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1.
In this study, nanocrystalline Ni powders and thermally sprayed coatings, containing ultrafine AlN particles, were synthesized
and characterized. The results indicated that the presence of AlN particles in the powders drastically decreased the dimension
of agglomerates formed by cryomilling and increased the surface roughness of the agglomerates. The AlN phase was broken down
into ultrafine particles of approximately 30 nm in size. These particles were dispersed in the Ni matrix and enhanced the
development of a nanocrystalline structure in the Ni matrix during cryomilling. Selected-area diffraction patterns, obtained
from transmission electron microscopy (TEM) and X-ray mapping with scanning electron microscopy (SEM), confirmed the presence
of AlN particles in the coatings. The presence of AlN particles also led to an increase in the amount of NiO phase that was
distributed in the coating, in the form of ultrafine, round particles. AlN particles increased the microhardness of the Ni
coating by approximately 60 pct. Indentation-fracture results also indicated that the fine, dispersed AlN particles raised
the apparent toughness of the Ni coating. The synthesized Ni coatings containing ultrafine AlN particles were characterized
as equiaxed nanocrystalline grains with an average size of 24 nm, in which twins were observed. The increase in microhardness
resulted from both grain refinement and the presence of ultrafine particles. The latter played the primary role in strengthening. 相似文献
2.
Commercially available, gas-atomized CoNiCrAlY powder was cryomilled to produce powder with nanocrystalline grains. The cryomilled powder and conventional gas-atomized powder were thermally sprayed using the HVOF process to prepare two coatings with fine-grain (~15 nm) and coarse-grain (~1 μm) microstructure, respectively. The two coatings were isothermally oxidized in air at 1000° C for up to 330 hr. The morphology and composition of the oxide scales formed on the two coatings were compared with each other. The results indicate that, while a fine-grain microstructure can promote the formation of a pure alumina layer on the coating by increasing the Al diffusion rate toward the surface, it can also accelerate the Al depletion by increasing the Al diffusion rate toward the substrate, which results in the formation of non-alumina oxides after long-term oxidation. The mechanisms governing the oxide formation are discussed in terms of atomic diffusion and thermodynamic stability. 相似文献
3.
Dongming Liu Yuhong Xiong Ying Li Troy D. Topping Yizhang Zhou Chris Haines Joseph Paras Darold Martin Deepak Kapoor Julie M. Schoenung Enrique J. Lavernia 《Metallurgical and Materials Transactions A》2013,44(4):1908-1916
The influence of tooling material, i.e., graphite and WC-Co, on the microstructure of a spark plasma sintering (SPS) consolidated, nanostructured aluminum alloy is studied in this paper. The results show that tooling selection influences microstructure evolution, independent of process parameters. The influence of tooling on microstructure is rationalized on the basis of the following factors: heating rate, electrical current density, localized heating, and imposed pressure. A theoretic framework, based on the physical properties of graphite and WC-Co, is formulated to explain the observed behavior. 相似文献
4.
5.
Ajdelsztajn L. Tang F. Schoenung J. M. Kim G. E. Provenzano V. 《Journal of Thermal Spray Technology》2005,14(1):23-30
Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a
metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has
been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to
form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized
at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation
resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result
of the formation of a continuous Al2O3 layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and
avoids the formation of mixed oxide protrusions present in the conventional coating.
The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8 2003, Basil R. Marple and Christian Moreau, Ed., ASM International,
2003. 相似文献
6.
Jichun Ye Leonardo Ajdelsztajn Julie M. Schoenung 《Metallurgical and Materials Transactions A》2006,37(8):2569-2579
Dense, bulk nanocrystalline aluminum 5083 alloy was fabricatedvia a combined technique: cryomilling (mechanical milling at cryogenic temperature) to achieve the nanocrystalline Al 5083 powder
and spark plasma sintering (SPS) to consolidate the cryomilled powder. The results of X-ray diffraction analysis indicate
that the average grain size in the SPS consolidated material is 51 nm, one of the smallest grain sizes ever reported in bulk
Al alloys produced by powder metallurgy derived methods. In contrast, transmission electron microscopy (TEM) analysis revealed
a bimodal grain size distribution, with an average grain size of 47 nm in the fine-grained regions and approximately 300 nm
in the coarse-grained regions. Nanoindentation was used to evaluate the mechanical properties and the uniformity of the consolidated
nanocrystalline Al 5083. The hardness of the material is greatly improved over that of the conventional equivalent, due to
the fine grain size. The mechanisms for spark plasma sintering and the microstructural evolution are discussed on the basis
of the experimental findings. 相似文献
7.
J. He J. Ye E. J. Lavernia D. Matejczyk C. Bampton J. M. Schoenung 《Journal of Materials Science》2004,39(23):6957-6964
In most studies related to milled powders, the grain size1 is analyzed via X-ray diffraction (XRD) experiments, and a transmission electron microscopy (TEM) image with high magnification, if provided, is used primarily to confirm the results obtained by XRD experiments. This widely used approach is reasonable in light of the difficulties associated with TEM sample preparation. The present study, however, addresses the hypothesis that such an approach may not be valid when there is an inhomogeneous distribution of grains present. TEM examination, carried out in carefully prepared Al-7.5 wt% Mg samples, in which a global region is observable by TEM, provided the opportunity for quantitative analysis of grain size in cryomilled powders having an inhomogeneous distribution of grain sizes. The cryomilled Al-7.5 wt% Mg had a bimodal grain microstructure of 77% (area fraction) fine grains in the range of 10 to 60 nm and 23% coarse grains of approximately 1 m. The results show that the XRD analysis yields a grain size that is close to that present in the fine-grained regions (i.e., 10–60 nm). The present study also systematically investigated the influence of the nine possible combinations of the Cauchy (C) and the Gaussian (G) approximations on the calculated grain size value, and the results show that the CC-CC approximation resulted in the largest calculated grain size, the GG-GG generated the smallest one, and the CG-CG, the approximation recommended by Klug and Alexander [1], led to a calculated grain size that is approximately equal to the average one from the CC-CC and GG-GG approximations. The maximum possible fluctuation of grain size values stemming from the various approximations is 38%. 相似文献
8.
Light-emitting diodes (LEDs) are advertised as environmentally friendly because they are energy efficient and mercury-free. This study aimed to determine if LEDs engender other forms of environmental and human health impacts, and to characterize variation across different LEDs based on color and intensity. The objectives are as follows: (i) to use standardized leachability tests to examine whether LEDs are to be categorized as hazardous waste under existing United States federal and California state regulations; and (ii) to use material life cycle impact and hazard assessment methods to evaluate resource depletion and toxicity potentials of LEDs based on their metallic constituents. According to federal standards, LEDs are not hazardous except for low-intensity red LEDs, which leached Pb at levels exceeding regulatory limits (186 mg/L; regulatory limit: 5). However, according to California regulations, excessive levels of copper (up to 3892 mg/kg; limit: 2500), Pb (up to 8103 mg/kg; limit: 1000), nickel (up to 4797 mg/kg; limit: 2000), or silver (up to 721 mg/kg; limit: 500) render all except low-intensity yellow LEDs hazardous. The environmental burden associated with resource depletion potentials derives primarily from gold and silver, whereas the burden from toxicity potentials is associated primarily with arsenic, copper, nickel, lead, iron, and silver. Establishing benchmark levels of these substances can help manufacturers implement design for environment through informed materials substitution, can motivate recyclers and waste management teams to recognize resource value and occupational hazards, and can inform policymakers who establish waste management policies for LEDs. 相似文献
9.
Jichun Ye Zhonghoon Lee Byungmin Ahn Steve R. Nutt Jianhong He Julie M. Schoenung 《Metallurgical and Materials Transactions A》2006,37(10):3111-3117
Cryomilling was successfully employed to fabricate particulate B4C-reinforced Al matrix nanocomposite powders. In order to investigate the microstructural evolution during cryomilling, composite
powders were milled for different times. These powders were collected from the milling chamber and the microstructures were
characterized to reveal the formation mechanism for this nanocomposite. The microstructural evolution, including the morphology
and size of the milled composite powders, the size and distribution of the B4C, and the dimension of the Al grains, is discussed on the basis of the experimental results. 相似文献
10.
The objectives of the present study are to estimate future quantities of electronic waste (e-waste) for which appropriate infrastructure needs to be established, and to the estimate the total cost for e-waste recycling in California. Estimation of the future amounts of e-waste as a function of time is critical to effective e-waste management. To generate estimates, we use a time-series materials flow analysis model (MFAM). The model estimates future e-waste quantities by modeling the stages of production, usage, and disposal. We consider four scenarios for the estimation of future e-waste generation in order to consider the effects of exportation outside the State of California and of user preferences to store or to recycle the e-waste. These efforts were further investigated through the use of sensitivity analysis. The results of the present research indicate that the outflow (recycling) amount of central processing units (CPUs) will increase and will reach approximately 8.5 million units per year in 2013, but the outflow (recycling) of cathode ray tube (CRT) monitors will decrease from 2004 in California because of the replacement of CRT monitors by liquid crystal display (LCD) monitors. In 2013, the cost for CPU recycling will be 1.7 times higher than that in 2005. But for CRT monitors, the cost for recycling in 2013 will be negligible. After the State of California enacted the ban on landfill disposal of e-waste, recycling became the most common end-of-life (EOL) option in California. Also, after 2005, the State of California will need more than 60 average-capacity materials recovery facilities (MRFs), to recycle the number of personal computer systems generated, which represents an investment in capital of over 16 million dollars. 相似文献