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  • 11
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 80 (1993), S. 995-998 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The technique of perturbed angular correlation of gamma rays (PAC) was applied to study the martensitic phase transition of a shape-memory-effect alloy Ni49.9Ti50.1 doped with111In/Cd probe atoms. Spectra measured above and below the martensite starting temperature,M s ≈ 340 K, exhibit quadrupole interactions of probes at cubic and noncubic lattice sites, consistent with the respective crystal structures. Unlike spectra measured belowM s those measured above exhibit a large frequency distribution attributed to lattice displacement waves observed in diffraction studies. Analysis tentatively suggests that the waves are dynamic and not static. BelowM s, at 290 K, a static quadrupole interaction was observed with coupling frequency ω1=34 Mrad/s and asymmetry parameter η=0.18, increasing to ω1=40.5 Mrad/s and η=0.34 at 77 K.
    Type of Medium: Electronic Resource
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  • 12
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 80 (1993), S. 1257-1261 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A new method has been developed to measure properties of equilibrium, or thermal, defects in intermetallics using the technique of perturbed angular correlations of gamma rays (PAC). After quenching, thermal defects are detected microscopically by distinctive nuclear hyperfine interaction signals produced when they localize next to probe atoms present in high dilution. Using a Schottky defect model and applying the law of mass action to defect equilibria, a linear Arrhenius temperature dependence is predicted for a “normalized” monovacancy site fraction. We have observed such linear dependences in PAC experiments on NiAl, CoAl and TiAl using the111In probe. Features of the method are summarized, of which the most important are the abilities to discriminate between different defects and to determine absolute vacancy concentrations, formation enthalpies and entropies. Extremely large formation entropies have been obtained for NiAl, CoAl and TiAl, suggesting vacancy concentrations of ∼ 15% at the melting temperatures.
    Type of Medium: Electronic Resource
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  • 13
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 92 (1994), S. 949-953 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract High-purity Fe powder was mechanically milled under argon at ambient temperature using an SPEX 8000 mill. The local atomic and magnetic structure was studied using57Co/Fe Mössbauer and111In/Cd perturbed angular correlations (PAC) spectroscopies. After 32 hours of milling, X-ray diffraction revealed effective grain diameters of 18 nm and energy-dispersive X-ray analysis indicated a Cr impurity concentration of ∼5%, presumably introduced by mechanical attrition of steel ball bearings used for milling. In addition to a spectral component very similar to bulk iron metal, the Mössbauer spectra exhibited hyperfine field shifts attributed to the Cr impurities. PAC spectra on Fe milled for 5 h, with no contamination, exhibited two components: (1) A slightly broadened magnetic interaction attributed to interior, defect-free sites of In/Cd probes with a mean hyperfine field slightly greater than in macroscopic grains. The defect-free site fraction grew appreciably during milling, even though In is essentially insoluble in Fe. (2) An indistinct signal due to mixed magnetic and quadrupole interactions attributed to probes at surface or other defect sites.
    Type of Medium: Electronic Resource
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  • 14
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 92 (1994), S. 955-958 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Ni and Fe powders in a 75∶25 atomic proportion were mechanically alloyed using a high-energy ball mill.57Fe Mössbauer measurements were made to determine the reaction mechanism for alloy formation by means of analysis of the evolution of the Fe hyperfine fields during milling. After a latent period of 2 hours, the spectral area of an Fe-like component decreased monotonically with milling time, disappearing after 8 hours. It was replaced by a well-resolved Zeeman pattern with hyperfine fieldH=29.1 T and outer lines of width 0.75 mm/s which is attributed to Fe in disordered Ni3Fe. The evolution of hyperfine fields rules out alloy formation by dissolution of Ni in the Fe matrix or of Fe in the Ni matrix, so that formation must occur by reaction of Ni and Fe at the interfaces between their grains.
    Type of Medium: Electronic Resource
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  • 15
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The quadrupole coupling frequency of an111In-defect bound state in Pt with ω0(295 K) = 200 Mrad/s was measured over the range 4 – 300 K using perturbed angular correlations. The temperature dependence exhibited positive curvature, unlike the T3/2 dependence observed in noncubic metals, and was fitted using a model which assumed that the temperature dependence was caused by vibrations of a resonant mode in the probe-defect complex.
    Type of Medium: Electronic Resource
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  • 16
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 60 (1990), S. 651-653 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The perturbation function of a Cd probe atom bound in a hydrogen-decorated vacancy complex in Pt is found to exhibit dynamical relaxation at 294 K, as measured by the technique of perturbed γ-γ angular correlations. Based on other characteristics of the complex, a model is proposed for the structure of the complex. In the undecorated state, it consists of a probe atom at the center of a tetrahedron of vacancies in the fcc structure. In the decorated state at low temperature, the probe atom returns to one of the four vacant lattice sites. The relaxation at room temperature is attributed to motion of the probe atom in a cage formed by the four vacant sites.
    Type of Medium: Electronic Resource
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  • 17
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 79 (1993), S. 745-748 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Site fractions of dilute111In/111Cd probe atoms in different lattice locations in quenched, equi-atomic TiAl are measured using perturbed angular correlations of gamma rays (PAC). The vacancy-free substitutional site fraction, observed via its nuclear quadrupole interaction, is observed to decrease with increasing quenching temperature, reflecting an increase in the thermal defect concentration. Assuming that the thermal defect is the Schottky defect, the law of mass action, and a binding energy of 0.2 eV between vacancies and the In probe, the experimental temperature dependence is analysed to yield a formation enthalpy of 4.7(4) eV and entropy of 25(4)k B for the Schottky defect. The vacancy concentration on one sublattice is given in terms of these parameters by the expression [v]=exp(S F/2k B) exp(−E F/2k B T), and is found, for example, to have the value 1.4% at 1350°C, a large value.
    Type of Medium: Electronic Resource
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  • 18
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Plastically deformed and p+ irradiated Ni have been studied using perturbed gamma ray angular correlations of111In. Isochronal annealing results in the temperature range 200 ≤T ≤ 700 K, and a comparison to earlier ion-implantation studies, lead to partial picture of defect migration and recovery in Ni.
    Type of Medium: Electronic Resource
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  • 19
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 15 (1983), S. 391-394 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using perturbed angular correlations we studied the formation of a cubic symmetry defect trap in Ni near 350 K following plastic deformation at 295 K. The trap has been identified structurally as a trivacancy trap. The evolution of the defect-free and cubic trap site populations in the course of isochronal and isothermal annealing experiments leads to the conclusion that the cubic trap forms by simple trapping, and that the capture radius of the cubic trap for additional defects is very small.
    Type of Medium: Electronic Resource
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  • 20
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using a source of181Tb, we have observed a substantial linewidth anomaly in the161Dy Mössbauer Effect just above the Curie temperature of Gd. Preliminary analysis of the anomaly is consistent with behavior expected for critical spin dynamics. Uncertainty in the Curie temperature prevents conclusive analysis of the linewidth divergence in terms of the dynamical exponent, z.
    Type of Medium: Electronic Resource
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