摘要:

AbstractSpinodal decomposition has been investigated in Cu–30Ni–2·5Cr and Cu–45Ni–10Cr (all wt-%) alloys, which lie at the Cu rich end and at the centre of the miscibility gap, respectively. Specimens were fast quenched from 950°C for the low Cr alloy and 1050°C for the high Cr alloy, and aged in the temperature range 300–800°C. The progress of spinodal decomposition was followed via hardness measurements, X-ray diffraction, and scanning and transmission electron microscopy. Modulation wavelengths were measured from both X-ray diffraction patterns and electron micrographs. It was found that during the initial stages of aging the modulation wavelength remained constant, whereas hardness increased continuously. After a certain aging time, the hardness remained constant at the peak value, and the modulation wavelength increased continuously. The results are consistent with current theories of spinodal decomposition and hardening.MST/1512

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.197

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe hot deformation behaviour of polycrystalline nickel has been characterised in the temperature range 750–1200°C and strain rate range 0·0003–100 s−1using processing maps developed on the basis of the dynamic materials model. The efficiency of power dissipation, given by [2m/(m+1)], where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of dynamic recrystallisation has been identified, with a peak efficiency of 31% occurring at 925°C and 1 s−1. The published results are in agreement with the predictions of the processing map. The variations of efficiency of power dissipation with temperature and strain rate in the dynamic recrystallisation domain are identical to the corresponding variations of hot ductility. The stress–strain curves exhibited a single peak in the dynamic recrystallisation domain, whereas multiple peaks and‘drooping’stress–strain curves were observed at lower and higher strain rates, respectively. The results are explained on the basis of a simple model which considers dynamic recrystallisation in terms of rates of interface formation (nucleation) and migration (growth). It is shown that dynamic recrystallisation in nickel is controlled by the rate of nucleation, which is slower than the rate of migration. The rate of nucleation itself depends on the process of thermal recovery by climb, which in turn depends on self-diffusion.MST/1524

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.206

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractAn experimental investigation of the residual stresses in three types of welded structural steel joint is presented. A total of twelve specimens was tested. In this investigation, the blind hole-drilling method of ASTM E837 was used–adapted and improvedfor determination of residual stresses.To accountfor the effect of local plasticity due to stress concentration near the drilled hole, new calibration coefficients were obtained by carrying out a series of calibration tests. Then, the residual stresses in the welded joints were determined using the new coefficients. The improved hole drilling method presented in this paper was found to be much less time consuming than the conventional sectioning (saw cutting) method for residual stress measurement. The final results obtained in this study showed good agreement with the residual stresses obtained by previous researchers using the sectioning method.MST/1486

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.212

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe metallurgical and mechanical properties of friction welds between titanium and AISI 304L stainless steel were examined. Joint tensile strength increased when high friction pressure (>196 MN m−2) and high upsetting pressure (294 MN m−2) were used during welding. Although the surface roughness of the titanium substrate had no effect on joint strength, decreasing the surface roughness of the AISI 304L material did increase the tensile strength of completed joints. As welded dissimilar joints had poor bend test ductility and failed in the interface region. Detailed microscopy and X-ray diffraction analysis confirmed that the poor bend ductility was caused by a combination of high hardness of the titanium material immediately adjacent to the joint interface, the presence of unrelieved residual strain at the joint interface, and intermetallic phases formed during the welding operation. Detailed transmission electron microscopy and X-ray analysis confirmed that a thin layer rich in intermetallics was present in the as welded joints. (FeNiCr)Ti phases were formed during seizure formation and disruption; this provided the necessary conditions for anomalously high rates of diffusion of titanium in stainless steel, and of iron, chromium, and nickel in titanium. Low temperature post-weld heat treatment (PWHT), involving heating to 500–600°C followed by immediate air cooling, reduced intermetallic precipitation, promoted stress relaxation, and facilitated complete bonding across the whole joint interface. This treatment markedly improved bend ductility and had a negligible effect on joint tensile strength. High PWHT temperatures (≥900°C) and long holding times at temperature markedly reduced joint tensile strength and bend ductility, owing to excessive formation of intermetallic phases at the joint interface.MST/1521

