摘要:

AbstractThe effect of silicon content on the microstructure and phases present in Ti–(20–23)Al–11Nb alloys has been studied in the temperature range 800–1270°C.Four phases,βo,α2, O, and a silicide, are formed. The parentβois ordered at 1270°C. At 1050°Cα2is formed which exhibits a higher silicon solubility than the parentβo. A peritectoid transformationβ0+silicide→α2is proposed. Assuming that niobium substitutes for titanium and silicon for aluminium, energy dispersive X-ray spectroscopic data suggest that theα2phase, unlike that in binary Ti–Al alloys, is highly stoichiometric and of the form (Ti+Nb)3(Al+Si). Similarly the silicide corresponds to binary Ti5Si3with the same site substitution as in theα2phase. The O phase is orthorhombic and similar in composition to theα2which it replaces: its formation is promoted by silicon.MST/3088

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.617

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractIsostatic diffusion bonds were produced between Ti–6Al–4V and Super Alpha-2 sheet materials at temperatures of 920–940°C and pressures of 6–10 MPa. The diffusion bonds were evaluated using optical microscopy, lap shear testing, and scanning electron microscopy of the lap shear fracture faces. The results of X-ray microanalysis carried out on the bond interface region showed that substantial diffusion of niobium and molybdenum had occurred from the Super Alpha-2 to the Ti–6Al–4V. This caused a local suppression of theβtransus in the Ti–6Al–4V alloy and on subsequent cooling theβphase decomposed to give a microstructure of fine acicularαphase and retainedβphase in the bond region.MST/3421

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.623

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractElectron probe microanalysis showed that Al–7Mg–Cu alloys possess serious segregation tendencies. The addition of copper promoted the segregation of magnesium and led to the formation of non-equilibrium eutectic. With an increase in the copper content of the alloys, the severity of the solute segregation increased. Homogenisation reduced the solute segregation significantly. During homogenisation, the non-equilibrium eutectic compound AlxCuyMgzgradually dissolved. Its dissolution behaviour depended on its copper content. Precipitates of AlxCuyMgzwith a comparatively low level of copper dissolved, while those with a high level of copper were less soluble and became divided into small blocks. The higher the copper content of the alloys, the larger and the greater in number the remaining AlxCuyMgzparticles. In the undissolved AlxCuyMgz, the concentration of copper increased and that of magnesium decreased. Two step homogenisation reduced the solute segregation and dissolved the non-equilibrium eutectic further.MST/3194

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.628

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

Abstracthe conclusions reached based on a grain/interphase boundary sliding controlled flow model for optimal structural superplasticity are verified using experimental data from three aluminium alloy systems. Isostructural strain rate–stress relationships could be calculated very accurately using five experimentally determined, physically meaningful constants (three of which could also be calculated from theory; two fully and one partially). The true activation energy for the rate controlling boundary sliding process, the variations of the internal stress distribution arising from sliding, the stress function that is proportional to the measured isostructural isothermal strain rate, and the apparent viscosity, were calculated. It is suggested that the basic units of microscopic boundary sliding in the three aluminium alloys examined have a common structure.MST/3077

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.635

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe mechanical properties of the high strength casting alloys A356, A357, A20l, and A206 are compared using the best current casting technology, to provide an accurate overview of these important alloys. The alloys were cast using precision sand moulds, via an uphill filling system incorporating a filter. Subsequent mould inversion was applied to later castings in A201 and A206 alloys. Aluminium chills acting against cooling fins were used to aid feeding by enhancing the temperature gradient. Mechanical properties were obtained for as cast, solution treated, and aged alloys, where aging was carried out to very long effective times, enabling assessment of alloy behaviour over a wide range of temperatures and times. Comparison with previously published data obtained in green sand and permanent moulds indicated that the faster cooling times in these moulds give somewhat improved properties.MST/3280

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.644

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractMagnesium based Mg–9Al–1Zn–5RE (RE=Y, La, Nd, Ce, or Pr) alloys with or without an addition of 1%Si were rapidly solidified by chill block melt spinning and splat quenching. The base alloy AZ91 (Mg–9Al–1Zn) was also rapidly solidified. Isochronal heat treatment for 1 h at 100–400°C showed that the microhardness of the ribbon maintained a similar level to that of the as spun alloy up to 300°C but decreased when heat treated at 400°C. Isothermal heat treatment for up to 24 h at 250–350°C showed that there were aging responses for the sample treated at 250°C while above this temperature, the microhardness decreased as the treatment time increased. The addition of 5% of RE elements to AZ91 displaced the Mg17Al12phase in AZ91 with fine dispersoids of Al2RE (RE=Y or Nd) or Al11RE3(RE=La, Nd, Ce, or Pr) in Mg–9Al–1Zn–5RE alloys. These Al–RE intermetallics remained fine and precipitated at the grain boundaries so restraining grain growth during heat treatment at up to 400°C. Although Mg2Si precipitates were found to be present in the silicon containing alloys after heat treatment at 400°C, their size was greater than those of Al–RE intermetallics, indicating that Mg2Si has a lower thermal stability than these Al–RE intermetallics. The relationship between microhardness and grain size is discussed.MST/3400

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.651

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractAlthough much research has been carried out on the erosion of materials in aqueous slurries, little attention has been given to defining transitions between erosion regimes in such environments. This is despite the large amount of work that has been carried out in this area in‘dry’high temperature corrosion environments. Defining transitions between regimes permits distinction between erosion and corrosion dominated behaviour, which can be an aid to materials selection and to process monitoring in such conditions. This paper describes the basis of a relatively simple theoretical method for evaluating the transitions between erosion and aqueous corrosion regimes. Models for solid particle erosion at normal impact are combined with those for aqueous corrosion to define regimes of damage in corrosion conditions varying from dissolution to passivation. This permits the construction of the erosion–aqueous corrosion map where the transitions between the regimes are shown as a function of erosion and aqueous corrosion variables. The rationale for the regimes suggested is discussed on the basis of research on erosion maps in oxidising conditions. The boundaries on the maps are compared with experimental data. Other issues addressed in this study include using the erosion–corrosion map to identify favourable and adverse operating conditions in addition to identifying the mechanism of damage.MST/3543

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.662

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe hot deformation characteristics of Inconel 625 alloy were investigated by torsion testing at constant strain rates in the range of 0·17–5·3 s−1, and testing temperatures in the range of 950–1150°C. Examination of the microstructures was carried out by optical microscopy. The flow stress in the torsion test showed a single peak in the flow stress–strain curves and indicated that dynamic recrystallisation took place during hot torsion. The strain to the peak stress increased with decreasing temperature and increasing strain rate. Analysis of flow stress data at the peak gave an activation energy for deformation of 400 kJ mol−1. The relationship between peak stress, temperature, and strain rate can be expressed by the usual potential equation between the stress and the Zener–Hollomon parameter Z=εexp(Q/RT). Flow stress data obtained in the present work compare well with data published for similar superalloys. Fully dynamically recrystallised microstructures are observed at high temperatures and the dynamically recrystallised grain sizes drexare related to the flow stressσ. The actual results of the present work together with those found in the literature can be fitted to a relationship (σ∞drex−0·75). A range of temperature (between 1050 and 1100°C) in which ductility was maximised, and above which ductility decreased, was observed.MST/3357

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.673

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractMicrostructural observations and measurements of the retained austenite content, hardness, austenite carbon content and unreacted austenite content are reported as a function of austempering time at 400, 375, 350, and 300°C after austenitising at 920°C for a ductile iron of composition (wt-%) Fe–3·49C–2·33Si–0·42Mn–0·004P–0·016S–0·25Cu–0·23Mo–0·035Mg. Solute profiles are presented which show solute segregation during solidification and that austenitising for 8 h does not remove the segregation. The segregation of Mo and Mn to intercellular areas is shown to result in stage I austempering reactions in the eutectic cell and intercellular region occurring at different austempering times. Evidence is provided of the occurrence of the stage II austempering reaction before the completion of the stage I reaction. The consequence of this sequence of changes is that the processing window is predicted to be closed for austempering temperatures of 400 and 375°C and open for temperatures of 350 and 300°C.MST/3392

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.679

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractComposites of three different tungsten carbide powders and two matrix alloys of the Co–Cr–C and Ni–B–Si type were cladded on to mild steel using a 6kW CO2laser. The effects of parameter variation, such as volume fraction of carbides within the matrix, beam intensity, and traverse speed on the coatings' morphology were investigated. Coatings with up to 40 vol.-% unmelted carbides, excellent bonding, and without cracks were obtained. This study has confirmed that, in the area of applying composite materials, laser powder cladding is technologically superior to conventional deposition welding using filler wire.MST/3409

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.8.691

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor