Equilibrium Defects and Concentrations in Nickel Aluminide
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that those are the lowest energy defects [9]. In an ordered single-phase alloy, the equilibrium defect must be some combination of elementary defects that preserves the local proportion of elements. Such 2 a combination made out of the observed structural defects is the triple defect, VNi+ NiAP. This combination has a priori to be considered the most likely equilibrium defect combination. Indeed, the triple defect is confirmed below to be the dominant defect below 1200 C, and its properties are determined. EXPERIMENTS NiAl(nlln) samples were made by arc-melting high-purity metals (m5N) together with "'In activity under argon. The "'In probe fractional concentration was below 10-8. Compositions were determined from the original Ni and Al masses after checking that there was negligible loss of mass during the melts. Since, as will be shown, there are great changes of vacancy concentrations with composition close to stoichiometry, special care was taken to determine compositions of such samples to better than 0.05 at.%. An oven was designed and built that uses resistive and electron-beam heating for PAC measurements to over 1300'C [10]. Measurements were made using a standard 4-detector PAC spectrometer and data were analyzed as described previously [1]. Each PAC spectrum was fitted to a superposition of quadrupole interaction signals described completely in ref. [6] or [10]. For purposes of this paper, the signals are grouped according to the number of Ni-vacancies in the closest atomic shell of probe sites: (1) a vacancy-free signal for probes with no vacancies in the first atomic shell, (2) a monovacancy signal for probes having one VNi in the first shell, and (3) a minor divacancy signal for probes with two neighboring VNi. Since no other signals were observed in these experiments, the sitefractions f 0, f 1 and f 2 of the grouped signals sum to unity. To illustrate, in Fig. 1 are shown PAC spectra for a sample with 50.14 at.% Ni. The room-temperature spectrum at bottom exhibits the monovacancy signal (period - 50 ns) with a site fraction of about 20% after quenching from high temperature. The remaining low-frequency signal corresponds to probes with no VNi in the first shell. The top four spectra are in thermal equilibrium. As can be seen, the site fraction of the 1V signal increases with temperature up to 1457 K. The 1508 K spectrum exhibits damping attributed to jumping of vacancies at rates of order 10 MHz, described in ref. [11].
1.0 0.5
1045K
1.0 0.5 0.5
1302K
•1. 0 0.5
Figure 1. PAC spectra of a NiAI sample with 50.14 at.% Ni measured at the indicated temperatures. The 296K spec-
57K
1.0
exhibits quenched-in vacancies.
•trum
0.5
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1.0•
0.5 0 0
100
200
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t (ns) KK8.7.2
ANALYSIS AND RESULTS Thermodynamics of site fractions and defect concentrations
For a fractional concentration [VI of vacancies on the Ni-sublattice, the site fraction for monovacancy complexes, f 1 ,can be expressed in terms of the site fraction for vacancy-free probes, fo, as f1 Ifo = 8[Vjexp(EB 1 /IkT)
(1)
us
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