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.219

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

Abstracthree steels having different silicon contents were prepared to study the microstructure and toughness of the thermally simulated heat affected zone (HAZ) in titanium killed steels. For a low silicon addition, the oxygen content in the molten steels decreased remarkably. This in turn caused a change in the inclusion phase from predominantly titanium oxide to titanium nitride (TiN), the change being accompanied by two major microstructural modifications. The austenite grain size became refined and the quantity of intragranularly nucleated acicular ferrite decreased. The microstructural change was found to cause coarsening of Charpy fracture surfaces and deterioration of HAZ toughness of the steels. The minor change of silicon content therefore has a profound influence on the properties of titanium killed steels.MST/1503

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.236

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractAn in situ fracture experiment was carried out in a scanning electron microscope to investigate plastic deformation and strain distribution in the process zone (PZ) located in the immediate vicinity of the crack tip in an Fe–3Si alloy (wt-%) under mixed mode loading conditions. It was observed that plastic deformation occurred by successive activation of a number of slip systems. The strain distribution and shape of the PZ were strongly dependent on the crystallographic orientation of the grain containing the crack tip. The distribution differed from that predicted using near tip blunting calculations and was best expressed by an exponential equation. Additional strain concentrations created by surface defects caused slight perturbations in the overall distribution. Crack propagation started along a coarse slip band which possessed the highest strain. It was found that the maximum strains in the PZ exceeded the uniaxial tensile fracture strain.MST/1404

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.245

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

Abstractypical Fe–Si sheets were cut using various techniques, reducing the material to ribbons having their grains oriented parallel to the longitudinal axis. The magnetic and magnetoelastic properties in this direction reveal differences in the effects produced by conventional mechanical cutting in comparison with laser cutting. In particular, the latter technique results in both the highest saturation magnetisation level and the lowest values of coercive field, hysteresis loss, and saturation magnetostriction. It can be inferred from the results that laser cutting is effective in decreasing both induced stresses and defects on the cut edges.MST/1446

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.252

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

Abstracthe initiation and extension of slip lines and the microscopic yielding behaviour have been investigated in tensile deformation of as cast and prestrained ferrite spheroidal graphite cast irons. Slip lines were observed at a small strain of 0·36% and cracks appeared in the graphite nodules before a strain of 0·71%. This suggests that microscopic yielding had occurred and that the dynamic continuity between the graphite nodules and the matrix was diminished during the initial stages of deformation before macroscopic yielding. The values of Young's modulus of the graphite nodules and the ferrite matrix were estimated to be 300 and 180 GN m−2, respectively. The continuous yielding behaviour is attributed to the graphite nodules acting as initiation sites of plastic deformation.MST/1507

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.257

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractHigh silicon content in nodular cast iron leads to a large amount of retained austenite (γ) during isothermal transformation in the bainitic zone by inhibiting the precipitation of carbides. The presence ofγphase results in high toughness, with an optimum fracture toughness KIcof about 85 MN m−3/2for a 0·2% proof stress of 1000 MN m−2being observed at 30% retained austenite. The presence of martensite after transformation for short times and of coarse carbides after transformation for long times greatly reduces the toughness. For lower bainite having austenite volume fractions less than 30%, optimum fracture toughness is obtained when the fracture is predominantly transgranular ductile. For austenite volume fractions greater than 30% in the upper bainite region,γ⇛α′(martensite) transformation induced plasticity occurs, leading to superior toughness compared with conventional cast irons. In this case the fracture, although predominantly ductile, also contains some intergranular zones due to the transformation of austenite to martensite in the plastic zone ahead of the crack, and the strain hardening exponent for the tensile tests increases with strain, also indicating strain induced transformation of austenite to martensite.MST/1537

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.263

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe behaviour of high modulus carbon fibre reinforced Al–Si composites after thermal exposure at 773 K for various times has been investigated. The composites were fabricated via hybridisation of a small volume fraction of SiC particles using a pressure casting process. The longitudinal tensile strength (LTS) of the composites increased to a maximum of 899 MN m−2when exposed for 216 h, then decreased as the exposure continued. The transverse tensile strength (TTS) of the composites increased monotonically with increasing treatment time, suggesting that interfacial bond strength was improved by the thermal exposure. It was found that fibre pull-out played an important role in the LTS of the composites and that moderate fibre pull-out resulted in higher values of LTS. The results of single fibre tensile tests show that long term thermal exposure at 773 K only slightly degraded the fibres compared with the fibres in the as cast composites. The fibre pull-out behaviour and the microstructural observations of the fibre surfaces gave some evidence that chemical interactions occurred in the fibre/matrix interfaces.MST/1528

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.3.275

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor