Perturbed angular correlation of gamma rays (PAC) was applied to measure concentrations of point defects in quenched and milled intermetallic compounds.  Two systems with high ordering energies were studied, PdIn and NiAl, with principal results obtained for PdIn.  The defects, lattice vacancies and antisite atoms, were detected by quadrupole interactions induced at nuclei of nearby In/Cd probe atoms.  Measurements on annealed, quenched and milled samples are compared to identify quadrupole interaction signals with specific configurations of defects near probe atoms.  Müller and Hahn had previously identified signals with a Pd-vacancy in the first near-neighbor shell of the In probe and a Pd-antisite atom in the second shell.  Six new signals are identified in this work, including two distinct configurations of two Pd-vacancies in the first shell, a Pd-vacancy in the fourth shell, an In-vacancy in the second or third shell, and an In-antisite atom in the first shell.  After quenching, observation of both Pd and In-vacancies after quenching from 1100-1500 K demonstrates that the high temperature equilibrium defect is the Schottky vacancy pair.  An activation energy of 0.91(18) eV  for formation of a Schottky pair was determined.  After milling, the site fraction of probes having one Pd-vacancy in the first shell increased rapidly and saturated at a value of about 20 %.  This implies a Pd-vacancy concentration of about 3.3 at.%, with a corresponding enthalpy of the order of 3 kJ mole-1, making a significant fraction of the excess enthalpy in the milled intermetallics.  After milling NiAl, a Ni-vacancy concentration between 1 and 10 at.% was observed.  A comparison is made of defect combinations observed after mechanical milling of PdIn and NiAl by PAC and of other intermetallics by Mössbauer spectroscopy.  Dominant defects observed to be produced by milling are the Schottky pair for PdIn, triple defect for NiAl and FeRh and antisite atom pair for FeAl. 

A search was made to detect the possibility that individual impurity atoms in two-phase, binary alloys nucleate embryonic crystals. The alloy system studied was Ni-Al and the impurities were 111 In probe atoms. Local surroundings of the probes characteristic of the crystal phases were detected through quadrupole interactions with 111 Cd daughter nuclei using the method of perturbed angular correlation of gamma rays. Analysis of site fractions of the probes led to two alternative interpretations that could not be distinguished on the basis of the present measurements: (1) 111 In probes in the two-phase domain between Ni2Al3 and NiAl segregate to Ni2Al3 with a segregation energy of about 0.10 eV; or (2) 111 In probes nucleate embryonic crystals of Ni2Al3, with the driving force for nucleation being an attraction between In atoms and Ni vacancies that is known to be strong in NiAl.

Thermal defects in B2 FeAl samples with compositions between 49.5 and 54.7 at.% Fe were investigated using perturbed angular correlation of gamma rays (PAC). Vacancies on the Fe-sublattice were detected through quadrupole interactions induced at adjacent 111 In/Cd probe atoms on the Al-sub-lattice. Five high-frequency quadrupole-interaction signals were detected (greater than 50 Mrad/s) that are attributed to complexes involving 1, 2, 3, 4 and (with less certainty) 5 Fe-vacancies in the first neighbor shells of the probes. These attributions are based on (1) a comparison between measured quadrupole interaction parameters and point-charge calculations of electric-field gradients for possible vacancy-probe complexes; and (2) numerical simulation of the evolution of site fractions of probes in the complexes at lower temperatures. Measurements were made at temperatures up to 950 C. Assuming that the equilibrium high-temperature is the triple defect (2 Fe-vacancies and one Fe-anti-site atom), measurements over the range 600-900 °C yield a formation enthalpy of 1.1(1) eV for the triple defect. Below about 600 °C, Fe-vacancies are quenched-in with a fractional concentration of the order of 1 at.% close to stoichiometry. However, quenched-in vacancies continue to jump over short distances and trap next to the impurity probes atoms in day-long measurements down to 200 °C. Simulations of site fractions below 700 °C were used to determine binding enthalpies of vacancies with probe complexes. Binding enthalpies obtained for the first four vacancies were 0.23, 0.23, 0.15 and 0.18 eV. Simulations in the range 200-700 °C suggest a negative value for the formation entropy.  The negative value indicates either that triple defects stiffen the B2 lattice or that repulsive defect-defect interactions become important at the high defect concentrations in FeAl.

An unstable f.c.c. Fe50Cu50 solid solution was prepared using high-energy ball milling of an elemental powder mixture.  It was used as a parent alloy for isothermal annealing or low-energy ball milling (LEBM), both at 373-523 K.  Alloy evolution was analysed by X-ray diffraction, Mossbauer spectroscopy and differential scanning calorimetry.  LEBM significantly enhanced the initial decomposition rate, but the decomposition process was complex and not monotonic.  At and below 423 K, the initial decomposition was completely reversed at later times.  The results are interpreted in terms of an effective-temperature model proposed by Martin for irradiated alloys.  A dynamic phase diagram in the effective temperature is presented, and used to explain the presence of a two-phase region under all milling conditions.  The non-monotinic behavior is suggested to be a result of coupling between phase evolution and mechanical properties, leading to a time dependence of the effective temperature.

Perturbed angular correlation of gamma rays was applied to determine properties of equilibrium defects in B2 NiAl near the stoichiometric composition. Point defects were detected through quadrupole interactions they induce at In probe atoms on the Al sublattice. Well-resolved signals were observed for probe atoms having zero, one or two Ni-vacancies (V Ni ) in the first neighbor shell. The fractions of probes in different sites are analyzed using a thermodynamic model to determine defect properties as follows. The equilibrium high-temperature defect is determined to be the triple defect combination (two V Ni and one Ni-antisite atom) through the variation of the vacancy concentration with composition and not, for example, the Schottky vacancy pair. The binding enthalpy of V Ni with a probe atom was determined to be in the range 0.18-0.24 eV. Site fractions were measured for three samples having 50.03, 50.14 and 50.91 at.% Ni at temperatures up to 1300 C. Vacancy concentrations were deduced from the site fractions and binding enthalpy. The equilibrium constant for formation of the triple defect was determined as a function of temperature from the vacancy concentrations and sample compositions. The formation enthalpy was found to be in the range 1.65-1.83 eV, depending on the binding enthalpy. The formation entropy was found to be -3.2(4) k B . The large, negative value of the formation entropy probably cannot be explained in terms of a binding entropy, and we speculate that triple defects harden the B2 lattice, perhaps by disrupting the well-known 1/3 <111> Îsoft modeâ lattice instability in B2 and bcc materials.

Using perturbed angular correlation of gamma rays (PAC), nuclear relaxation has been detected at high temperature in highly ordered B2 intermetallics that is attributed to stochastic motion of vacancies near 111 In/Cd probe atoms. The relaxation is of quadrupole interaction signals due to transition-metal vacancies in the first atomic shells of the probes. Possible relaxation mechanisms are reorientation of the vacancy with respect to the probe, detrapping of the vacancy, or trapping of a second vacancy. The relaxation reaches values of 10 MHz at temperatures of 1500K, 1200 K and 1170 K, respectively, for NiAl, FeAl and PdIn. For NiAl and FeAl, the onset of relaxation is abrupt, suggesting an activation enthalpy for the associated vacancy motion of several electron-volts. For PdIn, the relaxation has an activation enthalpy of 1.00(19) eV, a value significantly lower than might be expected naively on the basis of diffusion data. This difference gives insight into vacancy motion and diffusion mechanisms in PdIn.

The fractional concentration of Ni-vacancies in NiAl at high temperature has been determined for compositions between 50 and 53 at.% Ni from measurements using perturbed angular correlation of gamma rays (PAC). The vacancies were detected by quadrupole interactions induced at nearby 111 In/Cd impurity probes present on the Al sublattice in high dilution. One set of measurements was made at high temperature. A second set made after rapid quenching exhibited an enhancement of Ni-vacancy site fractions that is attributed to diffusion and trapping of vacancies during quenching. The composition dependence of the enhancement was analyzed assuming that the enhancement is proportional to the root-mean-square diffusion length during quenching. The experimental dependence on composition is found to be consistent with bulk diffusion data. The methodology developed clarifies how local environments of impurity probes are modified during rapid quenching. In addition, it is shown that there is an enormous increase in the mobility of vacancies with increasing deviation of the composition from stoichiometry. Diffusion mechanisms that can explain the data trends are discussed.

Mössbauer measurements were made on FeRh containing 52 atomic percent (at.%) Fe after mechanical milling for different times in a high-energy SPEX 8000 vibrator mill. Hyperfine fields are compared with fields for annealed ferromagnetic (F) samples having the B2 structure in the range 52-58 at.% Fe. For annealed F samples, hyperfine field shifts of ö1.66 T and ö1.12 T were detected at majority FeFe and minority FeRh probes due to FeRh antisite atoms in, respectively, the first and second atomic shells. Analysis of dipolar fields indicates that the magnetization lies along the <110> direction. The transformation from F B2 phase to a metastable paramagnetic fcc phase was observed that was half complete in 8 minutes. Analysis of spectra for the milled F B2 phase show that a second point defect was produced by milling that induces shifts of +6 T and +8.5 T, respectively, at majority and minority probes. Many-spectra fits were made under different defect models that led to the conclusion that milling produces point defects in the triple-defect configuration: 2 Fe-vacancies and 1 Fe-antisite atom. Defect concentrations were determined and show that the fractional concentration of vacancies on the Fe-sublattice increases linearly with milling time, reaching 3 at.% after 8 minutes, a very large value. The rate of transformation from B2 to fcc phase appears to be independent of the concentrations of point defects in the B2 phase, indicating that the transformation is purely stress-induced, as in a martensite transformation.

Point defects in NiAl and other structural intermetallics have been studied using perturbed angular correlation of gamma rays (PAC), by which signals are detected from antisite atoms and/or lattice vacancies in the first few atomic shells of probe atoms.  For samples of quenched NiAl having 50 to 54 at.% Ni, site-fractions of Ni-vacancies (proportional to the vacancy concentration) were found to be independent of composition, indicating that the equilibrium, high-temperature defect is the Schottky vacancy pair and not the so-called triple defect.  An effective formation enthalpy of 1.11(4) eV was measured for Ni-vacancies after equilibrating and quenching samples from temperatures in the range 700-1400 C.  The formation enthalpy of the Schottky pair is two times larger. Quenched-in vacancies exhibit striking, novel behavior at low temperature:  vacancies start to become mobile at about 350 C in 15-minute anneals, as observed by trapping at the probe atoms, but are only able to anneal out appreciably at temperatures of about 700 C.  This behavior is attributed to different mobilities of Ni and Al vacancies.  Recent data suggests that the Al-vacancy becomes thermally activated at about 350 C, converting into a Ni-vacancy and Ni-antisite atom when it comes near the probe atom.  The Ni-vacancy becomes mobile only at about 700 C. Thermal activation of motion of quenched-in and confined vacancies at low temperature, observed here for the first time, offers an explanation for the well-known transition from brittle-to-ductile mechanical behavior in NiAl near 300 C.  The explanation is supported by  a study of vacancy interactions with Zr solutes that is briefly described.  Results for other B2 intermetallics are also briefly described.

Point defect concentrations in milled samples of stoichiometric FeAl were obtained by Mûssbauer measurements. Mûssbauer spectra consisted of a single absorption dip with broadening attributed to unresolved hyperfine interactions. A local environment model was used to analyze spectra that allowed for defect-induced changes in monopole interactions at Fe probe atoms due to elementary defects in the two nearest atomic shells. It was found that pre-existing vacancies on the Fe sublattice disappear rapidly in the first five minutes of milling, with a corresponding rapid increase in the concentration of antisite-atom pairs. This behavior is very different to that observed in this laboratory for PdIn, using PAC, where vacancies are the principal defect produced. The difference is attributed to a lower energy of ordering for FeAl. A stationary state was observed after 10 minutes of milling, in which the mole fraction of antisite atoms, the average lattice parameter, and the monopole interactions of defect-free probes all ceased to change further. The mole fraction was observed to become stationary at about 10 atomic percent.

Local configurations of point defects were detected with atomic-scale resolution in mechanically-milled PdIn and NiAl through nuclear quadrupole interactions they induce at neighboring sites of 111 In probe atoms, using the technique of perturbed angular correlation of gamma rays (PAC). For PdIn, signals were detected which could be attributed to Pd-vacancies, Pd-antisite atoms and In-vacancies, as in previous studies of annealed or quenched PdIn. In addition, a new signal has been observed which is attributed to high-energy In-antisite atom defects next to the In probe. Concentrations of Pd-vacancies and In-antisite atoms were determined from measured site fractions and monitored as a function of milling time. For PdIn, milling in a WC vial under argon using a Spex 8000 mill leads to a Pd-vacancy concentration that increases from zero and saturates after 30 minutes at a value of 3.5(5) at.%. Such large vacancy concentrations make a contribution to the stored excess enthalpy of about 4.4 kJ mol -1 . The In-antisite atom concentration reached ~4 at.% after 2 hours of milling, providing evidence of gradual disordering of the B2 structure. Spectra for NiAl exhibit larger vacancy site fractions attributed to binding with the impurity In probe.

We are applying perturbed angular correlations (PAC) to study point defects near 111 In probes in ordered B2 alloys. Systems studied include NiAl, CoAl, FeAl, CoGa and PdIn Ten different point defect configurations have been observed in annealed, quenched or mechanically-milled samples. The present status of this work is summarized. Systematics of hyperfine interactions of vacancy defects are then examined. It is found that quadrupole interactions due to first-shell transition-metal vacancies vary in good approximation as the reciprocal cube of the lattice-parameter. Quadrupole interactions due to configurations of two first-shell vacancies exhibit deviations from predictions of a simple point-charge model, which can be explained by small amounts of local lattice relaxation of the probe atom toward the vacancies, of order 0.02 nm.

Perturbed angular correlations of gamma rays (PAC) is being applied to study defects in ordered intermetallic alloys. Vacancies on both Pd and In sublattices in the B2 system PdIn were detected after quenching through quadrupole interactions induced at nearby 111 In probe atoms. Fractions of probe atoms having each type of neighboring defect were observed to increase monotonically with quenching temperature over the range 825-1500 K. For compositions close to 50.15 at.% Pd, nearly equal site fractions were observed for Pd and In vacancies, indicating that the Schottky vacancy-pair defect is the thermal defect at high temperature. The formation enthalpy of the Schottky defect was determined to be 1.3(2) eV through analysis of quenching data from in the range 825-1200 K. Above 1200 K,  however, the vacancy concentration was observed to saturate at a value of 1.4(2) atomic percent, perhaps due to breakdown of the law of mass action for high defect concentrations.

The interaction between hydrogen atoms and lattice vacancies in metals has been studied with atomic-scale resolution using perturbed angular correlations of gamma rays (PAC).  The hydrogen atoms and vacancies have been detected through changes in nuclear quadrupole interactions at adjacent 111In/Cd probe nuclei.  Results are summarized for Pt samples which had been doped with complexes of probe atoms containing 1-4 vacancies.  Hydrogen (H) was introduced by cathodic charging.  Transformations were observed between H-free and H-decorated complexes.  During charging, signals were identified with complexes decorated first with one and later with more than one H atom (possibly two).  During desorption of H, decorated sites were observed to transform into undecorated sites. Binding enthalpies of H to the vacancy complexes were determined from changes of site fractions during desorption and were all found to be close to 0.25 eV.  Extremely large changes in the quadrupole coupling parameters of the 3V and 4V complexes were observed when H trapped, but not of the 1V and 2V complexes.  This is attributed to atomic reconstruction of the multivacancy complexes; that is, to large local lattice relaxations.  This indicates that the relative stability of small multivacancy complexes against rearrangement is small.  Detailed structural models of the complexes are described.