摘要:

AbstractInvestigations ofωphases formed by isothermal aging in Ti–20 wt-%Mo alloy and by deforming in tension in Ti–14 wt-% Mo alloy were carried out using transmission electron microscopy (TEM). Fourωphase variants in the same region of Ti–20Mo alloy single crystal aged at 623 K for 28·8 ks were observed using dark field TEM imaging. The microstructure of theωparticles was observed using high resolution electron microscopy (HREM). Dark field images were taken by using the diffraction spot due to respectiveωphase variants and with the incident beams parallel to [l01], [313], and [311]. It was found that allωparticles grew in the〈111〉direction and were ellipsoids. The growth direction was the same as the direction of motion of atoms for formingωparticles according to a model proposed by previous authors. This result was confirmed by HREM images of theωparticles observed using dark field TEM. The volume fractions of respectiveωphase variants were estimated using a conventional method. Theωparticles were also observed, using dark field imaging, in the plastically deformed Ti–14Mo alloy. Although fineωparticles were observed in the as quenched state of this alloy, it was found that particle growth occurred only in the twin bands that formed as a result of the tensile deformation.MST/1364

ISSN:0267-0836    

DOI:10.1179/026708392790169858

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe coarsening kinetics of solid and liquid silver particles in a nickel matrix have been investigated at temperatures in the ranges 850–935°C and 1000–1100°C. The silver particles and nickel matrix grow in a coupled manner that corresponds to a scaling enlargement. In both temperature ranges the observed growth data correspond to a fourth power rate law, but the rate constants are several orders of magnitude greater than those predicted by classical theories of Ostwald ripening. The accelerated coarsening kinetics are attributed to a coalescence mechanism resulting from the coupled migration of particles and grain boundaries. In the range where the particles are solid a kinetic model gives close agreement with the experimental data, the controlling mechanism being boundary diffusion. In the range where the particles are liquid, however, interpretation of the data is more problematic; an activation energy comparable to that for volume diffusion suggests either that diffusive flow between the leading and trailing edges of the particles controls the kinetics, or that transport through the liquid is controlled by transfer across the liquid/solid interface.MST/1496

ISSN:0267-0836    

DOI:10.1179/026708392790169876

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractRegular grain oriented silicon iron, produced by a two stage cold rolling technique, has been the subject of many investigations owing to its economic significance as sheet steel for transformer cores. In particular, .many investigations of the texture development through the various processing stages have been carried out by calculation of the orientation distribution functions. The final processing stage is a high temperature box annealing, by which discontinuous grain growth (secondary recrystallisation), controlled by a fine dispersion of second phase particles, leads to a very sharp Goss texture. To investigate the conditions for secondary recrystallisation, material from two stages of processing has been examined: the primary recrystallised material and the material just before the start of secondary grain growth. Utilising the etch pit method–a well established and powerful technique for single orientation determination–in addition to grain size measurements, both applied to scanning electron micrographs, grain size and orientation distributions of Goss oriented and other matrix grams were measured and compared for the two types of material. The potential of this technique and the interpretation of the results m the context of the material used are discussed.MST/1513

ISSN:0267-0836    

DOI:10.1179/026708392790169911

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe available data on minimum strain rate (‘secondary creep’) in creep tests and on maximum stress (first peak stress) in hot deformation tests obtained forγiron and forγFe–C alloys have been reviewed and allocated in the form of an equationɛ=4·6×1017(σ/E)3·0exp (α%C) sinh (60·137σ/T) exp (−32 714/T), whereɛis the strain rate (s−1),σis the stress (MN m−2), E is the elastic (Young's) modulus (MN m−2) according to E=230 950−99·04T, %C is the per cent carbon content, and T is the absolute temperature. The range of validity is from approximately Z′=ɛexp (32 714/T) exp (−α%C)=103to 1014s−1(where Z′is a modified Zener–Hollomon parameter), which covers the temperature range from 900 to 1400°C. The factorαdescribing the influence of carbon content is given by the expressionα=2·3−2·1 log (60·137;amp;#x03C3;/T). Hence, the softening of austenite by carbon decreases with increasing stress and decreasing temperature. At very highσ/T ratios, the factor ;amp;#x03B1; may even become negative reversing the effect of carbon to strengthening. A procedure is suggested by which the given equations can be applied also for the description of primary creep.MST/1386

ISSN:0267-0836    

DOI:10.1179/026708392790169830

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractRecrystallised and unrecrystallised Al–Li–Cu–Mg–Zr alloy 8090 sheet and Al–Cu alloy 2024 sheet have been tested to determine their relative resistance to creep cracking. Creep cracking in recrystallised 8090 sheet occurred at much lower stress intensity factors K, at higher rates for a given value of K, and at lower temperatures, compared with unrecrystallised 8090 or 2024 sheet. For recrystallised sheet, significant rates of creep cracking were observed at temperatures as low as 60°C, at K values of ~10%Kc(the critical value of K for overload fracture). Creep crack growth was predominantly intergranular for recrystallised 8090 and 2024 sheet, and a mixture of intersubgranular and transgranular for unrecrystallised 8090 sheet. The fractographic features, as well as other observations, suggest that 8090 alloys contain low melting point sodium-rich phases which are not found in conventional alloys. It was concluded that the presence of these phases and the continuous, intergranular crack paths approximately normal to the applied stress in recrystallised 8090 sheet were responsible for its poor resistance to creep cracking.MST/1482

ISSN:0267-0836    

DOI:10.1179/026708392790169803

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe recrystallisation behaviour has been studied in a cold worked 2014 Al–Al2O3particulate reinforced composite which was heat treated to produce two markedly different initial matrix microstructures. The composite was cold deformed to a reduction of either 30 or 50% and then annealed for various times at temperatures in the range 100–500°C. The microstructural development during recrystallisation was studied using optical and transmission electron microscopy (TEM). The recrystallisation kinetics were monitored via hardness measurements and the results correlated with the microstructural analysis. It was observed that the kinetics and temperature of recrystallisation were dependent on the original heat treatment. A detailed TEM investigation indicated that nucleation occurs preferentially at individual and clustered Al2O3particles, but the progress of recrystallisation was controlled by the distribution of dispersoid particles in the matrix. Recrystallisation proceeded rapidly in the composite containing large stable dispersoids. In contrast, when precipitation occurred during annealing, recrystallisation was severely retarded and was controlled by the fine precipitate dispersion.MST/1566

ISSN:0267-0836    

DOI:10.1179/026708392790169894

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe microstructure of a cast aluminium alloy A356 reinforced with 15 vol.-%SiC particles has been investigated using analytical microscopy. It is shown that the morphology of the silicon phase, as well as that of the Al–Si eutectic structure, which are the features of this alloy system, are dramatically changed by the presence of the SiC particles. Significant effects of grain refinement werefound to occur not only on the primary aluminium grains, but also on the primary and eutectic silicon phases. Twinning and dislocations were often observed within that silicon phase which was situated adjacent to the SiC particles. Microcracks were also observed at the SiC/silicon interface. The presence of such microcracks suggests that a stress concentration had developed at the silicon/SiC particle interfaces, probably as a result of the thermal expansion mismatch occurring between the silicon and SiC particles. The microcracks which consequently develop are formed as a result of the poor silicon–SiC particle bonding. Other intermetallics, notably Mg2Si and FeSiAl5, which exhibit cubic and tetragonal symmetries respectively, were also identified as being present in the microstructure.MST/1460

ISSN:0267-0836    

DOI:10.1179/026708392790169885

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractTwo different preheating treatments were used to vary the Al6Mn dispersoid characteristics in AA 5182. High temperature followed by slow cooling produced coarse, needlelike dispersoids which allowed rapid recrystallisation following hot rolling, whereas a low temperature preheat produced afine dispersion of low aspect ratio particles which retarded recrystallisation. Long, unrecrystallised bands persisted in the hot rolled material subected to the low temperature preheat even after holding for 24 h at the final pass temperature. Micro-orientation determinations within the bands showed that they were predominantly of the brass (Bs) orientation, {110}〈112〉, confirming the results of X-ray bulk texture analyses. The results are considered conclusive evidence of the widely reported higher resistance to recrystallisation of the Bs orientation compared with other orientations in rolled high stacking fault energy fcc metals. The Bs oriented bands could be removed by annealing at a temperature substantially greater than that experienced in the final hot rolling pass.MST/1475

ISSN:0267-0836    

DOI:10.1179/026708392790169795

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractA model developed by Sellars and Dutta, for strain induced carbonitride precipitation under isothermal conditions, is extended to precipitation under continuous cooling conditions. It is applied here to laboratory simulations of the stretch reducing stage of a seamless tube mill. The model predictions are compared with experimental observations of the contrasting rolling behaviours of Ti–V and Nb–V steels and excellent agreement is obtained. It is shown that in order to design dynamic recrystallisation controlled rolling (DRCR) stretch reducing mill schedules, the critical strain for dynamic recrystallisation must be achieved well before the onset of strain induced precipitation. If, however, precipitation begins before the critical strain is reached, then‘pancaking’occurs. Thus, Ti–V steels are more suitable for DRCR processing, because strain induced precipitation takes place only at very low temperatures in these materials. As a result, rolling can be carried out at relatively low temperatures, which results in very fine dynamically recrystallised austenite (and therefore ferrite) grain sizes.MST/1443

ISSN:0267-0836    

DOI:10.1179/026708392790169821

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractPrediction of the evolution of grain size in hot rolling of Al–5Mg alloy (AA 5056) is described in this paper. Published equations that relate microstructure to processing conditions in aluminium are incorporated into a thermomechanical model of hot rolling. Detailed modelling of the distribution of temperature, strain, and strain rate (Zener–Hollomon parameter) allows prediction of through thickness variation in the microstructure. The model described in this paper was developed to examine the effects of rolling process parameters (roll speed, reduction, strip entry temperature, and interpass time) on the extent of recrystallisation and grain size in the resulting microstructure.MST/1439

ISSN:0267-0836    

DOI:10.1179/026708392790169902

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor