Energy of a wedge-shaped nanotwin calculated in terms of a dislocation mesoscopic model

The total energy of a wedge-shaped micro- and nanotwin is calculated in terms of a dislocation mesoscopic model. The total energy of the twin is represented as a sum of the elastic energy, energy of interaction between twinning dislocations, and stacking-fault energy of partial dislocations of the wedge-shaped twin. It is found that the evolution of the twin is controlled by the energy of interaction between twinning dislocations: in the case of a microtwin, it is five orders of magnitude higher than the elastic energy and six orders of magnitude higher than the stacking-fault energy. In the case of a nanotwin with the number of twinning dislocations at the twin boundary less than 20, all the three energies listed above are of the same order of magnitude. Therefore, all the components of the total energy contribute to the origination of a wedge-shaped twin. As the length of the twin increases with its width and the number of twinning dislocations at twin boundaries fixed, the total energy modulo grows although the density of twinning dislocations at twin boundaries decreases. This indicates that long-range stress fields due to twinning dislocations play an important part in the evolution of a wedge-shaped twin.     

State of stress at a crystal surface deformed by a concentrated load in the presence of a wedge twin

A procedure for the calculation of the stress fields near a wedge twin located at a crystal surface is developed using a dislocation macroscopic model. The problem is solved for the case of a concentrated load applied to the surface of a crystal with a twin. The concentrated load is found to increase the level of stresses near the wedge twin and to cause their localization at the twin boundary that is closer to the load application point.     

Molecular dynamics simulation of the formation of twin boundaries during agglomeration of nanoparticles

The processes controlling early stages of agglomeration of nanoparticles have been investigated by the molecular dynamics method. It has been established that the formation of boundaries with twin misorientation is the main mechanism of structural relaxation during primary agglomeration of nanoparticles. It has been shown that an increase in the temperature leads to an increase in the number of twin boundaries and that their mutual arrangement depends on the misorientation of the nanoparticles. In the case where twin boundaries are noncoplanar, structure relaxation results in the formation of pentagonal twin boundaries. The role of twinning in the formation of interfaces upon compaction of nanoparticles has been discussed.     

Dislocation model for an incoherent nonthin twin

A condition for the equilibrium of a nonthin twin is obtained from a dislocation model using the approximation of a continuous distribution of twinning dislocations at twin boundaries. The model of a nonthin twin is shown to be applicable to the elastic and inelastic stages of twinning. The theory of a thin twin is found to be a particular case of the developed dislocation model of a nonthin twin.     

Effect of dislocation distribution in twin boundaries on the nucleation of microcracks at the twin tip

The effect of dislocation distribution in the boundaries of an arrested twin on the nucleation of microcracks at its tip is investigated. The twin is simulated by a double step pileup (cluster) of twinning dislocations located in adjacent slip planes. The equilibrium equations for dislocations are solved numerically. Clusters with different total numbers of dislocations and with different ratios of the numbers of dislocations at the upper and lower twin boundaries are considered. The formation of microcracks as a result of coalescence of head dislocations according to the force and thermally activated mechanisms is analyzed. The equilibrium configurations of a single twin boundary and of the twin are calculated. It is found that the condition for microcrack formation at the twin tip considerably depends on the ratio of the numbers of dislocations in twin boundaries. In the limit, this condition coincides with the condition of crack formation at the tip of a single twin boundary with the same total number of dislocations. It is shown that thermally activated formation of a microrack corresponds to lower values of the critical stress.     

Functional twin boundaries

Functional interfaces are at the core of research in the emerging field of ‘domain boundary engineering’ where polar, conducting, chiral, and other interfaces and twin boundaries have been discovered. Ferroelectricity was found in twin walls of paraelectric CaTiO₃. We show that the effect of functional interfaces can be optimized if the number of twin boundaries is increased in densely twinned materials. Such materials can be produced by shear in the ferroelastic phase rather than by rapid quench from the paraelastic phase.     

Shear at twin domain boundaries in YBa2Cu3O7-x

The microstructure and strain state of twin domains in YBa2Cu3O7-x are discussed based upon synchrotron white-beam x-ray microdiffraction measurements. Intensity variations of the fourfold twin splitting of Laue diffraction peaks are used to determine the twin domain structure. Strain analysis shows that interfaces between neighboring twin domains are strained in shear, whereas the interior of these domains are regions of low strain. These measurements are consistent with the orientation relationships of twin boundaries within and across domains and show that basal plane shear stresses can exceed 100 MPa where twin domains meet. Our results support stress field pinning of magnetic flux vortices by twin domain boundaries.     

Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries

Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from 1 to 5 providing twin boundary spacing in the range 8.5–2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. The occurrence of twin–twin interaction during tensile deformation and dislocation–twin interaction during compressive deformation has been discussed.     

Direct characterization of boron segregation at random and twin grain boundaries

Boron distribution at grain boundaries in hot-deformed nickel is directly characterized by the time-of-flight secondary ion mass spectrometry. The segregations of boron are observed at both the random and twin grain boundaries. Two types of segregations at random grain boundaries are observed. The first type of segregation has a high intensity and small width. Its formation is attributed to the incorporating of dislocations into the moving grain boundaries. The second type of segregation arises from the cooling induced segregation at the dislocations associated with the grain boundaries. The segregation at twin boundary is similar to the second type of segregation at random grain boundaries.     

Slip transfer at coherent twin boundaries in Ni3Al and ordered Cu3Au

Slip transfer at coherent twin boundaries in L1₂-ordered alloys has been investigated by analysis of plastically deformed Cu₃Au and Ni₃Al specimens in a transmission electron microscope. A comparison between the ex situ deformed material and previous in situ experiments is made. In ordered Cu₃Au, antiphase boundaries induced by slip transfer have been found at a coherent twin boundary (CTB), similar to the in situ deformed material. On the other hand, in Ni₃Al superlattice intrinsic stacking faults (SISFs) were detected at CTBs which have not been observed in in situ deformed material. A possible mechanism for SISF formation is discussed. The transfer of slip, and the associated creation of stacking faults at CTBs in both materials is described in terms of absorption of superdislocations in the boundary and the general criteria for slip transfer at grain boundaries.     

Effect of Al and Cd impurities on the bauschinger effect during the twinning of a zinc crystal

A quantitative study has been made of the Bauschinger effect at various twin boundaries in crystals of pure zinc and of zinc containing Al and Cd impurities. In the pure zinc, the Bauschinger effect is strongest at stretched twin boundaries, while in Zn + Al and Zn + Cd crystals the effect is much weaker at twin boundaries and is approximately the same at different types of boundaries. The stress is a stronger than linear function of the displacement of twin boundaries in the impure crystals for both forward and reverse loading, because of an increase in the friction force acting on the twinning dislocations during motion along the shear plane. An increase in the Al content from 0.1% to 0.3% (by weight), which leads to the appearance of a finely dispersed precipitate, has essentially no affect on the magnitude of the Bauschinger effect.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 59–63, December, 1969.     

Production of materials saturated with twin boundaries by compacting electrolytic microparticles

A technique is proposed for producing nanostructured materials saturated with twin boundaries by compacting (pressing) small pentagonal electrolytic particles. The concentration of twin boundaries in the formed compound is estimated.     

GGA+U方法研究ZnO孪晶界对V_(Zn)-N_O-H复合体对p型导电性的影响

采用基于密度泛函理论的广义梯度近似平面波赝势方法,探究四种ZnO-Σ7(1230)孪晶界中V_(Zn)-N_O-H复合体的电子结构和p型导电机理.计算结果表明,在ZnO-Σ7(1230)孪晶界中,N掺杂后会与锌空位(V_(Zn))、氢填隙(Hi)等点缺陷结合,进而形成V_(Zn)-N_O-H复合体,并出现在孪晶中的晶格应变集中区.此外,四种孪晶界中孪晶GB7a有利于V_(Zn)-N_O-H离化能降低,从而使其表现出浅受主特征.分析显示特殊的孪晶结构导致了氮替位(N_O)与近邻的O原子间距离缩短,阴离子之间发生相互作用,导致禁带中的空带能级下降,降低了电子跃迁所需能量.这一结果也说明GB7a孪晶界中的V_(Zn)-N_O-H可能成为N掺杂ZnO材料的p型导电的来源之一.     

Effect of twin boundary spacing on deformation behavior of nanotwinned magnesium

The effect of twin spacing and temperature on the deformation behavior of nanotwinned magnesium is investigated using molecular dynamics simulation. The results indicate that there is a pronounced shift in the mechanical behavior of nanotwinned magnesium when twin spacing is smaller than 2.9 nm, and that the yield strength decreases with increasing temperature. The results show that at relatively high temperatures, a strength softening can be observed when twin spacing is larger than 7.8 nm. This study demonstrates that the yield strength is associated with the dislocation storage ability of nanotwinned magnesium and the repulsive force between twin boundaries and dislocations.     

Change in the Bauschinger effect during the relaxation of twinned crystals

Changes taking place in the quantitative characteristics of the Bauschinger effect in wedge-shaped twins are studied in relation to the period for which these are held before untwinning. All the measurements are made in the cleavage plane of single-crystal zinc and Zn-Cu alloys. The Bauschinger displacement of the twin boundaries first increases slightly and then starts diminishing during the delay period. In the course of untwinning, the region of easy untwinning on the (b) relationship diminishes with time in the softened zone; the extent of the softened zone remains constant during the relaxation of the crystals. The suppression of the Bauschinger effect takes place more intensively if the crystal is held under load before untwinning. The change in the quantitative characteristics of the Bauschinger effect may be explained by the diffusion of impurities to the twinning dislocations and dislocation sources and the consequent blocking of these, and also by the relaxation of stresses in the twin boundaries as a result of plastic shears and dislocation reactions between complete and twinning dislocations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 113–117, April, 1971.     

Novel magneto-oscillatory behavior of confined electrons at a twin boundary in ZnSe and at an interface in GaAs/AlGaAs heterostructures

Electronic properties of confined electrons at naturally formed twin boundaries in ZeSe have been investigated by microwave absorption measurements. Cyclotron resonance signal of the confined electrons was observed. On the cyclotron resonance peak, novel oscillation of the absorption intensity was observed. Similar oscillatory behavior was also observed in GaAs/AlGaAs heterostructures near cyclotron resonance filed at which new Shubnikov–de Haas-like oscillation was observed by Zudov. The oscillatory structure has a period not in inverse magnetic field but magnetic field. The origin of the oscillation observed is caused by Rayleigh interference of two-dimensional electron gas.     

Magnetically modulated microwave absorption in twin boundaries of a high-temperature superconducting single crystal

We propose a model of magnetically modulated microwave absorption (MMMA) in twin boundaries of high-temperature superconductors which assumes formation of superconducting current loops closed by two Josephson junctions. It well explains the origin and properties of the periodic signal in HTS monocrystal.     

Anomalous diffusion in presence of twin boundaries

After implantation of the ⁷¹Ge radiotracer in the Mg-1.8 wt% Al magnesium alloy, tracer diffusion coefficients were measured using both the serial sectioning and the residual activity technique in the temperature range from 448 to 848 K. Anomalously high diffusivities were observed at high temperatures where volume diffusion usually predominates. A contribution of high diffusivity paths to the total diffusion flux can be a possible explanation of both phenomena. Based on the high fraction of twin boundaries confirmed by electron microscopy, a simple microstructure model is proposed.     

Experimental characterization of quantum-correlated twin beams and a quantum channel with twin beams

We report the generation and characterization of quantum-correlated twin beams and their application to a quantum channel. Compared with our previous results, the characterization of twin beams and the quantum channel have been studied with 7.0-dB intensity difference squeezing. The measured inference variance of twin beams is decreased to 0.40 ± 0.02. For the quantum channel, the bit error rate will be 0.035 by using 7.0-dB intensity difference squeezing compared with our previous result of 0.067 by using 4.9-dB intensity difference squeezing.