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Nanocrystalline high melting point compound-based materials 总被引:2,自引:0,他引:2
R. A. Andrievski 《Journal of Materials Science》1994,29(3):614-631
The different preparation methods of ultrafine powders of high melting point compounds (carbides, borides, nitrides, and oxides with melting temperatures higher than 2000C) are reviewed. Some properties of these powders are discussed and compared. The consolidation behaviour of these compounds in the nanocrystalline (nc) state is described in detail. Compaction by hot pressing, including high pressures and high temperatures, sintering, and high-energy consolidation methods, is analysed. The microstructure, recrystallization, mechanical and physical properties of nc-carbides, nitrides, and oxides are characterized. Special attention is focused on relationships between structure and properties. 相似文献
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Nanocrystalline materials: A study of WC-based hard metals 总被引:12,自引:0,他引:12
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《Nanostructured Materials》1998,10(5):679-689
The search for superhard materials with Vickers hardness H ≥ 40 GPa (about 4000 kg/mm2) concentrates mainly on polycrystalline diamond, cubic c-BN and C3N4 or the substoichiometric CNx. This approach is based on the theoretical strength which is proportional to the bulk modulus. However, the practically achievable strength (and hardness) of engineering materials is two to four orders of magnitude smaller because their failure occurs due to flaws, and it is determined by their microstructure. Therefore, an alternative approach deals with the design of materials with an appropriate microstructure, such as heterostructures.Recently, we have developed new superhard nanocrystalline composites (Me = Ti, W; V,…). These materials consist of ≤ 4 nm small nanocrystals of a hard transition metal nitride embedded into < 1 nm thin matrix of amorphous silicon nitride. Unlike pure nanocrystalline metals and the heterostructures which show softening when the crystallite size or lattice period decreases below 5–6 nm, the hardness of our composites strongly increases with decreasing crystallite size in that range and approaches the hardness of diamond. In this paper we shall briefly summarize the concept for the design of these materials and experimental results achieved so far. New results to be reported concern the surprising structural stability of these composites and a discussion of the possible origin of the superhardness. 相似文献
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M E McHenry M A Willard H Iwanabe R A Sutton Z Turgut A Hsiao D E Laughlin 《Bulletin of Materials Science》1999,22(3):495-501
Conventional physical metallurgy approaches to improve soft ferromagnetic properties involve tailoring chemistry and optimizing
microstructure. Alloy design involves consideration of induction and Curie temperatures. Significant in the tailoring of microstructure
is the recognition that the coercivity, (H
c) is roughly inversely proportional to the grain size (D
g) for grain sizes exceeding ∼0·1−1 μm (where the grain size exceeds the Bloch wall thickness,δ). In such cases grain boundaries act as impediments to domain wall motion, and thus fine-grained materials are usually harder
than large-grained materials. Significant recent development in the understanding of magnetic coercivity mechanisms have led
to the realization that for very small grain sizesD
g<∼100 nm,H
c decreases sharply with decreasing grain size. This can be rationalized by the extension of random anisotropy models that
were first suggested to explain the magnetic softness of transition-metal-based amorphous alloys. This important concept suggests
that nanocrystalline and amorphous alloys have significant potential as soft magnetic materials. In this paper we have discussed
routes to produce interesting nanocrystalline magnets. These include plasma (arc) production followed by compaction and primary
crystallization of metallic glasses. A new class of nanocrystalline magnetic materials, HITPERM, having high permeabilities
at high temperatures have also been discussed. 相似文献
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《Materials Letters》2006,60(25-26):3170-3174
The nature of the ZrO2 thin films in the Zr/ZrO2 electrode plays a vital role to build Zr/ZrO2 high temperature–pressure chemical sensors, when the electrode is utilized to be in situ measuring electro-chemical properties of hydrothermal fluids. Zr metal was oxidized with a NaCO3 melt to form a thin film of ZrO2 on the surface of Zr, then an oxidation–reduction electrode was performed. The electro-chemical properties of the Zr/ZrO2 electrode were tested from room temperature to high temperatures and high pressures. By using SEM, EPMA and HRTEM, analyses of topography, chemical composition and structure of the ZrO2 thin films revealed that Zr/ZrO2 interface is divided in to 5 zones from the outmost zone to the center: 1) prismatic and oxygen-rich ZrO2, 2) ZrO2; 3) oxygen-rich Zr; 4) oxygen-bearing Zr; 5) Zr metal. Especially, the outmost oxygen-rich ZrO2 zone of the thin films is composed of nanometer-sized monoclinic crystals with high oxygen content, when Zr/ZrO2 electrode is conducted to make a good high temperature chemical sensor. 相似文献
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V. V. Vavilova N. A. Palii Yu. K. Kovneristyi V. N. Timofeev 《Inorganic Materials》2000,36(8):783-787
Annealing Fe-P-Si amorphous alloys was found to produce nanocrystalline particles and raise the microhardness of the alloys
by a factor of 2 to 3. The most significant strengthening was observed in the alloys containing the smallest amounts of Si
and P and the largest amount of the α-Fe-based phase. As shown by x-ray diffraction and electron microscopy, the alloy consisting
entirely of nanocrystalline phases with a particle size of about 25 nm crystallizes in three steps. 相似文献