摘要:

AbstractExtension of the logarithmic law of mixtures enables equations to be formulated which describe the dielectric behaviour of particulate polymeric composites containing conductive particles. These equations give the permittivity and dielectric loss of the composite as a function of the permittivity of the polymeric matrix, the volume fraction and aspect ratio of the inclusions, and the frequency of the applied field. The proposed equations were tested with experimental data obtained over a wide range of frequencies and temperatures from composites consisting of epoxy resin and aluminium powder. Satisfactory agreement was observed when the volume fraction of the inclusions was small, but at higher values discrepancies appeared which are attributed to the intrinsic weakness of the logarithmic law of mixtures, on which the proposed equations are based.MST/3167

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.533

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe information concerning solid fraction with respect to temperature in the mushy range is very important to solidification models which employ the effective specific heat method. The computer aided cooling curve analysis (CA–CCA) method is used in this study to measure the relationships between solid fraction (fs) and temperature/time for aluminium alloys of different composition for various cooling rates. The present study examined near pure aluminium, A356.2, A390, and A413 (near eutectic) alloys. The results of the measurements and analyses show that a rather large temperature range is observed near the end of solidification for both near pure aluminium and A413 alloys. This mushy range becomes longer as cooling rate increases. For the A356.2 and A390 alloys, the solidus temperature, liquidus temperature, eutectic temperature, and maximum solid fraction at the eutectic temperature decrease as cooling rate increases. This is not true, however, for cooling rates higher than 9·5 K S−1. It is also known that afunctional relationship of fswith temperature is very convenient when it is applied to a solidification model. Two non-linearity factors neand npare required to define the function; neand npare found to increase as cooling rate increases. The relationship between neand npand cooling rate can also be obtained. A reasonable estimation of the solid fraction datafor the cooling rate not measured can then be made.MST/3262

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.539

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe effect of flow localisation and specimen thickness on the superplastic deformation of the commercial alloy Suprall 100 (Al–6Cu) is discussed. Optimal flow was accompanied by the formation and spread of intense‘shear bands’along grain/interphase boundaries. The spacing between the shear bands depended on both the strain rate of deformation and the specimen thickness. Superplasticity drastically decreased when the specimen thickness fell below about 10 grain diameters. The apparent activation volume was afunction of strain rate but was independent of specimen thickness, while the strain rate sensitivity index depended on both the strain rate and specimen thickness. The results are explained in terms of the theory that as well as being influenced by specimen geometry, the degree of superplasticity depends on the extent to which cooperative boundary sliding and boundary shear band formation are present during flow.MST/3076

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.545

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractStudies of minor phase evolution in a 12CrMoVNb martensitic steel, creep tested for durations of up to 100000 h at 550°C and 600°C, have revealed the presence of a hitherto unknown complex nitride precipitate having a tetragonal unit cell similar to that of Z phase, previously only reported in certain niobium bearing austenitic steels. This paper describes the electron diffraction and X-ray microanalysis studies leading to the discovery of this phase, and the conclusion that it is a new form of Z phase enriched in vanadium. A modified crystallographic model is proposed for this new complex nitride phase, as are the precipitation sequences leading to its formation.MST/3575

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.552

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractIn this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions was investigated by generating and evaluating the short crack data under different loading conditions for the six principal specimen orientations. Short cracks showed a strong anisotropic behaviour in which they grew muchfaster in the transverse and short transverse orientations than in the longitudinal orientations in all fatigue processes. This was due to the relatively high content of the elongated inclusions. Inclusion induced anisotropy is also observed in the percentages of fatigue life of the initiation and growth periods of short cracks which may result in inaccurate fatigue life predictions. Inclusion induced anisotropy was the result of strain intensification at the matrix/inclusion interface.MST/3250A

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.557

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractLow cycle fatigue (LCF) behaviour as a function of test direction was studied for a quaternary Al–Li–Cu–Mg alloy. The analysis of the variation in fatigue life with plastic strain amplitude∆ɛp/2 or with average stress amplitude∆σ/2 or with plastic strain energy per cycle∆Wp, revealed bilinear power law relationships in all the test directions. The transition strain values in the Coffin–Manson plots were seen to match closely with those obtained in the cyclic stress response as well as with the cyclic stress–strain relationships of the alloy. The observed bilinear behaviour in these LCF properties was attributable to a change in deformation as well as to the deformation assisted fracture mode. The alloy revealed significant in plane anisotropy in the LCF properties. The observed anisotropy was found to result from the combined effects of strong crystallographic texture and grain fibering.MST/3016

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.563

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractAt high temperatures, materials display rate dependent plasticity, which in global terms is controlled by the glide and climb of dislocations. Classical descriptions of the flow behaviour under hot working conditions generally take no account of the internal microstructure, leading to apparent rather than actual values in the constants considered. An expression which takes into account the initial microstructure is proposed in the present work. In order to study the flow behaviour of a commercial medium carbon steel containing vanadium, titanium, and aluminium, uniaxial hot compression tests were carried out. True strain rates ranged from 10−4to 10 s−1and testing temperatures varied from 1150 to 850°C. Various austenitising conditions were employed in order to produce different initial microstructures. The results obtained were then correlated with the initial austenite grain size.MST/3265

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.579

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractRoom temperature fracture toughness and tensile tests were carried out on metal matrix composites based on the aluminium alloy 7075, and also on monolithic 7075. The particulate reinforcements used were SiC in three nominal sizes: 5, 13, and 60μm. Three aging conditions were studied: peak aged, underaged, and overaged conditions of equivalent matrix micro hardness values.The addition of 5 and 13μm particles increased the 0·2% proof stress and tensile strength, and reduced the ductility, compared with the monolithic material. Composites containing 60μm particles had lower 0·2% proof stress and tensile strength, and very low tensile ductility. In all cases the toughness of the composites was lower than that of the unreinforced material. However, in contrast to the tensile ductility, the material containing 60μm particles was the toughest of the composites. The failure mechanism is believed to be one of particle fracture and/or decohesion at low applied stress intensities, followed by ductile failure of the matrix material. A model to predict toughness from tensile ductility and nominal interparticle spacing is proposed which is consistent with the observed experimental results.MST/3217

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.586

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractMetal matrix composite layer formation by means of laser alloying using 6μm particle size SiC powder (SiCp) preplaced on titanium surfaces in a nitrogen environment produced golden coloured tracks and a complete solution of SiCpin the melt zones under a range of processing conditions. The melt layers consisted of dendrites at the top (titanium nitride based) followed by threadlike particle structures (titanium silicides), and the sizes of dendrites and the threadlike particles werefound to increase with increasing laser power density. The surface layer of the dendrites developed a hardness 4·5–9 times that of the base metal (150 HV), and the deep underlying threadlike structures had a plateau of hardness of aboout 2·8–4 times the base hardness. The metal matrix composite layers were found to be 2–4 times thicker than those produced previously in a helium environment under similar processing conditions. The exothermic reactions due to the formation of titanium nitride, titanium carbide, or titanium carbonitride along with titanium silicide during laser melting of SiCpcoated titanium surfaces under a nitrogen environment are considered to be responsible for the greater melt depth and complete dissolution of ceramic particles, by increasing the temperature of the melt.MST/3208

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.595

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractTransient liquid phase (TLP) bonding can be used for joining advanced materials containing secondary strengthening phases, with the advantage of producing isothermally solidified joints, free from precipitates and defects. However, this study shows that when TLP bonding an oxide dispersion strengthened alloy MA957, initial melting and homogenisation of the liquidfiller metal results in parent metal dissolution and the loss of any dispersion strengthening phases from the joint region. A theoretical study shows that the extent of dissolution is determined by the thickness of the inter layer used, and it is suggested that thinner interlayers in the form of sputter coatings will reduce dissolution significantly. Experimental results show that 2μm thick sputter coatings based on the Fe-B-Si ternary system can be deposited by rf sputtering techniques. The TLP bonds produced are free from intermetallic precipitates and pores and show no agglomeration of the dispersion strengthened phase, and this is attributed to the lower volume of boron and silicon deposited in the coatings.MST/3454

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.7.603

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor