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The correlations between structural features, critical current density and pinning landscape of YBa2Cu3O7-δ and MgB2-based materials produced in the forms of bulk melt-textured (MT-YBCO) and polycrystalline (MgB2) blocks and wires with excellent superconducting characteristics were revealed. The impurity oxygen distribution, higher borides inclusions and carbon doping are most important for the critical currents and pinning forces in MgB2, while the twin density and the density of Y211 inclusions are relevant in MT-YBCO.

Comprehensive experimental results of fully oxidized (up to YBa2Cu3O6,9-7) melt-textured YBaCuO materials with different microstructures are presented. These microstructures are built respectively: (1) with a high dislocations density but almost without twins (after high temperature treatment at 2 GPa) and (2) with a high twin density, but practically free from dislocations and stacking faults (after high temperature oxygenation at 10-16 MPa). It is shown that for attaining high critical current densities and fields of irreversibility (jc(H‖c, 0 T)=9·104 A/cm2, H irr=9.7 T at 77 K), a high twin density in YBa2Cu3O6.9-7 matrix of MT-YBCO is required. The density of twins in fully oxidized materials depends on the distances between Y2BaCuO5 inclusions, larger twin densities are related to shorter distances between inclusions. The influence of phase composition of the initial powder mixtures on the distances between Y2BaCuO5 inclusions have been characterized and discussed.

Different core structures have been evidenced in silicon which control, respectively, low and high temperature deformation. Indeed perfect shuffle dislocations have been found to build deformation microstructures in the temperature range and stress where silicon is usually brittle. In contrast, the plastic regime is controlled by dissociated glide set dislocations. This paper aims at relating the plastic properties of silicon to the core structure of dislocations and in particular to the existence and the properties of these two core structures.

Silicon micropillars obtained by FIB from Floating Zone silicon single crystal have been deformed in situ in a SEM between room temperature and 400 °C. Pillars with axis were studied in order to promote single slip deformation. Mechanical data as a function of pillar diameter and surface deformation microstructures observed by SEM are reported. Yield stresses as a function of temperature are compared with those resulting from high-pressure tests (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Type Ib synthetic diamonds from De Beers with a low dislocation density have been deformed under confining pressure under pressure of 5 GPa in a multianvil apparatus between 900 and 1200 °C and in a D-DIA experimental set up at 1100 °C and strain rate of 7 × 10−5 s−1. TEM observations of diamond deformed at 1100 °C, the lowest deformation temperature where plasticity was unambiguously evidenced are reported in this work. Two types of dislocations have been found to coexist: dissociated and perfect. No large stacking faults resulting from the dissociation splitting blow up under large stress have been observed. Numerous dipoles are found in the microstructure which can be built with perfect or dissociated dislocations. These results are discussed in relation with dislocation core configuration calculations.

4H-SiC intrinsic homoepitaxied single crystals have been nano indented at room temperature using a spherical indentor and the related deformation microstructures have been analyzed by Transmission Electron Microscopy. Dislocations are lying in the basal plane but have been found to be perfect, in contrast with observations made at higher temperature. Although such a change in deformation mechanism has been observed in other semiconductors such as Silicon and Indium Antimonide, it was unexpected in a very low stacking fault material such as SiC.

Compression tests under confining pressure are performed with a Paterson press in order to deform 4H-SiC plastically below the brittle-to-ductile transition temperature. Deformation microstructure in this regime is analysed using conventional and high resolution transmission electron microscopy and is compared with literature. Leading partial dislocations with carbon core and silicon core are observed on basal plane as well as perfect dislocations out of the basal plane. Double stacking faults are evidenced in the deformed microstructure. These original observations indicate that the deformation microstructures of 4H-SiC in the brittle domain are more complex than previously reported. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The deformation microstructure of single crystals of 4H-SiC resulting from microindentations on a prismatic surface was investigated by TEM. Indentations were performed at 400 and 675°C, i.e. below the brittle to ductile transition temperature of 4H-SiC (temperature close to 1100°C). TEM analysis reveals dissociated dislocations as well as extended stacking faults in the basal plane. In addition, perfect edge dislocations are observed on prismatic planes. From the observations, it is assumed that perfect dislocations are nucleated in the prismatic plane and cross-slip on the basal one where they dissociate.

Melt-textured YBa2Cu3O7 samples were deformed under high pressure–high temperature (HP–HT) conditions (P = 2 GPa, T = 900 and 800°C, t = 15 min). During the HP–HT treatment, samples were in contact with annealed zirconia, preventing oxygen loss, in such a way that the orthorhombic superconductor phase was preserved at the end of the experiment. The deformed microstructure, investigated by transmitted electron microscopy, exhibited original features characterized by a very low twin density, a memory effect on the dislocations shape and a non-classical faulted loops distribution in the matrix. A mechanism, based on the interaction between twin and gliding dislocations, is proposed to explain the nucleation of these aligned loops.

Icosahedral Al–Pd–Mn quasicrystalline alloys are known to be extremely brittle at low and intermediate temperatures when deformed with conventional tests. Using specific deformation conditions, such as high confining pressure techniques, allowed plastic deformation of this non-periodic structure at temperatures in the brittle domain. Surprisingly, low temperature deformation microstructures revealed the presence of dislocations, whose motion essentially implicates climb mechanisms. The aim of this study is to use a shear experiment technique, specially designed to favour glide processes, in order to check if glide may occur solely, without any climb component. The results confirm the difficulty to promote pure glide processes and support the idea that both glide and climb are activated.

Microstructures of Al–Pd–Mn quasicrystalline specimens deformed at low temperatures were studied by transmission electron microscopy. Analysis of the corresponding microstructures indicates that plastic deformation takes place by dislocation glide and climb. Faulted dislocation loops were observed. Tentative explanations are given to explain nucleation and growth of these dislocation loops.

AlPdMn quasi-crystalline specimens were deformed at low temperatures using a high-confining-pressure device. The microstructures, observed in each sample by transmission electron microscopy, depend on the deformation temperature and show that plastic deformation is accompanied by dislocation movements. A detailed analysis of the dislocation configurations shed light on the low temperature deformation mechanisms. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Plastic deformation of Al-Pd-Mn icosahedral quasicrystals has been achieved at room temperature using a high-confining-pressure medium. The deformation microstructure, investigated by transmission electron microscopy, is characterized by long straight bands of dislocations. A detailed analysis of the dislocation configurations indicates that the plastic deformation is controlled by dislocation glide.

The microstructure of 4H–SiC single crystals deformed under very high stresses and at low temperature (T ≤ 350°C) have been analysed by transmission electron microscopy. Depending on deformation conditions, large stacking faults, weakly dissociated perfect dislocations and possibly undissociated perfect dislocations have been observed. From these results, it appears that the nucleation of partial dislocations, which is a limiting process in deformation of 4H–SiC below ~1000°C, can be helped by very high stress. Consequently, it is not excluded that a deformation mechanism by undissociated perfect dislocations can take place under very high stress, as observed in Si and GaAs.

The microstructure of subboundaries (SBs) occurring in single domain YBa2Cu3O7 (Y123) melt textured composites is studied by transmission electron microscopy. The form of SB dislocation networks is controlled by the properties of constituting dislocations. For arbitrary SB configurations, it can be parametrised using the generalised Frank's formula, allowing the calculation of dislocation densities, and an estimation of the SB critical current. Besides the underlying dislocation networks, SBs may develop mesostructures like faceting and stepped interfaces accommodating the deviation from low index planes. Implications of the observed microstructures on the bulk critical currents are addressed.

The evolution of the microstructure of melt-textured YBa2Cu3O7−Y2BaCuO5 (123–211) composites in O2 at 100 bar and 600°C has been characterized by transmission electron microscopy (TEM). Under these conditions the 123 phase is brought out of its stability field, within a region of the phase diagram where the weakly superconducting YBa2Cu4O8 (124) polytype is stable against a CuO + 123 mixture. The nucleation of 1/6 (301) stacking faults is not restricted to the 211–123 interfaces, as found in samples processed at 350 and 450°C but is strongly enhanced in the bulk matrix. Simultaneously, matrix decomposition into a Ba-Cu oxide, 211 and presumably CuO occurs at 211 interfaces, which is accompanied by an exaggerated growth of the stacking faults. After 12 h at PO2 = 100 bar and 600°C, the whole volume imaged by TEM appears to be covered by large stacking faults overlapping along the c axis. It is found that, when the 211 interfaces become saturated by stacking faults, new stacking faults have to nucl...

Several thermomechanical treatments have been used in order to create specific microstructures of dislocations in melt textured grown (MTG) YBa 2 Cu 3 O 7 m i /Y 2 BaCuO 5 ceramic superconductors. To by-pass the extreme brittleness of those materials, suitable techniques have to be used in order to increase substantially the dislocation density, in the orthorhombic phase, without promoting crack nucleation. The main features of microstructures obtained using e cold isostatic pressing f , high oxygen pressure anneals and plastic deformation under confining pressure are presented. The effects of applied stresses and phase instability on dislocation configurations are discussed together with the dislocation interactions with mirror twin boundaries. Consequences of those microstructures on the physical properties are also addressed with reference to the signature of each type of the defects.

This paper concentrates on dislocation plasticity and microstructural features in ceramic materials. The consequences of some features of the dislocations are examplified and compared with those of metallic and semi conducting materials.

Dislocation microstructures induced by room-temperature microhardness tests have been investigated in silicon nitride. Surface analysis of the residual indent by atomic force microscopy reveals intragranular slip bands and demonstrates that room-temperature plastic deformation involves dislocation motion as well as cross-slip events. Cross-slip events have been found to occur between {1010} prismatic planes. Transmission electron microscopy shows that dislocations have a Burgers vector b = [0001] and are located along the screw direction. Based on these observations, specific dislocation core configurations are discussed.

Macroscopic plastic deformation of silicon single crystals has been achieved at RT and 150°C under a confining pressure of 5 GPa in a multi-anvil apparatus. TEM observations of a sample deformed at 150°C show that the deformation microstructure is built with dislocations with 1 2 a 1 1 0 Burgers vector in (1 1 1) planes which are undissociated. They appear as mainly aligned along the screw orientation and 〈1 1 2〉 orientations at 30° from the Burgers vector. Those features are analysed in the light of perfect dislocation generation in the shuffle set.

Microstructures associated with room temperature indentation subsequently annealed at 450°C have been analyzed by TEM in Y and Nd based melt textured ceramic superconductors. It is shown that stress relaxation around indentations occurs by the nucleation of stacking faults with R = 1/6〈031〉 displacement vector in Y based material. Differently relaxation occurs in NdBaCuO by the nucleation of perfect dislocations with 〈100〉 Burgers vectors which can be locally dissociated. A model is proposed to explain these microstructures and is related to the difference in energies of stacking faults with R = 1/6〈031〉 displacement vector in these compounds.

Dislocation microstructures induced by plastic deformation at room temperature in Si have been investigated by TEM. Plastic deformation has been obtained by using two types of technique: deformation under a confining pressure of 5 GPa in an anisotropic multi-anvil apparatus and by surface scratching. The TEM observations show common features in the two deformation substructures which are characteristic of high stress-low temperature deformation. The deformation microstructures are built with dislocations with a/2110 Burgers vector in (111) planes which are undissociated. Such dislocations are mainly aligned along the screw orientation and 112 orientations at 30° from the Burgers vectors as well as along 132 orientations at 41° from the Burgers vector. The occurrence of those Peierls valleys confirms that different dislocation core configurations from those usually dealt with at higher temperatures have to be taken into account when dislocations are nucleated at very high stresses.

Intragranular plasticity induced by indentation is studied in β Silicon Nitride between room temperature and 700°C by Transmission Electron Microscopy (TEM). It is shown that all the dislocations resulting unambiguously from plastic deformation have a Burgers vector b = [0001] and a dominant screw character. This can account for a large Peierls stress along the [0001] direction and a larger mobility of edge dislocation segments as compared to screw ones. Dislocations bowing out of screw direction are found to glide in [1010] prismatic planes. Cross-slip is evidenced from the dislocation microstructure observations. It is found to occur between {1010} prismatic planes at 20°C whereas {1120} is found to be a new deviation plane at 700°C. Those observations are discussed in relation with the possible core structure of [0001] screw dislocations.

The deformation microstructures of Si single crystals have been studied by transmission electron microscopy (TEM) in the temperature range of deformation where “dislocation trails” are evidenced. The main features of the deformation microstructures are characterized by dislocations dipoles and their debris. These microscopic observations are discussed in relation with the mesoscopic properties of dislocation trails as well as the possible transformation between two different core structures of dislocations in silicon.

Single domain YBa/sub 2/Cu/sub 3/O/sub 7/-Y/sub 2/BaCuO/sub 5/ melt textured ceramic composites have revealed a very rich microstructure, which has usually impeded, by using standard measurements, to evaluate the contribution of each defect to the enhancement of the critical current. We have measured the in-plane magnetoresistance anisotropy and the anisotropic in-plane inductive critical currents and we show that together with the microstructural TEM analysis, the contribution of the different extended pinning centers can be separated. These results have allowed us to infer the kind of microstructure modifications required to improve the critical current. In particular, we present an isostatic pressing deformation technique as a very promising post-processing treatment to strongly increase the critical currents of these composites

The influence of dislocations on pinning of vortices has been investigated in melt-textured YBa 2 Cu 3 O 7− x and YBa 2 Cu 3 O 7− x ceramics. The dislocations, introduced by plastic deformation at room temperature in the orthorhombic phase, are characterized using transmission electron microscopy. dc and ac magnetic measurements revealed that the plastic deformation of melt-textured samples induced a decrease of the activation energy for flux motion. This effect has been related in a preceding paper to the microcracks created in important density along the (001) planes during the deformation process. The comparison of these results with those obtained on a ceramic material confirms this interpretation.

This paper considers the plasticity of InSn under high stress, at low temperatures. The studies cover the plasticity of InSh in the temperature range between 20 and 200°C and under stress which is applied by superimposing uniaxial pressure on a confining hydrostatic pressure. The analysis of the deformed substructures by transmission electron microscopy revealed the existence of different deformation mechanisms at different temperatures, and consequently. stress regimes At temperatures T ≥ 50°C the dominant deformation mechanism is the glide of perfect dislocations, while at room temperature the deformation can be attributed to partial dislocations. At lower temperatures. screw dislocalions become dominant, a fact that demonstrates the small mobility of these dislocations. Extended stacking faults introduced by the dissociation of β-type segments demonstrate the higher mobility of the 90 β partial dislocations conspired to the mobility of the 30 β ones. In contrast, the absence or extended dissociation for the 60 α-type dislocations suggests that the 90 α and the 30 α partial dislocations have the sortie mobility An additional interesting issue arises from the observations showing a small difference in mobility between the 30 α and the 30 β partial dislocations The significance of these observations is discussed and the results are correlated to similar observations in other III-V compounds.

The microstructure of YBa2Cu3O7−x −Y2BaCuO5 melt-textured composities deformed in the secondary and tertiary creep regimes has been investigated by transmission electron microscopy. The high density of Y2BaCuO5 precipitates plays an important role in the microstructural development as pinning sites for gliding dislocations. In the secondary regime, trapped dislocations are dissociated, leaving a stacking fault with displacement vector [½—δ 0 ⅓]. A second stacking fault, ⅙〈301〉, is typically associated with the former stacking fault. At this stage, the deformation microstructure is dominated by diffusive processes between precipitates interconnected by the trapped dislocations. In the tertiary stage, dislocation multiplication is the main factor controlling the microstructure, which is characterized by a dramatic increase in the density of perfect dislocations with Burgers vectors 〈100〉 and 〈110〉. Since deformation is performed above the orthohombic-to-tetragonal transition temperature, the sample...

The microstructure and superconducting properties of directionally solidified YBa2Cu3O7/Y2BaCuO5 composites made to creep at 930°C, have been investigated using transmission electron microscopy, and dc and ac magnetic measurements. It is found that the plastic deformation of high critical current samples induces a shift of the irreversibility line toward lower fields and temperatures as a result of a decrease of the activation energy for flux motion. Such a superconducting behaviour is directly correlated with the observed microstructure, which is dominated by microcracks and large stacking faults that decrease the vortex correlation along the c axis.

International audience; Plastic deformation of a 4H-SiC wafer has been produced by nanoindentation at room temperature. The superficial layer of the specimen in the indented area has been lifted out thanks to a specific focussed ion beam micromachining method and the deformation microstructure close to the imprints has been investigated by means of both conventional and high-resolution transmission electron microscopy (TEM). The analysis of the images revealed the presence of various types of extended defects. Perfect dislocations and single stacking faults bounded by isolated Shockley partial dislocations have been observed on the basal plane. Perfect dislocations have also been evidenced out of the basal plane. These results highlight the competition between various activated systems involved during the plastic deformation of 4H-SiC in the brittle regime.

This chapter is devoted to the structure and properties of dislocations obtained at high stress in silicon, which have been shown to have different core structures from the ones observed at low stress and high temperature. TEM observations of high-stress deformation microstructures revealing the existence of a specific type of dislocations, which exhibit a non-dissociated core and are expected to lie in the shuffle set, are reported together with relevant atomistic computations. The results of experiments and computations are confronted in order to obtain a comprehensive view of the high-stress deformation regime. The transition between the two deformation regimes is also discussed.

The effect of high‐temperature creep under different conditions in melt‐textured YBa2Cu3O7–Y2BaCuO5 composites, with high critical currents before deformation, is investigated. Transmission electron microscopy studies show that the low‐temperature oxygenation process causes a strong modification of the as‐deformed microstructure and leads to a clear degradation of the superconducting performances. This behavior of the superconducting properties is well correlated with the formation of extended planar defects such as large stacking faults and microcracks that severely diminish the vortex cutting length, thus enhancing the thermal activation effects on the critical currents and the irreversibility line.

Conference: International Cryogenic Materials Conference Location: Univ Wollongong, Wollongong, AUSTRALIA Date: FEB 10-13, 2004; International audience; MT-YBCO samples oxygenated under controlled oxygen pressure exhibited at 77 K a critical current density j(c) = 85 kA cm(-2) in zero field and more than 10 kA cm(-2) up to 5 T field when the external magnetic field was perpendicular to the ab-plane of Y123, and a j(c) = 23 kA cm-2 in zero field and jc close to I kA cm-2 in 10 T field when the magnetic field was perpendicular to the c-axis of Y123. The microstructure of these samples contained an unusually high density of twins (about 30 twins mum(-1)) as well as a lot of stacking faults around Y211 inclusions. Using quasi-hydrostatic high pressure-high temperature (HP-HT) treatment we may vary the twin and dislocation densities in the material by changing the sample orientation in high pressure apparatus, while the oxygen content of Y123 phase as well as the lattice parameters remain unchanged. The microstructure of the material in the case where the highest pressure has been applied in the direction perpendicular to the ab-plane of Y123 is characterized by a very low twin density, perfect dislocations stepped along (110) directions and small faulted loops corresponding to CuO intercalating in the matrix. For this material j(c) = 10 and 8 kA cm(-2) in zero field were observed (when the external magnetic field was perpendicular to the ab-plane and perpendicular to the c-axis of Y123, respectively). High pressure-high temperature treatment causes an increase in the material density (up to near the theoretical one), microhardness and fracture toughness.

We present a study of flux pinning by in-plane partial dislocations in melt-textured grown ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}/{\mathrm{Y}}_{2}{\mathrm{BaCuO}}_{5}$ (123/211). The in-plane dislocations are generated by high oxygen pressure (HOP) postprocessing treatment as confirmed by transmission electron microscopy (TEM). In order to characterize the dislocation density and the perimeter to surface ratio (PSR) of the associated stacking faults a large number of TEM micrographs have been analyzed. We demonstrate that the evolution of the microstructure correlates with the oxygen absorbed by the samples during different HOP treatments. Two regimes of oxygenation activity are identified: first, the low oxygenation regime where the PSR of the stacking faults stays almost constant but the in-plane dislocation density is increased near the 123/211 interfaces and second, the high oxygenation regime where the stacking faults extend over the entire 123 matrix inducing a drastic enhancement of the dislocation density and a strong decrease in the PSR. Two contributions to the critical current density have been identified and quantified: the weak pinning and the correlated disorder pinning. We demonstrate that the correlated disorder comes from 123/211 interfaces and that this remains invariant with the HOP process. On the other hand, the weak pinning contribution is associated to the in-plane dislocations and this contribution strongly correlates with the HOP treatments. The temperature and field dependencies of the in-plane dislocations' contribution to the critical currents are in agreement with a pointlike single vortex pinning mechanism. Finally, we demonstrate that the dislocation pinning can be counterbalanced by wide stacking faults inducing a downward shift of the irreversibility line.

Examples of plastic deformation related to dislocation movements in ceramic materials are reported. Specific properties of dislocations and associated microscopic mechanisms in some ceramic materials (Gd3Ga5O12, s Si3N4, ZrO2, A1N, SiC) that have been investigated at LMP are reviewed in relation with lattice structure and bonding. Mechanical behaviour related to the observed microstructures is discussed and compared to that of metallic and semiconductor materials.

Melt textured single domain ReBa2Cu3O7 (Re = Y,Nd) samples were post-processed, either under high oxygen pressure (PO2 ≤180 bar) or under cold isostatic pressing (CIP) conditions. We show that CIP processing can lead to critical current enhancements up to 100% at 77K. The analysis of the evolution of the microstructure by TEM allows us to attribute the enhanced pinning to 1/6[031] partial dislocations which are distributed anisotropically within the (001) plane. In Nd123/422 composites we demonstrate that high oxygen pressure annealing enhances interface pinning in parallel with a reduction of the fishtail anomaly due to the presence of Nd-Ba antisite defects.

[1] The curvature of dislocation lines can serve as a probe for the resolved shear stress in multianvil deformation experiments. This method is very well adapted to deformation experiments in the multianvil apparatus where the microstructure (and in particular the dislocation density) can be largely determined by early deformation stages. On the contrary, individual dislocation movements can adjust more rapidly to stress level changes and can thus record the last stress level borne by the specimen before it is quenched. The method has been applied to dense specimens of wadsleyite and ringwoodite deformed at 15 to 22 GPa and 1000 to 1500–1600°C. The resolved shear stress is very similar in wadsleyite (400 MPa) and ringwoodite (350 MPa) deformed at 1000°C showing that these minerals exhibit very similar rheological properties under these conditions.

An enhancement of the critical currents in the H‖c configuration, reaching 100% at 77 K, has been obtained in melt textured YBa2Cu3O7/Y2BaCuO5 composites after a cold isostatic pressing process (300 °C, PAr=200 MPa). A transmission electron microscopy analysis of the microstructure demonstrates that a high density of nanometric stacking fault loops, with anisotropic growth within the (001) plane, is generated. The improvement of critical currents is attributed to the flux pinning enhancement due to the increased length of the 1/6〈031〉 partial dislocations surrounding nanometric stacking faults. We stress that this technique has high potential for the large scale processing of high critical currents superconducting ceramic composites.