L.E. Rehn

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.

Substrate effects on the structure of epitaxial PbTiO3 thin films prepared on MgO, LaAlO3, and SrTiO3 by metalorganic chemical‐vapor deposition

Epitaxial PbTiO3 films were prepared by metalorganic chemical‐vapor deposition on MgO(001)‐, SrTiO3(001)‐, and LaAlO3(001)‐oriented substrates. Four‐circle x‐ray diffraction, transmission electron microscopy, Rutherford backscattering (RBS) channeling, and optical waveguiding were performed to characterize the deposited films. Epitaxial, single‐crystal films were obtained on all three substrate materials under the same growth conditions. However, the defect structure of the films, including grain tilting, threading dislocation density, and 90° domain formation, was strongly dependent on the choice of substrate material. Films grown on MgO(001) and LaAlO3(001) (pseudocubic indices) substrates are nominally c‐axis oriented; however, the PbTiO3 grains in the film form a fourfold domain structure, with the grains tilted ∼0.6° and ∼0.7°, respectively, toward the [100] directions (cubic or pseudo‐cubic) of the substrates. In addition, these films contain a significant volume fraction of 90°‐domain (a‐axis) stru...

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.

Recent progress in understanding ion-beam mixing of metals

Abstract Recent progress in three fundamental areas related to the ion-beam mixing of metal targets is reviewed: (1) molecular dynamics simulations of the temporal development of energetic displacement cascades; (2) the substantially increased mixing efficiencies that have been reported in many systems at intermediate irradiation temperatures; and (3) amorphization during ion irradiation. Significant advances in our fundamental understanding have occurred in all three of these areas in the past few years. Mass transport at various times during the evolution of energetic displacement cascades in Cu and Ni has been calculated recently using fully dynamic computer simulations. These results, discussed in section 2, strongly support the conclusion drawn from earlier experimental work that a majority of ion-beam mixing occurs at low (1–2 eV) atomic recoil energies during the cascade cooling phase. In section 3, characteristic differences are identified between radiation-enhanced diffusion, that is mass transport due to freely-migrating vacancy and interstitial defects and the enhanced ion-beam mixing that has been observed in many materials at intermediate temperatures. It is argued here that true radiation-enhanced diffusion should be observable only at substantially higher temperatures. Finally, recent measurements of the shear elastic constant, the lattice parameter and the long-range order parameter during ion-induced amorphization of highly-ordered intermetallic compounds are summarized in section 4. This new information indicates that ion-induced amorphization is preceded by a first-order phase transformation triggered by an elastic instability and reveals several parallels between solid-state melting and amorphization phenomena.

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.

Growth of aluminum nitride thin films on Si(111) and Si(001): Structural characteristics and development of intrinsic stresses

We have grown aluminum nitride thin films by ultrahigh vacuum reactive sputter deposition on Si(111) and Si(001) substrates. We show results of film characterization by Raman scattering, ion beam channeling, and transmission electron microscopy, which establish the occurrence of epitaxial growth of wurtzitic aluminum nitride thin films on Si(111) at temperatures above 600 °C. In contrast, microstructural characterization by transmission electron microscopy shows the formation of highly oriented polycrystalline wurtzitic aluminum nitride thin films on Si(001). Real‐time substrate curvature measurements reveal the existence of large intrinsic stresses in aluminum nitride thin films grown on both Si(111) and Si(001) substrates.

Mechanical properties and microstructure of TiC/amorphous hydrocarbon nanocomposite coatings

Using the techniques of reactive magnetron sputter deposition and inductively coupled plasma (ICP) assisted hybrid physical vapor deposition (PVD)/chemical vapor deposition (CVD), we have synthesized a wide variety of metal-free amorphous hydrocarbon (a-C:H) and Ti-containing hydrocarbon (Ti-C:H) coatings. Coating elastic modulus and hardness have been measured by the technique of instrumented nanoindentation and related to Ti and hydrogen compositions. We show that both metal and hydrogen compositions significantly influence the mechanical properties of Ti-C:H coatings. The microstructure of Ti-C:H coatings is further characterized by transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES) spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. XANES spectroscopy and high-resolution TEM examination of Ti-C:H specimens shows that the dissolution limit of Ti atoms in an a-C:H matrix is between 0.9 and 2.5 at.%. Beyond the Ti dissolution limit, precipitation of nanocrystalline B1-TiC cluster occurs and Ti-C:H coatings are in fact TiC/a-C:H thin film nanocomposites. Measurements of the average Ti bonding environment in TiC/a-C:H nanocomposites by EXAFS spectroscopy are consistent with a microstructure in which bulk-like B1-TiC clusters are embedded in an a-C:H matrix.

Characterization of microstructure and mechanical behavior of sputter deposited Ti-containing amorphous carbon coatings.

Abstract We report on the characterization of microstructure and mechanical properties of sputter deposited Ti-containing amorphous carbon (Ti-aC) coatings as a function of Ti composition. Ti-aC coatings have been deposited by unbalanced magnetron sputter deposition, in an industrial-scale four-target coating deposition system. The composition and microstructure of the Ti-aC coatings have been characterized in detail by combining the techniques of Rutherford backscattering spectrometry (RBS) and hydrogen elastic recoil detection (ERD), transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. At Ti compositions 8 at.%, XANES and EXAFS data indicate that the average Ti atomic bonding environment in Ti-aC coatings resembles that in cubic B1-TiC, consistent with TEM observation of precipitation of TiC nanocrystallites in the a-C matrix. Beyond the Ti dissolution limit, the Ti-aC coatings are nanocomposites with nanocrystalline TiC clusters embedded in an a-C matrix. A large scale, quasi one-dimensional composition modulation in the Ti-aC coatings was observed due to the particular coating deposition geometry. Elastic stiffness and hardness of the Ti-aC coatings were measured by instrumented nanoindentation and found to vary systematically as a function of Ti composition. Unlubricated friction coefficient of Ti-aC coatings against WC–Co balls was found to increase as the Ti composition increases. As Ti composition increases, the overall mechanical behavior of the Ti-aC coatings becomes more TiC-like.

Microstructure and mechanical properties of Ti–Si–N coatings

A series of Ti-Si-N coatings with 0 < Si < 20 at.% were synthesized by inductively coupled plasma assisted vapor deposition. Coating composition, structure, atomic short-range order, and mechanical response were characterized by Rutherford backscattering spectrometry, transmission electron microscopy, x-ray absorption near-edge structure spectroscopy, and instrumented nanoindentation. These experiments show that the present series of Ti-Si-N coatings consists of a mixture of nanocrystalline titanium nitride (TiN) and amorphous silicon nitride (a-Si:N); i.e., they are TiN/a-Si:N ceramic/ceramic nanocomposites. The hardness of the present series of coatings was found to be less than 32 GPa and to vary smoothly with the Si composition.

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.