摘要:

The possibility for superdislocations in L12-ordered alloys to cross-slip from (111) to (111) planes has been considered. The stress to induce homogeneously this cross-slip is very high. Cross-slip can be locally activated, however, over an energy barrier of modest height, which is independent of the antiphase boundary energy but depends only on the anisotropy factorA.There are similarities to the kink-pair mechanism in the Peierls potential.

ISSN:0950-0839    

DOI:10.1080/09500838908206352

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The existence of icosahedral twins has been established in Al–10at.% Mn alloy. By a stereographic approach a close resemblance to the decagonal phase is pointed out. The simulation of twin diffraction patterns has been done based on the projection formalism. The physical significance of twinning in terms of hyperdimensional projection is discussed.

ISSN:0950-0839    

DOI:10.1080/09500838908206353

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The crystal lattice of the S′ phase in a 2024 aluminium alloy with 4·43 wt% Cu and 2·00 wt% Mg, and the precipitate–matrix orientation relationship are directly illustrated by the [100] lattice image of both the S′ and matrix phases. Two types of the precipitation morphology of the S′ phase, which correspond to homogeneous and preferential nucleation respectively, are also shown.

ISSN:0950-0839    

DOI:10.1080/09500838908206354

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

High-resolution transmission electron microscopy has been used to image the atomic structure of the (2S2)γaustenite-martensite interface. By imaging along [101]γ∥[111]α, the interface was viewed edge-on and seen to consist of facets on the close-packed (111)γ, planes. From the correspondence of atoms in the close-packed planes across the austenite-martensite interface, the magnitude of the shear can be analysed as (a/24)<112> on every close-packed plane in the plane of projection. Comparison with theory indicates that this is an (a/12)<112> Burgers vector out of the plane of projection. Hence, each atomic facet can be viewed as a structural ledge containing an (a/12)<112> transformation dislocation.

ISSN:0950-0839    

DOI:10.1080/09500838908206355

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

A quantitative model is proposed to explain the indentation size effect (ISE) often observed in the hardness response of hard brittle materials, namely that hardness is observed to increase with decreasing indentation size. The model is based on a mixed elastic/plastic materials deformation response whereby plastic deformation occurs in a discrete manner progressively to relieve stresses created by elastic flexure of the surface at the edges of the indentation. During unloading of the indenter, recovery of the elastic increment of deformation, which precedes each new band of plastic deformation, results in the indentation appearing smaller than expected, particularly as the indentation sizes decrease to approach the scale of the plastic deformation band spacing. The model fits observed experimental data well and analysis of hardness/size data in this way is shown to allow both a bulk hardness value and a characteristic deformation band scale to be calculated for a given sample.

ISSN:0950-0839    

DOI:10.1080/09500838908206356

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Samples of rapidly solidified Ti–25 Al–0·4 Er(at.%) have been consolidated by hot isostatic pressing and deformed subsequently in compression at room temperature to ≈3% strain. In addition to dislocations with Burgers vectors, b, given by b=⅓<1120> gliding on {1010}, activity of these dislocations has also been observed on (0001). This result is in contrast to that expected of binary Ti3Al of nominal purity where glide of dislocations with b=⅓<1120> on basal planes does not contribute significantly to the production of strain. This result has been attributed to the internal gettering of interstitial impurities resulting from the addition of Er. It is proposed that interstitial elements such as oxygen, carbon and nitrogen enhance the anisotropy of the charge distribution about Ti atoms, and so the consequence of removing these elements by gettering is to reduce the degree of directionality of bonding between the Ti atoms, and therefore the large Peierls stress associated with glide of both screw and edge segments of <1120>(0001). It is noted that it is not possible to resolve the expected superpartials of dislocations with b=⅓<1120> lying on (0001), which places an upper limit of ≈2nm on their separation, which is to be compared with a separation of ≈10nm for these dislocations when contained in {1010) planes; this difference in separation serves to illustrate the anisotropy of antiphase boundary energy in the compound.

ISSN:0950-0839    

DOI:10.1080/09500838908206357

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Dislocations in samples of a heat-treated and quenched two phase Ti-46at.% Al alloy have been characterized, and those with Burgers vectors, b, given by b=½<1l0] and b=½<112] have been observed. The microstructure of deformed samples is characterized mainly by dislocations with b=½<1l0]. These dislocations have high Peierls stresses in TiAl of nominal purity, because of the directionality of bonds between the Ti atoms. Therefore, the present observations have been interpreted on the basis that the phase Ti3Al getters the interstitial elements from the TiAl, since the solubility of these elements in the former phase is significantly larger than in the latter. It is proposed that the removal of interstitials from TiAl in this way decreases the degree of directionality of bonding between the Ti atoms, and so reduces the anisotropy in Peierls stresses caused by these directional bonds. A relatively large number of twins have also been observed in the lamellae of TiAl, and this may be interpreted on the same basis, namely that the mobility of dislocations with b=⅙<112], the twinning dislocations, is also increased when the concentration of interstitial elements is reduced.

ISSN:0950-0839    

DOI:10.1080/09500838908206358

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The crack tip opening displacement (CTOD), δp, in the plastic zone at a crack tip has been determined according to the dislocation-free zone (DFZ) theory. From this, the total CTOD, δT, can be obtained. It follows that the total CTOD range, ΔδTunder an applied cyclic stress can be achieved through a transposition from monotonic loading to cyclic loading. Under the assumption that when Δδr≥ Δδcrita fatigue crack can propagate, where Δδcritis the critical CTOD range which corresponds to the fatigue crack propagation threshold ΔKth, then a very simple relationship between ΔKthand the yield strength, σ, is obtained, namely

ISSN:0950-0839    

DOI:10.1080/09500838908206359

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Using small-angle X-ray diffraction, we demonstrate in this Letter that alternate-layer Langmuir-Blodgett films of tricosanoic acid/1-docosylamine and docosanoic acid/1-docosylamine retain their ‘as-deposited’ structure. Both of these non-centrosymmetrical bilayer structures exhibit a smaller tilting of their hydrocarbon tails than that observed for the corresponding centrosymmetric monocomponent systems. The electron-density profiles obtained for the alternate-layer systems reveal that the points of contact for the ends of the hydrocarbon chains are shifted from the centre of the unit cell and that the density maxima, corresponding to the positions of the polar head groups, possess an asymmetrical shape.

ISSN:0950-0839    

DOI:10.1080/09500838908206360

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

By means of Monte Carlo simulation techniques the relaxation of a packet of charge carriers migrating in a Gaussian density of states of width σ has been studied. The relaxation time determines the transition from dispersive to non-dispersive transport seen in a time-of-flight signal. For a 10 μm thick sample under a bias field of 105V cm−1the critical disorder parameter σ/kT turns out to be 4·4. Consistency with experimental results from polymers is found.

ISSN:0950-0839    

DOI:10.1080/09500838908206361

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor