摘要:

AbstractThe ductility of aluminium is attributed to the low elastic shear modulus of this fcc metal. Its trivalency endows a strong electrostatic resistance to shear, but this is almost entirely offset by a negative band structure contribution, due to a special Brillouin zone feature, with the result that the C44shear component is very small. Several effects are associated with the addition of transition metals, for example in trialuminides such as Al3Ti. Outstandingly, the band structure contribution is severely reduced, because the large scattering cross-sections of the transition metal atoms greatly reduce the Brillouin zone effect. As a result, C44becomes large and the alloyed crystal is thereby embrittled. Because similar scattering occurs in dilute random solutions of transition metals in aluminium, there is a direct link between electrical resistivity and brittleness in these materials.MST/1527

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.981

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThis paper reports measurements of large sample populations of grain boundary geometries–both grain misorientations and boundary plane orientations–in annealed nickel. Two heat treatment schedules are represented: one in which a specimen is heated at 1000°C for 1 h (the‘fast’specimen) and another which combines this treatment with a slow heating and cooling cycle (the‘slow’specimen). There was a marked difference between the proportions of‘geometrically special’boundaries in the two data sets, with the slow set containing almost double the proportion of coincident site lattice boundaries (CSLs) observed for the fast set. Furthermore, of the CSLs in the slow set, half were tilts or twists (mostly asymmetrical tilts), compared with less than a quarter of the CSLs in the fast set. The interpretation of these data is that kinetic factors have a strong influence on the evolution of populations of‘special’boundaries.MST/1431

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.985

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe cyclic stress response characteristics of Al–Li alloy 2090 aged to contain fine ordered precipitates were studied over a range of plastic strains. The specimens were cycled using tension–compression loading under total strain control. The peak aged alloy showed softening at all plastic strain amplitudes. The degree of softening was observed to be greater at higher amplitudes than at lower amplitudes. The presence of shearable matrix precipitates in the alloy results in a local decrease in resistance to dislocation movement, leading to a progressive loss of ordering contributions to hardening. The observed softening behaviour is attributed primarily to dislocation–precipitate interactions, and is a mechanical effect. The fracture behaviour of the alloy is discussed in terms of competing mechanistic effects involving plastic strain amplitude, response stress, intrinsic microstructural features, and deformation behaviour.MST/1309

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.991

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe influence of prior surface condition and of a shot peening treatment on the bending fatigue strength of a standard Si–Cr spring steel (SS 2090) has been investigated. This steel was initially hardened and tempered to a hardness of 52–54 HRC. After shot peening, compressive residual stresses had been introduced into a surface layer of depth ~0·3 mm, with the maximum value of ~1000 MN m−2being found close to the surface. The effect of this treatment was to increase the fatigue limit by ~40% to 890 MN m−2. Coincident with this increase was a change in the site of fatigue initiation from a surface to a subsurface location beneath the compressive residual stress layer. The initiating inclusions, which were 20–40μm in size, were analysed and found to be Al2O3. At stress amplitudes greater than the fatigue limit, initiation was invariably found to occur at the surface and was not always due to inclusions. Inclusion initiated failure has been modelled using the size and spatial distribution of inclusions in the test bars in addition to the variation of applied and residual stresses through the section. A crack propagation criterion based on linear elastic fracture mechanics is used, assuming that propagation is controlled by stress intensity threshold value. It is assumed that small cracks exist at oxide inclusions from the beginning of the fatigue life and that failure is associated with the propagation of one of these cracks.MST/1392

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.998

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe cleavage fracture strengths and Charpy impact transition temperatures of plain C–Mn steels (0·12%C and 1–1·4%Mn) having ferrite–pearlite microstructures have been determined for ranges of grain size and grain boundary carbide thickness. Using appropriate heat treatments, ferrite grain size and carbide thickness were varied independently. Refining the ferrite grain size or grain boundary carbide thickness increased the fracture strength and decreased the impact transition temperature. Of the current theories for brittle fracture, an equation recently derived by Petch was found to give the most satisfactory agreement with the experimental data for cleavage strength.MST/1424

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.1005

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractAn Al–Mg–Si alloy 6063 was heat treated in the underaged, peak aged, and overaged conditions. The microstructures of the alloy in the different aging conditions were examined using transmission electron microscopy. Tensile and fatigue tests were carried out and the resulting fractures were studied using scanning electron microscopy. It has been found that the alloy shows different tensile and fatigue properties and different modes of fracture under different aging conditions. The results have been explained in terms of slip distribution and grain boundary strength.MST/1188

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.1010

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effect of residual stresses, induced by cold water quenching, on the morphology of fatigue crack fronts has been investigated in a powder metallurgy 8090 aluminium alloy, with and without reinforcement in the form of 20 wt-%SiC particles. Residual stress measurements reveal that the surface compressive stresses developed in these materials are significantly greater than in conventional metallurgy ingot 8090, because surface yielding occurs on quenching. The yield stresses of the powder route materials are greater than those of ingot produced 8090 and hence greater surface stresses can be maintained. In fatigue, severe crack front bowing is observed in the powder formed materials as a result of the reduction of the R ratio (minimum load/maximum load) by the compressive residual stresses at the sides of the specimen, causing premature crack closure and hence reducing the local driving force for fatigue crack growth∆Keff. This distortion of the crack fronts introduces large errors into measurements of crack growth rate and threshold values of∆K.MST/1370

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.1015

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractAn attempt was made to analyse the phase structure of thin layers of electrodeposited Cu–Cd alloys using anodic linear sweep voltammetry combined with a potentiostatic pulse technique (ALSV–PPT). The results were compared with data obtained using conventional techniques for phase structure analysis. It was found that application of the conventional X-ray technique (Guinier analysis) is limited, because the grain size of the alloy constituents is too small for compounds to be revealed by X-ray peaks. Electrodeposited alloys could be successfully analysed using that method only after subsequent thermal treatment. Diffraction patterns from transmission electron micrographs of very thin foils (of the order of 20–50 nm) of electrodeposited 75Cd–25Cu (at.-%) alloy revealed the presence of CuCd3intermetallic compound having a grain size of ~20 nm. By comparing all the results, the distribution of the various intermetallic compounds present at various stages in the electrodeposition of thin (up to 10μm) layers of Cu–Cd alloys could be established. Combined ALSV–PPT was found to be more sensitive than either of the other methods employed.MST/1206

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.1021

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe adhesion of thin ceramic films to metal substrates is an important issue in a wide variety of technological areas; one of which is the corrosion of metal alloys at high temperature. Consequently, there is great interest in exploring the measurement of adhesion between ceramic films and metal substrates and the factors that control such adhesion. In this study, five methods for assessing the adhesion of thermally grown oxide scales to their substrates have been evaluated to establish their relative usefulness. Simple, well characterised model oxidation systems (high purity nickel, Ni–0·1 wt-%Cr, and Ni–0·1 wt-%Al alloys) have been used. The experiments showed that practical adhesion was measured most successfully using a double bending beam configuration and quantified in terms of the normal applied stress for catastrophic interfacial failure. Although the more convenient standard scratch test gives only semiquantitative results, these are nevertheless useful for comparisons within a given system. None of the techniques was capable of measuring the fracture toughness of the interface. The degree of practical adhesion of the oxide scales was found to decrease in the substrate order: high purity nickel>Ni–0·1Al>Ni–0·1Cr. The practical adhesion to the two alloys was sensitive to oxide thickness and there is a critical thickness (about 1·7μm for Ni–0·1Cr and 4·8μm for Ni–0·1Al) above which the practical adhesion is very low. This critical condition is probably the result of relaxation of residual compressive stress in the oxide by buckling of the scale.MST/1467

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.1031

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA numerical analysis program, based on slab analysis, was developed to calculate the stresses and strains that occur in the component layers of a composite sheet in the roll bite during the fabrication of clad metal (e.g. silver clad phosphor bronze) by roll bonding. Results calculated using the perfect plastic and strain hardening models were compared with experimental results: the results obtained using the strain hardening model were found to be in better agreement with values of rolling force and thickness of component layers measured after roll bonding. The program requires input data such as roll radius, initial thickness of the specimen, initial cladding thickness fraction, reduction ratio, coefficients of friction between rolls and material and between component layers, and front and back tensions. The coefficients of friction were evaluated indirectly by comparison of the measured rolling force with that calculated using slab analysis. Measured coefficients of friction varied with reduction ratio and initial thickness, but were independent of rolling speed.MST/1334

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.11.1042

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor