摘要:

AbstractIntercept length distribution curves were calculated or determined graphically for a variety of polygons and shapes as an initial step in ascertaining the nature of intercept distributions for statistical grain sections, an intermediate goal being to provide a method of calculating section diameters from the more easily measured intercept distributions determined by lineal analysis, but with the ultimate aim of determining the spatial distribution. The following shapes were selected: an ellipse, an equilateral and an irregular triangle, a square, a 60°rhombus, a rectangle, a pentagon, a hexagon, an irregular seven sided polygon, and an irregular five sided polygon having convex sides. If the intercept length is designated as q and the distribution function as f(q), it is concluded that f(q) of a statistical grain section would have the following characteristics: f(q) finite at q=0, df(q)/dq≠0 at low values of q, f(q) a maximum at q

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.585

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractAn attempt is made to model the transition from upper to lower bainite in steels, based on the hypothesis that bainitic ferrite grows with a supersaturation of carbon in solid solution. The theory involves a comparison between the time required to reject the excess carbon into the residual austenite by diffusion and the time required to obtain a detectable degree of cementite precipitation in the bainitic ferrite. If the precipitation process is relatively rapid, then it is assumed that lower bainite is obtained. The results are found to be in broad agreement with published experimental data.MST/1095

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.592

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe effect of prior austenite grain size on the crack propagation behaviour of tempered martensitic steels having tensile strength of about 2 GN m−2was studied in hydrogen gas at pressures in the range from 98 to 784 kPa using modified compact tension specimens. The crack propagation rate da/dt in hydrogen decreased as the prior austenite grain size increased from 45 to 450μm. The dependence of da/dt on hydrogen pressure at a given applied stress intensity was examined. The permeation of hydrogen from the crack tip surface was estimated to decrease with increasing grain size. However, the fractographic study suggested that the degree of embrittlement of grain boundaries increases with grain size. Consequently, the inverse effect of grain size on da/dt may be caused by a decrease of the average concentration of hydrogen along grain boundaries at the crack tip with increasing grain size.MST/1060

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.604

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe effects of extrusion processing parameters on the mechanical properties of an AA 8090 alloy were monitored using a combination of hardness, tensile, andfracture toughness tests, and using light, transmission electron microscopy, and scanning electron microscopy. It was found that variations in the processing parameters affect the tensile properties to a greater extent in the as extruded condition than in the heat treated condition. In the former, the property changes occur as a result of both variation of grain structure and the solutionising effect during the process. In the latter, the tensile properties are controlled by the precipitation processes that occur, and the toughness remains essentially unaffected by changes in the processing conditions. Improved combinations of strength, ductility, and toughness are achieved when the material is subjected to suitable preaging treatments, which modify the precipitate morphology within the microstructure; the fracture surface characteristics of both tensile and fracture toughness test specimens reflect the microstructural changes.MST/1115

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.611

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThis paper presents the results of a study of the effect of interface microstructure and related mechanical properties on the overall mechanical behaviour of an Al alloy metal matrix composite (A356) containing 10 vol.-%SiC particles. Two different interface microstructures were obtained by using as received or oxidised SiC particles. For the oxidised particle composites, TEM observations show a spinel phase (MgAl2O4) formation at the Al/SiC interface, whereas for as received particle composites very few chemical reactions are observed. The particle oxidation reduces the mechanical strength (0·2% proof stress (σ0·2 and ultimate tensile strength (UIS)) and increases ductilityɛfconsiderably. In addition, it is evident from tensile tests carried out in an SEM that these two different types of overall behaviour are a result of different failure processes: particle rupture in the as received particle composites and interface decohesions in the oxidised particle composites.MST/1105

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.621

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractExtruded rods have been produced from gas atomised powders of pure magnesium and a magnesium alloy containing 2 wt-%Si. It has been found that the microstructures of the powders vary with the powder size. The static mechanical properties at ambient temperature show significant improvement compared with their ingot equivalent. The tensile properties of both materials increase with increase in the extrusion temperature, even though the refinement in the microstructure in terms of average grain size is more pronounced at lower temperatures. This is shown to result from the retention of porosity in the low temperature extrudates. The nature of pore formation has been explained on the basis of the parent powder characteristics and the extrusion parameters. The mechanical properties of the alloys produced are stable after exposure to high temperature for short periods but the high temperature mechanical properties do not show any significant improvement. The extrudates have a recrystallised grain structure and develop a texture equivalent to that of conventionally extruded materials.MST/1083

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.629

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractDiffusion welding of titanium cylinders to an alumina cylinder (Ti/Al2O3/Ti) was carried out at temperatures above theα–βtransition temperature of titanium. Titanium and alumina were welded under the following conditions: maximum heating temperature greater than 1400 K, holding time of 30 s, heating (cooling) rate of 1–10 K s−1, welding pressure of 1·6 MPa, and an air atmosphere. The shear strength of Ti/Al2O3/Ti welds was very low because of the residual stress at the welded interface. Alumina cylinders were welded using titanium foil as an insert (AI2O3/Ti foil/Al2O3). The shear strength of Al2O3/Ti foil/Al2O3welds increased slightly with the decrease of foil thickness, and attained a maximum value of 82 MN m−2. When the foil thickness was less than 10μm, the welds failed at the welded interface, but the welds with foil of 100μm thickness failed in alumina close to the interface.MST/1114

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.641

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe Pd containing brazing alloys were invented as filler materials that (i) had good spreading and gap filling properties, (ii) did not badly erode the high temperature metals for which their use was intended, (iii) were ductile, and (iv) did not contain volatiles, which made their use in radio valves attractive particularly for glass–metal seals, where the Ag–Cu eutectic was a favoured alternative, but could lead to cracking of parent metal during the brazing cycle unless preplated with Ni. The work reported here traced the mechanism of attack of Fe–Ni alloy by Ag–Cu eutectic with three modifications containing nominally 5, 10, and 25 wt-%Pd. A static beam loading technique was used in conjunction with scanning electron and optical microscopy, electron probe microanalysis, X-ray (XPS), and Auger (AES) spectroscopy. The alloys containing 5%Pd or less were hot short, but were ductile when cold bend tested; those containing 10 and 25%Pd were ductile at all temperatures tested. Ductility was associated with a dark phase which concentrated and spread as the Pd content was increased. It appeared that liquid metal embrittlement was responsible for cracking found in the lower Pd content alloys. Analyses using AES and XPS showed that Cu was migrating into the Fe–Ni alloy leading to alloying with Ni and weakness at grain boundaries for the Pd free braze. This was accompanied by the presence of Fe and Ni in the braze, with Ni being predominant. As Pd was increased progressively, the ratio of Fe/Ni changed, with Fe becoming predominant. Metallography showed that the Ag–Cu eutectic braze caused pitting and tunnelling at grain boundaries typical of stress corrosion. Little evidence of this was found in alloys with 5%Pd and it was concluded that general corrosion was increased progressively as a result of an increasing take up of Fe by the Pd, hence removing the susceptibility to intergranular cracking.MST/1117

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.645

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe resin transfer moulding process has been identified as possessing good potential for the manufacture of components at low to medium production rates, but its application to high volume manufacture has yet to be demonstrated. The establishment of minimum cycle times is prerequisite to this aim; hence, a fundamental study has been carried out to establish the relative importance of several process variables regarding individual process times and the overall cycle time. This has been based on the use of simple modelling techniques in combination with a series of experiments using an instrumented plaque mould. The results of the two methods are compared and the relevance of each variable to process design for high volume manufacture is discussed.MST/1251

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.656

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractMoulded microporous samples composed of calcium silicates, xonotlite, and tobermorite, wereformed by a one-step hydrothermal synthesis reaction. The properties obtained are: 0·131–0·191 kg m−3density, 0·51–0·90 MN m−2compressive strength, 0·25–0·40 MN m−2flexural strength, 0·0462 Wm−1K−1(24°C) thermal conductivity, and 1·93% linear shrinkage under firing temperature 800°C for 24 h. The process developed was made possible by the use of accelerating admixtures, which include magnesium acetate and gypsum.MST/1133

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.7.666

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor