摘要:

AbstractNumerical simulations of the stress fields of dislocation networks were carried out. Various types of dislocation configuration including single dislocations, dipoles, and interface dislocations were taken into consideration. The numerical description of dislocation statics with high spatial resolution provided a useful diagnostic means of scrutinising assumptions which are usually incorporated into analytical models of strain hardening and flow stress. It was shown that interface dislocations generated long range stress fields as predicted using analytical models, but contrary to the predictions of many models, their contribution to the total dislocation density was not negligible. The simulations confirmed that an adequate description of local stress fields, and hence of strain hardening and flow stress, can only be obtained if both the local densities of dislocations, and their character and arrangement are taken into account. Since the statics of complicated dislocation arrangements can no longer be treated analytically, because of the long range stress fields of dislocations, numerical methods are considered an important additional tool for the theoretical treatment of flow stress.MST/3256

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.281

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe growth kinetics ofγ′precipitates in Ni based wrought superalloys was investigated, using a computer aided X-ray analysis method. It was found that the growth process obeys the Lifshitz–Slyozov–Wagner law. Regression formulae describing the liquidus, solidus, andγ′solvus temperatures as functions of the overall composition were established. The heat treatment window as a function of composition and its effect on homogenisation and retained hardness were determined. The high retained hardness of homogenised Al + Ti rich alloys hinders their commercial use as Ni base wrought superalloys.MST/3206

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.290

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractMicrostructural characterisation of a cast Al–Si alloy was carried out after laser surface alloying with Ni and Ti. When Ni rather than Ti was used for alloying, more severe segregation of the alloying element in the surface alloyed layer was observed. This segregation was attributed to the higher density of Ni. The phases found in the alloyed layer in both cases could be explained by assuming the ternary systems to be equivalent to the corresponding pseudobinary systems, i.e. Al–Si–Ni and Al–Si–Ti ternary systems being equivalent to (Al, Si)–Ni and (Al, Si)–Ti pseudobinary systems, respectively. Segregation of Ni in the alloyed layer resulted in the formation of high Ni-containing phases and low Ni-containing phases in close proximity, whereas the uniform distribution of Ti yielded a single primary phase in the alloyed layer.MST/3216

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.295

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe surface oxides formed during oxidation of Al alloys for automobile body panels were analysed using electron spectroscopy for chemical analysis, Auger electron spectroscopy, and energy dispersive X-ray spectrometry. Two commercial AA 6000 alloys were studied. The alloys were oxidised in dry air at room temperature, 300°C, 400°C, and 520°C. The thicknesses and the compositions of the oxides formed on the matrix and on the Si, Mg2Si, and Al–Fe–Si particles were determined. The surface oxides consist primarily of Al2O3and MgO. The content of MgO increases with temperature. The activity of Mg is decreased owing to formation of Mg2Si at the metal/oxide interface during oxidation. A model for the formation and growth of MgO nodules at high temperatures is proposed; during the initial stages of oxidation Al2O3is formed, and after prolonged exposure the oxide cracks and Mg evaporates. Magnesium oxide is formed on the surface by condensation and oxidation of Mg. One piece of evidence for this mechanism is that MgO is found on the non-Mg containing Si and Al–Fe–Si particles.MST/3348

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.306

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe modifying effect of strontium on the intermetallic phases found in 6061 alloy was investigated in a commercial product. Qualitative and quantitative approaches were used to assess the microstructure. It was shown that strontium altered the platelikeβ-AlFeSi phase (Al5FeSi) to the Chinese scriptα-AlFeSi compound (Al8Fe2Si). The influence of the strontium on both the T4 and T6 heat treatments was examined. The use of strontium in 6061 homogenisation allowed reductions in the homogenization time of the alloy as it promoted the formation of the more desirableα-AlFeSi phase in the as cast condition and had no negative effect on the mechanical properties of the end products.MST/3176

