Physical chemistry of the powder metallurgy of beryllium: Chemical characterization of the powder in relation to its gra
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INTRODUCTION
THE metallurgy of beryllium, including all conceivable aspects of its production by powder metallurgy (P/M) techniques and specific properties of the particulate or dense products, is extensively covered in the literature,UJ leaving very little room for original ideas and novel observations. Nevertheless, in the course of a research program intended to understand the thermal behavior of beryllium P/M parts and in particular to find the origin of residual high temperature swelling and temperature induced porosity (TIP), several new experimental facts were established. They mainly deal, first, with the surface characteristics of beryllium in the powder state and, second, with the chemical transformation of bulk and surface impurities of the powder particles along the successive steps of the densification process and during subsequent thermal treatments. The present article is restricted to the first point. The chemical characterization of beryllium powders has already been reported in the literature,t2.3I but the subject was not addressed from the standpoint of the quantitative analysis of the surface state as in the present article. Indeed, here, emphasis is put on the dependence of the chemical characteristics of the powder on its granularity expressed in terms of surface area. The interest of such a procedure is that valuable quantitative information may thus be inferred about the average phase composition and equivalent thickness of the contamination and passivation layer on the surface of the particles. And this can be obtained without resorting to the (usually exclusive) use of spectroscopic sur-
PIERRE BRACCONI, CNRS Research Assistant, is with the Laboratory for the Reactivity of Solids associated with the CNRS, University of Burgundy, 21004 Dijon, France. LAURENT BUISSON, formerly Doctorate Student, Laboratory for the Reactivity of Solids, is Engineer, University of Burgundy. CHRISTIAN BONNET, R & D Manager, is with the Atomic Energy Board, CEA, 21120 Valduc, France. Manuscript submitted April 20, 1995. METALLURGICALAND MATERIALSTRANSACTIONSA
face analytical techniques, Auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS), the application of which to fine particulate matter, especially when coupled with ion etching, faces fundamental limitations linked to the roughness of the surface of a bed of powder. Accordingly, XPS is only being used here for comparison. For the sake of clarity, all results dealing with the densification and the dense products are excluded from the present article. This, however, does not mean that specific measurement on those dense materials may not be used as a reference, where justified. The relevant granular properties of the powder fractions are described in Section II, followed by a short identification of the analytical techniques used. The results are presented, and discussed when necessary, in Section IV, which is structured as follows: the results regarding light and metallic elements are dealt with successively; next, the stoichiome
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