Monte Carlo simulations of the isothermal long-range order (LRO) relaxation in A3B system with Ll2 superstructure have been performed within a model based on a vacancy jump mechanism between nearest neighbour lattice sites. The studies aimed at the explanation of the origin of two simultaneous LRO relaxation processes experimentally observed in Ni3Al. An effect of pair interaction energies and saddle-point energies (assigned to jumping atoms) was studied. The preference of vacancies for the A-sublattice (face centres), commonly postulated for Ni3Al, occurred only for a narrow range of pair-interaction energies within the domain corresponding to Ll2 ordering. It was found that the appearance of the fast relaxation process is definitely correlated with the efficiency of B-atom jumps, as well as with the values of the saddle-point energies. The results lead to a microscopic model of the “order-order” relaxation in Ll2 superstructure and yield new indications for the choice of pair-interaction energy parameters applied in Monte Carlo simulations of atomic migration in Ll2 intermetallic compounds.

The temperature dependence of disordering of Cu3Au during ion irradiation has been investigated by computer simulations using MONTE CARLO methods. The investigation was motivated by peculiar experimental observations that the initial disordering rate begins to decrease above ~300 K, which can be explained by vacancies just becoming mobile at that temperature, but then at ~ 475 K, it reaches a minimum and then increases rapidly as the temperature is further increased, up to the order-disorder temperature. The present simulation shows that this behavior can be understood in terms of temperatures dependencies in both atomic mixing in the cascade and the efficiency of vacancy annealing of disorder as the vacancies diffuse out of the cascade.

The elementary diffusion jump of Fe atoms in the ordered intermetallic alloy FeAl is studied in a coordinated effort of atomistic experimental techniques, Monte Carlo simulation and abinitio electron theory. The experiment demands that the elementary diffusion jump is a jump into an antistructure site on the Al sublattice which is occupied for a much shorter time than the sites on the Fe sublattice. The diffusion path can be followed by Monte Carlo simulations which can perfectly explain the experiments. Since ab-initio theory yields a very low concentration of Al vacancies it is suggested that correlated jumps of two atoms prevent the creation of a fully developed Al vacancy.

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 111In/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.

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 111In/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 NiAI, FeAl and PdIn. For NiAl and FeAI, 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 interdiffusion coefficient is related via the modified Darken equation to the tracer diffusivities of the constituents. The thermodynamic factor Φ entering the Darken equation for the interdiffusion coefficient is expressed in terms of the energy parameters which describe the ordering process in the binary alloy. Our calculations for Fe-Al alloys yield the dependence of Φ on the long-range order parameter. These calculations are performed with the parameters extracted from diffuse X-ray scattering data on FeAl.

Using Monte Carlo simulation, we model the effect of interaction of vacancies on their diffusivities in ordered structures of B2-type. The activation barriers of atom jumps to vacancy are calculated by the static relaxation technique using interatomic potentials for both vacancy exchanges: with nearest-neighbour atoms and next-nearest-neighbour atoms. This is done for all possible positions of the second vacancy. Knowing these barriers it is possible to calculate the jump rates and to model the vacancy migration. The calculations were done for a system with parameters similar to NiAl. The modeling allows to inter the existence of a new diffusion mechanism in ordered structures of B2-type, which could be attributable to dynamic pair of vacancies and vacancy exchanges with next-nearest-neighbor atoms.

Classical diffusion measurements in intermetallic compounds are often complicated by low diffusivities or low solubilities of the elements of interest. Using secondary ion mass spectrometry for measurements over a relatively shallow spatial range may be used to solve the problem of low diffusivity. In order to simultaneously obtain measurements on important impurity elements with low solubilities we have used ion implantation to supersaturate a narrow layer near the surface. Single crystal NiAl was implanted with either 12C or both 56Fe and 12C in order to investigate the measurement of substitutional (Fe) versus interstitial (C) tracer diffusion and the cross effect of both substitutional and interstitial diffusion. When C alone was implanted negligible diffusion was observed over the range of times and temperatures investigated. When both Fe and C were implanted together significantly enhanced diffusion of the C was observed, which is apparently associated with the movement of Fe. This supports one theory of dynamic strain aging in Fe alloyed NiAl.

Kinetic properties of γ-phase (Cu5Zn8) growth in the binary system Cu-Zn were studied at 160-270°C. It has been found that γ-phase grows in diffusional-kinetic regime. Both kinetic coefficients of interfacial reaction rate β and diffusion coefficients D have been determined taking into account Kirkendall effect. Arrhenius equations for the kinetic and diffusion coefficients have been obtained, with the high values of “dynamic” diffusion coefficients and low activation energies for both diffusion (QD) and interfacial reaction (Qβ). The mechanism of Zn diffusion in non-stoichiometric Cu5Zn8 phase is discussed.

Measurements of solute GB diffusion in Cu and Ag polycrystals were carried out in Harrisons type-B and type-C kinetic regime using 75Se, 195Au, 63Ni radioisotopes. In the B-regime at high temperatures, the GB diffusion parameter sδDGB was determined (s being the GB segregation factor, δ the GB width and DGB the GB diffusion coefficient), while in the C-regime at low temperatures DGB values were measured directly. The investigations of these differently behaving solute–solvent systems on Cu and Ag basis yield information on solute GB diffusion and segregation in relation to solute–solvent atomic binding and solute–vacancy interaction in the bulk and in GBs. The GB diffusion of 64Cu and 195Au in a series of near ∑ = 5, Θ = 36.9° (310) [001] Cu tilt GBs and of 71Ge in a series of near ∑ = 7, Θ = 38.2° (123) [111] Al tilt GBs in dependence on the tilt angle Θ and on the temperature was studied. A characteristic minimum in the GB diffusion parameter and a maximum in the activation energy was observed at the ideal CSL GBs.

We consider the problem of solute diffusion and segregation in the grain boundaries moving during a phase transformation in the framework of Cahn's impurity drag model. The concept of a dynamic segregation factor for the diffusion along moving grain boundaries is introduced. The difference between static and dynamic segregation factors may cause the apparent difference of the triple product of the segregation factor, grain boundary width and grain boundary diffusion coefficient for stationary and moving grain boundaries. The difference between static and dynamic segregation is experimentally verified for the Cu(In)-Bi system, for which the parameters of static segregation are well-known. It is shown that the complications associated with the dynamic segregation may be avoided during the study of the discontinuous ordering reaction. From the kinetics of this reaction, the activation energy of the grain boundary self-diffusion can be determined.

Simultaneous grain boundary self-diffusion of iron and cobalt in FeCo was investigated in the B-kinetic regime by the radiotracer method combined with a serial sectioning technique over the temperature range 873-1133 K, which includes the A2-B2 bulk order-disorder transition temperature. This transition leads to a decrease in both iron and cobalt mass transport mainly owing to a change in the pre-exponential factor, but not in the diffusion activation energy. Moreover, the breaking point in the Arrhenius plots is observed below the bulk order-disorder transition temperature. In view of recent surface segregation studies on Fe-Co, and according to the Fe-Co phase diagram, the results are discussed assuming an iron segregation, which decreases the A2-B2 transition temperature in grain boundaries.

Location of diffused 57Co atoms in single crystals, bicrystals and polycrystals of pure iron and Fe72Al28alloy were investigated by means of emission Mössbauer spectroscopy. To interpret the results, first principles calculations of iron atom magnetic moments and hyper-fine field were carried out. From comparison of M6ssbauer spectra of single crystals with those of bicrystals and polycrystals, an information about grain boundary positions occupied by diffusing atoms is obtained. It is shown that about 5% of the diffusing atoms at the {112} grain boundary of iron are located at the positions either having impurity atoms in the nearest neighbourhood or characterized by larger atomic spacing in comparison with the bulk. In the Fe72Al28 a dominating portion of diffusing atoms have different surrounding than in grain volume. An enrichment of grain boundaries by aluminum could explain their hyperfine parameters.

The co-deposition of two metals using a molecular beam epitaxy (MBE) technique at various growth temperatures (TG) yields single-crystal alloy thin films with a columnar microstructure whose structural and magnetic properties can be different from those of the corresponding equilibrium bulk alloys. A general overview of results obtained in hcp Co-Ru and hcp or fcc Co-Pt thin films grown on a hcp (0002) substrate will be presented. Around the A 3B composition (A = Co, B = Ru or Pt) the films display a composition modulation along the growth direction whose amplitude is strongly dependent of TG passing through a maximum at respectively 600 and 650 K. This long range order (LRO) that does not exist in equilibrium phases is explained as resulting from the competition between two phenomena occurring simultaneously during the growth process: a surface effect driven by surface interactions and surface diffusion that tends to enrich the surface layer in one element (Co, Ru or Pt segregation) and a bulk effect driven by bulk interactions and bulk diffusion that tends to restore the bulk equilibrium phase when the bulk diffusion becomes efficient during the growth time. A thermally activated model that takes into account both surface and bulk diffusion during the growth time reproduces quite satisfactorily the TG dependencies of the LRO in Co3Ru and Co3Pt as well as that of the uniaxial magnetic anisotropy in CoPt3.

At high TG ( > 800 K) or after ex situ anneals, the diffusion of the Ru buffer through the grain boundaries of the columnar microstructure, that occurs before the inside grain diffusion, isolates the columns magnetically and is thought to be responsible of the large observed coercive field and of a substantial modification of the magnetic domain shapes.

Instabilities arise out of dynamic events and can lead to nonequilibrium (self-organization) processes. Ion irradiation is by nature a nonequilibrium process and hence formation of structures, metastable or otherwise is to be expected. Recently, it has been theoretically predicted that metals under ion irradiation can lead to dissipative structure formation arising out of radiation damage and their subsequent annealing. The possibility of direct observation of such structures in metals, under irradiation, is however reduced due to nonavailability of a large concentration of defects (mainly point defects) at any point of time. In this experimental presentation we show that this can be overcome through the involvement of microstructural imperfections which rearrange during irradiation. Employing microstructurally impure specimens of Fe and Ni, it is shown that heavy ions dissipate their electronic energy to modify atomic arrangements at the microstructure. The increased concentration of defects (atomic rearrangements), amenable to statistical decay is shown to produce structures in the 4-probe resistivity measurements which we assign to dissipative structure formation.

Many irradiation effects in Fe-Cr-Ni alloys such as radiation-induced segregation, radiation-enhanced diffusion, and void swelling are known to vary with bulk alloy composition. The development of microstructural and microchemical changes during irradiation and during post-irradiation annealing is determined by the rate of diffusion of point defects and alloying elements. To accurately predict the changes in grain boundary chemistry due to radiation-induced segregation and post-irradiation annealing, the composition dependence of diffusion parameters, such as the migration energy, must be known. A model has been developed which calculates migration energies using pair interaction energies, thereby accounting for the effect of composition on diffusivity. The advantages of this calculational method are that a single set of input parameters can be used for a wide range of bulk alloy compositions, and the effects of local order can easily be incorporated into the calculations. A description of the model is presented, and model calculations are compared to segregation measurements from seven different iron-chromium-nickel alloys, irradiated with protons to doses from 0.1 to 3.0 dpa at temperatures between 200°C and 600°C. Results show that segregation trends can be modeled using a single set of input parameters with the difference between model calculation and measurement being less than 5 at%, but usually less than 2 at%. Additionally, model predictions are compared to grain boundary composition measurements of neutron irradiated 304 stainless steel following annealing. For the limited annealing data available, model calculations correctly predict the magnitude and time scale for recovery of the grain boundary composition.

We present a scanning tunneling microscopy (STM) study, performed at different elevated temperatures, on the influence of Sb surface segregation on the morphology of the (111) surface of a Cu-0.45 at.% Sb solid-solution single-crystal; the surface was initially cleaned at room temperature by Ar+ ion sputtering. Unexpectedly, when the temperature is increased from room temperature to 380°C, the typical (111) surface morphology, obtained after sputtering, evolves in two very different manners that depend on the heating rate. If the heating rate is rapid (approximately a few minutes), it evolves to a structure with large terraces, whereas if it is slow (about 10 hours) the morphology does not evolve -- i.e., it is frozen. These unexpected results are interpreted in terms of excess subsurface point defects (vacancies and self-interstitials) created during ion bombardment, which are mobile and can mediate Sb diffusion at low temperatures. This is mainly on step edges, but the point defects can precipitate out of solution, for a rapid heating rate, thereby forming small secondary clusters in the near-surface region, in which Sb atoms are trapped.

Kinetics of growth and lateral spreading of an intermetallic layer along a free surface during surface interdiffusion in binary systems have been studied both theoretically and experimentally. By comparison of experimental data on intermetallic growth kinetics with the proposed theory, the surface diffusion constants have been calculated for intermetallics Cd21Ni5 and Cu5Zn8, and mechanisms of surface diffusion have been discussed.

Diffusivities of Cr in icosahedral quasicrystals of Al-Pd-Mn system have been measured by the radioactive tracer method in combination with a precision grinding technique. The temperature dependence of the diffusivities follows the Arrhenius equation in the investigated temperature range from 850 to 1107 K. The activation energy Q = 235.5 ± 7.8 kJmol-1 and pre-exponential factor D0 = 1.2 × 10-2 m2s-1 were evaluated. The diffusion behavior is discussed in terms of a vacancy diffusion mechanism.

Diffusion mechanisms and point defects in GaAs and related III-V compounds are discussed. An understanding of the As sublattice situation has been arrived at fairly recently and is presently tentative. Understanding of the Ga sublattice situation has become more acceptable in that experimental results are consistently explained by the Fermi-level effect and the As4 pressure effect. On the Ga sublattice, though controversies still exist, some are readily resolved by noting the role of the electric field produced by semiconductor electrical junctions, physical junctions, and surfaces.