摘要:

AbstractThe formation ofθin A l–3Cu–0·1Be alloy has been investigated using a resistivity technique and transmission and scanning electron microscopy, and the results are compared with those reported in an earlier investigation of an AI–3Cu alloy. The formation ofθobeys the Avrami equationξ=1−exp(−ktn), whereξis the fraction of excess solute precipitated at time t, and k and n are parameters dependent on nuclei density and growth mechanism, respectively. The rate ofθformation in Al–3Cu alloy is significantly increased by the addition of beryllium, and this is manifested in the earlierθ′→θtransition and in the faster overall transformation rate. However, the values for the growth constant (n=0·72±0·07) and activation energy (108·5 kJ) are the same (within experimental error) for both alloys, indicating that the addition of beryllium does not significantly affect theθgrowth mechanism. A beryllium-enhanced nucleation rate is proposed to account for the higher precipitate particle counts and k values (which include nuclei density) obtained for the Al–3Cu–0·1Be alloy.MST/492

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.1

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractPrecipitation reactions occurring within two Al–Li–Zn–Mg–Cu alloys have been investigated and are discussed in terms of the phase equilibria of simpler alloy systems. The influence of solute supersaturation and free vacancy concentration upon precipitation is highlighted.MST/556

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.7

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractAn investigation of subcritical crack growth in C–Mn steels at 360°C has been conducted to provide data relevant to the assessment of the long-term integrity of cold-formed bends in boiler integral pipework systems. The material was obtained from bends that exhibited hardnesses towards the top and bottom of the usually observed range and which had been in service for approximately 63000 h. Crack growth rates were found to be a function of specimen displacement rates, and despite evidence that growth may have occurred under non-steady-state conditions, good correlation of the data was obtained using the C*parameter. Material hardness was found to be a major factor in determining resistance to cracking, although the trends observed in creep and crack growth properties suggest that its effect may be reduced at long exposure times.MST/509

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.14

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractA new approach to creep, termed theθ-projection concept, is illustrated by reference to high-precision data obtained for polycrystalline copper. Over wide ranges of stress and temperature, the increase in creep strainεcwith time t can be described precisely using the equationεc=θ1[1−exp(−θ2t]+θ3[exp(θ4t)−1] whereθ1andθ3define the extent of the primary and tertiary stages with respect to strain andθ2andθ4quantify the curvatures of the primary and tertiary stages of creep respectively. Once the stress and temperature dependences of the fourθ-parameters and the creep ductility are evaluated, the complex stress and temperature dependences of the minimum creep rate and the rupture life, as well as the exact form of the deformation mechanism map expected for polycrystalline copper, can be predicted directly without invoking any change in creep mechanism.MST/504

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.23

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractA viscoplastic constitutive model with an evolving internal state variable, called hardness, has been developed for commercially pure aluminium. One application of such a constitutive model is in process modelling where hardness distributions may be predicted throughout the workpiece. This paper assesses the accuracy with which microhardness measurements of quenched specimens correlate with the hardness predicted by the constitutive model for various imposed thermomechanical histories. Using axisymmetric compression, different hardness values are achieved by various tests, both underdeveloped (increasing hardness) and overdeveloped (decreasing hardness) structures being produced during deformation. The steady state flow stress and hardness for a particular strain rate and temperature may be achieved with less strain by first deforming at a high strain rate and then decreasing the strain rate. The constitutive model accurately predicts the amount of prestrain required at the higher strain rate. Differing initial structures are achieved by using as-extruded and fully recrystallized materials. Using microhardness measurements as a method of characterizing the initial structure, the constitutive model is able to predict the subsequent flow curve and evolution of microhardness.MST/572

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.28

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractThe plastic strain ratio r of Ti–6Al–4 V sheet was measured after superplastic strain at 925°C and the effects of sheet thickness and testpiece shape onrwere investigated. Ther-values were related to directionality in the microstructure and increased with superplastic strain towards unity, representing increasingly isotropic behaviour, as the microstructure became more equiaxed. Ther-values were also influenced by the constraining effect of the testpiece heads on the width strain in the gauge length. This effect was reduced as the length-to-width ratio of the undeformed gauge length was increased.MST/465

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.36

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractThe solubility of nitrogen in molten iron and molten iron alloys has been analysed on the basis of statistical thermodynamics developed for the analysis of very dilute interstitial solutions in which solute–solute interaction is assumed to be negligible. Nitrogen atoms in molten iron appear to be stabilized to an extent similar to those inα-iron. The present analysis also shows that substitutional alloying elements vanadium and chromium may function as trapping sites for nitrogen atoms in molten iron. The importance of an appropriate statistical model is pointed out for this type of analysis.MST/479

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.43

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractA mathematical model has been devised for the calculation of the heat flux from a cast cylindrical body to a metal chill during solidification. The model is based on the temperature measured in the chill close to the casting/chill interface. The heat flux as a function of time was determined for Al–Si alloys having different contents of alloying element and for chills with different initial temperatures. It was found that an increased silicon content as well as rising initial chill temperature cause a reduction in the heat flux. A further mathematical model is presented, for the calculation of the temperature field during solidification as a function of the heat flux and the thermophysical properties of the cast material, which in turn are functions of the temperature. This model yields the temperature in the casting as a function of the distance from the chill, in good approximation to the temperature actually measured in the casting. In addition, the local solidification time is obtained as a function of the distance from the chill. From metallographic specimens taken along the axis of the cylinder at specified distances from the chill, the mean dendritic arm spacingλwas calculated. It was found that the relationship betweenλand the local solidification time is given by the equationλ= ASitf0·43, where ASiis a constant which varies inversely with the solute content.MST/313

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.49

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractA fabrication process for SiC whisker reinforced aluminium composites is being developed. Theftrst step involves preforming the SiC whisker and squeeze-infiltrating the molten metal into the preform. Secondary fabrication can involve a conventional hot metal working practice such as extrusion. Microscopic observations indicate that the distribution of whiskers is very homogeneous. In an aluminium alloy (AA 6061) matrix composite containing 17 vol.-% SiC whiskers, the Young's modulus is>100 GPa and the tensile strength increases by ~ 50–70% above that of the matrix to 460 and 529 MN m−2in the cast and extruded products, respectively. The strength at elevated temperature and the wear resistance are also improved. These enhanced properties make the composite potentially suitable for a number of automotive applications.MST/574

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.57

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractThere is a requirement for on-line monitoring of elements in molten aluminium and one possible approach is to use electrochemical sensors which produce a voltage that is related to the concentration of an element in the melt. The principle of electrochemical sensors is outlined and examples given for hydrogen, lithium, and sodium determination in molten aluminium using sensors based upon calcium hydride, lithium silicate–phosphate, and sodiumβ-alumina, respectively: sensors could also be used to control the Hall–Heroult cell and possible examples are the measurement of the NaF/AlF3ratio in the cryolite bath, the evolved CO/CO2ratio from the anode, as well as the concentration of sodium and lithium in the aluminium tapped from the cell.MST/571

ISSN:0267-0836    

DOI:10.1179/mst.1987.3.1.61

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor