A. Gilbert

Robert Isaac Jaffee

George T. Hahn

There appear to be two ways in which interstitials promote brittle behavior in Groups V-A and VI-A refractory metals: (1) By locating at the grain boundaries in the form of segregates of precipitates, promoting grain-boundary rupture and crack initiation; and (2) In solid solution by inhibiting dislocation movement and reducing the resistance to crack propagation. Both embrittling effects operate in Group VI-A metals; whereas, only the latter is significant in the Group V-A at the levels of purity currently achieved. The problem of reducing the low-temperature brittleness of Group V-A metals is readily solved through the purity route. Purification of Group V-A metals to interstitial levels well below the solubility limits may be readily accomplished either in the initial purification of the base metal or during melting or consolidation. There are three possible routes to solving the problem of the embrittling effect of interstitials on the Group VI-A metals: (1) the purity route, (2) through working, and (3) through alloying (thermodynamically through strong scavengers, by additions which modify the electron bonding, and via grain-refining dispersions). (Author).

The Effects of Solutes on the Ductile-to-brittle Transition of Refractory Metals

Norman S. Allen

The breadth of scientific and technological interests in the general topic of photochemistry is truly enormous and includes, for example, such diverse areas as microelectronics, atmospheric chemistry, organic synthesis, non-conventional photoimaging, photosynthesis, solar energy conversion, polymer technologies, and spectroscopy. This Specialist Periodical Report on Photochemistry aims to provide an annual review of photo-induced processes that have relevance to the above wide-ranging academic and commercial disciplines, and interests in chemistry, physics, biology and technology. In order to provide easy access to this vast and varied literature, each volume of Photochemistry comprises sections concerned with photophysical processes in condensed phases, organic aspects which are sub-divided by chromophore type, polymer photochemistry, and photochemical aspects of solar energy conversion.Volume 34 covers literature published from July 2001 to June 2002. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading authorities in the relevant subject areas, the series creates a unique service for the active research chemist, with regular, in-depth accounts of progress in particular fields of chemistry. Subject coverage within different volumes of a given title is similar and publication is on an annual or biennial basis.NOW AVAILABLE ELECTRONICALLY - chapters from volumes published 1998 onwards are now available online, fully searchable by key word, on a pay-to-view basis. Contents pages can be viewed free of charge. Visit www.rsc.org/spr for full details.

Photochemistry

C. N. Reid

G. T. Hahn

The first year's work on this program is reported. It is concluded that there is good evidence for brittle fracture induced by mechanical twinning in single crystals and polycrystalline Mo-C with grain sizes larger than 0.40 mm. However there is no evidence for such failures in Mo-C of grain size smaller than 0.25 mm, or in high-purity Cb. The effect of recrystallization texture on twinning is demonstrated, and the orientation dependence of twinning in Cb crystals is reported; a most unusual plastic instability is described in crystals of one orientation. It is shown that twinning in Mo-35Re occurs in bursts, which form in less than 2 mu sec; nucleation of twins occurs only at small strains but lateral growth takes place up to at least 12% strain.

Investigation of the Relationship of Twinning to Brittle Fracture of Refractory Metals

Volume 26

Royal Society of Chemistry

D. Bryce-Smith

D. J. Maykuth

Various alloying additions have been discovered which render unalloyed chromium much less susceptible to low-temperature embrittlement as well as to nitridation in air at elevated temperatures. These include additions of the Group IIIA metals, magnesia, and carbides based on the Groups IVA and VA metals. Of these additions, only the carbides contribute significantly to the hot strengthening of chromium. The combination of selected carbides and solid-solution-strengthening elements such as tungsten, molybdenum, and/or tantalum, has resulted in experimental alloys which retain useful strengths at temperatures through 1316 C (2400 F). These high strengths are achieved at some sacrifice in the low-temperature ductility of chromium. Also, despite the improvements afforded in the oxidation and nitridation resistance of chromium through alloying, no alloys are available which are capable of service in long-time exposures in air above 982 C (1800 F) without suffering some property degradation.

Chromium and Chromium Alloys

Compiled by teams of leading authorities this Specialist Periodical Report on Photochemistry aims to provide an annual review of photo-induced processes.

Nektarios Vlachopoulos

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