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排序方式: 共有4047条查询结果,搜索用时 250 毫秒
61.
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63.
Isothermal oxidation behavior and the nature of oxide layer formed during oxidation of FeCo–2V alloy were characterized in the temperature range of 500–600 °C. Oxidation kinetics of the alloy follows a parabolic rate law. SEM and XRD studies indicate the formation of an iron rich outer oxide layer and an inner solute rich layer containing cobalt and vanadium rich oxides. The oxidation mechanism of the FeCo–2V alloy is similar to that of low alloy steels. During the initial stages, preferential oxidation of iron and cobalt occurs at the alloy surface and leads to the formation of a solute rich inner layer. Continued oxidation occurs through oxidation of iron and cobalt at the outer layer and internal oxidation of inner layer. The iron rich oxide layer formed at the surface on oxidation of FeCo alloy is semi-conducting in nature and may not provide the necessary insulating barrier required at the surface to minimize eddy current losses during A.C. applications. 相似文献
64.
Recent progress in the understanding of high-temperature creep of alloys is discussed in the context of theoretical modeling
and its application to alloy development. Emphasis is placed upon those engineering alloys specifically designed for high-temperature
applications, such as precipitation and dispersion-strengthened (DS) alloys and metal-matrix composites (MMCs). Currently,
these theoretical models use one of two different approaches, (a) a phenomenological approach, which is used in such models
as those based on the internal stress concept, and those based on empirical creep equations; and (b) micromechanical models
that are based on dislocation mechanisms and the interactions of dislocations with solute atoms, second-phase particles, and
other reinforcements such as fibers. All these theoretical models have a common goal, namely, the understanding of high-temperature
strengthening mechanisms and the relationship between high-temperature strength and the micromechanical mechanisms during
high-temperature plastic deformation of the alloys. These theoretical studies can provide information that is useful in alloy
design and processing, such as the selection of alloy chemistry, and the optimization of phase microstructural features (e.g.,
reinforcement amount, shape, size, and distribution; matrix grain size; and matrix and reinforcement interfaces) by optimization
of processing methods.
L. Shi, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22901, USA 相似文献
65.
The oxidation in air of an austenitic Fe-Mn-Cr steel containing 17.8 Mn, 9.5 Cr, 1.0 Ni, 0.27 C, and 0.03 N was studied over the range 700–1000°C. Oxidation of surface-abraded samples at low temperatures, 700–750°C, resulted in only Mn
2O3 containing dissolved chromium, except at corners, where large nodules containing spinel and manganowustite formed. The Mn2O3 layer grew into the substrate forming a globular-type film. This growth mode was the result of slow interdiffusion in the alloy after the cold-worked surface layer had been recrystallized and/or consumed, as evidenced by the formation of a ferrite layer subjacent to the scale and by the instability of the planar interface. No internal oxidation was observed beneath the Mn2O3 film at either 700 or 750°C. Samples oxidized in thehigh-temperature region, 800–1000°C, exhibited vastly different behavior, forming thick stratified scales at long times (24 hr), the scales consisting of a very thin outer layer of Mn2O3 (with appreciable iron in solution), Fe-Mn spinel beneath the outer layer, and a thick inner layer of manganowustite and a chromium-containing spinel. No chromium was found in the outer two layers. A thin layer of nearly pure Fe2O3 formed between Mn2O3 and the outer spinel.
Quasiparabolic kinetics were observed. The high-temperature rates were about 103 to 104 times greater than at low temperatures at the transition temperature. The rapid rates at high temperatures were attributed to manganowustite growth. However, oxidation of an electropolished sample at 750°C, from which the superficial cold-worked layer had been removed, formed scales similar to those observed at high temperatures at comparable rates. A difference by a factor of over 104 existed between the oxidation rate of the electropolished sample and the surface-abraded sample at 750°C. The much slower oxidation rate of the latter is attributed to greatly enchanced manganese diffusion through the high dislocation-density, cold-worked layer. Short-time tests at 800°C revealed an incubation period during which a thin protective layer of Mn2O3 formed. The incubation period corresponded to the recrystallization time of the cold-worked layer. Subsequently, nodular growth occurred which was associated with internal oxidation. The nodules, consisting of spinel and manganowustite, eventually linked up to form a thick, stratified scale. Comparison of the scale structures with calculated phase diagrams of composition versus oxygen activity (at constant temperature), showed that the protective films formed at low temperatures were due to kinetics factors, involving enhanced manganese diffusion through the cold-worked layer, rather than to thermodynamics. A model for the breakdown of protective films is proposed which involves internal oxidation. 相似文献
66.
Varol O. Kayhan Christopher J. Davis Rosann Webb Collins Anol Bhattacherjee 《Information Systems Management》2013,30(2):168-178
Two mechanisms that verify knowledge contributions in electronic repositories are expert-governance and community-governance. Our goal is to examine repository users' perceptions of the conditions under which these mechanisms verify knowledge contributions. Qualitative data show that perceived credibility of experts, perceived ownership of content, and experts' (meticulous) execution of governance functions are salient for expert-governance, and the perceived involvement of community members, and community members' (continuous and collective) execution of governance functions are important for community-governance. 相似文献
67.
We have attempted to bring together two areas which are challenging for both IS research and practice: forms of coordination and management of knowledge in the context of global, virtual software development projects. We developed a more comprehensive, knowledge-based model of how coordination can be achieved, and\illustrated the heuristic and explanatory power of the model when applied to global software projects experiencing different degrees of success. We first reviewed the literature on coordination and determined what is known about coordination of knowledge in global software projects. From this we developed a new, distinctive knowledge-based model of coordination, which was then employed to analyze two case studies of global software projects, at SAP and Baan, to illustrate the utility of the model. 相似文献
68.
The origin of work-function changes induced in polymer/metal structures (polyacrylic acid (PAA), ammonium and sodium polyacrylate) upon ammonia exposure is studied. To understand and explain the mechanism systematic measurements of uncoated and polymer coated metals are performed in Kelvin Probe (KP) setups. Additional data have been acquired through gravimetric and electrochemical experiments (cyclic voltammetry (CV)). The results obtained confirm that the field effect sensitive processes are taking place at the metal/polymer interface and allow their modelling. The intrinsic sensitivity of metallic samples upon ammonia exposure is influenced by the presence of coating polymers, by their ability to sorb water and to keep the metal surface wet or dry. 相似文献
69.
俞莎 《数字社区&智能家居》2009,(32)
我国高等教育的快速发展促使大校合并、校区扩建,VPN技术建设校园网使教学资源得到充分优化利用的一种虚拟教育环境。随中国互联网客户逐年增多,IPv4地址问题日趋严重,互联网向IPv6过渡已是大势所趋。本文从VPN和IPV6升级技术、实施方案等方面讨论下一代跨校区数字化校园网实现策略与研究。 相似文献
70.
卢友敏 《数字社区&智能家居》2009,(9)
随着网络应用的迅猛发展,网络拥塞成为一个不可避免的问题,拥塞控制已经成为当前网络研究的重点。为了使网络得到充分的利用,研究者提出了多种拥塞控制机制,并且得到不断的改进。该文通过比较各种典型控制算法的优缺点,来对网络拥塞控制机制作个综合阐述。 相似文献