and morphological texture. Specifically, a semi-crystalline polymer is modeled as a two-scale composite, assumed to be, at the larger length scale, an aggregate of randomly distributed grains that, at the smaller length scale, are made up of alternating layers of an amorphous and a crystalline phase. The model incorporates finite elasticity for the amorphous phase and crystallographic hardening for the crystalline phase. The instantaneous effective response of this composite is determined by means of multi-scale homogenization methods, consisting in the use of a “linear comparison composite” (LCC) with the same internal structure as the actual nonlinear composite, with local properties that are optimally chosen via suitably designed variational principles. The effective properties of the resulting two-scale LCC are obtained through a “sequential” homogenization procedure, involving the exact solution for the effective behavior of the lamellar grains and a self-consistent estimate for the aggregate. The latter results are also used to establish evolution laws for the appropriate internal variables in the material. The predictions of the model for the macroscopic response and texture evolution in high-density polyethylene are confronted with available experimental results and compared with those of earlier models.     

Ca、Sr对汽车用镁合金组织与力学性能的影响

以Mg-Al-Mn合金为研究对象,通过微合金化的方法研究了Ca、Sr对汽车用耐热镁合金常温力学性能与高温力学性能的影响,并分析了合金在固溶时效热处理后的时效硬化特征,观察了合金铸态、固溶态和时效态下的显微组织演变。     

Precipitation and solute distribution in an interrupted-aged Al–Mg–Si–Cu alloy

Quantitative analysis of the precipitate species and solute distribution was carried out on Al–Mg–Si–Cu alloy 6061 aged to peak hardness using a conventional T6 heat treatment and the so-called T6I6 heat treatments. In this latter, a dwell period at reduced temperature (65°C) is introduced into the T6 ageing cycle (at 177°C or 150°C) which modifies the microstructure and results in the simultaneous improvement of both tensile properties and fracture toughness. Analysis of three-dimensional atom probe data reveals that the superior mechanical properties of the T6I6/177 temper are achieved by a combined effect of a greater consumption of solute atoms by precipitates, an increased number density of fine precipitates and the presence of greater fractions of the effective strengthening precipitates in the final microstructure. Three types of precipitates were found to be characteristic of the peak aged conditions: β′′ precipitates, Guinier–Preston zones and Mg–Si(–Cu) co-clusters. The composition of the strengthening precipitates was found to vary over a wide range for the different heat treatment schedules, corresponding to a variation in the number density of stable nuclei, without any accompanying change in their morphology. All precipitates were found to contain substantial quantities of aluminium. The results also indicate that the strengthening precipitates are preferentially formed from Si-rich nuclei that contain Cu atoms, as opposed to Cu-free nuclei.     

Electron irradiation-enhanced core/shell organization of Al(Cr,Fe, Mn)Si dispersoids in Al–Mg–Si alloys

The age hardening 6061-T6 aluminium alloy has been chosen as structural material for the core vessel of the material testing Jules Horowitz nuclear reactor. The alloy contains incoherent Al(Cr, Fe, Mn)Si dispersoids whose characterization by energy-filtered transmission electron microscopy (EFTEM) analysis shows a core/shell organization tendency where the core is (Mn, Fe) rich, and the shell is Cr rich. The present work studies the stability of this organization under irradiation. TEM characterization on the same particles, before and after 1 MeV electron irradiation, reveals that the core/shell organization is enhanced after irradiation. It is proposed that the high level of point defects, created by irradiation, ensures a radiation-enhanced diffusion process favourable to the unmixing forces between (Fe, Mn) and Cr. Shell formation may result in the low-energy interface segregation of Cr atoms within the (Fe, Mn) system combined with the unmixing of Cr, Fe and Mn components.