摘要:

AbstractThe formation of austenite during the continuous heating of steels was investigated using neural network analysis with a Bayesian framework. An extensive database consisting of the detailed chemical composition, Ac1and Ac3temperatures, and the heating rate was compiled for this purpose, using data from the published literature. This was assessed using a neural network, with the aim of modelling the austenite start and finish temperatures. The results from the neural network analysis were consistent with what might be expected from phase transformation theory.MST/3373

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.453

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractMetastable COxCU100−xalloys with extended cobalt solubility (5≤x≤20), which can not be obtained by equilibrium methods, were prepared by melt spinning. The separation of the cobalt rich phase upon heating was monitored by X-ray diffraction and calorimetric measurements. The volume fraction of cobalt rich particles can not be determined using X-ray diffraction if the cobalt rich particle size is below a critical value. The separation of the cobalt rich phase seems to be a three step process including nucleation, growth, and coarsening. The non-isothermal kinetics of nucleation and growth of the cobalt rich phase were analysed in terms of a modified Kissinger model. The heat release during phase separation is closely related to the amount of cobalt dissolved in the copper matrix.MST/3258

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.464

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractSpun cast alloy HP50–Nb tubing retired from service in a steam reformer after approximately 8 years of service had stress rupture properties suggesting very short remaining life. Microstructural examination, however, indicated little or no creep damage (cavitation) but did reveal extensive microstructural modification. The microstructure, mechanical properties, and stress rupture properties of the ex service material were determined and compared with virgin as cast material. The microstructural changes that occurred during service included the formation of phases rich in alloying elements such as chromium and niobium that would otherwise be expected to contribute to high creep strength. The creep life of the service exposed material was correspondingly short. The creep ductility of the service exposed material, however, was significantly higher than that of the as cast material. The implications of these findings for remaining life assessments of reformer furnaces using these materials are discussed.MST/3207

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.469

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractCastings of 70–30 cupronickel (NES 824) often contain an undesirable amount of porosity, which leads to a high rejection rate for components made from this type of alloy. The possibility of removing such porosity by the application of hot isostatic pressing (hipping) to the castings has been investigated. Part of this investigation involved the hipping of artificial porosity specimens of controlled geometry and a comparison of the results obtained with the predictions of certain well known models for pressure assisted sintering. The results obtained were complicated by the effect of the specimen design on the rate of compaction which is not taken into account in any of the models. However, where the pores are truly internal the degree of agreement with both models is reasonable. It is evident that for internal porosity the removal of porosity in a typical industrial hipping operation occurs during the initial application of pressure and temperature rather than during the subsequent isothermal stage. This applies for up to 10% porosity in the form of a single hole but may not be true of large castings with uneven distributions of pores.MST/3268

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.477

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe dry sliding wear and friction behaviour of cast Al-Si alloy and composites containing 5·0, 8·0, 12·0, and 16·0 vol.-% alumina short fibres was investigated by means of a block on ring (bearing steel) type wear rig. The volume wear rate, the coefficient of friction, and the contact surface temperature of both matrix alloy and metal matrix composites (MMCs) were measured over a load range of 20–150 N for various sliding distances. Comparisons were made between the wear properties of MMCs produced by the centrifugal force infiltration route and by the squeeze infiltration method. The experimental results show that the addition of reinforcing alumina fibre in Al-Si alloy not only reduced the wear rate, but also effectively suppressed the transition to the severe wear rate regime which occurred in an unreinforced Al-Si alloy in the load range of 100–150 N. The coefficient offriction of the composites was higher than that of the unreinforced Al-Si alloy at applied loads below 100 N, but there was almost no difference above 100 N with a mean value of about 0·40. The contact surface temperature increases with increasing applied loads, and it can reach 520 K at a load of 150 N. Examination of the worn surfaces by SEM identified that the presence of alumina fibres suppressed the transition to the severe wear rate regime.MST/3204

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.483

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractA Zn–22 wt-% Al alloy was superplastically deformed in the optimal range to obtain experimental evidence based on scanning electron and transmission electron microscopy for mesoscopic grain/interphase boundary sliding and plane interface formation that facilitates the cooperative boundary sliding process. It was seen that such localised shear surfaces had formed in the entire specimen bulk in an interconnected manner. At boundaries that formed part of a cooperative shear surface the triple junctions had migrated to a near 1800 orientation. Mesoscopic grain/interphase boundary sliding appeared to be accommodated by local boundary migration brought about by diffusion at intercrystalline boundaries and lattice dislocation motion at interphase interfaces in accordance with the requirements of the model presented in Part 1.MST/3075

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.489

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractHemisphere free bulging of a superplastic 8090 Al–Li sheet was carried out, with particular emphasis given to the superplastic behaviour over the low strain regimeɛ=0−0·7. Various pressurising cycles, including constant pressure, constant strain rate, and multiple strain rate bulging, were performed to characterise the superplastic behaviour in terms of strain rate variation, thickness distribution, and evaluation of the strain rate sensitivity (m) of the sheet during biaxial bulging. For constant strain rate bulging, a modified Ghosh and Hamilton (GH) model and a model developed by the present authors (HL) have been used to simulate the necessary pressure profiles. Two different constitutive equations, extracted from uniaxial tensile tests, were used in the study. The modified GH model appliedσ=Kεmas the constitutive equation, i.e. considering only the m value in the simulation, while the H L model incorporated the strain hardening exponent (n) additionally into the constitutive equation for the simulation: i.e. the equationσ=εɛnεmwas used. The HL model can also be applied to simulate the multiple stage strain rate forming route. Based on the analyses, the material superplasticity characteristics obtained from uniaxial tensile tests were seen to be reliably applicable to equibiaxial sheet bulging. The equibiaxial straining condition was not obeyed exactly except at the pole. With the consideration of initial work hardening contribution towards the constitutive equation, the simulated results seemed to agree better with the experimental data. With further straining, the rate sensitivity increased and work hardening decreased, thereby a multiple strain rate bulging path, including a high initial rate followed by several lower strain rates, would be able to fully utilise the higher values of m and n during each straining stage. The thickness uniformity as well as the total forming time could thus be improved and shortened.MST/3107

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.495

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractConstant stress tensile tests have been carried out and cavitation resulting from tensile superplastic flow has been examined using metallographic and densitometric techniques in high strain rate superplastic Al–Mg–Si/Si3N4(p)and Al–Cu–Mg/Si3N4(p)composites (where‘p’denotes particulate). The elongations to failure depended strongly on the applied stress, and the elongations decreased rapidly at high stresses within the range having the same stress exponent of 2. Cavities developed parallel to the tensile direction at low stresses, but perpendicular to the tensile direction at high stresses. The development of cavities perpendicular to the tensile direction is probably responsible for low elongations in the high stress range. The rates of increase of the cavity volume for the composites were much lower than that for a superplastic 7475 alloy. This can probably be attributed to the presence of a discontinuous liquid phase for the composites.MST/3354

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.505

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractCompact tension specimens having a columnar grain orientation inclined to the notch plane at an angleθin the range 0–60°were prepared from a centrifugally cast HK40 tube. Constant load creep crack propagation tests were carried out on these specimens at 871°C. It wasfound that the creep cracks tended to advance along the columnar grain boundaries, and the crack propagation resistance increased with increasingθ. The dependence of creep crack propagation rate da/dt on the columnar grain orientation can be expressed as da/dt=A[KIcos2(θ/2)]m, where A and m are constants and KIis the stress intensity factor. A good correlation between the creep crack propagation rate and the path independent integral C* confirms that the integral C* is an appropriate controlling factor for creep crack propagation in cast HK40 alloy.MST/3244

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.513

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe fatigue properties of Al-1OMg (wt-%) castings have been shown to be improved by hot isostatic pressing (hipping) with a level of improvement highly dependent on the initial quality of the castings. Only castings with low oxide inclusion levels were found to show a significant improvement infatigue propertiesjollowing hipping. To reduce the oxide inclusion content a casting technique has been developed in which the molten aluminium is tapped from the bottom of an induction melting furnace. The average fatigue life (at a test stress amplitude of 106 MN m−2) increased from 213×103to 1016×103cycles following adoption of the bottom tapping technique. Hipping of the bottom tapped castings produced a further increase in the average life to at least 15×106cycles. These substantial improvements in fatigue properties can be attributed to a reduction in the size and quantity of oxide inclusions in the bottom tapped castings. Unless the oxide inclusion content of the castings can be maintained at a sufficiently low level the optimum properties will not be obtained after hipping and the repeatability in the mechanical properties of hipped castings, necessary for confidence in design, will be poor.MST/3186

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.6.518

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor