摘要:

AbstractPolycrystalline diamond films were deposited on to Si3N4coated silicon substrates using thermal chemical vapour deposition. The defects on the amorphous Si3N4layer played an essential role in diamond nucleation. After deposition, very few diamond crystallites were found on the untreated Si3N4coating, while a diamond film had formed on the ultrasonic diamond powder treated Si3N4coating. During the deposition process, the substrate temperature drastically affected the integrity of the intermediate Si3N4layer. At a substrate temperature above 850°C, the Si3N4layer began to degrade. The lower CH4concentration produced better diamond films in terms of crystallinity and purity. The adhesion of the diamond film to the Si3N4coated silicon substrate and the friction coefficient between the diamond stylus and the diamond film decreased as the CH4concentration increased. The corrosion resistance of the diamond film on the Si3N4coated silicon substrate was much stronger than that of the Si3N4film on the silicon substrate.MST/3177

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.1

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractSpecimens of the Bi–Cd–Pb ternary eutectic were solidified unidirectionally at very low speeds and quenched to form a representative solid/liquid interface for subsequent study. This eutectic was found to be of the Bi(faceted)–Cd(faceted)–Pb2Bi (non1aceted) type. A‘double binary’microstructure was formed on freezing, consisting of binary quasiregular regions composed of Bi and Pb2Bi and binary regions of broken lamellar Cd–Pb2Bi microstructure. Comparison with systems studied previously suggests that a faceted–faceted–non-faceted ternary eutectic of primarily lamellar form will tend to assume a double binary microstructure on freezing, the non-faceted phase will be common to the two binary structures, and each binary structure will take the form corresponding to that expected for a binary eutectic of similar volume fraction of faceting phase.MST/3146

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.7

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe microstructure and solidification behaviour of an Al–Mg alloy with different Mn additions have been investigated using optical microscopy and an electron microprobe. The results showed that small additions of Mn had little influence on the solidification behaviour. The microstructure consisted of primary (α-Al dendrites, the binary eutectic compound Al8Mg5, and the insoluble constituents Mg2Si and Al6(Mn,Fe). In the alloy with an intermediate level of Mn, after the eutectic reaction L→α-Al+Al6(Mn,Fe), the balance of Mn atoms further segregated to the finally solidified zone and formed platelike Al6(Mn,Fe). In the alloys with the highest level of Mn, the primary phase became Al6(Mn,Fe) with a block or‘Chinese script’structure. However, the finally solidified zones could be regarded as the binary Al–Mg system, and solidification terminated with the eutectic reaction L→α-Al+Al8Mg5. In addition, the cooling rate had a remarkable influence on the solidification behaviour and microstructure.MST/2016

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.12

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe annealing of supersaturated solid solutions has been studied in a number of rapidly solidified AI-transition metal alloys using transmission electron microscopy. Two very different decomposition processes have been found. The first involves the generation and migration of dislocation networks which provide short circuit diffusion paths to the grain boundaries for the solute atoms, where they precipitate as the equilibrium phase. Typical examples are Al–Mo and Al–V The second mechanism involves the precipitation of metastable quasicrystals in the grains before the equilibrium phase is formed, also within the grains. This process has been observed in Al–Cr and Al–Fe alloys. The different mechanisms are discussed in terms of the ability to generate dislocations and the stability of quasicrystal precipitates in Al–transition metal alloys. Stable supersaturated solid solutions containing quasicrystals exhibit strong strengthening behaviour. The significance of these observations is discussed in terms of the development of advanced Al alloys for high performance applications.MST/3356

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.19

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe effects of superheat and powder particle size fraction on the characteristics of Al–5Cr–2Zr (wt-%) alloy confined nozzle atomised powder particulate has been investigated. The median powder particle size decreased from 62 to 38μm with increase in superheat from 140 to 300 K for the atomising conditions studied, with a broadening of the size distribution and the increasing presence of large irregular powder particles at the lowest superheats. Essentially featurelessα-Al based microstructures in splat caps and small sized powder particles were replaced by cellularα-Al with increasing amounts and sizes of primary L12Al3Zr and Al13Cr2inclusions with increasing powder particle size. Refinement of theα-Al cell and intermetallic inclusion sizes in a fine compared with a coarse powder size fraction was consistent with the expected powder relationship between these variables. The decreased microhardness for coarser powder particle size fractions was attributed to the formation of increasingly coarse distributions of the primary aluminides at the expense of retention of Zr and Cr in extended solid solution. A notable age hardening response after 1h at 350–450°C for the 25–50μm powder size fraction was consistent, however, with retention of 0·5–1·0 wt-%Zr in solid solution in this fine size fraction of the powder.MST/3163

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.25

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe alloys Mg–6Si and Mg–4Si–4Zn (wt-%) were extruded at 573 K and the mechanical properties investigated by tensile testing at strain rates from 2×10−5to 2×10−2s−1and at temperatures in the range 673–773 K. The Mg–Si alloy behaved as M type at any given strain rate. The Mg–Si–Zn alloy behaved as A type at strain rates above 2×10−4s−1. The activation energy for the deformation process was higher than those for lattice self diffusion and for the chemical interdiffusion of Zn in Mg. This is probably related to the cross-slip of dislocations. For the Mg–Si–Zn alloy, superplastic behaviour was found at strain rates below 2×10−4s−1. The dominant superplastic deformation process is likely to be grain boundary diffusion controlled grain boundary sliding.MST/3192

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.35

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractSteels used in the construction of modern power plant contain relatively large concentrations of alloying elements. The martensite start temperature of such steels is important during fabrication, and can not be estimated using conventional empirical methods. It is demonstrated in the present paper that a recent theory due to Ghosh and Olson can be applied to predict both published and new experimental data on the martensite transformation temperatures of power plant steels. However, the method should not be used in circumstances where the steels containδferrite or undissolved carbides before transformation.MST/3252

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.40

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe microstructures obtained by diffusion bonding a low alloy steel AISI 4130 to nickel alloys (Inconel 625 and Hastelloy G3), using different techniques (hot uniaxial pressing, hot plane strain compression, hot isostatic pressing (hipping), and coextrusion) have been studied by light microscopy, scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). The interdiffusion of various elements through the interface has been determined using microanalysis in both the SEM and the STEM. In all instances profuse precipitation of M23C6carbides, resulting from the diffusion of carbon from the steel into the superalloy, is clearly visible in a region close to the interface on the superalloy side. For specimens processed by hot pressing and coextrusion, this region shows a work hardened substructure composed of cells and dislocations. By contrast, in specimens processed by hipping a poorly developed substructure, with few dislocations associated with grain boundaries and carbides, is observed. The thickness of the precipitation region increases with increasing temperature and bonding time. Carbides precipitate preferentially at the grain and twin boundaries, although for the higher temperatures used in hipping, the precipitation inside the grains is also important. On the steel side, a precipitation free austenite band is observed in all specimens analysed. This band is the result of the diffusion of nickel and chromium from the superalloy to the steel, thus stabilising austenite at room temperature. From the experimental concentration profiles of different elements, the corresponding diffusion coefficients have been calculated using diffusion theory, and compared with data reported in the literature. Good agreement between these data and the experimental values can be observed.MST/3200

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.45

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe acoustic emission behaviour during stage II Paris fatigue crack growth in parent and weld specimens of AISI type 316 stainless steel has been reported. Substages IIa and IIb, occurring during stage II fatigue crack growth, could be distinguished by a change in the acoustic emission activity, corresponding to a sharp change in the crack growth rate. The transition point in the plot of acoustic emission cumulative ring down count v. number of cycles agrees well with that of the crack growth rateνnumber of cycles. The transition from stage IIa to IIb is found to occur at a crack growth rate da/dn=3×10−7m cycle−1, which is in reasonable agreement with the values reported in the literature. The high acoustic emission activity during stage IIa is attributed to the extensive cyclic plastic deformation and the increasing size of the cyclic plastic zone with∆K (the cyclic stress intensity factor) under the plane strain conditions prevailing during stage IIa. The low acoustic emission activity during stage IIb is attributed to the reduction in the mean free path for dislocation movement and the decrease in the size of the cyclic plastic zone under the plane stress condition prevailing during stage IIb. The higher acoustic emission activity in weld specimens compared with the parent metal is attributed to the combined influence of cyclic plasticity, residual stress induced microcracking, and roughness induced crack closure phenomena.MST/3201

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.56

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractA method for determining the surface yield point and surface work hardening exponent by means of X-ray stress analysis and strain gauge techniques is proposed in the present paper. The reliability of the method is discussed. The test results of the annealed medium carbon high strength steel 40Si–Mn–Cr–Mo–Ni–V show that the proof stress corresponding to 0·1% permanent effective strain of the electrolytically polished surface is about 100 MN m−2less than the bulk yield point of the material, and the work hardening induced by shot peening raises the 0·05 and 0·1% surface proof stresses by about 400 MN m−2. The proof stresses of the shot peened surface under both bending–tension and bending–compression loading even exceed the ultimate tensile strength of the material.MST/3114

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.1.59

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor