Some Defect Types in Intermetallics and their Consequences
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Some Defect Types in Intermetallics and their Consequences Robert W. Cahn Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, England ABSTRACT This short introductory overview focuses on point defects and their practical implications, on antiphase domains, and also on a feature that is not always perceived as a defect – imperfect long-range order. All these defect types have consequences for mechanical behavior of various kinds; imperfect LRO also affects the superconducting behavior of A15s. Other unfamiliar defects in intermetallics, to be outlined, include ‘rattling atoms’ in certain thermoelectric materials. Geometry and dynamics of dislocations in intermetallics are left to other contributors. INTRODUCTION In this short overview of recent work on defects in intermetallics, I shall focus first on zero-dimensional and two-dimensional defects, to the exclusion of one-dimensional defects. With regard to point defects, most recent research has been concerned with constitutional vacancies and with mechanical hardening resulting from high vacancy concentrations. Research on novel thermoelectric materials with the skutterudite crystal structure also centres on a kind of point defects in these materials. The most important two-dimensional defects are antiphase domain boundaries, and here again there has been some recent research. Much of the overview centres on the consequences of imperfect long–range order, brought about by heat–treatment, irradiation or mechanical milling; research on this theme has recently accelerated. POINT DEFECTS The most disputatious issue in recent years has been the question whether true constitutional vacancies exist at all, or whether all vacancy concentrations are to some degree determined by thermal activation. The issue was introduced by Ren, Otsuka and Kogachi in Japan [1] and fleshed out in more detail by Ren and Otsuka [2]. The starting-point for the debate was a series of ultraprecise measurements of vacancy concentrations in offstoichiometric NiAl by Kogachi and his collaborators [3, 4]; this compound is the one in which constitutional vacancies (i.e., vacancies whose concentrations are purely defined by alloy composition) were first reported in 1937, and in which their concentrations are particularly high in the Al-rich regime. (Kogachi also studied CoAl and FeAl). Figure 1 shows the essentials of the argument. The figure shows vacancy concentrations measured after equilibration of Ni0.5–xAl0.5+x at 800–1000K. The thin line labelled ‘BT line’ represents the vacancy concentrations expected for perfect constitutional status, i.e. in the absence of any Al antisite atoms in the Ni sublattice. The dotted lines for various temperatures were calculated from the Ren/Otsuka theory, involving a quantity, ∆HBV, that
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Figure 1. Comparison between experimental and calculated composition dependence of the vacancy concentration at different temperatures for Ni0.5–xAl0.5+x . Measurements from [2]. The line labelled ‘BT–line
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