摘要:

To study the grain boundaries of the technologically important RE13.75Fe80.25B6(RE = Pr or Nd) permanent magnets, we developed a new method using small-angle dark-field imaging based on inner-potential differences at the grain boundaries, combined with high-resolution electron microscopy and nanoscale chemical analysis. In both the Pr- and Nd-based melt-quenched die-upset magnets we consistently observed a very thin Fe-rich amorphized intergranular phase at the boundaries, and this finding may have an impact on our understanding of the coercivity mechanism of these materials. Although the value of the method was demonstrated for transition metal-rare earth intermetallics, it may also be applied to grain-boundary studies in other materials.

ISSN:0950-0839    

DOI:10.1080/09500839508241011

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Using high-resolution electron microscopy (HREM) the core structure of a1/2(112) superdislocation in TiAl is examined and observed to have dissociated into an extrinsic stacking fault and an intrinsic stacking fault, bounded by three1/6(112) partial dislocations. The core structure is non-coplanar, with the extrinsic stacking fault lying on two adjacent [111] planes bounded by a dissociated b =1/6(112) partial dislocation whose deiocalization can pin the dislocation core and may lead to the formation of ±1/6(112)dipoles.

ISSN:0950-0839    

DOI:10.1080/09500839508241012

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The effect of grain size and strain on the flow stress of polycrystals has long been studied. However, the effect especially for polycrystalline iron, still cannot be fully explained by most of the models. In this paper, a unified equation based on a one-parameter approach has been proposed to predict the relation between flow stress and grain size at various strains. The predictions are found to be consistent with the published experimental data.

ISSN:0950-0839    

DOI:10.1080/09500839508241013

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Electron microdiffraction experiments have been used to study a fine metastable precipitate (designated X) that forms as platelets on the [111]αplanes of the alloy Al-2.5 wt% Cu-1.5 wt% Mg-0.5 wt%Ag, aged at 513 K. Microdiffraction patterns observed from individual platelets indicated that they have a hexagonal structure with the unit-cell parametersa= 0.496-nm andc= 1.375 nm, and an orientation relationship of (0001)× ||(111)αand [1010]× ||[110]α. Although earlier work by Bergman, Waugh and Pauling on similar alloys suggested that the platelets had a b.c.c. T-phase structure, the present results indicate that X phase is not in fact an isomorph of the T-phase that has been observed in the ternary Al-Zn-Mg and Al-Cu-Mg alloy systems.

ISSN:0950-0839    

DOI:10.1080/09500839508241014

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

High-resolution electron microscopy has been used to study the structure of copper-rich precipitates in an Fe-1.5wt% Cu alloy irradiated at 295°C with 2.5 MeV electrons to a dose of 3.1 × 1023m−2(1.4 × 10−2(displacements per atom (dpa)) at a dose rate of 4.1 × 1017m−2s−1(2 × 10−9dpa s−1). Most of if not all the precipitates smaller than about 8 nm in diameter were found to have a twinned 9R structure similar to that seen in thermally aged model alloys. Some precipitates larger than about 8 nm in diameter appeared to have transformed wholly or partly to 3R or f.c.c. The results confirm that the usual assumption that the main effect of irradiation is to enhance the diffusion of copper, and hence to accelerate the diffusion kinetics, is substantially correct. The smaller size at which the 9R-3R-f.c.c. transformation seems to occur under irradiation may be due to incorporation of vacancies in the precipitates or to nucleation of small dislocation loops at the precipitate-matrix interface, either of which may modify the critical size at which the 9R-3R transformation is triggered.

ISSN:0950-0839    

DOI:10.1080/09500839508241015

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new shape of copper precipitates in silicon has been observed to form after a heat treatment with rapid cooling. Its structure and epitaxial relationship have been determined by high-resolution electron microscopy. The influence of the experimental conditions (cooling rate and dislocation density) on the appearance of this kind of precipitate is discussed.

ISSN:0950-0839    

DOI:10.1080/09500839508241016

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

X-ray diffraction provides an optimum resolution of about 10−4Å−1in reciprocal space. Using synchrotron radiation, very weak diffraction phenomena can be studied with this resolution. Careful examination of the prominent Bragg reflections of a decaprismatic Al70Co15Ni15needle reveals contributions both from a perfect decagonal single crystal and from the orthorhombic (5, 7) approximant of the decagonal phase. The decagonal phase is established by strong quasicrystal reflections perfectly obeying the decagonal metrics. The coexisting approximant state gives rise to a characteristically structured background close to the quasicrystal reflections and to additional weak reflections. Regarding exact peak positions, the weak reflections cannot be described as satellite reflections originating from a long-range superstructure to the decagonal phase.

ISSN:0950-0839    

DOI:10.1080/09500839508241017

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Percolation theory has been applied to variable-range hopping in insulating cuprates, where an energy-dependent density of states is taken into consideration. Agreement between experiment and theoretical calculation suggests that the shape of the density of states in the charge transfer gap in superconducting cuprates can be deduced.

ISSN:0950-0839    

DOI:10.1080/09500839508241018

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The dynamic behaviour of small atomic clusters consisting of eight atoms and ten atoms on an Ir(001) surface has been studied using a field ion microscope. It has been found that eight-atom and ten-atom clusters are energetically favoured to form ring configurations with central lattice vacancies in the clusters. The fact that the atomic-ring configurations with fewer nearest-neighbour bonds are more thermally stable than those dense cluster structures with more nearest-neighbour bonds demonstrates convincingly the general believed non-additivity of bond energies at the metal surface.

ISSN:0950-0839    

DOI:10.1080/09500839508241019

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new theoretical framework and its experimental verification are presented in order to understand and generalize the dielectric response for relaxor ferroelectrics. By considering the fluctuating nanodomains of relaxors as fluctuating ‘giant dipoles’ we argue that the relaxor effect is indeed a low-frequency dispersion (LFD) phenomenon extended to the r.f. range. Because of the presence of disorder in relaxor ferroelectrics, configurational tunnelling plays an important role; we show that all the conditions necessary for LFD exist in the relaxors and at the same time we provide sufficient reasons to establish why LFD in relaxors is extended as well as prominent in the r.f. range. Our model explains the importance of random electric fields for exhibiting relaxor behaviour. Finally we substantiate our model by showing that dielectric constant and dissipation values as well as dispersion in lead magnesium niobate relaxor ferroelectrics are enhanced in a humid atmosphere in agreement with many well known adsorbate systems showing LFD.

ISSN:0950-0839    

DOI:10.1080/09500839508241020

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor