An Investigation of Point Defects in NiAl Using Positron Annihilation Techniques
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A first set of specimen was used for the quenching treatments. For these experiments the samples were heated for 30 minutes to the desired temperatures, TQ, and afterward rapidly quenched into a water bath. After quenching from 1600'C the samples were subjected to isochronal annealing treatments in air for 30 minutes - in steps of 50'C - up to 1250'C. Some of the well-annealed specimens (Ni 485A1515 , Ni5oAl50 and Ni 3A147) were irradiated at a temperature T < 230 K with 3 MeV protons to a total dose of about 7.1x101 p/cm 2 and with 2 MeV electrons at 77 K to a total dose of lx 018 e/cm 2. Positron lifetime spectra were recorded using a conventional fast-fast spectrometer with a time resolution (FWHM) of about 205 ps. The positron source itself was prepared in the well known sandwich configuration. The lifetime spectra were numerically analysed by the computer program PFPOSFIT [17] taking into account a source correction due to the contribution of positrons annihilating in the Al foil supporting the positron source. The Doppler-broadening of the 511 keV annihilation line was measured using an intrinsic Ge detector with a resolution (FWHM) of 1.18 keV at 497 keV. The numerical analysis of the spectra was performed by determining the shape parameter S (the ratio between the counts in a central part of the spectrum and the total counts). All annihilation measurements, both lifetime and Doppler-broadening, were performed at room temperature. Details of the experimental set-up and the analysis procedure as well as the physical background can be found elsewhere [181. RESULTS When positrons annihilate in a defect free sample the so called bulk lifetime, TB, is observed. If open volume defects like vacancies are present the positrons might be captured and annihilate from this bound state with a vacancy lifetime, TrV, which is longer than the bulk lifetime. The two-state-trapping model [19] describes this situation for the single defect case. At defect concentrations in the range 10- < Cv < 10- the annihilation characteristics depends sensitively on the number of defects. If the total defect concentration is high, all positrons get trapped and saturation trapping is observed. This means that all positrons annihilate from this bound states. Stoichiometric Dependencies
190 185 180 .
0,490 0,480 in
o18.
0,470
175 -0,460 170
1 0
165 160
. 47
48
49c., 50 at.-%J,51
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Figure 1: Mean lifetime T (0) and Sparameter (1-) as function of Ni concentration for well annealed NiAI samples.
0,450 0,440
In Figure 1 the positron lifetime T and the S-parameter for the annealed samples as a function of the Ni concentration are shown. Clearly visible is the decrease of the lifetime and the S-parameter on the Ni-rich side; a result which is in good agreement with earlier results [11, 12, 15]. In comparison to other intermetallic compounds like FeAl [20, 21], these lifetimes are rather high suggesting annihilations in vacancy like defects. This finding is also in accordance with theoretical calculations giving a bulk lifetime of
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