Structure and stability of Σ3 grain boundaries in face centered cubic metals

Σ3 grain boundaries form as a result of either growth twinning or deformation twinning in face centered cubic (fcc) metals and play a crucial role in determining the mechanical and electrical properties and microstructural stability. We studied the structure and stability of Σ3 grain boundaries (GBs) in fcc metals by using topological analysis and atomistic simulations. Atomistic simulations were performed for Cu and Al with empirical interatomic potentials to reveal the influence of stacking fault energy on the morphology of the twinned grains. Three sets of tilt Σ3 GBs were studied with respect to the tilt axis parallel to ?111?, ?112?, and ?110?, respectively. We showed that Σ3{111} and Σ3{112} GBs are thermodynamically stable and the others will dissociate into terraced interfaces regardless of the stacking fault energy. The morphology of the nano-twinned grains in Cu is predicted from the above analysis and found to match with experiments.     

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     

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.     

Dislocations in Fe-3% Si alloy single crystals deformed at a higher rate

The method of etching dislocations is used to study the distribution of dislocations and twins in Fe-3% Si alloy single crystals prepared from the melt after plastic deformation with higher speed. The crystals are deformed by twinning in the 〈111〉 directions along the {112} planes and by slip in the 〈111〉 directions along the {110} planes. The results prove that the dislocations causing plastic deformation move in the {110} planes during both fast and slow deformation. The difference in the slip surfaces during fast and slow deformation is explained by the different number of cross slips per unit dislocation path.     

Lattice reorientation in the shear bands of {112}〈131〉 crystallites in an Fe-3%Si alloy

Subregions (0.1 μm) with the {110}〈113〉 orientation form in shear bands in grains with the {112}〈131〉 orientation in a deformed (? ≈ 50%) polycrystalline Fe-3%Si alloy sample. The relationship between the matrix and the subregions in the shear bands is described by a special misorientation close to Σ5. It is assumed that the subregions that have a {110}〈hhl〉 orientation and special misorientation Σ5 with the surrounding matrix and form in the shear bands of crystallites with orientations other than {111}〈112〉 can serve as anomalous growth nuclei during heat treatment because of a high density of special Σ5 boundaries.     

Evolution of dislocation boundaries during cold deformation of microcrystalline titanium

The evolution of the misorientation spectrum of titanium during cold deformation via uniaxial tension was analyzed. The initial stage of plastic flow (true strain e ～ 0.1) is characterized by the intense formation of low-angle boundaries (LABs). The axes of rotation of most LABs are close to 〈001〉. This fact can be treated as a consequence of predominant prismatic slip during plastic deformation of titanium. An increase in strain to e ～ 0.5 and ～ 1.0 is accompanied by both a gradual increase in the mean misorientation angle of LABs and an increase in the fraction of high-angle boundaries (HABs). Twinning occurs over the entire deformation range under study: twinning on the {112}〈\(\overline 1 \overline 1 \)3〉 system is predominant at the initial stage, and twinning on the {102}〈\(\overline 2 \overline 1 \)3〉 system is predominant at the final stage. It is found that the misorientation spectrum of HABs forming during deformation is dominated by 15°, 30°, and 90° boundaries.     

γ-TiAl金属间化合物面缺陷能的分子动力学研究

运用分子动力学方法，对γ-TiAl金属间化合物的面缺陷能(层错能和孪晶能)进行了研究. 计算得到γ-TiAl不同滑移系(或孪生系)的整体堆垛层错能曲线，结果表明，γ-TiAl较一般fcc晶体结构的金属可动滑移系(孪生系)的数量减少，在外界条件下呈脆性. 研究孪生系(1/6)〈112〉{111}的弛豫的整体堆垛层错(GSF)能和整体孪晶(GTF)能曲线，对不稳定层错能γ_usf、稳定层错能γ_sf和不稳定孪晶能γ_usf值进行分析，可以预知， γ-TiAl的主要变形机理为孪生系(1/6)〈112〉{111}的孪生和普通滑移系(1/6)〈110〉{111}的滑移，以及超滑移系(1/2)〈011〉{111}的滑移. 关键词： γ-TiAl')" href="#">γ-TiAl 堆垛层错能 孪晶能 分子动力学     

应力诱发NiAl单晶马氏体相变的分子动力学模拟

利用嵌入原子势（EAM）,对NiAl单晶在外应力作用下的动态拉伸过程进行了分子动力学模拟.应力-应变曲线分析以及原子构型分析表明外应力诱发NiAl合金发生了马氏体相变,原子结构由B2相转变为L1₀相.通过研究原子构型的演化过程,发现马氏体相变是通过多个{110}孪晶面的扩展和湮灭作用来完成的.同时探讨了马氏体相变的微观机理. 关键词： 马氏体相变 NiAl 分子动力学模拟     

Dislocation formation and twinning from the crack tip in Ni3Al: molecular dynamics simulations

The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations.Owing to the unstable stacking energy,the [01ˉ1] superdislocation is dissociated into partial dislocations separated by a stacking fault.The simulation results show that when the crack speed is larger than a critical speed,the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip;subsequently the super intrinsic stacking faults are formed in adjacent {111} planes,meanwhile the super extrinsic stacking faults and twinning also occur.Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning,which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.     

New deformation twinning mechanism generates zero macroscopic strain in nanocrystalline metals

Macroscopic strain was hitherto considered a necessary corollary of deformation twinning in coarse-grained metals. Recently, twinning has been found to be a preeminent deformation mechanism in nanocrystalline face-centered-cubic (fcc) metals with medium-to-high stacking fault energies. Here we report a surprising discovery that the vast majority of deformation twins in nanocrystalline Al, Ni, and Cu, contrary to popular belief, yield zero net macroscopic strain. We propose a new twinning mechanism, random activation of partials, to explain this unusual phenomenon. The random activation of partials mechanism appears to be the most plausible mechanism and may be unique to nanocrystalline fcc metals with implications for their deformation behavior and mechanical properties.     

Effect of cyclic stresses on the microstructures of hexagonal close packed metals

Slip band extrusions are formed in cadmium, magnesium and titanium, but not in zinc. The extrusions form preferentially in untwinned crystals. Filamentary growths occur at {10¯12} and {11¯21} twin interfaces during cyclic twinning.Possible dislocation interactions at these twin interfaces are described. The dislocation debris produced during cyclic strain in the slip bands and by cyclic twinning is shown to be similar and composed of a high density of dipole loops. It is therefore concluded that the occurrence and distribution of slip band extrusions in metals and the formation of twin boundary filamentary growths can be accounted for by a model based upon the glide of interstitial type dipole loops. Vacancy type loops will then cause crack nucleation in slip bands and deformation twin boundary regions.Twin boundary debris can also cause the observed fragmentation of twins by acting as a barrier to twin boundary movement.The author is grateful to Dr. A. G. Crocker, University of Surrey, for many discussions on the twinning mode in h.c.p. metals and to P. J. E. Forsyth for his interest and encouragement. The paper is published by permission of the Controller, H. M. Stationery Office. Crown copyright is reserved.     

Comparison of different displacive processes in bcc crystals

A set of many-body potentials of Finnis-Sinclair type is used for modelling of several displacive processes in the bcc structures. The twinning process on the {112} plane and bcc-hcp transformation by Burgers mechanism are considered. The energy barriers on both paths are compared and Peierls stresses for partials responsible for movements of the twin and bcc-hcp interfaces are estimated.     

Peierls-nabarro barrier and stress of a/2 〈111〉 edge dislocation in α-Fe

The Peierls-Nabarro barrier and stress of thea/2〈111〉 edge dislocation on {112} and {110} plane inα-Fe at O K is calculated within the Peierls-Nabarro model. The method proposed by Nabarro is used, however, the sine force law is replaced by more general force laws based on two central interionic potentials inα-Fe. The values of the Peierls-Nabarro stress corresponding to one of the chosen interionic potentials, 3·5×10^?4 μ and 1×10^?4 μ on {112} plane (in the twinning direction) and on {110} plane, respectively, seem to be good estimates of the stress necessary to move edge dislocations inα-Fe at O K.     

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.     

Spinodal twinning of a deformed crystal

We propose the possibility of a spinodal mechanism for deformation twinning in addition to the nucleation and growth mechanism assumed in all existing studies of twinning, using the thermodynamic stability analysis of a homogeneously deformed crystal by examining its energy landscape as a function of strain along the twinning direction obtained from first-principles calculations. Twinning occurs continuously owing to thermodynamic instability with respect to twinning at large shear strains, whereas it can only take place through the nucleation and growth mechanism at small shear strains.     

Grain refinement and strengthening of austenitic stainless steels during large strain cold rolling

The microstructure/texture evolution and strengthening of 316?L-type and 304?L-type austenitic stainless steels during cold rolling were studied. The cold rolling was accompanied by the deformation twinning and micro-shear banding followed by the strain-induced martensitic transformation, leading to nanocrystalline microstructures consisting of flattened austenite and martensite grains. The fraction of ultrafine grains can be expressed by a modified Johnson-Mehl-Avrami-Kolmogorov equation, while inverse exponential function holds as a first approximation between the mean grain size (austenite or martensite) and the total strain. The deformation austenite was characterised by the texture components of Brass, {011}<211>, Goss, {011}<100>, and S, {123}<634>, whereas the deformation martensite exhibited a strong {223}<110> texture component along with remarkable γ-fibre, <111>∥ND, with a maximum at {111}<211>. The grain refinement during cold rolling led to substantial strengthening, which could be expressed by a summation of the austenite and martensite strengthening contributions.     

CVD金刚石薄膜孪晶形成的原子机理分析

采用X射线衍射技术、电子背散射衍射技术和扫描电镜分别观察了不同甲烷浓度条件下沉积的CVD自支撑金刚石薄膜的宏观织构、晶界分布和表面形貌. 研究了一阶孪晶在金刚石晶体{111}面生长的原子堆垛过程. 结果表明，由于一阶孪晶〈111〉60°的取向差关系以及{111}面的原子堆垛结构，使{111}面上容易借助碳原子的偏转沉积产生一阶孪晶. 低甲烷浓度时，碳原子倾向于在表面能较低的{111}面沉积，为孪晶的形成提供了便利，且高频率孪晶使薄膜织构强度减弱. 甲烷浓度升高使生长激活能较小的{001}面成为主要前沿生长面，因而只有〈001〉晶向平行薄膜法向的晶粒能够不断长大，因此孪晶形核概率明显减小. 另外，在薄膜中发现二阶孪晶，并对二阶孪晶的形成进行了分析. 关键词： 金刚石薄膜 孪晶 原子机理 取向差     

Thermal stretching of defective nanowires: the coupled effects of vacancy cluster defects,operating temperature,and wire cross-sectional area

Extensive atomistic simulations of the thermal stretching of defective nanowires (NWs) were performed using the embedded-atom molecular dynamics modeling approach. The nucleation and propagation of dislocations are described via quantitative dislocation-based analyses. The investigation focuses on the coupled effects of various vacancy cluster (VC) defects, operating temperature, and wire cross-sectional area on the mechanical properties and plastic deformations of defective NWs. With increasing internal stress of a stretched wire, a rapidly moving dislocation loop that transferred atoms to fill up the original vacancy cluster before the wire yielded was found (i.e. it carried the vacancies away from the inside of the wire and formed a notch at the wire edge). The heterogeneous nucleation of dislocations from the notch site propagated along the {111}〈112〉 partial dislocations and formed stacking faults or perfect dislocations on the {111} activated planes. Simulation results show a decreasing yield strength with increasing VC size for a given wire sectional area and temperature. Quasi-linear decreasing Young’s moduli were observed with increasing operation temperature. For a given operation temperature, NW Young’s modulus increased with increasing NW size. Two typical deformation regimes under various operation temperatures were found: (i) a high-temperature-induced pre-melting phenomenon and a thermal softening effect caused low-stress plastic flow and rapid pillar-necking deformation, and (ii) step-wise glides, slip bands, and cross-slips proceeded along the activated glide planes in the low-temperature hard-brittle structure. These two regimes were thoroughly characterized via the evolutions of microscopic dislocations and the changes of true stress. For operation at high temperatures, the ultra-thin 1/5-type pentagonal ring chains exhibit a relatively robust structure, which can potentially be used as building blocks and components for high-temperature nanoelectromechanical systems (NEMS) devices in the future.     

Stress-induced martensitic transformations in [001] crystals of titanium nickelide and its relation to mechanical twinning in the <Emphasis Type="Italic">В</Emphasis>2-phase

Methods of transmission electron microscopy were used to investigate the mechanisms of plastic deformation of TiNi(Fe, Mo) [001] crystals on compression in the interval of stress-induced martensitic transformations. It has been revealed that the formation of {113} and {114} twins of the В2-phase by the В19′→В2 reverse martensitic transformation over another path is, alongside with mechanical (100) twinning of В19′ martensite, an important deformation mechanism in this interval.