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.315

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe present paper concerns the microstructure of electron beam (EB) butt welded 2219 aluminium alloy. Crack and porosity free joints were obtained. The EB was also used to carry out in situ post-weld treatments. As a consequence, the microhardness was raised significantly to values very similar to that of the untreated alloy after five pass EB treatments. The contribution to the enhanced microhardness is thought to be related to the early precipitated CuAl2phase and immobile dislocations. The microstructure is discussed in terms of effects induced specifically by EB processing. This investigation shows that the in situ post-weld treatments can be regarded as a useful procedure for improving both microstructure and properties of EB welded joints.MST/3084

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.323

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe effect of different contents of silicon on the transformation kinetics of austenite in a ductile cast iron austempered at 400°C has been investigated. The amount of silicon varied from 1·55–3·48 wt-%. The number of nodules/mm2and the carbon content in the original austenite Cγ°, were similar for all of the chemical compositions studied. The effect of silicon on the rate of formation of bainitic ferrite, the product morphology, and the segregation was studied. The transformation of austenite occurs in two stages: in the first stage a bainitic ferrite and retained austenite microstructure forms; in the second stage austenite decomposes, increasing the volume fraction of ferrite and carbide. It was observed that increasing silicon content greatly increased the time for completion of the first stage; the volume fraction of bainitic ferrite decreased with increasing silicon content, whereas the volume fraction of retained austenite increased slightly. The morphology of bainitic ferrite did not change significantly. The‘processing window’, i.e. the time elapsed between the end of the first stage and the beginning of the second, was closed for all the silicon contents and temperatures studied.MST/3101

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.329

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe influence of thermomechanical treatments on the microstructural stability of a 15 Cr–2·2Mo–15Ni Ti modified austenitic stainless steel was studied. The thermomechanical treatments involved imparting cold work followed by aging in the temperature range 873–1123 K. The cold work was variedfrom 15 to 22·5% at intervals of 2·5%. The influence of cold work level on recrystallisation kinetics was studied by determining the activation energy for recrystallisation. In the temperature range 873–1023 K the stability of the cold worked microstructure was enhanced by the retardation of recovery and recrystallisation by TiC precipitation. The retardation was more effective for 20% cold work than for other cold work levels. At temperatures of 1073 and 1123 K the degree of retardation of recovery and recrystallisation by precipitates was reduced owing to coarsening of the precipitates. Higher activation energy for recrystallisation and hence slower recrystallisation kinetics were observed for 20% cold work compared with other cold work levels. The optimum cold work level resistant to recovery and recrystallisation was identified as 20%.MST/3228

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.338

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe continuous cooling transformation behaviour of a niobium microalloyed steel is studied by combining multipass deformation and continuous cooling torsion testing and by monitoring the variation of flow stress as afunction of temperature during cooling. Deformation above the non-recrystallisation temperature Tnrslightly increases the Ar3temperature and so encourages the austenite to ferrite transformation. Further deformation below Tnrboth enhances the Ar3temperature and promotes the transformation significantly. Decreasing the interpass time and increasing the pass strain below Tnrincreases the Ar3temperature. A method is proposed to estimate the stored deformation energy from pass to pass as the deformation proceeds below Tnrbased on the course of the mean flow stress curve and on the concept of shear deformation energy. The effect of the process variables, mainly the interpass time below Tnron the stored deformation energy and consequently on the Ar3temperature is discussed.MST/3198

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.345

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractAn evaluation model is proposed for the strain and strain rate of a localised deformation band produced by an orthogonal cutting test upon a high strength low alloy steel tempered at 600°C after quenching. It can be used to determine the strain and strain rate at the first deformed zone (FDZ) in the chip and also at the second deformed zone (SDZ) at the bottom of the chip. These zones were severely deformed at high strain rates up to 4·6×104−4·2×106s−1, with strains as high as 1190–2760%. X-ray diffraction showed that the SDZ consisted of ferrite and retained austenite which was not detected in the matrix. Transmission electron microscopy and X-ray diffraction showed that cell structures in the SDZ and FDZ were ofnanocrystalline‘grains’with a high dislocation density at the‘grain’boundaries. The X-ray diffraction analysis served to check the validity of the strain evaluation model.MST/3105

ISSN:0267-0836    

DOI:10.1179/mst.1996.12.4.351

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor