Metastable structures in metallurgy

Metastable structures or, more accurately, configurationally frozen metastable structures are no novelty in metallurgy. Indeed, much of the traditional practice of metallurgy has centered on the formation, characterization, understanding and control of structures which are either compositionally, topologically and/or morphologically metastable. However, in the past two to three decades we have seen a great upsurge in the production and study of new metastable structures in metallurgy, as well as in other condensed phase sciences. This upsurge reflects developments in the techniques of melt quenching, condensation and irradiation of materials, as well as in the kinetic understanding of structure evolution; and it has brought us nearer to making the concept of “ultramolecular engineering” viable. Among the new materials produced are glassy metals, highly super-saturated crystalline alloys and new alloys with exceptionally high interfacial densities. An overview of these new developments will be offered, following a discussion of the principles of metastable structure synthesis.     

Boron and boron carbide coatings by vapor deposition

The Bureau of Mines investigated the formation of boron and boron-carbide coatings by vaporphase reactions. Optimum parameters were determined for hydrogen reduction of boron trichloride and for the formation of boron-carbide coatings on graphite by reaction with the deposited boron. At 1300°C, about 85 pct of the boron was deposited. Tungsten substrates did not react with the boron deposit; other substrates reacted to various extents. The hydrogen reduction of boron tribromide was briefly investigated. Boron carbide was deposited at 1300°C by adding methane to the boron trichloride-hydrogen feed gas. The chemical composition of the vapor-deposited boron carbide approximated B₄C. A method of etching B₄C was developed to study its microstructure. When boron was deposited on graphite at 1500°C, very hard, uniform, strongly adherent coatings of B₄C were formed that might be useful in applications such as rocket nozzles and chemical reaction and processing vessels.     

Metastable phases in the Tl-Sn alloy system

The structures of metastable Tl-Sn alloys prepared by rapid quenching (splat cooling) to —190°C have been investigated. Over a large part of the total composition range single phase alloys were obtained. Four new metastable phases with relatively simple, elementlike structures were found: α₁ (tetragonal), ω (hexagonal), γ (hexagonal), and γ₁ (not determined in detail); α₁ represents the rarely found transition between the A1 and A2 structure types. Crystalchemically, the new phases are in agreement with previously established general rules for the occurrence of stable and metastable B-metal phases.     

火焰气相沉积法合成纳米颗粒生长机制及影响因素

介绍国内外对纳米粒子在高温火焰中的生长机理研究进展,以及在制备过程中火焰结构、气体流速和浓度、先驱物种类等影响因素对颗粒粒径的影响.     

Effects of prehafnizing on morphological development of a chemical vapor deposition aluminide coating formed on single-crystal Ni-based superalloy

We examined a sequential Hf doping procedure, which consisted of (1) “prehafnizing” the surface of a single-crystal Ni-based superalloy (RENé N5) with HfCl₄ and H₂, and (2) aluminizing with AlCl₃ and H₂, as a means of incorporating Hf as a dopant in the aluminide coating matrix. The prehafnized layer on RENé N5 substrate significantly altered the growth behavior and therefore the morphology of the resulting aluminide coating. With the prehafnizing step, the coating layer became much thinner with a significant amount of Hf incorporated as Hf-rich phases (Hf₂Ni₇, Hf₃Ni₇, and/or Hf₈Ni₂₁). However, the Hf-rich phases segregated to the coating surface and retarded the inward Al diffusion required to form the β-NiAl coating matrix. The sequential Hf doping procedure provided a mechanism to incorporate a significant amount of Hf in the coating, but did not produce a uniform distribution of Hf as a dopant. The results were compared to those observed for a continuous doping procedure that was previously studied, and were discussed in the context of understanding the limitations of these procedures.     

化学气相沉积制备粉体材料的原理及研究进展

采用化学气相沉积法(chemical vapor deposition,简称CVD)不仅可以制备金属粉未,也可以制备氧化物、碳化物、氮化物等化合物粉体材料.该法是以挥发性的金属卤化物、氢化物或有机金属化合物等物质的蒸气为原料,通过化学气相反应合成所需粉末,因其制备的粉末纯度高,比表而积大,结晶度高,粒径分御均匀、可控,在粉体材料制备方面的应用日趋广泛.该文主要介绍CVD技术制粉的形成机理和研究进程.CVD法制粉主要包括化学反应、晶核形成、粒子生长以及粒子凝并与聚结4个步骤.按照加热方式不同,CVD技术分为电阻CVD、等离子CVD、激光CVD和火焰CVD等,用这4种技术制备超细粉末各有其优缺点,选择合适的气源,开发更为安全、环保的生产工艺,以及加强尾气处理是使CVD法制备超细粉体材料付诸于工业应用的重要保证.