摘要:

AbstractThe implementation of the theory of diffusion-controlled growth of ferrite plates in plain-carbon steels is critically assessed. It is found that the use of empirically extrapolated diffusion coefficients, phase boundaries, and thermodynamic functions leads to errors in calculations of growth rate. The errors become most important for low transformation temperatures, leading to exaggerated growth rates. Ways of avoiding these difficulties are suggested, and a new analysis of experimental data indicates that the lengthening of Widmanstatten ferrite plates in Fe–C alloys occurs at a rate which is influenced by the diffusion of carbon in the austenite ahead of the interface, assuming that the plates adopt a tip radius consistent with the maximum growth velocity. However, there is a systematic discrepancy between theory and experiment: plate-growth theory seems to underestimate the lengthening rate by some 5μm s−1. This may have something to do with the lath shape of Widmanstatten ferrite, but an analysis using needle–growth theory does not resolve the problem for data obtained at low lengthening rates. In general, plate–growth theory gives a better explanation of experimental data. The growth of bainite sheaves occurs at a rate much faster than expected from carbon diffusion-controlled growth. If the maximum-velocity hypothesis is incorrect (as it is for dendritic solidification), the above-mentioned discrepancies would be larger.MST/192

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.497

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe technology associated with the use of semiconductors in monitoring the composition of the atmosphere has been advancing rapidly in recent years. The twin demands of remote process control and concern for the environment have produced requirements for sensing different gases in a wide range of applications. In this paper the authors review the state of the art of semiconductor gas sensor development and anticipate the directions in which future progress is likely to be made.MST/221

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.505

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe high cost and lengthy preparation time of conventional ceramic bend-bar strength specimens is holding back the development of sintered silicon nitride ceramics having good high–temperature properties. The equipment now available for high–temperature sintering enables discs of high–strength sintered silicon nitride to be fabricated in a wide range of formulations. The use of disc specimens has led to a considerable increase in the rate and volume of research into mechanical properties and behaviour in a high-temperature environment. This research has demonstrated the value of using disc specimens to reduce the time and cost of programmes for developing materials suitable for gas-turbine and other demanding applications.MST/232

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.510

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe size distributions ofγprecipitates in a Ni-based alloy containing 6·75 wt–% aluminium have been measured, using dark-field electron microscopy, at five temperatures ranging from 600 to 825°C and the corresponding volume fractions have been assessed. From these distributions, the average particle sizes were obtained and plotted against time according to the kinetics laws proposed by theory for diffusion–controlled growth, interface-controlled growth, and growth in the transition region where neither process dominates. The particle-size distributions were reconstructed in terms of the ratio of the actual particle size/average size, and the resulting distributions were compared with those predicted by theory for the three controlling conditions. Although the values of activation energy obtained from the plots of particle size against annealing time are equally consistent for the three processes, the agreement between the predicted distributions and the experimental distribution is distinctly better for growth in the transition region than for the two alternative processes.MST/167

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.516

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe hot ductility behaviour of high-purity iron-base alloys was determined by hot tensile tests. The observed ductility trough in the 850–1100°C temperature range was found to be a product of the interaction between the hardening effect of AIN precipitates and the intergranular segregation of sulphur. The dissolution of AIN leads to an improvement in hot ductility and the temperature for complete dissolution of AIN coincides with the onset of dynamic recrystallization.MST/171

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.520

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractAn experimental study has been made of the manner in which microfractures initiate and grow along adiabatic shear bands formed in the titanium alloy Ti–6AI–4V by the normal impact of hard steel spheres at velocities up to 340 m s−1. It is suggested that a critical shear strain must be exceeded along the shear bands for microvoids to nucleate, or to cause significant local thermal softening in the bands, leading to the formation of single voids or arrays of voids and smooth-sided cracks when the stress state became predominantly tensile. The final shape of the micro fractures within the shear bands and the morphology of the resultant fracture surfaces are explained in terms of the density of void nucleation sites and the tensile-stress state across the shear bands.MST/179

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.526

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractVoid nucleation at second-phase particles has been studied in round–notched, cylindrical tensile specimens of an ingot–cast AI–Li quaternary alloy. The volume fraction and distribution of voids has been determined as a function of strain history for samples in the solution–treated, peak–aged, and overaged conditions, and values of the stress to form cavitation calculated. The intermetallic phase responsible for void nucleation has been identified as being Fe–and Cu–rich; voids nucleate following the fracture of these particles. A mechanism of failure is proposed based on stress concentration at the particle/matrix interface where coplanar slip bands impinge.MST/78

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.531

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe traditional design approaches to fatigue at notches, based on stress level–endurance relationships, are briefly reviewed. It is shown, by considering crack propagation from notches and invoking a change in control mode from notch plasticity to crack-tip plasticity, that a critical stress condition can be obtained which must be exceeded if the crack is to propagate to failure. The traditional techniques are then reinterpreted and explained by this propagation method. An example is given of crack growth from a sharp defect at a weld toe. It is shown that the integration of an elastic fracture mechanics growth law can reproduce stress range–cycles to failure data for this situation. There are, however, complexities of stress analysis and crack shape. A simple treatment of residual stresses affecting the threshold and slow–growth regimes, shows some promise as a technique for accounting for residual stresses.MST/70

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.537

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractCold–worked and annealed copper rods were deformed at room temperature in a four–die rotary swaging machine. The outer parts of the cold–worked rods were work softened, whereas the annealed rods were inhomogeneously work-hardened, with a strength increase from the surface to the core. Both effects can be explained as being a consequence of a cyclic deformation during rotary swaging.MST/148

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.544

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractIn this investigation, macrostructural and microstructural imperfections, formed in the fibrous Cd–CuCd3eutectic during unidirectional solidification, were characterized and the factors important in their formation were studied. The effect of growth rate R, imposed thermal gradient G, and natural convection on the defect morphology were investigated, and the important physical factors that determine the manner in which microstructural defects form were considered. Convection was found to play a minimal role in defect formation. The most important variable was found to be the ratio G/R, which determined the solid/liquid interface shape during solidification. When low values of G/R were used, eutectic cells were formed. For intermediate values of G/ R, the microstructure was found to contain features analogous to nodes in single–phase materials. Only when high values of G/R were used were all microstructural defects eliminated, producing a nearly perfect structure of parallel CuCd3rods in a matrix of cadmium. The morphology of branching and curving rods, which were found to be the primary microstructural defects, indicates that both the solid/solid interfacial free energy and its anisotropy are important factors in rod branching, and a branching mechanism consistent with the observations made was proposed. Anisotropy in the solid/solid interfacial free energy was considered to account for the formation of blades; however, kinetic considerations were required to account for the growth dependence of the rod-to-blade transition observed in this eutectic.MST/130

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.7.550

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor