摘要:

AbstractTechnological advances are extending the applications for bonded ceramic-metal components and demanding more rigorous performance characteristics. The techniques available for fabricating high–integrity joints for structural applications have been reviewed and attention is drawn to the factors that effect both direct and indirect bonding using liquid–and solid–phase materials. Two processes still largely in the developmental stage–fusion welding and diffusion bonding–have been considered, as well as the more established processes such as brazing. It is suggested that active metal brazing and indirect diffusion bonding will be among the techniques to be further developed and used more extensively in future.MST/205

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.657

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractGlide in elemental semiconductors or III–V compounds takes place by dissociated 1/2〈110〉dislocations on {111} glide planes. Structural models and valence force calculations suggest that the cores of the basic partial dislocation may be reconstructed. Kinks may be reconstructed or have dangling bonds. The former will be associated with fairly shallow levels, the latter with deep acceptor and donor levels in the bandgap. Deep–level transient spectroscopy, electrical, optical, and electron paramagnetic resonance data give information on energy levels and concentrations of dangling bonds and deep levels, but the identification with particular sites on the dislocations is very difficult. The electronic states also lead to the dislocations acting as recombination centres, and give rise to photoplastic effects. There are strong interactions with impurities, which have a profound effect on the electronic properties. Dislocation velocities at relatively low temperatures are controlled by the Peierls force, and motion occurs by the generation and motion of double kinks. There is a pronounced dependence of dislocation velocity on doping, for both elemental semiconductors and III–V compounds. A recent theory attributes the doping effect to a dependence of the concentration of charged dangling bond kinks on the Fermi level, and of the kink velocity on the charge state. The doping effect is also reflected in mechanical properties, for example the variation of yield stress with temperature, rosette diameters, and radial cracking behaviour around hardness indentations, and values of hardness. In III–V compounds different mobilities ofα–andβ–dislocations give rise to anisotropies in hardness, rosette configurations, and associated cracking, and for {111} indentations different hardness values, slip patterns, and cracking behaviour result on the group III and V crystal faces.MST/269

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.666

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe coefficients of equilibrium partition of solute elements between austenite and liquid iron were evaluated thermodynamically for Fe–C–base quaternary systems. The validity of the calculation was examined in comparison with the measured coefficients of solute elements in some quaternary systems. The equilibrium partition coefficient of the third element i in an Fe–C–base quaternary system is affected by two factors: the effects of the fourth element j on the interaction between carbon and the third element and the interaction between the third and fourth elements. Most of the combinations of the third and fourth elements in Fe–C–i–j quaternary systems showed a relatively small interaction between the elements i and j except the combination of chromium and titanium, which exhibited an explicit influence of titanium on the partition coefficient of chromium. It was concluded that the partition coefficients in Fe–C–base multicomponent systems differed little from those in Fe–C–base ternary systems, with a few exceptions.MST/220

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.678

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractPlastic deformation in ductile metals is limited by a mechanism in which voids, nucleated at second–phase particles, grow and coalesce to form a crack. The results of a finite–element solution for a spherical elastic inclusion in a plastically deforming matrix are discussed. These results have been used in conjunction with experimental work using notched tensile specimens to generate multiaxial states of stress from which the local conditions leading to decohesion of the inclusion/matrix interface were determined. An important feature of these results is the statistical distribution of the interfacial strength. This distribution is bimodal, showing the presence of both weakly and strongly bonded particles. The latter have a modal strength of about seven times the initial yield stress and the weakly bonded particles behave as pre–existing voids. Experiments in plane and axisymmetric states of strain indicate that while the stress state is of relevance to ductile failure, the remote strain state is not. The absence of a macroscopic strain state effect is explained in terms of the statistical distribution of the voids nucleated from the population of randomly distributed inclusions. The stress and strain concentrations possible in local patches of high porosity have been investigated by a finite-element approach based on the mechanics of a dilating continuum to determine void growth in the porous aggregate and the local conditions at failure.MST/69

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.684

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractIn this paper are reported two recent investigations into crack initiation at notch roots using techniques developed for remote monitoring of crack growth in high–temperature water environments. In the first, a notched compact–type specimen of a carbon steel, SA 333 Gr. 6, was monitored for crack initiation (defined as a 0·076 mm crack at the notch root) in a variety of water chemistries and testing conditions. The mechanical conditions at the notch root have been analysed using the Neuber–notch method, enabling a direct comparison to be made with the strain–controlled fatigue data curves used for a smooth specimen in the ASME Boiler and Pressure Vessel Code, Section III. In the second investigation a different method was considered for monitoring crack initiation. A modification of the electrical–potential technique, called the reversing dc electrical potential method, was used to obtain quantitative information on the initiation and early growth of small surface cracks in notched bars of a high–strength alloy at elevated temperature. Results obtained by the method are presented and discussed.MST/72

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.691

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractTheoretical models are developed to describe the concentration of dissolved species in a corrosion–fatigue crack. The models are based on two separate reaction processes: the cathodic reduction of oxygen on the crack walls, and the anodic dissolution of metal at the crack tip. The concentrations of species in a trapezoidal crack are calculated for a range of crack depths from 0·25 to 20 mm, and for different minimum/maximum load ratios, stress–intensity factors, and frequencies, assuming single–edge–notch geometry. The relative importance of diffusion and fluid flow as functions of these parameters is analysed. The influence the geometry of the specimen has on the composition of the solution is determined, and the use of parallel–sided cracks is discussed. The implications of the conclusions for corrosion–fatigue crack growth are assessed.MST/73

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.700

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractAlthough notched specimens have been much used in studies of hydrogen embrittlement, precracked–specimen testing has become more widespread in the last 15 years or so. However, recent work has revived interest in the use of notched specimens for fundamental studies. The background for, and recent results from, the use of notched specimens is described, and current problems in the interpretation of fracture behaviour are discussed. Of particular interest are the questions relating to the appropriateness of assuming a critical, local hydrogen concentration for fracture and to the degree of fracture control by stress, strain, or some combination. An encouraging development is the recent convergence in interests between micromechanism studies by materials scientists and work on the parameters of microfracture by the mechanics community.MST/74

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.711

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractAir–cooled turbine vanes made from nickel–and cobalt–base superalloys undergo distortion, cracking, burning, and material degradation in operation. The complex geometry of the parts and the compositional and microstructural heterogeneity of cast alloys impose limitations on the selection of repair methods. Selective chemical stripping of the diffusion coatings; elimination of cracks and restoration of dimensions by joining processes involving both welding and brazing techniques; and formation of coatings by pack cementation are the major processes employed. These processes may give rise to defects that significantly diminish the integrity of the parts.MST/104

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.719

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractUnusually large anisotropy of strength has been observed in commercially produced sheets of a duplex stainless steel, Fe–1·5Mn–22Cr–5·5Ni–3Mo–0·15N. The austenite and ferrite phases in this material are highly elongated and possess unusually sharp preferred crystallographic orientations. Both phases are in a recrystallized condition and have almost equal hardnesses. On the basis of the present measurements and a crystallographic analysis of plastic flow it is concluded that the observed anisotropy cannot be due to composite reinforcement in the duplex structure but is rather a result of texture hardening.MST/214

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.728

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe characterization of rapidly solidified powders, their extrusion, and the evaluation of mechanical properties of the extrudates have been carried out for an Al–5Fe–7 Mn alloy. In the atomized powder, a metastable non–stoichiometric phase has been identified which transforms to the equilibrium (FeMn)Al6phase on heating at elevated temperatures (<400°C). It has been found that inhomogeneous dispersoid distribution and coarsening of phases during preheat causes deterioration of mechanical properties, both at room temperature and at elevated temperatures. It is suggested that control of atomization parameters and use of rapid heating of compacts may be essential to improve the mechanical properties.MST/174

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.9.737

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor