Internal Dislocation Research Articles

Formation of Ni<sub>3</sub>Nb and Influence of Y<sub>2</sub>O<sub>3</sub> on Mechanical Properties of Nano-Grained Ni20Cr20Fe5Nb1Y<sub>2</sub>O<sub>3</sub> Alloy

The effects of adding Y2O3, and the precipitation of Ni3Nb by heat treatment, on the mechanical properties of mechanically alloyed Ni20Cr20Fe5Nb alloy were studied. The addition of Y2O3 caused an increase in the tensile strength at room temperature, 400°C and 600°C. The difference in the tensile strength between the Ni20Cr20Fe5Nb and Ni20Cr20Fe5Nb1Y2O3 alloys decreased gradually with increasing test temperature. The tensile strength of the Ni20Cr20Fe5Nb1Y2O3 alloy at relatively low temperature was increased by the addition of Y2O3, but decreased abruptly at temperature above 600°C. This seems to result from a change in the deformation mechanism due to the ultra-fine grain size, that is, grain boundary sliding is predominant at temperatures above 600°C while internal dislocation movement is predominant at temperatures below 600°C. Following the conventional heat treatment of the solution and subsequent aging, only a small amount of δ(Ni3Nb) phase was formed in the Ni20Cr20Fe5Nb alloy, whereas in a previous report it was indicated that a large amount of γ″(Ni3Nb) was formed in IN 718 alloy. The small amount of δ(Ni3Nb) phase formed in the present case is due to the exhaustion of the Nb content resulting from the formation of NbC during consolidation.

Internal stresses and dislocation structure of large single crystals of germanium for IR optics

The thermoelastic stresses that appear during crystallization have been theoretically estimated for single crystals of germanium grown in the shape of a disk. It is shown that there is a correlation between the stress distribution and the dislocation structure of large single crystals of germanium obtained by the Stepanov method and by directed crystallization.

Characterization Of Dislocation InUnderground Mass Using Coupled Modeling

This paper deals with an application of coupled modeling in the identification of dislocation occurring in coal mines, threatening workers in the underground structures. The bumps can be induced by different circumstances. One of them is an accumulation of energy in unpredictable dislocations. The measurement on site for learning the position of bumps is very expensive and unreliable. One of the most reliable approaches is physical modeling, which enables one to carry out parametric studies and after certain results from these models one can assess the most probable concentration of stresses. On the other hand, the stresses are measured in a very difficult way, so that numerical analysis should be prepared. In numerical analysis the contact problem in limit state estimation is based on the data from physical modeling. Physical modeling seems to be the best for linear analysis. This is not the case in our study and great efforts are needed to estimate the real behavior of the material. The support of physical modeling mathematical formulation and numerical treatment can lead us to the location of bumps. In general, a large iteration should be used to solve the strongly nonlinear problem in all subdomains ranged in boundaries given by possible dislocations, and on the interfacial boundaries (dislocations). In order to eliminate some principal directions of iteration the physical modeling is used and the numerical processes become bearable. On the other hand, more dislocations can be sought by this modeling and the only restriction needed is to know that no internal dislocations are probable. It means that every discontinuity induces a statically determined or undetermined problem. A typical example from practice verifies the theory based on the back of the analysis. © 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 51, www.witpress.com, ISSN 1743-3533 (on-line) Computational Methods and Experiments in Material Characterisation II 333

Theoretical Models and Experimental Techniques in Nondestructive Evaluation of Concrete

When evaluating concrete strength, common opinion is that adequate precisions can be achieved only by a particular or even total destruction. However, such methods are not always applied, besides they are very laborious. The NDE methods have a number of merits, when compared with destructive ones: a possibility to find cracks and hidden flaws in concrete; besides, they show good results in testing materials of other types, such as metals and composites. At the same time, application of NDE methods to concretes is difficult because of their complex internal structure. No existing theory can predict these properties of the transmitted wave. Therefore, the main goal of the present work is to propose a theoretical model enabling the wave penetration of ultrasonic wave through a medium with multiple internal obstacles to be described adequately. Practical applications of this ultrasonic method is toward the evaluation of mechanical properties of concrete, where the influence of internal dislocations, such as pores and cracks, is of significant importance.

Dislocation structure and surface relief in fatigued metals

The geometry of the persistent slip markings (PSMs) produced by cyclic loading in two stainless steels, one with fcc structure and the other with bcc structure, were assessed using atomic-force microscopy (AFM) and high-resolution scanning electron microscopy (SEM). The internal dislocation structure in both steels was studied using transmission electron microscopy. The bands of the dislocation structure corresponding to cyclic strain localization were identified in both materials and were correlated with PSMs.

Cyclic deformation of silicon single crystals: correlation between dislocation microstructure and mechanical behaviour during linear hardening

A model is proposed to describe cyclic deformation of Si single crystals during the stage of linear hardening observed before stress saturation. The model starts from a wall and channel microstructure of given period, assuming that plastic strain is carried by screw dislocations moving in channels. Internal stresses and dislocation mobility are taken into account. Annihilation of screw segments by cross-slip is compensated by production of new ones from edges bulging out of the walls. Assuming a reasonable distribution of edge dislocation segments, the model satisfactorily describes the shape of stress-strain cycles when walls thicken. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Generation of dislocations in annealed silicon wafers under applied stress

Generation of dislocations during heat treatment under applied stress was studied in silicon samples that underwent different multi-step anneals. Various types of oxygen precipitates (microdefects) were produced by multi-step anneals – as evidenced by TEM. These microdefects act as internal sources of dislocations when samples are further heat-treated under applied stress. The mechanical strength is remarkably sensitive to the microdefect type. The most efficient internal dislocation sources are large stacking faults and dislocation dipoles. The highest mechanical strength was found in samples containing only small platelet precipitates. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

오스테나이트계 스테인리스강의 크리프 변형중 내부응력과 운동전위밀도의 평가

To investigate the change of internal stress and mobile dislocation density in the creep, stress relaxation test was examined from each strain range Mobile dislocation density increased until it reached minimum creep rate but after that, it decreased Internal stress did not change until it reached minimum creep rate but after that, it decreased The stress relaxation rate is fast and approached zero later 1 5 seconds, which were begun in the stress relaxation When the applied stress is large, the internal stress is large It is cleared that dislocations glide viscously which N passes by cutting Cr atom rather than typical viscosity movement by the evaluation of mobility of dislocation in STS310J1TB.

Evolution of the defect structure in helium implanted SiGe∕Si heterostructures investigated by in situ annealing in a transmission electron microscope

The evolution of the He-implantation induced defect structure in SiGe∕Si heterostructures is observed during in situ annealing at 650 and 800°C within a transmission electron microscope. The He implantation and annealing results in the formation of He precipitates below the SiGe∕Si interface, which at first show a platelike shape and subsequently decay into spherical bubbles. The coarsening mechanism of the He bubbles is revealed as coalescence via movement of entire bubbles. The nucleation of dislocation loops at overpressurized He platelets and their propagation into the heterostructure could be observed as well. We found distinctly different velocities of the dislocations which we attribute to glide and climb processes. The in situ experiments clearly show that the He platelets act as internal dislocation sources and play a key role in the relaxation of SiGe layers.

On the role of residual internal stresses and dislocations on Barkhausen noise in plastically deformed steel

The dependence of Barkhausen noise on elastic and plastic deformations, achieved in tension and in compression, has been investigated both in Armco iron and a low carbon steel. These materials exhibit quite different behaviours, especially with regard to the effect of plastic deformation: a tensile plastic deformation (>1%) induces a marked increase in Barkhausen noise for Armco iron while it induces a steep decrease in the low carbon steel. The comparison between the tensile and compressive behaviours, as well as between the elastic and plastic regimes of deformation enables us to attribute these effects to two underlying mechanisms, i.e. effect of residual internal stresses through magneto-elastic coupling and dislocation–domain wall interaction. In Armco iron, the latter mechanism seems to have the strongest influence on the Barkhausen noise, while in the low carbon steel the influence of residual internal stresses prevails.

Dislocation String Model and Sweep-up Vacancy

To verify the Granato–Lücke (G–L) theory, the internal friction and dislocation core current of NaCl single crystals are measured. Vacancies that move in-phase with dislocation cores and vacancies that undergo phase delay by dislocations are observed. Dislocations sweep up vacancies on the slip plane with the break-away strain amplitude. The accuracy of the G–L theory is demonstrated by subtracting the increase in friction due to sweep-up vacancies from the measured values. Corrections could also be performed for samples in which the number of vacancies had been changed by irradiation with gamma rays. The dislocation core of an irradiated sample is found to be negatively charged by an excess of vacancies in the small strain amplitude region.

Internal stress and heterogeneous dislocation structure in creep

The present work deals with the problem of microstructural interpretation of internal stress measured in high-temperature creep by the strain-transient dip-test technique. The development of microstructure in the course of creep is considered a transition from uniformly distributed dislocations to a well-developed substructure. The substructure is supposed to have a composite character that consists of cell or subgrain boundaries with few dislocations in cell or subgrain interiors. Model equations for the relation of internal stress to the parameters of the dislocation structure are discussed and examined with reference to experimental data. The evolution of internal stress in creep, evaluated using different formulae, is compared with the evolution of macroscopically measured internal stress. The use of applied-stress dependences of microstructure parameters permits quite realistic estimates of the values of internal stress in steady-state creep.

Microstructure and Nanostructure of Deformation Processed Cu-Nb and Cu-Ag Filamentary Composites/ Mikro- und Nanostrukturen von umgeformten Cu-Nb und Cu-Ag Faserverbundwerkstoffen

Microstructural features of Cu-Nb and Cu-Ag composites are examined and compared. Microstructural scale of Cu-Ag was finer than that of Cu-Nb because of the initial finer cast structure of Cu-Ag. The morphological change of filaments can be associated with the slip behavior during drawing process and the spheroidization during intermediate annealing. The microstructures of both composites are too fine and the interfacial areas are too large to maintain stable internal dislocation structures. Most dislocations are gradually absorbed at the interfaces as the draw ratio increases. Interfacial dislocations are necessary to accommodate the mismatch through the interface both in Cu-Nb and Cu-Ag. The coherent orientation relationship between filaments and the matrix and the presence of interface dislocations supports that Ag and Nb filaments are partially coherent with Cu matrix. Since the lattice mismatch through the interface in Cu-Ag is larger that that in Cu-Nb, interfacial dislocations are more closely spaced in Cu-Ag.

Microstructure of precipitated Au nanoclusters in MgO

Gold nanoclusters dispersed in single crystal MgO have been prepared by ion implantation at 975 K and subsequent annealing at 1275 K for 10 h. The morphological features, size, and crystallographic orientation of the Au nanoclusters with respect to the MgO matrix, as well as the interface structure between the Au nanoclusters and MgO, have been investigated using transmission electron microscopy. During annealing, the Au clusters nucleate coherently in the MgO lattice, leading to an epitaxial orientation relationship of [010]MgO//[010]Au and (200)MgO//(200)Au that is maintained for all the Au clusters. Above a critical size of ∼5 to 8 nm, a coherent-semicoherent interface transition is observed for the Au clusters in MgO. This critical cluster size is larger than the critical size ∼3 nm based on energetic considerations. This discrepancy is discussed with respect to the point and extended defect structures at the interface between the Au clusters and the MgO matrix. The Au clusters larger than this critical size exhibit faceting on the {001} planes and internal dislocations. It is further suggested that the density of quantum antidots should depend on the size of the Au clusters.

Effect of lattice defects on the thermal conductivity of β-Si3N4

Two types of β-Si 3 N 4 were sintered at 1900 °C one for 8 h and the other for 36 h by using Yb 2 O 3 and ZrO 2 as sintering additives. The latter specimen was further annealed at 1700 °C for 100 h to promote grain growth. The microstructures of the sintered materials were investigated by SEM, TEM, and EDS. The thermal conductivities of the specimens were 110 and 150 Wm −1 K −1 , respectively. The sintered material which possessed 110 Wm −1 K −1 had numerous small precipitates that consisted of Yb, O and N elements and internal dislocations in the β-Si 3 N 4 grains. In the sintered material with 150 Wm −1 K −1 neither precipitates nor dislocations were observed in the grains. The microscopic evidence indicates that the improvement in the thermal conductivity of the β-Si 3 N 4 was attributable to the reduction of internal defects of the β-Si 3 N 4 grains with sintering and annealing time as the grains grew.

A Flux‐Tube Tectonics Model for Solar Coronal Heating Driven by the Magnetic Carpet

We explore some of the consequences of the magnetic carpet for coronal heating. Observations show that most of the magnetic flux in the quiet Sun emerges as ephemeral regions and then quickly migrates to supergranule boundaries. The original ephemeral concentrations fragment, merge, and cancel over a time period of 10-40 hr. Since the network photospheric flux is likely to be concentrated in units of 1017 Mx or smaller, there will be myriads of coronal separatrix surfaces caused by the highly fragmented photospheric magnetic configuration in the quiet network. We suggest that the formation and dissipation of current sheets along these separatrices are an important contribution to coronal heating. The dissipation of energy along sharp boundaries we call, by analogy with geophysical plate tectonics, the tectonics model of coronal heating. Similar to the case on Earth, the relative motions of the photospheric sources will drive the formation and dissipation of current sheets along a hierarchy of such separatrix surfaces at internal dislocations in the corona. In our preliminary assessment of such dissipation we find that the heating is fairly uniform along the separatrices, so that each elementary coronal flux tube is heated uniformly. However, 95% of the photospheric flux closes low down in the magnetic carpet and the remaining 5% forms large-scale connections, so the magnetic carpet will be heated more effectively than the large-scale corona. This suggests that unresolved observations of coronal loops should exhibit enhanced heating near their feet in the carpet, while the upper parts of large-scale loops should be heated rather uniformly but less strongly.

Surface vertical displacements, potential perturbations andgravity changes of a viscoelastic earth model induced by internal point dislocations

Using the viscoelastic correspondence principle, we utilize the surface coseismic spheroidal deformation fields (i.e. vertical displacements, potential perturbations and gravity changes) of SNREI earth models caused by four typical types of point dislocation, derived by Sun & Okubo (1993), to deduce the fundamental formulas for spheroidal fields relevant to viscoelastic earth models. In computations, we employ a strike-slip dislocation on a vertical plane buried at the bottom of the lithosphere to estimate the maximal viscous relaxation responses to this kind of source that possibly exist on the surface of the earth. We take the seismic moment as 1022N m, which is characteristic of an average large earthquake. The numerical results demonstrate that, if we take the viscosity as 1019 Pa s in the asthenosphere, and 1021 Pa s in the other mantle layers, the rates of surface vertical displacements and gravity changes within about 2.5° for the 10 postseismic years are respectively 1.5–8.1 cm yr−1 and 4.0–14.9 μgal yr−1 : the viscous relaxation for this mantle viscosity profile proceeds much faster than for a constant mantle viscosity of 1021 Pa s.

Influence of forming parameters on static recrystallization behaviour during hot rolling aluminium alloy 5083

In this paper, the influence of rolling parameters (i.e. rolling temperature, roll speed, roll temperature, friction and the ratio of the mean thickness to the contact length in the roll gap Hm/L) on static recrystallization (SRX) behaviour is studied by the combination of the finite element method (FEM) with the Taguchi experimental method. The FEM is first applied to simulate a single pass laboratory rolling experiment by the use of both empirical and physical models. A new approach is used to generate the mean value of the Zener-Hollomon parameter, which is necessary for the prediction of the volume fraction recrystallized (XV) when the empirical SRX model is used. A physical model which considers the density of recrystallization nuclei and the total stored energy is also applied based on the prediction of internal dislocation density, subgrain size and misorientation. The predicted XV and the recrystallized grain size at the centre and subsurface fit well with experimental measurements from the literature. Then, the Taguchi method is applied to design an orthogonal experimental table, L9({34}), which indicates that there are four parameters, each parameter has three levels and a total of nine test runs need to be conducted. These nine virtual experiments are analysed by the use of FEM. The predicted results are then analysed by the use of the Taguchi method from which the influence of each rolling parameter on XV is given. The studies show that rolling temperature has the greatest influence on XV for the centre point whilst friction is the most important parameter on the determination of XV within the surface region. The roll temperature and roll speed have little influence on XV for both the centre and subsurface point.

Surface potential and gravity changes due to internal dislocations in a spherical earth-II. Application to a finite fault

SUMMARY We present a numerical formulation for computing elastic deformations caused by a dislocation on a ¢nite plane in a spherically symmetric earth. It is based on our previous work for a point dislocation (Sun & Okubo 1993). The formulation enables us to compute the displacement, potential and gravity changes due to an earthquake modelled as spatially distributed dislocations. As an application of the ¢nite-fault dislocation theory, we make a case study of the theoretical and observed gravity changes. The computed results are in excellent agreement with the observed gravity changes during the earthquake. The gravity changes in the near ¢eld can reach some 100 ]gal, which can be easily detected by any modern gravimeter. In the far ¢eld they are still signi¢cantly large:jdgj > 10 ]gal within the epicentral distance h 1 ]gal within h 0:1 ]gal within h 0:01 ]gal globally. We also calculate the geoid height changes caused by the 1964 Alaska earthquake and by the same earthquake with revised parameters and an assumed barrier. We ¢nd that the earthquake should have caused geoid height changes as large as 1.5 cm.

Microstructure of precipitated Au Nanoclusters in Single Crystal MgO

ABSTRACTGold nanoclusters dispersed in single crystal MgO were prepared by ion implantation and subsequent annealing at 1000 °C. The morphological feature, size, crystallographic orientation of the Au nanoclusters with respect to the MgO matrix, and the interface structure between the Au nanoclusters and MgO were investigated using transmission electron microscopy. During annealing, the Au clusters nucleate coherently in the MgO lattice, leading to an epitaxial orientation relationship of [010] MgO// [010]Au and (200)MgO//(200)Au that is maintained for all the Au clusters. A critical size for the coherent-semicoherent interface transition is observed to be in the range from ∼5 to 8 nm for Au clusters in MgO. Au clusters larger than this critical size exhibit faceting on the {001} planes. The precipitated Au also exhibits internal dislocations.