Vacancy Properties in Ordered NiGa And NiAQ
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VACANCY PROPERTIES IN ORDERED NiGa AND NiAQ S.M. KIM AECL Research, Canada
Chalk River
Laboratories,
Chalk River,
Ontario,
KOJ
JO,
ABSTRACT The recently formulated theory of vacancy formation in CsCQ-type ordered alloys has been applied to 8-NiGa and fi-NiAQ. The observed Nisublattice vacancy concentrations in these compounds could be well described by the theory. For the first time it is also shown that structural vacancies most likely exist in Ga-rich NiGa and AQ-rich NiAQ at absolute zero temperature. GENERAL While studying the structure of nickel-aluminum alloys, Bradley and Taylor discovered that in AQ-rich 8-NiAQ (CsCQ structure) the excess AQ atoms do not substitute in the usual way on the Ni sublattice, but instead vacancies are created on this sublattice [1]. Such vacancy populations, determined by composition and not temperature, have since been distinguished as structural vacancieo. Over the last fifty years, apparent structural vacancies have been observed not only in NiAQ but also in a number of other B2 intermetallic compounds such as NiGa, CoGa, CoAQ, FeAQ and PdIn (see e.g. Ref. [2]). A number of theoretical calculations have also been advanced in the past on the vacancy properties in ordered alloys [3-9]. However, none of these calculations could show the presence or absence of structural vacancies in p-NiGa and .8-NiAQ. In recent publications [8,9], we have shown that the widely-believed structural vacancies may not necessarily exist in all these intermetallic compounds, and that in case of CoGa and FeAQ thermal vacancies can account satisfactorily for the observed composition dependence as well as temperature dependence of vacancy concentrations. This theory was based on the pairwise bond-energy model, similar to all the other previous theoretical calculations. We have pointed out that structural vacancies can exist at T = 0 K only in an AB alloy rich in B atoms where the bond energy between two A atoms, CAA, is greater than that between two B atoms, 6 AA > 'EB, and that C., is repulsive, - (2c2 + SCAA) < EBB < 0, where c is the ordering energy, S= CAB - (CAA + CBB)/ , and 6 is the excess of B over A atoms as a fraction of the total number of atoms, 2N (see the next section). When the bond energies between the two atoms in CoGa and FeAQ were determined by fitting the theory to the observed composition dependence and temperature dependence of vacancy concentrations, we found that the bond energy between two B atoms, 6Ga.a and fAlAt, in these alloys were attractive rather than repulsive. Thus it was concluded that structural vacancies were highly unlikely in these compounds, and that the vacancy concentrations observed at room temperature were frozen-in thermal vacancies. The measurements of the lattice parameter and length changes in CoGa and FeAQ had already shown that vacancies become immobile below about 530°C in CoGa and below about 7000C in FeAQ [10, 11]. In this paper the theory, which is outlined in the next section, is applied to NiGa and NiAQ in order to determine whether the theor
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