P.R. Okamoto

Radiation-induced segregation in binary and ternary alloys☆

Abstract A review is given of our current knowledge of radiation-induced segregation of major and minor elements in simple binary and ternary alloys as derived from experimental techniques such as Auger electron spectroscopy, secondary-ion mass spectroscopy, ion-backscattering, infrared emissivity measurements and transmission electron microscopy. Measurements of the temperature, dose and dose-rate dependences as well as of the effects of such materials variables as solute solubility, solute misfit and initial solute concentration have proved particularly valuable in understanding the mechanisms of segregation. The interpretation of these data in terms of current theoretical models which link solute segregation behavior to defect-solute binding interactions and/or to the relative diffusion rates of solute and solvent atoms the interstitial and vacancy migration mechanisms has, in general, been fairly successful and has provided considerable insight into the highly interrelated phenomena of solute-defect trapping, solute segregation, phase stability and void swelling. Specific examples in selected fcc, bcc and hcp alloy systems are discussed with particular emphasis given to the effects of radiationinduced segregation on the phase stability of single-phase and two-phase binary alloys and simple Fe-Cr-Ni alloys.

Segregation of alloying elements to free surfaces during irradiation

Abstract Recent transmission electron microscopy examinations of a number of face-centered-cubic and body-centered-cubic metals and alloys irradiated by heavy ions or by high-energy electrons have shown thatdynamic interactions of displacement damage with impurities and alloying elements lead to segregation and/or to the formation of second phases at internal surfaces such as voids. To date, the phenomenon has been observed in an experimental 18Cr8Ni1Si stainless steel, in commercial 316L stainless steel, in vanadium and in nickel. In the electron irradiated Fe18Cr8Ni1Si alloy, analysis of the segregation-induced strain field around the voids indicates that during irradiation minor substitutional alloying elements with negative and positive size factors segregate towards and away from the void surface respectively. Preliminary Auger spectroscopy analysis indicates that a similar segregation phenomenon occurs at the external irradiated surface in nickel-ion bombarded 18Cr8Ni1Si stainless steel. These results suggest that undersized substitutional elements may tend to preferentially interchange positions with oversized solutes in interstitial sites, and that transport by interstitials may dominate segregation to defect sinks.

A theory of radiation-induced segregation in concentrated alloys☆

Abstract A new and simple theory for radiation-induced segregation in concentrated alloys is presented. The coupling between defect fluxes and atom fluxes is accounted for by the concept of preferential migration of vacancies and interstitials via A-atoms or B-atoms in a binary A-B alloy. Similarly, atom fluxes are partitioned into those occurring via vacancies and via interstitials. This approach permits expression of the defect fluxes and atom fluxes in terms of partial diffusivity coeffi- cients and concentration gradients of defects and alloy components. The time and space dependence of the defect concen- trations and composition of a binary alloy is described by a set of three coupled partial differential equations containing four partial diffusivity coefficients, i.e., those of A-atoms and B-atoms diffusing via vacancies and via interstitials. The set of differential equations has been integrated for some model binary alloys with complete miscibility, utilizing the geometry of a thin foil. The sample calculations are in good qualitative agreement with the general features of radiation-induced segregation as deduced from experiments. The temperature, dose and dose-rate dependencies of segregation in concentrated alloys are found to be similar to those predicted by the Johnson-Lam model for dilute alloys.

Physics of Crystal-to-Glass Transformations

Publisher Summary This chapter focuses on the relationship between melting and solid-state amorphization. It discusses that amorphization is a disorder-driven melting process, occurring below the glass transition temperature, and that a unified approach to heating- and disorder-induced melting is found in the generalized version of the Lindemann melting hypothesis. This hypothesis assumes that the melting of a defective crystal occurs when the sum of the static and thermal mean-square displacements reaches a critical fraction of the interatomic spacing, which was shown to be equivalent to a generalized T o -concept resulting from a disorder-induced softening of the shear modulus. Comparisons with available experimental data and with the predictions of microscopic defect-mediated melting models were made to establish the validity of the generalized Lindemann melting criterion. The chapter explores that the disorder-induced melting concept provides a new thermodynamic approach to understand other materials problems, including brittle fracture and stress-corrosion cracking. Stress-corrosion cracking is viewed as premelting of high-energy grain boundaries, because of the combined effects of applied stresses and the segregation of insoluble impurities in lowering the melting temperature of grain boundaries to the ambient temperature. Stress-induced melting may also occur in the vicinity of moving crack tips. However, as revealed by atomistic simulations, the local melting is a transient phenomenon at elevated temperatures and, hence it is observable only at temperatures below the glass transition temperature where the liquid phase persists indefinitely as a glass.

Effect of solute misfit and temperature on irradiation-induced segregation in binary Ni alloys☆

Abstract Four solid-solution, binary alloys of 1 at.% Al, Ti, Mo and Si in Ni were irradiated with 3.5-MeV Ni + ions at temperatures between 385 and 660° C. Auger analysis of the solute concentration as a function of depth shows that the oversize solutes, Al, Ti and Mo, are depleted near the irradiated surface, whereas the undersize solute, Si, is enriched. The magnitude of this irradiation-induced segregation in the Ni-1 at.% Si alloy is sufficient to cause precipitation of a surface layer of Ni 3 Si after a total dose of 5 dpa near 600° C; the segregation diminishes at both lower and higher temperatures. The observed temperature dependence is in qualitative agreement with a recently proposed theory of irradiation-induced solute segregation, but quantitative differences exist.

Solute segregation and precipitation under heavy-ion bombardment

Abstract Solute segregation and precipitation in dilute Ni-base binary alloys during heavy-ion bombardment were studied using a kinetic model. Spatially-dependent defect—production rates corresponding to 75-keV and 3-MeV Ni+ ion bombardment and various defect—solute interactions were considered in the calculations. For strong interstitial—solute binding, solutes in the peak-damage region are transported to the sample surface and into the bulk beyond the damage range, resulting in a solute depletion at the damage peak. However, for a vacancy—solute binding energy of ~0.05 eV, the solute-segregation trends are reversed from the strong interstitial—solute interaction case. With larger vacancy—solute binding energies, solute enrichment occurs at the surface. The present theoretical predictions are qualitatively compared with recent experimental measurements of the temperature and spatial dependence of radiation-induced segregation of undersize and oversize solutes in Ni-base alloys.

Disorder-induced amorphization

Many crystalline materials undergo a crystalline-to-amorphous (c-a) phase transition when subjected to energetic particle irradiation at low temperatures. By focusing on the mean-square static atomic displacement as a generic measure of chemical and topological disorder, we are led quite naturally to a generalized version of the Lindemann melting criterion as a conceptual framework for a unified thermodynamic approach to solid-state amorphizing transformations. In its simplest form, the generalized Lindemann criterion assumes that the sum of the static and dynamic mean-square atomic displacements is constant along the polymorphous melting curve so that c-a transformations can be understood simply as melting of a critically-disordered crystal at temperatures below the glass transition temperature where the supercooled liquid can persist indefinitely in a configurationally-frozen state. Evidence in support of the generalized Lindemann melting criterion for amorphization is provided by a large variety of experimental observations and by molecular dynamics simulations of heat-induced melting and of defect-induced amorphization of intermetallic compounds.

Solute redistribution processes in ion bombarded alloys

Abstract An overview of radiation-induced solute redistribution process in ion bombarded alloys is presented and some applications to damage correlation studies are described. The type and amount of solute redistribution which occurs and its temporal and spatial dependence are affected not only by the target material and irradiation conditions, but also by the mass and energy of the incident ion. Because of the sensitivity of radiation damage effects in alloys to changes in composition, an understanding and an appreciation of the magnitude of these redistribution effects are essential for a proper interpretation and correlation of damage microstructures produced in alloys by different bombarding ions. Furthermore, once understood, these redistribution effects can be used to quantify differences in the calculated atomic displacement rates and the net production rates of freely migrating defects for different ions. Such information is a prerequisite for quantitative damage correlation, and requires a quantitative understanding of the dose-rate dependence of solute redistribution effects. Recent experimental and theoretical studies which provide the basis for such applications of solute redistribution effects are discussed.

Effects of solute segregation and precipitation on void swelling in irradiated alloys

Abstract A model of radiation-induced segregation of substitutional solutes has been used to study the effects of solute segregation and precipitation on void swelling behavior in metals and alloys. If the binding energies between point defects and solute atoms are sufficiently large, significant solute segregation takes place during irradiation, giving rise to a solute depletion or enrichment at defect sinks and a nonuniform distribution of defect-trapping centers in the matrix. The swelling reduction, which results from trapping of point defects by minor additions of alloying elements, is affected by radiationinduced solute redistribution in alloys. The effects of either dominant interstitial-solute interactions or dominant vacancy-solute interactions on segregation and swelling were studied. The present calculations are in good qualitative agreement with experimental results.

Fundamental aspects of ion beam surface modification: defect production and migration processes

Abstract Ion beam modification of metals is generating increasing scientific interest not only because it has exciting technological potential but also because it has raised fundamental questions concerning radiation-induced diffusion processes. In addition to the implanted species, several defect production and migration mechanisms contribute to changes in the near-surface composition of an alloy during ion bombardment, e.g. atoms exchange positions via displacements and replacement sequences, preferential sputtering effects arise, and radiation-enhanced diffusion and radiation-induced segregation occur. The last two defect migration mechanisms are of particular significance since they can alter the composition to depths which are much greater than the implanted ion range. By altering various parameters such as irradiation temperature, ion mass, energy and current density and by changing the initial alloying distributions, a rich variety of near-surface composition profiles can be created. We have utilized changes in ion mass and energy and irradiation temperature to distinguish defect production from defect migration effects. Experimental results are presented which provide a guide to the relative efficiencies of different mechanisms under various irradiation conditions.