Thermal Defects in B2 Iron Aluminide
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EXPERIMENT Samples of Fel+2xAll_2x with x in the range -0.005 to +0.047 (49.5 to 54.7 at.% Fe) were prepared by arc-melting metal constituents (purity m5N) with 111 In activity under argon in an arc-furnace. Sample masses were typically 100 mg and In concentrations were at the part-per-billion level. Samples were then annealed for one hour at 1200 'C followed by cooling to room temperature over about 10 hours. Measurements were made in a standard 4-detector PAC spectrometer and data were analyzed as described previously [3]. In Fig. 1 are shown PAC spectra for a sample with 51.25 at.% Fe measured at different temperatures. For this composition, the deviation from stoichiometry implies KK4.2.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
a structural defect concentration of FeAI equal to 2.5 at.%. The spectra exhibit superpositions of quadrupole precession patterns for probe atoms in different local environments. The amplitude of each signal is equal to the fraction of sites in the corresponding environment. Based on its large size and valence and other arguments, the In probe should be found only on the Al-sublattice. For a probe atom without nearby defects, the point symmetry is cubic and the quadrupole frequency is zero. A low frequency signal is clearly visible in the spectra measured at 700 and 500 'C, but not at lower temperature. The high frequencies observed in Fig. 1 come from one or more point defects in the closest atomic shell. From systematics of quadrupole frequencies in a range of similar B2 compounds [4], all signals having frequencies greater than -50 Mrad/s can be attributed to different numbers of VFe in the closest atomic shell around the probes. If the spectra were to exhibit simple thermal activation of defects with increasing temperature, then the vacancy-free (low-frequency) signal would be most prominent at low temperature where there would be fewer defects. However, just the opposite is observed, from which we conclude that there must be a large quenched-in vacancy concentration and attractive interactions between Fe vacancies and the oversized PAC probes.
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Fe 51.25 AI48.75
0.5 1 'IL e12A4
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Figure 1. PAC spectra of FeAI (left) measured at the indicated temperatures, with corresponding frequency amplitude spectra on the right.
The 700 'C spectrum is dominated by signals with fundamental frequencies of -0 Mrad/s (vacancy-free), 141 Mrad/s (1V complex), and 276 Mrad/s (2V complex). (The attributions are justified below). The site fraction of the 2V signal is somewhat larger at 500 'C. At 300 'C, the 2V signal has increased greatly at the expense of the lV and a new 180 Mrad/s signal attributed to a 3V complex is observed. Finally, at 100 'C, the IV and 2V signals have become quite small, the 3V signal has increased and a new 70 Mrad/s signal has appeared that is attribute
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