Crystallographic nature of deformation bands shown in Zn and Mg-based long-period stacking ordered (LPSO) phase

Formation of curious deformation bands has been reported as one of the deformation mechanisms occurring in an Mg-based long-period stacking ordered (LPSO) phase. The origin of the deformation band is still unknown, and the possibility of the deformation kink band and/or the deformation twin has been discussed. To clarify this, the crystallographic nature of deformation bands formed in the LPSO phase was examined by scanning electron microscope–electron backscatter diffraction (SEM-EBSD) pattern analysis. The results were compared to those of the deformation kink bands formed in hcp-Zn and deformation twins formed in hcp-Mg polycrystals. The deformation bands in the LPSO phase was confirmed not to exhibit a fixed crystal orientation relationship with respect to the matrix, different from the case shown in the deformation twin. Instead, the deformation band in the LPSO phase showed three arbitrariness on its crystallographic nature: an ambiguous crystal rotation axis that varied on the [0 0 0 1] zone axis from band to band; an arbitral crystal rotation angle that was not fixed and showed relatively wide distributions; and a variation in crystal rotation angle depending on the position even within a deformation band boundary itself. These features were coincident with those observed in the deformation bands formed in Zn polycrystals, suggesting that the formed deformation bands in LPSO phase crystals are predominantly deformation kink bands.     

Extended planar boundary inclinations in fcc single crystals and polycrystals subjected to plane strain deformation

When fcc single crystals with high-symmetry crystal orientations are deformed to moderate strains by rolling, tension or channel die compression, long dislocation boundaries inclined to the extension axis form. Similarly, long dislocation boundaries are often found in grains embedded in polycrystals deformed in the same manner. These extended planar boundaries (EPBs) are characteristically -30-40° from the extension direction and contain the transverse specimen axis. The objective of the present article is to demonstrate that EPBs formed during plane strain deformation are parallel to equivalent slip planes, a pair of hypothetical slip systems used for analyses of the strain and crystal rotation components in place of the larger number of physical slip systems. The coincidence of EPBs and equivalent slip plane inclinations is shown to account for persistent observations of EPBs in the angle range -30-40° from the rolling direction, in rolled single crystals of various initial orientations. The tendency of EPBs towards tilt or twist boundary character can also be rationalized on the basis of the equivalent slip system concept and consideration of the dislocation types available to be incorporated into EPBs.     

A crystal plasticity model of a formation of a deformation band structure

The formation of deformation bands with the typically alternating sign of the misorientation across their boundaries is interpreted as spontaneous deformation instability caused by anisotropy of hardening. To analyse the nature of the fragmentation, a model of a rigid-plastic crystal domain deformed by symmetric double slip in a plane-strain compression is considered. The basic reason for the deformation band existence is that a local decrease in number of active slip systems in the bands is energetically less costly than a homogeneous deformation by multislip. However, such model of the bands predicts their extreme orientation and their width tends to zero. This trend is modified by hardening caused by a build up of the band boundaries and by a dislocation bowing (Orowan) stress. The model provides an explanation of observed orientation of the bands, their width and the significant change in the structural morphology seen as the band reorientation occurs at large strains. The predictions are in a favourable agreement with the available observations.     

Effect of microstructure on fracture in age hardenable Al alloys

ABSTRACT

In the present study, the fracture behaviour of AA6016 alloy was investigated during bending deformation. Wrap-bend tests were conducted and the material was subjected to different bend angles to study crack propagation. The average grain size of the as-received material is approximately 45?μm. The aspect ratio of the grains was changed from 0.53 to 0.40 during bending. The presence of deformation bands was observed during bending in both tensile and compressive regions of the sample. No orientation correlation was observed between the deformation band and its corresponding parent grain. The Schmid factor inside the deformation bands was higher than that of the parent grain, which indicates that the deformation bands accommodate strain during bending. The crystallographic texture evolved significantly during bending deformation. The strength of cube texture component decreases with increasing bend angle and new texture components formed during bending. These new texture components favour either single slip or duplex slip. A mixture of intra-granular and inter-granular fracture occurs during bending. It is observed that inter-granular crack propagation is predominantly favoured along high-angle boundaries, and grain boundary de-cohesion occurs in regions where the misorientation angle is greater than 40°. The formation of deformation-induced coincidence lattice site (CSL) boundaries is also observed during bending and it is shown that the volume fraction of CSL boundaries of Σ3 type increases with increasing bend angle. The current study shows that the formation of deformation-induced CSL boundaries of Σ3 type in AA6016 alloy can improve its inherent resistance to crack propagation during bending.     

数字剪切散斑干涉法研究铝合金中Portevin-Le Chatelier带的离面变形行为

作为Portevin-Le Chatelier(PLC)带的重要特征之一,其离面变形仍缺乏实验研究.本文提出使用数字剪切散斑干涉法研究GB6061铝合金中PLC带的离面变形.通过图像相减得到的条纹图,实时观察了PLC带的几何形貌和传播过程,并获得了PLC带离面位移分布.在1/15 s内,PLC带的最大离面位移为245 nm,位置偏向于PLC带传播的前沿.在条纹图中,PLC带传播前沿的亮条纹始终较窄.此外,实验还观察到PLC带位置变更和倾角转向的演化过程.实验表明,数字剪切散斑干涉法具有高灵敏度和防震性,是研究PLC带离面变形简便有效的方法.     

Hydrogen-induced strain localisation in oxygen-free copper in the initial stage of plastic deformation

Single crystals of oxygen-free copper oriented to easy glide of dislocations were tensile tested in order to study the hydrogen effects on the strain localisation in the form of slip bands appearing on the polished specimen surface under tensile straining. It was found that hydrogen increases the plastic flow stress in Stage I of deformation. The dislocation slip localisation in the form of slip bands was observed and analysed using an online optical monitoring system and atomic force microscopy. The fine structure of the slip bands observed with AFM shows that they consist of a number of dislocation slip offsets which spacing in the presence of hydrogen is markedly reduced as compared to that in the hydrogen-free specimens. The tensile tests and AFM observations were accompanied with positron annihilation lifetime measurements showing that straining of pure copper in the presence of hydrogen results in free volume generation in the form of vacancy complexes. Hydrogen-enhanced free-volume generation is discussed in terms of hydrogen interactions with edge dislocation dipoles forming in double cross-slip of screw dislocations in the initial stage of plastic deformation of pure copper.     

Correlation among grain boundary character,carbide precipitation and deformation in Alloy 690

The correlation among grain boundary character, carbide precipitation and deformation in the grain boundary engineering (GBE) treated Alloy 690 samples with and without pre-deformation aged at 715^oC for 15?h was analysed by scanning electron microscopy and electron backscatter diffraction. The fraction of low Σ coincidence site lattice (CSL) grain boundary was enhanced by GBE treatment. The fraction of Σ3 grain boundary decreased, and most of Σ9 and Σ27 grain boundaries disappeared in the deformed GBE samples. After aging treatment, bigger carbide precipitated at coherent Σ3 grain boundary, however, most of plate-like carbide precipitated at incoherent Σ3 grain boundary disappeared in the pre-deformed GBE samples. The larger carbide precipitated on the random grain boundary in the 5% pre-deformed sample, while smaller carbide can be observed in the 15% pre-deformed sample. During the in situ tensile test of the aged GBE samples, grain boundary carbide migrated with the grain boundary migration. The slip bands go across Σ3 grain boundary directly, but cannot go across other grain boundaries. The high density of carbide plate precipitated near incoherent Σ3 and Σ9 grain boundaries can resist the evolution of slip bands. Compared to the Σ3 and Σ9 grain boundaries, Σ27 and random grain boundaries are more easily to form microcrack during deformation. The initiation of grain boundary microcrack not only related to the character of grain boundary but also related to the character of nearby grain boundaries. The phase interface of carbide and matrix is another region to initiate the microcrack.     

Yield phenomenon in oriented polyethylene terephthalate

The object of this investigation was to determine the origin of deformation bands in one-way oriented polyethylene terephthalate (PET) and their relation to the upper yield point. The tensile yield point was measured at room temperature as a function of the angle (θ) between the original draw direction and the tensile axis. The effect of strain rate was observed. The growth and geometry of deformation bands were studied. The results showed that the formation of a deformation band in PET cannot be interpreted as the strength-limiting yield mechanism for all θ and that the viscoelastic behavior plays the primary part in determining the yield point of this semicrystalline polymer.     

Analysis of slip activity and heterogeneous deformation in tension and tension-creep of Ti–5Al–2.5Sn (wt %) using in-situ SEM experiments

The deformation behavior of a Ti–5Al–2.5Sn (wt %) near-α alloy was investigated during in-situ deformation inside a scanning electron microscope. Tensile experiments were performed at 296?K and 728?K (≈0.4?T _m), while tensile-creep experiments were performed at 728?K and 763?K. Active deformation systems were identified using electron backscattered diffraction-based slip trace analysis. Both basal and prismatic slip systems were active during the tensile experiments. Basal slip was observed for grains clustered around high Schmid factor orientations, while prismatic slip exhibited less dependence on the crystallographic orientation. The tension-creep experiments revealed less slip but more development of grain boundary ledges than in the higher strain rate tensile experiments. Some of the grain boundary ledges evolved into grain boundary cracks, and grain boundaries oriented nearly perpendicular to the tensile axis formed ledges earlier in the deformation process. Grain boundaries with high misorientations also tended to form ledges earlier than those with lower misorientations. Most of the grain boundary cracks formed in association with grains displaying hard orientations, where the c-axis was nearly perpendicular to the tensile direction. For the tension-creep experiments, pronounced basal slip was observed in the lower-stress creep regime and the activity of prismatic slip increased with increasing creep stress and temperature.     

Structural factors in deformation of crystalline polymers

The multiple α absorption of bulk-crystallized polyethylene (PE) was separated into the α₁ and the α₂ absorptions on the assumption that this α₂ absorption is associated with shear deformation of lamellar crystals, i.e., has the same characteristics as in single crystal mats. The separated α₁ mechanism is related to the molecular motions in the intermosaic block region. The α₁ process is very sensitive to static and dynamic deformation, whereas the α₂ process is not affected. Plastic deformation of bulk crystallized PE was analyzed in terms of true stress and true strain. The temperature dependence of the critical yield stress below 60°C showed the same magnitude of activation energy (26 kcal/mole) as that of α₁. The leading mechanism of deformation at lower temperatures is the breakdown of lamellar crystals into mosaic blocks. Compressive deformation of solid-state extrudates along the molecular axis, giving rise to kink bands, was analyzed with X-ray goniometry and in terms of the strain-rate dependence of the yield stress. The deformation of the crystals in the kink bands occurred by superposition of intercrystallite slip (α₁) and uniform shear deformation (α₂). It was concluded that consideration of intermosaic slip mechanisms (α₁), in addition to the shear deformation (α₂) and the interlamellar deformation (β), is effective and helpful to understand the deformation process of crystalline polymers.     

Slip band formation and mobile dislocation density generation in high rate deformation of single fcc crystals

The mechanisms for the nucleation, thickening, and growth of crystallographic slip bands from the sub-nanoscale to the microscale are studied using three-dimensional dislocation dynamics. In the simulations, a single fcc crystal is strained along the [111] direction at three different high strain rates: 10⁴, 10⁵, and 10⁶?s^??1. Dislocation inertia and drag are included and the simulations were conducted with and without cross-slip. With cross-slip, slip bands form parallel to active (111) planes as a result of double cross-slip onto fresh glide planes within localized regions of the crystal. In this manner, fine nanoscale slip bands nucleate throughout the crystal, and, with further straining, build up to larger bands by a proposed self-replicating mechanism. It is shown that slip bands are regions of concentrated glide, high dislocation multiplication rates, and high dislocation velocities. Cross-slip increases in activity proportionally with the product of the total dislocation density and the square root of the applied stress. Effects of cross-slip on work hardening are attributed to the role of cross-slip on mobile dislocation generation, rather than slip band formation. A new dislocation density evolution law is presented for high rates, which introduces the mobile density, a state variable that is missing in most constitutive laws.     

Orientation dependence of void growth at triple junction of grain boundaries in nanoscale tricrystal nickel film subjected to uniaxial tensile loading

Molecular dynamics simulation was performed in order to investigate the dependence of void growth on crystallographic orientation at the triple junction of grain boundaries in nanoscale tricrystal nickel film subjected to uniaxial tensile loading. The nucleation, the emission and the transmission of Shockley partial dislocations play a predominant role in the growth of void at the triple junction of grain boundaries. The orientation factors of various slip systems are calculated according to Schmid law. The slip systems activated in a grain of tricrystal nickel film basically conform to Schmid law which is completely suitable for a single crystal. The activated slip systems play an important role in plastic deformation of nanoscale tricrystal nickel film subjected to uniaxial tensile loading. The slip directions exhibit great difference among the activated slip systems such that the void is caused to be subjected to various stress conditions, which further leads to the difference in void growth among the tricrystal nickel films with different orientation distributions. It can be concluded that the grain orientation distribution has a significant influence on void growth at the triple junction of grain boundaries.     

Effect of twinning on recrystallisation textures in deformed magnesium alloy AZ31

The evolution of texture during the tensile deformation of Mg–3.1%Al–1.05%Zn was studied by microtexture and macrotexture techniques. The deformation texture is shown to be the direct consequence of the variant selection that occurs during primary (contraction) and secondary (extension) twinning. During subsequent annealing, the secondary twins have a higher rate of nucleation than the primary twins and expanded within the latter rather than in the matrix grains. As a result, the primary twins did not contribute significantly to the annealing texture.     

Energy barriers associated with slip–twin interactions

The energetics of slip–coherent twin boundary (CTB) interactions are established under tensile deformation in face centered cubic (fcc) copper with molecular dynamics simulations, exploring the entire stereographic triangle. The CTBs serve as effective barriers in some crystal orientations more than others, consistent with experimental observations. The resulting dislocation structures upon slip–twin reactions are identified in terms of Burgers vector analysis. Visualization of the dislocation transmission, lock formation, dislocation incorporation to twin boundaries, dislocation multiplication at the matrix–twin interface and twin translation, growth, and contraction behaviors cover the most significant reactions that can physically occur providing a deeper understanding of the mechanical behavior of fcc alloys in the presence of twin boundaries. The results make a distinction between deformation and annealing twins interacting with incident dislocations and point to the considerable role both types of twins can play in strengthening of fcc metals.     

Transition of deformation mechanisms in nanotwinned single crystalline SiC

ABSTRACT

The ability to experimentally synthesise ceramic materials to incorporate nanotwinned microstructures can drastically affect the underlying deformation mechanisms and mechanics through the complex interaction between stress state, crystallographic orientation, and twin orientation. In this study, molecular dynamics simulations are used to examine the transition in deformation mechanisms and mechanical responses of nanotwinned zinc-blende SiC ceramics subjected to different stress states (uniaxial compressive, uniaxial tensile, and shear deformation) by employing various twin spacings and loading/crystallographic orientations in nanotwinned structures, as compared to their single crystal counterparts. The simulation results show that different combinations of stress states and crystal/twin orientation, and twin spacing trigger different deformation mechanisms: (i) shear localised deformation and shear-induced fracture, preceded by point defect formation and dislocation slip, in the vicinity of the twin lamellae, shear band formation, and dislocation (emission) avalanche; (ii) cleavage and fracture without dislocation plasticity, weakening the nanotwinned ceramics compared to their twin-free counterpart; (iii) severe localised deformation, generating a unique zigzag microstructure between twins without any structural phase transformations or amorphisation, and (iv) atomic disordering localised in the vicinity of coherent twin boundaries, triggering dislocation nucleation and low shearability compared to twin-free systems.     

高温、高压、高应变速率动态过程晶体塑性有限元理论模型及其应用

对于高温、高压、高应变速率加载条件下的材料冲击变形行为,动态晶体塑性模型能够直接反映晶体中塑性滑移的各向异性及其对温度、压力和微观组织结构的依赖性,因而广泛应用于材料的动态冲击力学响应、微观结构演化以及动态损伤破坏的模拟。本文综述了高压冲击下动态晶体塑性有限元的理论模型,主要包括变形运动学、包含状态方程的超弹性本构模型和晶体塑性本构模型,涉及位错滑移、相变、孪生等塑性变形机制,以及层裂、绝热剪切带等动态破坏方式。     

Special misorientations in localized deformation regions in Fe-3%Si alloy single crystals

Extended regions located at an angle of 20° to the rolling plane are observed inside deformation bands in a (110)[001] Fe-3%Si alloy single crystal at a high strain (～60%). These regions were interpreted earlier as shear bands. The lattice orientation in these bands is close to (110)[001], and their habit plane is parallel to the {112} planes of the deformed {111}〈112〉 matrix. The misorientations between the bands and the matrix group around special misorientations Σ9, Σ19a, Σ27a, and Σ33a, which are characterized by close angles of rotation about axis 〈110〉. During primary recrystallization, the (110)[001] grains growing from the bands retain segments of the corresponding special boundaries with the deformed matrix.     

Remarks on the distortion of a cholesteric structure by a magnetic field

A cholesteric liquid crystal, located between fixed boundaries, does not necessarily alter the spatial period of the helical structure if a magnetic field acts perpendicular to the helical axis, whereas for an infinitely extended crystal the spatial period increases continuously with the field. For fields not too large, a thin sample of a cholesteric liquid crystal should show higher order reflection bands for light propagation parallel to the helical axis but no displacement of the main reflection band.     

Shear band formation in IF steel during cold rolling at medium reduction levels

The mechanisms of shear band formation in IF steel after cold rolling to ～50% reductions have been investigated using transmission electron microscopy. The observations revealed that shear bands were always parallel to a second set of microbands, where these exist, and contained within individual crystals, indicating that shear banding is controlled by orientation. Crystallographic analysis revealed that shear banding involves two mechanisms, dislocation glide and rigid-body rotation. In the first step, dislocation glide causes a rotation about the 〈211〉 axis to produce the so called ‘S’ band, which gives the shear band its crystallographic character. In the second step, when the most heavily stressed slip plane parallel to the shear band is of the form {110}〈111〉, rigid-body rotation continues about the 〈211〉 axis in the sheared zone and, then, a rotation about the transverse direction (TD) is promoted by the geometry of the sample. Using rigid-body matrix theory, the calculated orientations of shear bands are shown to be in agreement with experimental observations. The process outlined is capable of explaining how slip processes in grains that contain microbands, using either {110} or {112} slip planes, can produce crystallographic shear bands.     

单晶铜纳米线屈服机理的原子模拟研究

采用分子静力学方法模拟了〈100〉单晶铜纳米线的拉伸变形过程,研究了纳米线屈服的机理. 结果表明：1） 纳米线初始屈服通过部分位错随机激活的{111}〈112〉孪生实现,后继屈服通过{111}〈112〉部分位错滑移实现;2） 纳米线变形初期不同滑移面上的部分位错在两面交线处相遇形成压杆位错,变形后期部分位错在刚性边界处塞积,两者都阻碍位错滑移,引起一定的强化作用. 关键词： 纳米线 屈服 位错 分子静力学