共查询到20条相似文献,搜索用时 15 毫秒
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J. E. Wittig 《Metallurgical and Materials Transactions A》2002,33(8):2373-2379
Rapid solidification by twin-anvil splat quenching captures the initial nucleation and growth of the α → γ
m
massive transformation in titanium aluminides. Splat quenching Ti52Al48 and Ti50Al48Cr2 from the liquid at slightly below the melting point produces an equiaxed α solidification structure. Solid-state cooling rates that approach 106 K/s arrest the α → γ
m
massive transformation with 1- to 5-μm-sized γ
m
nuclei, especially in the Ti50Al48Cr2 alloy. Classical massive-transformation heterogenous nucleation occurs at α:α grain boundaries with an orientation relationship of [111]
γ
//[0001]
α
and
. The γ
m
nucleus then grows into the adjacent α grain without the orientation relationship by forming an incoherent α:γ interface with {111}
γ
facets. Orthogonal variants of the tetragonal c-axis in the γ
m
product suggest that the massive transformation initially produces an fcc structure which subsequently orders into the L10 phase. Nucleation of γ
m
is not only observed at α:α grain boundaries and triple points, but also within the α grains. The intragranular γ
m
nucleation, which is believed to be heterogeneous, occurs with the same orientation relationship as for the intergranular
nuclei. However, the intragrain nuclei do not form {111}
γ
facets and retain a curved α:γ
m
interface. Although analysis of the {111}
γ
faceted growth using weak-beam dark-field (WBDF) imaging shows no evidence for any type of misfit-compensating dislocations,
lattice imaging of the {111}
γ
facets with high resolution transmission electron microscopy (HRTEM) reveals that the planar interface exhibits a slight
curvature, produced by atomic steps of (111) planes. These experimental data have been used to estimate a ratio of ledge spacing
(λ) to ledge height (h) for the {111}
γ
facets as λ/h=41, which is similar to calculated values for a ledge growth mechanism of massive transformations in Cu-Zn and Ag-Cd alloys.
This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part
of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations
Committee. 相似文献
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Valerie Randle 《Metallurgical and Materials Transactions A》1990,21(8):2215-2221
This paper reports on an investigation into the early stages of recrystallization in nickel from the standpoint of the misorientation
between newly formed recrystallized grains and the surrounding deformed microstructure. For the case of recrystallization
“nuclei”(i.e., <l-μm diameter), half the sampled interfaces were close to low-gS coincident site lattice (CSL) misorientations, particularly
gS = 3. Established new grains(i.e., >l-μm diameter) tended to be either not low-gS CSLs or less close to a low-gS CSL than for the “nucleus” case. The interpretation
of these data is that the very earliest stage of recrystallization is facilitated at a boundary which is more mobile than
average, often a characteristic of a low-gS CSL. After nucleation of recrystallization has been established, the important
criterion for the recrystallization interface is a large capacity for dislocation absorption, which is associated with general
boundaries rather than CSLs. The number of boundaries bordering the new grains and their geometrical characteristics were
consistent with a model which relates sides per grain to grain boundary energy. 相似文献
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S. A. Lyamkin 《Russian Metallurgy (Metally)》2010,(2):105-107
Data on electron diffraction study of bismuth films during heating are presented for various partial oxygen pressures in the
gas phase. The formation of bismuth oxide phases is completed in several seconds. 相似文献
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In commercial aluminum with a purity of 99.4 pct, the formation and growth of recrystallization nuclei were studied by techniques
such asin-situ annealing in a high voltage electron microscope, transmission electron microscopy and light microscopy. Sample parameters
were the initial grain size (370 and 19 microns) and the degree of deformation (50 and 90 pct reduction in thickness by cold-rolling).
It was found that the initial grain boundaries and high angle boundaries within the original grains are preferential sites
for recrystallization nuclei, and that the effect of such sites is enhanced by the FeAl3 particles present in the commercial aluminum as impurities. The nucleation temperatures determined by high voltage electron
microscopy and transmission electron microscopy decrease markedly when the initial grain size is decreased both after 50 and
90 pct cold rolling; a less pronounced temperature decrease is obtained by increasing the degree of deformation. The size
of the recrystallization nuclei, the recrystallization temperature and the recrystallized grain size are reported for the
four sample states, and finally the structural and kinetic observations are discussed. 相似文献
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G. Y. Kim Ph.D. W. Y. Lee J. A. Haynes T. R. Watkins 《Metallurgical and Materials Transactions A》2001,32(3):615-624
The (100) surface of a single-crystal Ni alloy was aluminized as a function of time to study the development of the resulting
coating microstructure. A chemical vapor deposition (CVD) reactor, which was specially configured for short-term aluminizing
experiments, was used to prepare coating specimens at 1150 °C. After 5 minutes, γ′-Ni3Al particles ∼100 nm in size randomly nucleated on the alloy surface. Within 20 minutes, a coating layer consisting of preferentially
oriented, columnar β-NiAl grains was formed with the segregation of refractory elements (i.e., Ta and W) from the alloy to the coating grain boundaries. The lateral growth of the columnar grains was observed to be relatively
rapid for up to 45 minutes, but slowed considerably between 45 and 180 minutes. While the columnar nature of the coating did
not change significantly after 20 minutes, the surface features continually evolved, with the appearance of a small amount
of the γ′ phase, which coincided with the segregation of the refractory elements to the coating surface. 相似文献
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S Silvennoinen-Kassinen O Vainio J Karvonen M Kauppinen M Kallioinen 《Canadian Metallurgical Quarterly》1995,106(4):345-350
Nickel is the major cause of allergic contact dermatitis, and to increase our understanding of this immune reaction we studied changes in the expression of adhesion molecules on mononuclear cells during nickel stimulation in vivo and in vitro. Nickel-induced lymphocyte cultures were used in vitro, the cells being examined with monoclonal antibodies (Mabs) and by flow cytometry. Mononuclear cells from skin biopsies of in vivo cutaneous nickel reactions were studied with Mabs and immunohistochemistry. The expression of adhesion molecules in vitro was differential: the number of cells carrying CD11c, CD29, CDw49b, CDw49d, CDw49e, CDw49f, CD54, CD56 and ELAM-1 being significantly overrepresented among the nickel-induced lymphoblasts whereas the number of blasts carrying CD44 was underrepresented and those of CD11a, CD18, CD58 and LAM-1 remained unchanged. CD4+ cells gained adhesion molecules during nickel-induced blast transformation whereas CD8+ cells lost most of their adhesion molecules. The in vivo results were in agreement with the in vitro ones except that CDw49b, CDw49f, CD56 and ELAM-1 could not be detected in a 96-hour nickel reaction in vivo. In conclusion, the nickel allergic reaction favors the expression of certain adhesion molecules, and this expression is induced on CD4+ cells while CD8+ cells tend to lose such molecules. The changes were more sensitively detected with the in vitro method. 相似文献
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Maria Peña Carlos Ramos Arturo Bronson 《Metallurgical and Materials Transactions B》1996,27(2):271-276
The phase relations of a zirconium silicide/silica reaction couple have been investigated at 2273 K in air. After annealing times from 4 to 49 hours, the reaction couples, created by encapsulating zirconium disilicide in a quartz ampule, developed an interdiffusion zone and an inner core consisting of solidified zirconium silicide. The interdiffusion zone consisted of a silica layer, zirconia precipitates, and prior liquid silicide globules dispersed in prior liquid silica. Zirconia precipitates formed from the oxidation of the silicide melt between the protective silica layer and the two liquid regions of silicide and silica, as a result of oxygen diffusion. At 2273 K, the inner core consisted of liquid zirconium silicide (ZrSi)L, although the microstructure analysis indicated formation of proeutectic ZrSi2 and a eutectic microstructure of ZrSi2 and Si upon solidification. A constant oxygen potential developed within the interdiffusion zone and protected the silicide from oxidation even after 49 hours at temperature. 相似文献
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《Acta Metallurgica》1981,29(1):65-77
The recrystallization behaviour has been studied in a dispersion strengthened aluminiumalumina alloy containing a bimodal distribution of fine alumina particles (<0.1 /μm) and coarse FeAl3 particles (0.2–4 μm). The formation and growth of recrystallization nuclei were studied by in situ annealing in a high voltage electron microscope, conventional transmission electron microscopy and light microscopy. The parameters studied were the initial grain size and the degree of deformation (50 and 90% reduction in thickness by cold-rolling). It was found that recrystallization nuclei formed preferentially at the initial grain boundaries and at deformation bands; the effect of such sites was enhanced by the FeAl3 particles which, however, in isolation were not strong nucleation sites. The recrystallization kinetics in the aluminium-alumina alloy were retarded or accelerated due to the presence of alumina or FeAl3 particles respectively. However, the FeAl3 particles did not lead to a significant grain refinement ; the recrystallized grain size of the aluminium-alumina alloy was large. Finally, the structural and kinetic observations are discussed and related to results from a previous study of the initial stages of recrystallization of aluminium of commercial purity. 相似文献
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R. Mitra 《Metallurgical and Materials Transactions A》1998,29(6):1629-1641
Titanium silicide (Ti5Si3) and its composites show promise for applications at temperatures higher than 1000 °C. Dense Ti5Si3 was processed by reaction hot pressing of a TiH2/Si powder mixture, which involved decomposition of TiH2 into Ti and H2 at around 800 °C, a chemical reaction between Ti and liquid Si at 1500 °C forming Ti5Si3
in situ, and densification under pressure. The use of fine TiH2 particles led to the formation of a relatively fine-grained microstructure with fewer microcracks and higher hardness and
fracture toughness values than those expected for a coarse-grained Ti5Si3. The addition of 8 wt pct Al as an alloying element led to the formation of Al0.67Si0.08Ti0.25 and Al2O3
in situ and a solid solution of Al in Ti5Si3. Both alloying with Al and the addition of TiC as a reinforcement phase improved the room-temperature fracture toughness.
Fracture toughness measurements were performed by three-point bend testing of single-edge notch bend (SENB) specimens, as
well as by indentation techniques using different models, and the data have been compared. The role of different operating
toughening mechanisms such as crack deflection, bridging, branching, and energy dissipation through microcracracking have
been examined. The investigation has also shown that Ti5Si3 maintains a high yield strength value up to 1200 °C. 相似文献
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J. R. Rairden C. A. Neugebauer R. A. Sigsbee 《Metallurgical and Materials Transactions B》1971,2(3):719-722
The diffusion of chromium, nickel and chromium silicide into gold has been studied using thin film techniques. Electrical measurements of film diffusion couples were made at several temperatures and these data are related to the interdiffusion of the metals. It was found that the activation energy for resistance changes in the films is somewhat greater than one-half of that for bulk diffusion indicating the predominance of a grain boundary diffusion mechanism; however, a drastically lowered temperature coefficient of resistance also indicates some interdiffusion by lattice diffusion. Under the conditions of these experiments, the chromium, nickel, and chromium silicide diffuse from the substrate through the gold to the outer surface of the gold where they are oxidized. 相似文献
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S. A. Lyamkin 《Russian Metallurgy (Metally)》2009,(3):197-200
Metal lead films are studied by electron diffraction during heating depending on temperature T and oxygen partial pressure p
O2 in the gas phase. It is found that the formation of oxide phases begins from the α-PbO2 modification. As p
O2 increases or T increases, intermediate oxides nPbO2 · mPbO and Pb3O4 form and then transform into the β-PbO modification. 相似文献
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