摘要:

AbstractAn assessment has been made of the current interest and potential future of zirconia ceramics for structural applications. Interest in the mechanical properties of ceramics based on, or containing, ZrO2centres on the manipulation of the tetragonal to monoclinic martensitic phase transformation. It will be demonstrated that the maintenance of a metastable tetragonal phase is one of the most important factors in the optimum design of a strong and tough ceramic. There are several microstructural approaches that can be taken to achieve toughening based on either precipitate-hardened or particulate-dispersed systems, and a number of different toughening mechanisms can operate, either singly or together, to produce different combinations of properties. The potential engineering future of three ZrO2ceramics: partially stabilized zirconia, tetragonal zirconia polycrystals, and zirconia-toughened alumina, is examined in this context. The important relationship between microstructure and mechanical properties is explored for these materials and current applications are cited. The significance of grain boundaries and grain-boundary phases in affecting the way in which the microstructures evolve during sintering and/or aging, and hence in affecting the final properties of the ceramic, are highlighted. The properties of ZrO2ceramics in current use are then compared with other common engineering materials, and the problems of property reliability are stated. Finally, an opinion is expressed regarding the deficiency of present scientific effort in ceramics, and some predictions are made concerning the future of ZrO2ceramics.MST/201

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.417

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThermal analysis, metallography, and electron-probe microanalysis have been used to corroborate one of the conflicting constitutions of the AuSn–Pb–Sn partial ternary system. The phase equilibria consist of two ternary transition reactions at 13–0Au, 48–0Pb, 39–0Sn at.-%, 275°C and 8–5Au, 28–0Pb, 63–5Sn at.-%, 210°C, and a ternary eutectic at 3–5Au, 20–0Pb, 76–5Sn at.-%, 177°C. The solubility of Pb in the ternary system is substantial and between 275 and 177°C the system can dissolve up to 4 at.-% Au and 27 at.-%Sn. The maximum width of the AuSn primary phase field in the partial AuSn–Pb–Sn system is approximately 3 at.-%. and that of Sn approximately 2 at.-%.MST/82

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.433

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractActivation energies for impurity diffusion in solids are discussed. New approaches based on elasticity are outlined to predict values of activation energy from the atomic size, surface energy, and elastic modulus of the matrix and impurity. The activation energy for self-diffusion in the matrix must also be known. It is shown that reasonable agreement between the theoretical and experimental values of activation energy are obtained for copper, aluminium, nickel, and iron matrices. The elastic strain energy model is extended to facilitate predictions of vacancy–impurity binding energies. Again good agreement between experiment and theory is obtained, except in the case of aluminium matrices.MST/211

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.442

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractAn adiabatic calorimeter was used to measure the molar heat capacities and heats of transformation of iron alloys containing up to 14 at.-% Cr and 15 at.-% Co over the temperature range 700–1500 K. A Sucksmith magnetic balance was used to measure the magnetic properties of these alloys and of Fe–Co and Fe–Cr alloys previously investigated calorimetrically. In all but one of the ternary alloys theα→γtransformation occurred at a temperature below the Curie temperature (Tc), but the heats of transformation, corrected to the transformation temperature of pure iron (1184 K), correlated well with the difference between the extrapolated Curie temperature and 1184 K, as had been observed previously for alloys with clearly defined values of Tc. The fact that all observations fell on the same curve supported the proposal that a significant amount of magnetic order remains after theα→γtransformation for most iron alloys.MST/198

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.448

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractExperiments have shown that the amount of hydrogen occluded in iron for a given charging condition depends on both the ferritic grain size and the nature of the grain boundaries. The larger the relative misorientation between grains, the higher the occlusivity per unit grain-boundary area. With increasing grain size of the same grain–boundary nature, the boundaries are saturated more quickly, and thus the susceptibility to hydrogen damage is higher. In general, cold working increases the hydrogen uptake by increasing the defect density. When stressed parallel to the deformation axis, specimens cold worked to 20% deformation were shown to have a reduced ductility loss after hydrogen charging, thought to be a result of the development of texture.MST/143

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.454

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe accumulation of cavitation damage with increasing strain during the biaxial deformation of two superplastic aluminium-base alloys has been studied using densitometry and quantitative metallography. A model based on constitutive relationships for the diffusive and plastic growth of voids has been extended to account for cavity coalescence, a phenomenon commonly observed during superplastic deformation. Good agreement between the measured and calculated cavity growth rates was obtained for one alloy (AI 7475), while discrepancies observed for the second alloy (Supral 220) are explained in terms of the effects of continuous cavity nucleation.MST/189

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.461

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe tensile and bulging characteristics of Ti–6Al–4V alloy sheet have been investigated under superplastic conditions following various prior treatments. These prior treatments. consisted of cold rolling to different degrees of reduction and recrystallization. It was found that cold rolling markedly improves the superplasticity of the alloy, and even recrystallization, which causes grain coarsening, does not eliminate this effect. Moreover, the ratio of the total tensile elongation to the bulging strain to failure was found to decrease with increased prerolling.MST/172

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.466

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractFour aluminium alloys of different zinc/magnesium ratio have been studied under various extrusion conditions. The alloys were cast in steel book moulds and subjected to initial thermomechanical treatments. Studies were made of hot extrusions and cold hydrostatic extrusions and in each case the changes in the extrusion parameters were analysed. An attempt has been made to explain some of the extrusion defects which appeared in various extruded sections. The extrusion speed was found to be crucial, since sections developed surface cracks at higher speeds. The extrusion speed was also found to vary inversely with the extrusion ratio, with higher speeds at low ratios. A well defined solute–depleted weld zone was observed on each of the four faces of a square tube extruded using a porthole die. Thermal treatment was not found to improve this weak weld zone. Tubes extruded using a floating-mandrel die withstood pressure testing up to 550 MPa.MST/43

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.470

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractRecent studies of stress-relief cracking in low-alloy steels have focused attention on a novel mode of brittle intergranular fracture which occurs at elevated temperatures (300–650°C) in hard, coarse–grained heat–affected–zone microstructures. Fracture initiates at stress concentrators such as sharp cracks or inclusions, and can propagate under static loading at rates of 10−11−10−5ms−1to produce intergranular facets with very little associated plastic deformation. The stress-intensity parameter K has been used to characterize crack growth, and three regimes of behaviour have been observed: (i) a threshold region at growth rates of 10−11−10−10m S−1, (ii)a plateau region, in which growth rates are independent of K between 10−10and 10−8m S−1, and (iii) a region of highly K-sensitive crack growth between 10−8and−5m S−1. Independent Auger electron spectroscopy analyses have demonstrated that sulphur segregates locally to the high-temperature crack tip, giving rise to the embrittlement of a limited area of grain boundary. Together with other presegregated solutes, this enables brittle fracture to occur at high temperature, and the transfer of sulphur to the crack tip controls the rate of crack growth. Two models describing crack-tip sulphur segregation are currently proposed. In the first model, a quantitative analysis demonstrates that the crack-tip stress field will drive undersize solute atoms such as sulphur to the physical crack tip. In the second, the intergranular crack is modelled as a sharp cavity. Grain-boundary sulphides which are exposed by cavity formation become unstable and dissolve, saturating the cavity surface with sulphur, which is then drawn into the tip as part of the cavity growth process.MSTj77

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.475

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractResults are presented which show that for environment-sensitive cracking the changes in environment composition or potential that may occur in pits, crevices, or cracks are probably as important as the stress concentration associated with such discontinuities. However, with some solutions, especially those in which the solvated metal is very slightly soluble and the pH is effectively buffered, the changes of composition and potential in discontinuities will probably be small and less important than the stress concentration. When the stress concentration is appreciable, it is the attendant plastic strain, and even more so the rate of its manifestation, that is important, because of the time dependence of the associated corrosion-related reactions in environment-assisted crack growth.MST/65

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.6.480

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor