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.     

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.     

Influence of low-temperature annealing on the microstructure of cryogenically deformed copper

The microstructure formation during annealing of cryogenically deformed copper was studied. It was shown that the primary recrystallization was completed already after a one-hour annealing at 150°C. It was found out that grains with the crystallographic orientation close to {112}〈111〉, {4; 4; 11}〈11; 11; 8〉, {110}〈122〉, and {130}〈132〉 had an advantage in the growth, which, apparently, was associated with a relatively low energy stored during the deformation.     

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.     

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.     

Nanoscale anisotropic plastic deformation in single crystal aragonite

The nanoscale anisotropic elastic-plastic behavior of single-crystal aragonite is studied using nanoindentation and tapping mode atomic force microscopy imaging. Force-depth curves coaxial to the axis exhibited load plateaus indicative of dislocation nucleation events. Plasticity on distinct slip systems was evident in residual topographic impressions where four pileup lobes were present after indentation with a conospherical probe and distinct, protruding slip bands were present after indentation with a Berkovich pyramidal probe. A finite element crystal plasticity model revealed the governing roles of the {110}<001>slip system family, as well as the (100)[010], (100)[001], (010)[100], (010)[001], (001)[100] and (001)[010] systems.     

Orientation effects in interaction of screw dislocation with interstitial impurity atom in b.c.c. lattice

The cross-slip and pinning of a 1/2a〈111〉 screw dislocation in b.c.c. metals in the vicinity of an interstitial impurity atom are studied in dependence on crystal orientation. To this purpose, the interaction energy between the dislocation and an interstitial atom is calculated in an anisotropic elastic continuum and it is assumed that the screw dislocation moves microscopically on {112} or {110} planes between its stable configuration positions in b.c.c. lattice. It is found that the probability of induced cross-slip is orientation dependent. This result is used for discussion of orientation dependence of the change of CRSS due to increased carbon content which was experimentally determined for Fe-3.2% Si alloy single crystals in a previous paper (Blahovec J., Kade?ková S.: Czech. J. Phys.B 21 (1971), 846).     

Grain-boundary faceting at a = 3, [110]/{112} grain boundary in a cubic zirconia bicrystal

The atomic structure of a = 3, [110]/{112} grain boundary in a yttria-stabilized cubic zirconia bicrystal has been investigated by high-resolution transmission electron microscopy (HRTEM). It was found that the grain boundary migrated to form periodic facets, although the bicrystal was initially joined so as to have the symmetric boundary plane of {112}. The faceted boundary planes were indexed as {111}/{115}. The structure of the {111}/{115} grain boundary was composed of an alternate array of two types of structure unit: {112}- and {111}-type structure units. HRTEM observations combined with lattice statics calculations verified that both crystals were relatively shifted by (α/4)[110] along the rotation axis to form a stable grain-boundary structure. A weak-beam dark-field image revealed that there was a periodic array of dislocations along the grain boundary. The grain-boundary dislocations were considered to be introduced by the slight misorientation from the perfect = 3 orientation. The fact that the periodicity of the facets corresponded to that of the grain-boundary dislocations must indicate that the introduction of the grain-boundary dislocations is closely related to the periodicity of the facets. An atomic flipping model has been proposed for the facet growth from the initial = 3, {112} grain boundary.     

Plastic-strain localization in barium fluoride single crystals at elevated temperatures

The strain distribution is studied in BaF₂ crystals subjected to compression tests along [110] and [112] at a constant strain rate in the temperature range T = (0.22–0.77)T _m. At T > 0.5T _m, the plastic strain in deformed samples is found to be strongly localized in narrow bands, where the shear strain reaches several hundred percent. The degree of localization increases with temperature. Localized-shear microbands are shown to be oriented along {001}〈110〉 slip systems. The phenomenon of serrated yielding is detected, and stress jumps (serrations) are established to correlate with the formation of shear zones.     

Ni3(Tix,V1-x)合金系中的长周期结构

本文利用电子衍射及高分辨点阵象实验方法证实了Ni₃(Ti_x,V_1-x)合金系在铸态下存在有9R,10H,16H,20H,21H,44H等长周期结构以及2H结构。它们与体心四方基体相(结构与Ni₃V相同)及六角基体相(结构与Ni₃Ti相同)有固定取向关系:{001}_长周期∥{001}_(Ni3Ti)∥{112}_{Ni3V);〈100〉长周期∥〈100〉(Ni3Ti)∥〈110〉(Ni3V)。样品经长期高温退火处理后其它长周期结构均向9R结构转变。 关键词：}     

Change in the emitter surface during high-current-density field emission from the 〈111〉 trihedral angle of a reconstructed tungsten tip

Changes in the shape and emission characteristics of the atomically sharp trihedral 〈111〉 angle of a tungsten tip reconstructed in an electric field are studied by continuous-mode field desorption microscopy during high-current-density field emission. The main changes in the tip shape and the slope of the Fowler-Nordheim characteristic occur at an emission current of 1–5 μA. At a current of 50–100 μA taken from the angle, the tip shape and emission characteristics stabilize and remain unchanged in the range 0–150 μA. The new tip shape is characterized by the widening of the angle edges; the appearance of {112} and {001} plane steps in them; a decrease in the sizes of the {011} planes forming the angle faces; and the appearance of steplike transition regions between {011}, {001}, and {112} faces. These changes in the tip shape are related to the fact that the emission field is weaker than the electric field used for preliminary tip reconstruction, the weakening of the field by the space charge of emitted electrons, and a nonuniform temperature distribution in the tip.     

Oxidation of tungsten single crystals in the presence of a high-electric field

The effects of high-electric fields on oxidation of tungsten single crystals in 6 × 10^?4 torr of oxygen at 1200–1500 °K were studied by field emission and transmission electron microscopy. Exposure of field emitters to oxygen in the absence of a field resulted in the build-up of emitter tips. Oxidation under the application of a negative or positive field, on the other hand, involved plane faceting and formation of oxide crystallites. Plane faceting was recognized to occur on the {111} and the {112} regions, showing the facetings of the {111} and the {112} planes into the {110} planes, whereas, crystallite formation seemed to take place selectively on the {100} regions. It was suggested by field emission microscopy that negative fields have an additional effect which causes the growth of an oxide crystal on the (110) plane. Transmission electron microscopy of an emitter oxidized in a negative field actually revealed a tiny oxide crystal with a size of ～ 300 Å grown on the developed (110) plane. The crystal exhibited a triangular shadow image strongly indicating an external pyramid-like form.     

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     

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.     

Hydrogen diffusion in the elastic fields of dislocations in iron

The effect of dislocation stress fields on the sink efficiency thereof is studied for hydrogen interstitial atoms at temperatures of 293 and 600 K and at a dislocation density of 3 × 10¹⁴ m^–2 in bcc iron crystal. Rectilinear full screw and edge dislocations in basic slip systems 〈111〉{110}, 〈111〉{112}, 〈100〉{100}, and 〈100〉{110} are considered. Diffusion of defects is simulated by means of the object kinetic Monte Carlo method. The energy of interaction between defects and dislocations is calculated using the anisotropic theory of elasticity. The elastic fields of dislocations result in a less than 25% change of the sink efficiency as compared to the noninteracting linear sink efficiency at a room temperature. The elastic fields of edge dislocations increase the dislocation sink efficiency, whereas the elastic fields of screw dislocations either decrease this parameter (in the case of dislocations with the Burgers vector being 1/2〈111〉) or do not affect it (in the case of dislocations with the Burgers vector being 〈100〉). At temperatures above 600 K, the dislocations affect the behavior of hydrogen in bcc iron mainly owing to a high binding energy between the hydrogen atom and dislocation cores.     

bcc Fe中刃型位错的结构及能量学研究

基于位错理论，利用分子动力学方法建立了〈100〉{010}，〈100〉{011}，1/2〈111〉{011}和1/2〈111〉{112}刃型位错的芯结构，并计算了这四种刃型位错的形成能、位错芯能量和芯半径.计算结果表明：〈100〉{010}和〈100〉{011}刃型位错的形成能比1/2〈111〉{011}和1/2〈111〉{112}刃型位错的要高，这表明〈100〉刃型位错比1/2〈111〉刃型位错更难形成.而〈100〉{010}和〈100〉{011}刃型位错的芯半径比1/2〈111〉{011}和1/2〈111〉{112}刃型位错的小，这说明在1/2〈111〉刃型位错中位于奇异区的原子数多于〈100〉刃型位错，而这些原子要比完整晶体中的原子具有更大的活性.可见，1/2〈111〉刃型位错比〈100〉刃型位错更易运动，且〈100〉刃型位错在bcc Fe中难以形成. 关键词： bcc Fe 刃型位错 分子动力学模拟     

Growth and desorption of indium epitaxial layers investigated by high field microscopy

Growth of indium single crystals on tungsten field emission tips was carried out by deposition of indium from vapour in ultra high vacuum, using substrate temperatures in the range of 293–420 K. Two different tungsten tips were used as the substrate: a perfect W single crystal in one case and a bi-crystal with a distinct grain boundary in the other. No influence of the grain boundary on the epitaxial growth was found. Two orientation relationships were observed mostly: {111}In ∥ {110}W with 〈110〉In ∥ 〈111〉W and {111}In ∥ {100}W with 〈110〉In ∥ 〈110〉W. In the first case the growth was initiated by the indium nucleus created on the ledges of the {110}W plane. A field strength of 0.9 V/Å was found for the evaporation field of indium. The field strength of the desorption of In-W interfacial layer atoms was found to be 4.4–5.2 V/Å. A mechanism of the growth of indium crystals has been proposed.     

β—SiC外延层中晶体缺陷的观察

用腐蚀法研究了β-SiC外延层中的晶体缺陷。腐蚀剂为熔融氢氧化钾。三角形尖底蚀坑对应于位错。在β-SiC中的全位错为立方晶系的73°位错和60°位错。不同堆垛方式的β-siC生长层相遇时将形成{111}交界层错,其腐蚀图象为平行于<110>方向的直线。60°位错可分解为两个1/6<112>SchockLey不全位错,并夹着一片{111}层错构成扩展位错。三个1/6<110>压杆位错与三片{111}层错可构成层错锥体。正、反堆垛的β-SiC可形成尖晶石律双晶,双晶面为(111)。腐蚀法和X射线劳厄法证实了这种双晶的存在。 关键词：     

High field magnetoresistance of orientedcrystallites of AuPb2

High field magnetoresistance measurements have been made on samples of AuPb₂, which consist of islands of oriented AuPb₂ surrounded by eutectic mixture. The results obtained are very similar to what would be expected for single crystal samples. Open orbits are observed in 〈100〉, 〈110〉, 〈101〉, 〈112〉 and 〈211〉 and [001] directions. The results are consistent with predictions of the Nearly Free Electron Model.     

Growth and stability of carbon islands on platinum surfaces

Details of the growth of C(graphite) islands and their stability on Pt surfaces were studied by FEM, UHV-SEM and very high resolution scanning AES. Initial nucleation of the C occurs on dislocations in the curved high index surface areas. Above 1150 K these randomly distributed islands dissolve and face specific layers are formed on {110} which can extend along the 〈100〉 zones all the way to the {100} planes. The sequence of stability of graphite layers on Pt is: {110} > all other {hk0} on the 〈100〉 zone except {100} > {100} followed closely by {111}. Concerning this layer stability, epitaxial mismatch plays a subservient role to the dipole interaction between metal substrate and graphite layer.