摘要:

AbstractThe main problem to be solved before advanced engineering ceramics can become industrial reality is that components having reliable and consistent properties cannot be made, and rejection rates can be as high as 95%, in stark contrast to the situation with metals and polymers. This results from the small size of defect sufficient to cause failure in a ceramic and the variability in the population density of such defects. It is unlikely that sufficiently sensitive non-destructive testing and diagnostic techniques will become available, and so success will depend on the development of intrinsically reliable process technology. The nature and origin of defects in ceramics are discussed and related to raw material characteristics and process technology. The changes that are needed in raw materials and processes are outlined, and the various approaches to intrinsically reliable techniques (such as the colloidal processing of submicrometre powders, sol–gel methods, etc.) are critically assessed in the light of the need for industrially realistic processes. An attempt is made to assess likely future developments such as ceramic fibre reinforced ceramics.MST/409

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.329

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractThe general problems involved in the production of precision optical components are discussed with reference to plastics. The methods available for the manufacture of plastic optical parts are reviewed with particular reference to injection moulding. The design and manufacture of injection moulds are considered, and ways in which dimensional and surface finish requirements may be achieved are given. The criteria for producing optimum quality mouldings are listed, and the role of modern microprocessor controlled machines in producing quality moulded parts is emphasized. Methods for improving surface abrasion resistance and minimizing reflections are described. The optical, mechanical, physical, and processing properties of the plastics most favoured for optical components are given.MST/375

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.337

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractLiquid–solid and solid–state phase equilibria have been studied in the Ni–NiAl–Ni3Ta triangle of the Ni–AI–Ta system, using a combination of several experimental techniques. Five primary phases occur in this region, including the ternary compound,π, Ni6TaAI, which enters into equilibrium with each of the other four. Another compound, Ni8Ta, forms in the solid state by decomposition of Ta–rich Ni solid–solutions and occurs in equilibrium with theγ,π, andδ(Ni3Ta) phases. The extent of these different phase fields has been determined at 1250°C and particular attention has been paid to theγ-γ′solvus surface which has been shown to be accurately described by a second–order polynomial function of the atomic concentrations.MST/321

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.344

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractPrecipitation reactions in a series of Al–Li–Cu–Mg alloys, encompassing the composition ranges of the commercial Lital A and Lital B materials, have been studied to determine the role of solute and vacancy concentrations on homogeneous S-phase (Al2CuMg) precipitation. Homogeneous precipitation of S-phase only occurs when a critical combination of free vacancy and copper and magnesium supersaturation is achieved. Lithium reduces the free-vacancy concentration by strong binding between vacancies and lithium atoms or atom clusters. Precipitation ofδ′(Al3Li) releases the bound vacancies and thereby promotes S-phase formation and, in extreme cases, vacancy condensation, producing dislocation loops.MST/350

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.349

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractThe plastic anisotropy of textured polycrystalline materials has often been described by the r-value–the ratio of the width and thickness strains. The measured–-values of low-carbon steels for larger grain sizes are found to agree reasonably well with values calculated by the authors on the basis of the Taylor theory. In steels with small grain sizes the absolute level of r-value is not predicted particularly well, but the variation with testing direction (planar anisotropy) is predicted well. Calculations showed that in all cases the instantaneous anisotropy parameterρas a function of strain, when extrapolated to zero strain, agreed with the value predicted from the measured texture. With small grain sizes a sudden drop inρwas observed with increasing strain, this being complete within 2–3% of strain. The influence of grain size on the plastic anisotropy can be understood if it is assumed that the grain boundary resistance parameter K in the Hall–Petch relation depends on the contraction ratio–in a different way from the Taylor factor M(q). Measurements of the Hall–Petch constant in plane-strain compression support this view.MST/84

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.354

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractSlip distribution was varied in a series of Al–Mg–Si alloys by changing the amount of manganese-bearing dispersoid present and by under- and overaging, which also altered the grain–boundary precipitate-free zones and hence the grain–boundary strength. Dispersoids are shown to increase ductility by slip homogenization. Slip is more heterogeneous in underaged alloys, but these show greater ductility than overaged alloys because the grain boundaries are stronger. Work-hardening rates increase with dispersoid content, although for a given dispersoid content, the underaged alloys have higher work-hardening rates. This effect is interpreted in terms of the effect of aging upon the properties of the grain-boundary regions.MST/340

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.363

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractThe factors controlling the intergranular fracture of three 1Cr–0·5Mo steels, tested at 550°C, have been examined. Failure results from the nucleation and growth of grain-boundary cavities. It is shown that creep life is dependent on the maximum principal stress, and that variations in the rupture properties of the steels are controlled by their susceptibility to nucleate intergranular cavities. Increasing the metalloid element content and, in particular, increasing the austenitizing temperature from 930 to 1300°C resulted in an increase in the cavity nucleation rate and a concomitant decrease in the rupture life. The cavity nucleation rate was found to be dependent on the maximum principal stress and when this dependence is used in conjunction with a simple cavity diffusion growth model the stress-state dependence of rupture life and the effect of residuals and austenitizing temperature on fracture properties could be predicted. These results are discussed in terms of the material and fabrication factors and service conditions that designers and operators of high-temperature plants must consider so that the plant may be operated safely and efficiently.MST/81

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.368

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractAcoustic emission (AE) has been monitored in test pieces of 7010 aluminium alloy containing growing fatigue cracks. Both high-purity and commercial casts were studied in underaged, peak-caged, and overaged conditions, using tension–tension fatigue with R = 0·1 and 0·5, at crack growth rates in the range 0·1–3μm/cycle. The reduction of grip noise and discrimination of AE events with sources distant from the plane of the crack were achieved by careful design of test piece geometry and clamping, and by the use of two broadband point transducers with calibrated response. Analysis of the load distribution at which AE events occurred at the crack plane showed that most events occurred near peak loading. However, the frequency of detected AE events was sometimes greatly reduced by adding oil to the crack, implying that crack-face rubbing was one source of emission. This was confirmed by fractographic examination and suggests that the practice used by some workers of gating–out all but those signals generated close to peak load in order to eliminate rubbing signals is not necessarily effective. The frequency of events remaining after the removal of rubbing signals was shown to be consistent with the size of the plastic zone at the crack tip, the rate of crack growth, and the inclusion size/density distribution. It is estimated that the fracture of inclusions of area greater than ~40μm2could be detected by the testing system. The commercial cast contained more inclusions of this size than the pure cast, in proportion to the increased AE event frequency.MST/184

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.374

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractThe stress-relief cracking susceptibility of simulated heat affected zone (HAZ) microstructures in 9 wt-%Cr steel alloys has been assessed using a four-point bend test procedure. Five alloys were examined, in which the levels of molybdenum, phosphorus, and sulphur were deliberately varied. Stress–relief cracking was apparent only in HAZ microstructures heated to≥1320°C during simulation. In this condition, alloys containing no molybdenum were most susceptible, exhibiting a low-ductility intergranular mode of fracture during stress-relief heating between 550 and 650–C, and intergranular microvoid coalescence at higher temperatures up to the hold level of 735°C. Phosphorus was found to exacerbate cracking by the low-ductility mechanism, and was observed to segregate to grain boundaries together with a local enrichment of sulphur at crack tips. The presence of molybdenum in 9Cr–1Mo and 9Cr–2Mo alloys greatly reduced susceptibility to stress-relief cracking. The low-ductility fracture mode was completely eliminated in these alloys, despite high levels of phosphorus and sulphur, qnd the tendency to form intergranular cavities was reduced. These effects were associated with the formation ofδ-ferrite at grain boundaries during HAZ simulation, promoted by molybdenum additions. A range of low–alloy steels has previously been assessed using the same experimental procedure, and in comparison, the 9Cr–1 Mo and 9Cr–2Mo alloys were highly resistant to stress–relief cracking.MST/317

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.386

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor

摘要:

AbstractIn this paper the authors describe a new rapidly solidified alloy which is capable of meeting the projected requirements of the aerospace industry. Initial studies using splat quenched particulates were carried out on the Al–Cr–Zr system. Microhardness tests indicated that these alloys age–hardened between 350 and 400°C and showed excellent thermal stability. Further alloy development and studies of fabrication behaviour were carried out using air atomized powder. Powders were consolidated by either conventional or hydrostatic extrusion. Microstructural changes during fabrication were identified and correlated with mechanical property data. The alloy can achieve the requirements of the aerospace industry provided microstructural development is controlled during fabrication.MST/233

ISSN:0267-0836    

DOI:10.1179/mst.1986.2.4.394

出版商:Taylor&Francis    

年代:1986

数据来源: Taylor