摘要:

AbstractSilicon nitride powders of highαcontent were prepared from mixtures of very fine amorphous silica and carbon powders in flowing nitrogen at 1400–1500°C. The higher the reaction temperature within this range the lower the yield of silicon nitride in spite of a higher rate of SiO generation. The overall reaction rate increased with increasing nitrogen flowrate. An increase of carbon content of the starting mixtures increased the reaction rate by increasing the rate of SiO formation as well as the rate of Si3N4nucleation. It is thought that the effect of the carbon particle size on the reaction rate is more significant than that of the specific surface area. The amount of silicon carbide formed and theα/βratio in the reaction products were strongly affected by a change of the source of carbon used in the starting mixtures.MST/1346a

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.289

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractTwo low alloy structural steels having carbon contents of 0·2 and 0·4 wt-% and similar levels of Cr, Mo, and Ni have been studied to determine the effect of martensite morphology on mechanical properties of steels having a mixed structure of martensite and lower bainite. The 0·2% and 0·4%C steels were isothermally transformed at 693 and 593 K, respectively. In the 0·2%C steel, the lower bainite was acicular inform and partitioned the prior austenite. The martensite formed on quenching was of entirely lath morphology. The transformation products of the 0·4%C steel were similar, but a small amount of twinned martensite was also present. After tempering at 473 K, the values of strength, fracture ductility, and true notch tensile strength of the 0·4%C steel were considerably superior to those of the 0·2%C steel. This difference resultsfrom plastic restraining of the lower bainite by the stronger martensite of the 0·4%C steel during plastic deformation. The details are discussed in terms of the modified law of mixtures, fracture profiles, and true stress–strain analysis.MST/1261

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.299

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effect of surface decarburisation upon the fatigue strength in rotating bending of a through hardened low alloy steel and a carburised carburising steel was investigated. The relationships between surface carbon content, depth of decarburisation, microhardness, residual stress, surface finish, and fatigue limit were examined. Whereas the fatigue limit was found to be independent of the depth of decarburisation over the range investigated (up to 1 mm), linear relationships were found between fatigue limit and surface carbon concentration and micro hardness. Furthermore, the presence of residual tensile stresses at the surface were found to lead to a reduction of fatigue limit. The fatigue limit of the rough machined specimens was found not to be significantly lower than that of the polished specimens.MST/1179

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.307

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effect offerrite–pearlite content on the cleavage fracture behaviour of carbon steels was determined via plain tensile tests at temperatures from 12 to−196°C. It was found that, under conditions of uniaxial tension within the test temperature range, the cleavage fracture stress for pearlitic eutectoid steel is independent of temperature, whereas that for mild steel changes with temperature and exhibits a depression at low temperatures. An increase in the proportion of pearlite enhances the effect of pearlite colonies as obstacles to cleavage crack propagation. Only when the volume fraction of pearlite is comparable to that of ferrite does the steel begin to display the fracture features of pearlitic steel. Possible mechanisms manifesting different fracture characteristics for steels of various ferrite–pearlite contents are discussed with the aid of fractographic observations.MST/1075

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.311

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractSamples of a 9Cr–1Mo and a 2·25Cr–1Mo steel (both wt-%) have been heat treated to produce microstructures that are likely to form in the heat affected zone of a weld. Two treatments were employed for this purpose, involving quenching (i) from an artificially high austenitising temperature and (ii) from an abnormally low temperature, within the intercritical region. Some of the samples in each condition were given aging treatments at 550°C, which were designed to simulate exposure to typical in-service temperatures. The microstructures produced by each heat treatment schedule were characterised using transmission electron microscopy (TEM). The effects of each treatment on the mechanical properties of the steels were assessed using ambient temperature tensile testing and Charpy tests at various temperatures. The results were compared with those obtained from samples for which standard austenitising temperatures were employed. The higher austenitising temperature was found to have very little effect on the mechanical properties of either steel. The lower temperature treatment affected the properties of both steels–its most significant effect being to increase the ductile–brittle transition temperature (DBTT) of the 2·25Cr–1Mo steel and reduce that of the 9Cr–1Mo steel, such that the DBTT of each steel converged to a similar value. It also resulted in increases in the ductility and upper shelf energy (USE) of the 9Cr–1Mo steel, but had little effect on these properties in the 2·25Cr–1Mo steel. The difference in behaviour of each steel, including the different response to aging in each condition, is explained in terms of the microstructural changes caused by the different heat treatments.MST/1232b

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.315

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe influence of the cooling rate variation induced microstructural changes, present throughout the cross-section of commercial scale direct chill cast rolling ingots, on the substructural and thus flow stress characteristics during subsequent hot deformation has been investigated. Ingots of an Al–0·3%Fe alloy were cast by direct chill (DC) and sand casting processes and their behaviour compared with commercially produced DC cast ingots. The hot working behaviour of these ingots, of different initial as cast structures, was studied using elevated temperature tensile testing to establish the flow stress dependence on subgrain size and to assess the effect of temperature and initial as cast structure on hot ductility. The room temperature mechanical properties of hot rolled and subsequently low temperature annealed Al–Fe alloys (Al–0·5%Fe and commercially produced ETIAL O) having different as cast structures were also determined. Microstructures were characterised using optical and electron microscopy and hardness testing was performed to assist in explaining the tensile data. Significant variation in mechanical properties, flow characteristics, and hot ductility was found for samples machined from as cast and hot worked slabs; these samples had retained the wide range of particle distribution parameters which reflect the dendrite arm spacing of the as cast materials and are essentially governed by the conditions prevailing during solidification.MST/1266

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.321

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA bearing alloy consisting of Al–18·6Pb–8·8Si–1·4In has been produced by isostatically compacting mixed atomised alloy powders of Al–11Si and Pb–7In (all wt-%), followed by cold extrusion and rolling, with intermediate annealing treatments. The alloy was subjected to hardness tests and tensile tests and its tribological properties were evaluated using a miniature pin-on-disc machine. The properties of the alloy were found to compare favourably with those of other aluminium based bearing alloys. The alloy was roll bonded to a steel backer to which an intermediate layer of aluminium had been roll bonded. It is suggested that the alloy warrants further evaluation with a view to its possible commercial development.MST/592

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.330

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the<45μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al13Cr2intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellularαaluminium, (iii)αaluminium plus fine spherical precipitates rich in chromium and manganese, and (iv)αaluminium plus Al13Cr2primary intermetallic particles. The solidification of these powders is discussed in terms of solidification front velocity controlled by external heat flow and by the initial undercooling. Particles less than 10μm in diameter undercool significantly before solidification. Segregation free microstructures occur in the fine<1μm) particles, where the solidification front velocity exceeds the absolute stability velocity.MST/1247a

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.334

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe influence of structural changes occurring during the total processing cycle of AA 2014 alloy has been investigated and their contribution to the final extrudate structure and properties assessed. The importance of an adequate homogenisation treatment is demonstrated and it is also shown that, with the correct processing conditions, acceptable properties may be produced in both the T5 and T6 tempers. The influences of processing and of aging on the fracture properties are investigated in detail. It is shown that the homogenisation and aging sequences are critical for the production of damage tolerant material that is also sensitive to the extrusion parameters.MST/1231

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.341

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractAn investigation has been made of the laser surface alloying of titanium using a continuous feed of aluminium powder. By means of a continuous wave CO2laser operating at 1·8 kW power, with a beam diameter of 3 mm, ranges of traverse speed (from 3 to 20 mm s−1) and aluminium powder feed rate (from 0·03 to 0·11 g s−1) were used to produce a series of alloyed zones with aluminium contents in the range ~20–80 at.-%. Conditions for obtaining reasonable homogeneity and reproducibility of composition were determined.MST/1353

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.4.353

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor