Deformation experiments in the diamond-anvil cell: texture in copper to 30 GPa

The combination of the diamond-anvil cell, synchrotron x-ray diffraction in radial geometry and simultaneous Rietveld refinement with texture analysis allows us to quantitatively investigate the plastic deformation behaviour of materials at very high pressures. Our study of copper to 30?GPa shows in ideal experimental geometry a [110] fibre texture component, as is typical for axial compression of soft face centred cubic metals. Locally a plane strain texture develops which is energetically favoured (curling). A transition from compressional to plane strain/pure shear texture can be monitored by analysing individual images taken at different positions in the diamond cell.     

Zr-4合金压缩形变行为的研究

密排六方结构的Zr呈现弹塑性各向异性,轧制工艺会使材料内部产生晶间应力.准确地评估Zr合金内部的晶间应力分布并明确其微观形变机制,对其服役能力和使用寿命的准确评判具有重要的科学意义和应用价值.利用中子原位衍射技术结合弹塑性自洽(EPSC)模拟分析了Zr-4合金的压缩形变行为,加载方式为沿轧板厚度方向压缩.研究中辅以非原位的背散射电子衍射测试进行织构演化分析及透射电镜(TEM)测试分析缺陷形态.EPSC模拟可以定量地给出不同形变量下的形变机制,并且计算结果可由TEM实验佐证.研究表明:当形变量较小(0.55%)时,柱面{10ˉ10}?11ˉ20?(?a?型)滑移起主导作用;随着塑性形变量的增加,锥面滑移的作用增强,且锥面{10ˉ11}?11ˉ23?(?c+a?型)滑移的作用大于柱面{10ˉ10}?11ˉ20?(?a?型)滑移,少量的锥面{10ˉ11}?11ˉ20?(?a?型)和{10ˉ12}?11ˉ20?(?a?型)滑移也存在.     

Plastic deformation in mesocomposite materials under dynamic loading as applied to their joining with metals

The paper studies the effect of the amount and distribution pattern of nanoinclusions in a high-strength mesocomposite matrix on its plastic deformation under dynamic loading. The study is performed on mesocomposite specimens shaped as hollow thick-walled cylinders subjected to combined shear/compression loading with an explosive. It is found that homogeneous strain decreases with the growing volume fraction of nanoinclusions. The mechanical texture formed by the distribution of nanoinclusions in mesocomposite bars is shown to influence the deformation and cracking mechanisms. Additionally, the influence of structure is studied by computer simulation. The simulation has revealed that plastic deformation is rotational in the mesocomposite with chaotic structural distribution.     

Theoretical examination of ultrasonic pole figures via comparison with the results analyzed by finite element polycrystal model

The ultrasonic wave velocities in a polycrystalline aggregate are sensitively influenced by texture development due to plastic deformation. According to Sayer's model, it is possible to construct ultrasonic pole figures via the crystallite orientation distribution function (CODF), which can be calculated by using ultrasonic wave velocities. In the previous papers, the theoretical modeling to simulate ultrasonic wave velocities propagating in solid materials under plastic deformation has been proposed by the authors and proved to be a good agreement with experimental results. Generally, wave velocities are dependent upon the propagating wave frequency; hence to evaluate texture development via ultrasonic pole figures it is necessary to examine an influence of frequency dependence on the ultrasonic wave velocities. In the present paper, the proposed theoretical modeling is applied to the texture characterization in polycrystalline aggregates of FCC metals under various plastic strain histories via ultrasonic pole figures, and also the frequency dependence is examined by using Granato-Lücke's dislocation strings model. Then the simulated ultrasonic pole figures are compared with the pole figures analyzed by the finite element polycrystal model (FEPM). The good qualitative agreement between both results suggests the sufficient accuracy of our proposed theoretical modeling.     

关於固体的现实应力空间(续)

前文[1]综合四理论[2],[3],[4],[5]构成固体现实应力空间之一初步理论,大体反映固态静力学性质,对金属较对非金属固体反映得当,后者受范形变曲面有异于弥氏圆柱。总起来看,前文仅涉及原则概念,未触及具体问题。为使此理论对金属压力加工及材料试验研究有所帮助,本文进一步研究几个问题:1)由应力空间图形比较不同金属的静力学性质;2)受范形变效率及其计算;3)形变过程之轨迹;并得到一定数量或质量上的结论。同时,附带对前文[1]中一个实验记录图的错误作修正,包括在附录内。     

Effect of reversible torsion on the structure and mechanical properties of iron under severe plastic deformations in a Bridgman camera

The effect of the reversible direction of rotation of the movable anvil of the Bridgman camera (torsion under high quasi-hydrostatic pressure at room temperature) on the structure and mechanical properties of commercially pure iron (steel 08kp) is investigated. It is established that a change in the direction of rotation of the movable anvil under torsion substantially effects the structural characteristics of deformation fragments and dynamically recrystallized grains formed under severe plastic deformations. The effect of the method of deformation on the internal distortions of structure and on the mechanical properties is analyzed.     

Prediction of flow stress and textures of AZ31 magnesium alloy at elevated temperature

The viscoplastic behaviour of magnesium alloys at high temperatures leads to highly temperature-dependent mechanical properties. While at high strain rates a notable strain hardening response is observed, at low strain rates the material shows a smooth plastic response with negligible amount of hardening. This complicated behaviour is due to different deformation mechanisms that are active at different strain rate regimes, resulting in different strain rate sensitivity parameters. In this study we show, by utilizing both numerical simulations and experiments, that this behaviour can be predicted by a model that combines two deformation mechanisms, grain boundary sliding mechanism and dislocation glide mechanism. We discuss the importance of each deformation mechanism at different strain rate regimes based on the findings of modelling and experimental results for AZ3 magnesium alloy. By developing a model that includes the above-mentioned two deformation mechanism, the prediction of flow properties is expanded to a wide range of strain rate regimes compared to previous study. The obtained numerical findings for the stress–strain behaviour as well as texture evolution show good agreement with the experimental results.     

Texture transition in Al–Mg alloys: effect of magnesium

ABSTRACT

The evolution of deformation texture and microstructure in commercially pure Al (cp-Al) and two Al–Mg alloys (Al–4Mg and Al–6Mg) during cold rolling to a very large strain (true strain ε_t? ≈?3.9) was investigated. The development of deformation texture in cp-Al, after rolling, can be considered as pure metal or Copper-type, which is characterised mainly by the presence of Cu {112}<111>, Bs {110}<112> and S {123}<634> components. The deformation microstructure clearly indicates that deformation mechanism in this case remains slip dominated throughout the deformation range. In the Al–4Mg alloy, the initial slip mode of deformation is finally taken over by mechanism involving both slip and Copper-type shear bands, at higher deformation levels. In contrast, in the Al–6Mg alloy, the slip and twin mode of deformation in the initial stage is replaced by slip and Brass-type shear bands at higher deformation levels. Although a Copper-type deformation texture forms in the two Al–Mg alloys at the initial stage of deformation, there is a significant increase in the intensity of the Bs component and a noticeable decrease in the intensity of the Cu component at higher levels of deformation, particularly in the Al–6Mg alloy. This phenomenon indicates the possibility of transition of the deformation texture from Cu-type to Bs-type, which is concurrent with the addition of Mg. Using visco-plastic self-consistent modelling, the evolution of deformation texture could be simulated for all three materials.     

A portable high-pressure stress cell based on the V7 Paris–Edinburgh apparatus

We describe a new device, based on a V7 Paris–Edinburgh press, for torsional testing of material at pressures up to 7 GPa (extendable to 15 GPa). Samples are deformed using a simple shear geometry between opposed anvils by rotating the lower anvil, via a rotational actuator, with respect to an upper, stationary, anvil. Use of conical anvil profiles greatly increases sample dimensions more than other high-pressure torsional apparatus did. Samples of polycrystalline Zr (2 mm thick, 3.5 mm diameter) have been sheared at strains exceeding γ ～1.5 at constant strain rate and at pressures from 1.8 to 5 GPa, and textural development has been studied by electron microscopy. Use of amorphous-boron-epoxy gaskets means that nearly simple shear of samples can be routinely achieved. This apparatus allows study of the plastic and anelastic behaviour of materials under high pressure, and is particularly suited for performing in situ investigations using synchrotron or neutron radiation.     

Selective role of bainitic lath boundary in influencing slip systems and consequent deformation mechanisms and delamination in high-strength low-alloy steel

We elucidate here the deformation behaviour and delamination phenomenon in a high-strength low-alloy bainitic steel, in terms of microstructure, texture and stress evolution during deformation via in situ electron back-scattered diffraction and electron microscopy. Furthermore, the selective role of bainitic lath boundary on slip systems was studied in terms of dislocation pile-up and grain boundary energy models. During tensile deformation, the texture evolution was concentrated at {1 1 0}<1 1 1> and the laths were turn parallel to loading direction. The determining role of lath on the deformation behaviour is governed by length/thickness (l/t) ratio. When l/t > 28, the strain accommodates along the bainite lath rather than along the normal direction. The delamination crack initiated normal to (0 1 1) plane, and become inclined to (0 1 1) plane with continued strain along (0 1 1) plane and lath plane. This indicated that the delamination is not brittle process but plastic process. The lack of dimples at the delaminated surface is because of lack of strain normal to the direction of lath. The delaminated (0 1 1) planes were associated with cleavage along the (1 0 0) plane.     

Effects of texture,temperature and strain on the deformation modes of zirconium

Clock-rolled, high-purity, textured polycrystalline zirconium exhibits significant plastic anisotropy for compression along the through-thickness and in-plane directions and strong temperature dependence of flow stress for both orientations. Orientation imaging microscopy in a scanning electron microscope and defect analysis via transmission electron microscopy are used to characterize the defect microstructures as a function of initial texture, deformation temperature and plastic strain. The observed deformation mechanisms are correlated with the measured mechanical response.     

A general scheme derived from video-controlled stretching tests and WAXS for describing tensile deformations of polyethylenes

When polyethylene (PE) is deformed to large strains, the stress originates from both the viscous forces associated with the plastic deformation of the crystallites by slip and fragmentation processes and the entropic elastic forces arising from the stretching of the entangled amorphous regions. The dependencies of the relative weights of these processes on crystallinity were analyzed in a comprehensive study of A series of samples: high-density polyethylene (HDPE), low-density polyethylene (LDPE), and ethylene-vinylacetate copolymers. The comparison was based on measured true stress-strain curves at constant strain rates, on the recovery properties of the samples studied in tensile tests with included unloading-reloading loops, and on wide-angle X-ray spectroscopy (WAXS) measurements carried out to determine the related texture changes. It was found that, in spite of the large changes in the gross mechanical properties from solid-like to rubberlike behavior, there always exist four characteristic points at which the deformation behavior changes. These may be associated with (1) the onset of isolated slip processes, (2) a change into a collective activity of the slips, (3) the beginning of crystallite fragmentation, and (4) chain disentanglement. Increasing crystallinity leads to increasing stresses at these points: the related strains, however, remain essentially constant. The crystal texture is a function of the imposed strain only. Experiments support the novel picture of a granular substructure of the crystalline lamellae as a basic structural feature.     

Anvil design for slip-free high pressure deformation experiments in a rotational diamond anvil cell

A rotational diamond anvil cell is the most suitable deformation apparatus with which to investigate the rheological properties of deep-Earth materials at pressures similar to those found in the lower mantle and core. However, slip between the sample and piston is still a problem, since the slip prevents the attainment of a constant strain rate and interferes with the uniform deformation of a sample. In this paper, we report that using a diamond anvil with deep grooves results in a marked improvement in the coupling between the sample and the diamond anvils.     

Numerical study of stress and plastic strain evolution under compression and shear of a sample in a rotational anvil cell

The large-strain problem on the evolution of distribution of the components of stress tensor and plastic strain in a sample under compression and torsion in a rotational anvil cell was formulated and studied in detail using the FEM. Results are obtained for compression by different axial forces and torsion under two different constant axial forces. The effects of redistribution of the friction radial and torsional stresses and the load on a sample and gasket on the resultant fields are elucidated. Small pressure self-multiplication effect is revealed during torsion after compression below some critical force, and significant heterogeneity of all fields is found. Strong shear strain localization near the contact surface between sample and anvil is quantified. Results are compared with the simplified solution and available experiments. The results obtained are important for the determination of elastic and plastic properties of materials under high pressure and for the interpretation of kinetics of strain-induced phase transformations and chemical reactions.     

In situ rheological measurements at extreme pressure and temperature using synchrotron X‐ray diffraction and radiography

Dramatic technical progress seen over the past decade now allows the plastic properties of materials to be investigated under extreme pressure and temperature conditions. Coupling of high‐pressure apparatuses with synchrotron radiation significantly improves the quantification of differential stress and specimen textures from X‐ray diffraction data, as well as specimen strains and strain rates by radiography. This contribution briefly reviews the recent developments in the field and describes state‐of‐the‐art extreme‐pressure deformation devices and analytical techniques available today. The focus here is on apparatuses promoting deformation at pressures largely in excess of 3 GPa, namely the diamond anvil cell, the deformation‐DIA apparatus and the rotational Drickamer apparatus, as well as on the methods used to carry out controlled deformation experiments while quantifying X‐ray data in terms of materials rheological parameters. It is shown that these new techniques open the new field of in situ investigation of materials rheology at extreme conditions, which already finds multiple fundamental applications in the understanding of the dynamics of Earth‐like planet interior.     

Effect of deformation of diamond anvil and sample in diamond anvil cell on the thermal conductivity measurement

Studies show that the sample thickness is an important parameter in investigating the thermal transport properties of materials under high-temperature and high-pressure (HTHP) in the diamond anvil cell (DAC) device. However, it is an enormous challenge to measure the sample thickness accurately in the DAC under severe working conditions. In conventional methods, the influence of diamond anvil deformation on the measuring accuracy is ignored. For a high-temperature anvil, the mechanical state of the diamond anvil becomes complex and is different from that under the static condition. At high temperature, the deformation of anvil and sample would be aggravated. In the present study, the finite volume method is applied to simulate the heat transfer mechanism of stable heating DAC through coupling three radiative-conductive heat transfer mechanisms in a high-pressure environment. When the temperature field of the main components is known in DAC, the thermal stress field can be analyzed numerically by the finite element method. The obtained results show that the deformation of anvil will lead to the obvious radial gradient distribution of the sample thickness. If the top and bottom surfaces of the sample are approximated to be flat, it will be fatal to the study of the heat transport properties of the material. Therefore, we study the temperature distribution and thermal conductivity of the sample in the DAC by thermal-solid coupling method under high pressure and stable heating condition.     

Elastic and plastic deformation of NaCl and Ni3Al polycrystals during compression in a multi-anvil apparatus

Abstract

The compression behaviour in a multi-anvil apparatus of pure NaCl and of a foil of Ni₃Al embedded in a pressure medium of NaCl has been studied by energy-dispersive X-ray diffraction. At ambient temperature, the pressure and stresses, determined from line positions of NaCl, were constant throughout the sample chamber. Line positions and line widths of NaCl reflections were reversible on pressure release. A saturation of microstrains observed in NaCl at 2 GPa is thus attributed to brittle fracture setting in at uniaxial stresses of around 0.3 GPa. Ni₃Al polycrystals, in contrast, undergo extensive (ductile) plastic deformation above 4 GPa. The compression behaviour of both Ni₃Al and NaCl is identical to that previously determined in a diamond anvil cell. While a multi-anvil device thus has the advantage, compared with a diamond anvil cell, of constant pressure and stress throughout the sample chamber, microstrains in poly-crystalline samples arise in both devices. Samples in a multi-anvil apparatus thus need to be mixed with a pressure medium and to consist of essentially single crystals just as in a diamond anvil cell. Annealing experiments at high pressures confirm that the release of the uniaxial stress component in the pressure medium does not cause a release of microstrains in the embedded sample if the latter has been plastically deformed. Annealing for the purpose of attaining hydrostatic conditions in compression studies thus has to be carried out with care.     

Strain-rate-induced bcc-to-hcp phase transformation of Fe nanowires

Using molecular dynamics simulation method, the plastic deformation mechanism of Fe nanowires is studied by applying uniaxial tension along the [110] direction. The simulation result shows that the bcc-to-hcp martensitic phase transformation mechanism controls the plastic deformation of the nanowires at high strain rate or low temperature; however,the plastic deformation mechanism will transform into a dislocation nucleation mechanism at low strain rate and higher temperature. Furthermore, the underlying cause of why the bcc-to-hcp martensitic phase transition mechanism is related to high strain rate and low temperature is also carefully studied. Based on the present study, a strain rate-temperature plastic deformation map for Fe nanowires has been proposed.     

Mechanoluminescence induced by elastic and plastic deformation of coloured alkali halide crystals at fixed strain rates

Mechanoluminescence (ML) emission from coloured alkali halide crystals takes place during their elastic and plastic deformation. The ML emission during the elastic deformation occurs due to the mechanical interaction between dislocation segments and F-centres, and the ML emission during the plastic deformation takes place due to the mechanical interaction between the moving dislocations and F-centres. In the elastic region, the ML intensity increases linearly with the strain or deformation time, and in this case, the saturation region could not be observed because of the beginning of the plastic deformation before the start of the saturation in the ML intensity. In the plastic region, initially the ML intensity also increases linearly with the strain or deformation time, and later on, it attains a saturation value for large deformation. When the deformation is stopped, initially the ML intensity decreases at a fast rate; later on, it decreases at a slow rate. The decay time for the fast decrease of the ML intensity gives the relaxation time of dislocation segments or pinning time of the dislocations, and the decay time of the slow decrease of the ML intensity gives the diffusion time of holes in the crystals. The saturation value of the ML intensity increases linearly with the strain rate and also with the density of F-centres in the crystals. Initially, the saturation value of the ML intensity increases with increasing temperature, and for higher temperatures the ML intensity decreases with increasing temperature. Therefore, the ML intensity is optimum for a particular temperature of the crystals. From the ML measurements, the relaxation time of dislocation segments, pinning time of dislocations, diffusion time of holes and the energy gap between the bottom of the acceptor dislocation band and interacting F-centre level can be determined. Expressions derived for the ML induced by elastic and plastic deformation of coloured alkali halide crystals at fixed strain rates indicates that the ML intensity depends on the strain, strain rate, density of colour centres, size of crystals, temperature, luminescence efficiency, etc. A good agreement is found between the theoretical and experimental results.