摘要:

AbstractThe energy levels of the various physicochemical states of a hydrogen or carbon atom in iron are compared. Both types of atom segregate strongly to grain boundaries in a iron. The embedded atom theory shows that the grain boundary state of hydrogen is very similar to the chemisorbed state, so that when a fast crack passes along the boundary the atom can readily transfer from one state to the other, thus making no contribution to intergranular cohesion. By contrast, the carbon atom cannot gain its full chemisorption energy without making a jump from the grain boundary state, a process that is slow compared with that of fast cracking. As a result, the carbon atom contributes strongly to intergranular cohesion, estimated at about 2 eV per segregated atom.MST/1166

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.121

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractA precipitation–segregation mechanism of high temperature (reversible) temper embrittlement has been suggested on the basis of Auger electron spectroscopy, internal friction measurements, and scanning and transmission electron microscopy (SEM, TEM) of 0·3C–Si–Mn–Cr steel after various heat treatments. It was found that P segregation takes place during heating for quenching and tempering. The intensity of P segregation does not change when the steel transforms from a ductile (600°C tempered for 2 h with water cooling) to a brittle (600°C tempered with furnace cooling) state. Isothermal aging at 500°C for 200 h leads to further segregation of P, but the intensity remains unchanged after a reductilising treatment (600°C reheating for 2 h with water cooling). (The term reductilising is used to describe the treatment employed to attempt to return the material to its original ductile state.) The internal friction measurements and TEM examinations indicated that the temper embrittlement of the steel is always accompanied by precipitation of very fine particles of Fe3C(N) causing dead pinning of dislocations within the grains while the reductilised state corresponds to the re-solution of these fine particles back into the a solid solution which explains the reversibility of the phenomenon. Therefore, the P segregation is a necessary factor for grain boundary fracture of the steel in the temper embrittled state, but is still not sufficient for the actual appearance of this mode of fracture. The leading factor is the precipitation of Fe3C(N) particles on dislocations and a precipitation–segregation mechanism is suggested which can be used to explain all the specific features of high temperature temper embrittlement.MST/707

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.124

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe AuIn–Ge and AuIn2–Ge sections of the Au–Ge–In ternary systems are characterised as pseudobinary eutectic systems having a eutectic temperature of 488°C at 2·0 at.-% Ge and 522°C at 4·1 at.-% Ge, respectively. The AuIn–AuIn2–Ge partial system is considered to be a simple ternary eutectic system having a ternary eutectic temperature of 471°C.MST/940

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.134

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe effect of exposure to HCOOH and CH3COOH vapours on the dielectric behaviour of hot pressed AlN ceramic having different volume percentages of porosity is studied. Dielectric constant and dissipation factor both increase after exposure to acid vapours at humidity levels greater than atmospheric. The results indicate that the porosity plays an important role in the change of dielectric parameters on exposure to acid vapours. The exposure effects are found to be completely reversible as, after annealing, the exposed samples show behaviour similar to that of unexposed samples. The results are explained in terms of ionic conduction due to dissociation of these vapours in the presence of moisture.MST/1137

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.137

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe influence of specimen geometry on the strain rate sensitivity index–strain (m–δ) relationship has been determined for a Zn–5Al (wt-%) eutectic alloy at a strain rateɛof 0·025 min−1and a temperature of 250°C. It was found that all the m–δcurves were of the type mL=mmax>m0and>mFforδ0<δLm0for w0/l0between 0·15 and 0·39. When w0/l0=1(or w0=l0), the notch effect is eliminated and the total elongation at fracture will take its maximum value.MST/1034

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.141

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractIn this paper, the structural and magnetic properties of Co–P alloys produced by electrochemical deposition, using a cathode of Al instead of Cu, are investigated. It is shown that the following structures may be obtained: polycrystalline material with no texture, microcrystalline material with preferential orientation, and amorphous alloy, designated A, B, and C, respectively. Results obtained using saturation magnetisation, coercive field, saturation field, magnetostriction, and X-ray diffraction are in good agreement, which enables specific experimental conditions to be outlined in order to produce samples of structure A, B, or C with significant magnetic properties.MST/898

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.146

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThis paper presents results from studies using transmission and high resolution electron microscopy of as received and warm worked manganese partitioned dual phase steel. It is found that a growing manganese enriched austenite grain shares two distinctly different boundaries with ferrite: (a) a stationary coherent boundary, replacing a stretch of the original ferrite/ferrite boundary, and (b) a migrating non-coherent boundary. This finding gives full support to the previously described mechanism of the nucleation and growth of the manganese enriched austenite. The structure of the warm worked steel consists of recovered ferrite, strengthened by substructure, and fine martensitic lathes, separated by low angle boundaries, which have resulted from the transformation of deformed non-recrystallised austenite grains.MST/1106

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.151

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractLow temperature sensitisation (LTS) refers to sensitisation at temperatures below the typical range of sensitisation (773–1073 K). The effect of prior cold work (5–30% by rolling) on LTS is studied based on the techniques of oxalic acid etching and electrochemical potentiokinetic reactivation. A threshold level of cold work for the effect on LTS is observed.MST/1071

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.157

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe microstructural evolution and resulting mechanical properties of an Al–2·7Li–1·0Cr alloy produced by hot extrusion of inert gas atomised powder is reported. The response of the material to various heat treatments is also investigated. All of the extrudates exhibit a dynamically recovered substructure, but the extrusion temperature is shown to have a significant effect on the nature and distribution of the precipitates present. At the lower temperatures (<350°C) there is evidence ofδprecipitation together with an Al–Cr phase (probably Al7Cr). The higher extrusion temperatures yield greater quantities of the Al–Cr phase in a bimodal distribution, butδis no longer present. In general, the post-extrusion heat treatment involving a high temperature solution soak at 540°C tends to produce a similar microstructure in all the extrudates consisting of the bimodal Al–Cr precipitate distribution. The tensile strength levels achieved in the peak aged condition (500 MN m−20·2%PS and 530 MN m−2UTS) are comparable to the values reported for 7000 series alloys. However, the corresponding ductility (8% elongation) and fracture toughness values (7 MN m−3/2) are somewhat lower.MST/1057

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.161

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractSessile drop spreading tests have been carried out to evaluate the influence of braze and substrate chromium content on the wetting and flow of Ni–Cr–Si–B and Ni–Cr–P brazes on nickel and Ni–Cr substrates. The effect of filler metal chromium additions is examined in terms of fluidity processes, while the role of base metal chromium content is discussed in the context of oxide reduction and alloying phenomena.MST/1056

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.2.170

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor