Radiation-induced atom transport controls phase stability under irradiation. Quantitative understanding of the latter phenomenon requires measurement of all relevant components of atom transport as well as of all quantities which characterize the concurrent phases. These requirements can be met for sufficiently simple systems. The paper gives an overview on studies by means of electron microscopy, field ion microscopy with atom probe, small angle neutron scattering, and diffusion coefficient measurements by depth profiling with dynamical secondary ion mass spectroscopy, all for samples irradiated with electrons, protons or heavy ions of Cu-Ni-Fe.

Evaluation of the data revealed detailed insights into the counteraction of precipitate dissolution by atomic mixing and re-precipitation by radiation-enhanced interdiffusion as well as into the evolution path of phase changes. In Cu-Ni-Fe the interstitial transport mechanism changes the tie-line for the alloy decomposition substantially when compared with the unirradiated case. Proton irradiation to fluences above 0.1 dpa indicate evolution of long-range composition fluctuations superimposed on the periodically decomposed two-phase structure. The obtained information show the importance of a combined application of microscopic, diffraction, and direct methods for atomic transport measurements. 1. Introduction

Cascade size may affect phase stability under irradiation because of two distinct contributions: the replacement to displacement cross section ratio depends on the deposited energy density; ballistic jumps which tend to disorder ordered compounds occur by bursts (of size b), while thermal jumps which restore long range order occur one by one.

The latter effect cannot be handled by standard rate theory. A stochastic treatment of the problem, based on a Fokker Planck approximation of the relevant master equation is summarized. It is shown that the possible values of the long range order parameter under irradiation are not affected by the size b of the bursts, but that the respective stability of the former is b dependent. As a consequence, the stability diagram of phases under irradiation varies with b. Such a diagram is computed for the Ni4Mo system where three structures are competing: the disordered solid solution, Dia and D023. A broadening by 100K of the stability domain of the short range ordered structure to the expense of the long range ordered one is predicted when increasing b from 1 to 100.

The stochastic potentials introduced in the present treatment are by no means free energies of some constrained state. They can however be computed in a mean field type approximation.

The spontaneous collapse of energetic displacement cascades in metals into vacancy dislocation loops has been investigated by molecular dynamics (MD) computer simulation and transmission electron microscopy (TEM). Simulations of 5 keV recoil events in Cu and Ni provide the following scenario of cascade collapse: (i) atoms are ejected from the central region of the cascade by replacement collision sequences; (ii) the central region subsequently melts; (iii) vacancies are driven to the center of the cascade during resolidification where they may collapse into loops. Whether or not collapse occurs depends critically on the melting temperature of the metal and the energy density and total energy in the cascade. Results of TEM are presented in support of this mechanism.

Defects introduced into single crystals of Al,Cu, and Fe by low temperature fast neutron irradiation were examined by diffuse scattering of X-rays. Structure and correlations of interstitials and vacancies were investigated after irradiation and during annealing. The primary defect structure is characterized by the correlation of defects in defect cascades. Size distributions for the arrangement of interstitials and vacancies were obtained. The evolution of the damage structure during recovery was followed and found to depend on the primary damage state. In Al we find large and dilute cascades with a mean radius of R-90Å, whereas in Cu small and dense cascades with R-20Å are produced. During recovery in Cu dislocation loops of interstitial and vacancy type are formed. In Al agglomeration of interstitials does not lead to loop formation. In Fe rather dilute cascades with R=60Å are observed. Here only small agglomerates of interstitials evolve during recovery in stages I and III.

High-resolution transmission electron microscopy was used to investigate the structure and growth kinetics of early-stage radiation-induced defects in commercially pure Al. The majority of defects were determined to be single and multiple interstitial Frank-partial loops, although some perfect dislocation loops and vacancy loops were also observed. The growth rates of loops with final radii less than 8 nm were recorded in situ and were found to be approximately 0.2 nm/sec for an electron density of about 8×1018 electrons/cm2.sec. These results are similar to previous high-voltage electron microscope analyses performed on larger radiation-induced defects in Al alloys.

The elastic diffuse neutron scattering from two Al-1.75 at.% Cu single crystals with either the isotope Cu-63 or Cu-65, aged to contain type-I Guinier-Preston zones has been measured along a reciprocal lattice line and evaluated based on an expansion of the atomic displacement scattering up to terms of third order. It is shown that erroneous results for the short-range order term are obtained if the evaluation does not include data from the first Brillouin zone. The most reliable short-range order scattering extracted for both crystals is consistent with essentially single-layered Cu-rich zones.

Rietveld analysis of Bragg intensities from in-situ neutron powder diffraction of nearly stoichiometric Mn 0.9990 0 reveals the presence of several percent Mn interstitials. Similar to isostructural Fe1−x 0, the cation interstitials are tetrahedrally coordinated in the rocksalt structure (Fm3m).

The x-ray diffuse scattering from a single crystal of Cu3Au was measured in absolute units in a volume of reciprocal space. The measurement was made while the crystal was held at 703 K (approximately 35 K above the critical temperature for ordering) and the measured intensity was separated into components due to short range order (SRO) and strain. The Warren short range order parameters, pair interaction potentials and the first and second order static atomic displacements were obtained by Fourier inversion of these separated intensities. The relative interatomic pair potentials, out to the third shell, were found to be V2/V 1=-0.5 and V3/V 10 consistent with earlier determinations. The first order atomic pair displacements were found to be contrary to what has previously been assumed: Au-Au first neighbor pairs were found to have negative displacements on average and AuCu pairs positive displacements, compared to the average interatomic distance.

The integrated X-ray diffraction intensities of quasi-forbidden reflections in gallium arsenide, such as the 002, are particularly sensitive to stoichiometry variations. We show here that the integrated intensity as a function of wavelength exhibits a minimum whose position is dependent on the stoichiometry. Using synchrotron radiation to tune the wavelength, we have performed measurements on a horizontal Bridgman grown crystal. A shift in the minimum is observed between measurements made from different parts of the sample consistent with arsenic loss at the surface of the crystal.

Spherical strain centres produced by annealing of silicon crystals grown in a hydrogen atmosphere have been studied by X-ray section topography. Excellent agreement has been found between experimental images and those simulated by numerical solution of Takagi's equations. The contrast differences between images in four reflections are examined and recommendations made as to the most suitable conditions for the study of oxygen precipitates in processed silicon wafers.

Elevated temperature 1MeV electron irradiations have been carried out on a series of Fe-Cr alloys (1-10wt. % Cr) and an Fe-10 wt % Mo alloy. Results show that dislocation loop populations are similar in all the Fe-Cr alloys and in the Fe-10Mo alloy, and consist of an approximately equal distribution of a<100> and a/2<l 11> Burgers vector types. Differences in loop morphologies and behaviour may be related to the nature of the solute.

Interstitial and vacancy loops in ion-irradiated copper have been studied through the use of x-ray diffuse scattering in the “Asymptotic” scattering region near the (111) reflection. Diffuse scattering measurements were made on copper single crystals irradiated at room temperature with He, O, and Si ions, and the results have been analyzed in terms of size distributions of interstitial and vacancy dislocation loops. The measurements were made in an “off-symmetry” direction (along the Ewald Sphere) using a single angular setting of a linear position sensitive detector. The He ion-irradiation was found to produce mainly interstitial loops while the 0 and Si irradiations produced both vacancy and interstitial loops.

Measurements of x-ray profiles and diffuse scattering from (111) and (100) oriented single crystal Niobium films implanted with Nitrogen to average levels of 5 and 0.5 atomic percent are discussed. Theoretical analysis of the asymmetric Bragg profiles are used to determine the strain profile in the implanted films. The measured strain profile results from two factors: (i) depth distribution of implants and knock-on damage and (it) elastic constraints. Residual elastic strains develop due to the constraints imposed by a sapphire substrate. Comparison of the diffraction results with theoretical predictions of TRIM indicates thde presence of measurable knock-on damage in the films. Huang and Stokes-Wilson scattering measurements made using synchrotron radiation at the ORNL beanmline, Brookhaven National Laboratory, were used to reveal the identity of defects formed during the knock-on process.

X-ray diffraction line profiles have been described in an earlier work published by the authors as a convolution of three functions: particle size broadening, and two types of non-uniform strain broadening. The strain coefficients used in this approach are simply related to the strain coefficients obtained by Krivoglaz et.al, using a theory based upon correlated dislocations. This connection enables us to determine the dislocation density and the ratio of the correlation range to the mean particle size. Results for cold worked fcc and bcc materials as well as highly imperfect sputtered films of Mo are considered. Dislocations are highly correlated in cold worked metals, whereas correlation is much lower in sputtered films deposited at low temperatures. In each case, the dislocation density is high. An analysis of wear debris consisting of cubic Zirconia gave the highest dislocation density and correlation. The close similarity between the early work on cold work filings and wear debris provides insights on wear mechanisms.

Iron alloys (Fe, Fe-0.7Ni, Fe-0.7Ni-0.025P) were irradiated at 288°C to a fluence of 4.63×1019 neutrons/cm2. Analysis of the as-irradiated alloys showed irradiation-induced defects in the form of interstitial prismatic dislocation loops. The defect size decreased and the defect number density increased with increasing impurity content. The pure iron alloy contained dislocation loops predominantly with an a<100> Burgers vector. The Fe-Ni and Fe-Ni-P alloys, however, contained loops predominantly with an a/2<111> Burgers vector. Annealing at 500°C increased the size and decreased the number density of dislocation loops in all the alloys. The a<100> dislocation loops seemed to grow at the expense of the a/2<111> dislocation loops. The results are explained by a solute trapping mechanism.

Monoenergetic recoil of 29 eV on 111In atoms due to neutrino emission in combination with the microscoui PAC method is utilized to study single Frenkel pairs in materials. The 111In atoms serve as primary knock-on atoms and are simultaneously nuclear probes for PAC. In this way microscopic information on the Frenkel pair formation process and the thermal behavior of vacancies and interstitials in several fcc metals (Cu, Ag, Al) have been obtained. In semiconductor Ge an intrinsic defect - probably a monovacancy -is observed. No indication of Frenkel pair production was found in the intermetallic compound PdIn.

Pulsed nuclear magnetic resonance proved to be a complementary new technique for the study of moving dislocations in Al-Mg-Zn alloys. The NMR technique, in combination with transmission electron microscopy have been applied to study dislocation motion in A1-0.6 at% Mg-1 at% Zn and Al-1.2 at% Mg-2.5 at% Zn. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been determined as a function of strain. From the NMR data it is concluded that moving dislocations advance over a number of solute atoms in these alloys as described by Mott-Nabarro's model.

The microstructural and compositional changes produced in the near surface region of single crystals of α-A12O3 by ion-implantation with iron have been investigated by a number of complementary techniques, including Rutherford backscattering spectroscopy (RBS), conversion-electron Mdssbauer spectroscopy (CEMS), analytical electron microscopy (AEM), and scanning transmission electron microscopy (STEM). Iron precipitates, 1 to 3 nm in diameter, have been identified in as-implanted material at depths corresponding to the peak in the deposited iron concentration profile. The same techniques have been used to monitor the redistribution of iron and the corresponding changes in valence state during post-implantation annealing. Electron microscopy has been used to correlate RSB and CEMS results with the microstructural development.

Microstructures play an important role in sintering. Hlowever, most existing sintering theories are macroscopic and do not consider the topological constraints in a microstructure. To fully take into account the microstructural effect, Anderson et al. [l] developed a Monte Carlo statistical mechanics model to study the grain growth behavior of a fully dense system. lHere we developed a generalized model which allows both densification and grain growth to occur. The densification occurs through the diffusion of matter and is incorporated in the Monte Carlo procedure. Thus we can monitor both the microstructural evolution and the densification at the same time during heat treatment. To our knowledge, this is the first statistical mechanics model constructed for sintering. As a quantitative example, the sintering behavior of an isolated pore of various sizes and shapes at different grain growth rates is studied.

Electron energy loss spectroscopy of voidite defects in natural diamond shows that they are composed of nitrogen which is likely to be present in the form of solid N2 under high pressure (ca. 10 GPa). A similar examination of platelets has failed to reveal any nitrogen in them. It is proposed that the voidites are generated by a discontinuous precipitation mechanism as the platelets decompose. A similar study of the the chemistry of nanometre sized mineral particles present in most diamonds shows they are related to the earth's magma although there are notable differences.