N. Karpe

Ion irradiation induced grain growth in nanocrystalline Fe and Fe (Zr)

Abstract Thin films of nanocrystalline Fe and Fe95Zr5 were prepared by electron-beam evaporation. Grain growth induced by 500 keV Ar+ and Xe+ irradiation was investigated using dark-field electron microscopy and X-ray diffraction. After irradiation with different ions, a large difference in overall grain growth is found for similar amounts of displacement per atom (as estimated from TRIM computer simulations). Rapid grain growth is oberved only for 500 ke V Xe+ irradiation, which causes dense collision cascades. This observation provides new support for earlier suggestions of radiation induced grain growth being associated with diffusion during the quenching of atomic-collision cascades. In contradiction to earlier assumptions about irradiation induced grain growth, the grain-boundary mobility in nanocrystalline Fe is found not to be proportional to the elastically deposited energy FD. The data appear to be in agreement with a FD2 dependence as suggested from a thermal-spike model recently proposed by Alexander and Was.

The thermodynamic factor in interdiffusion: A strong effect in amorphous Ni-Zr

Abstract Simultaneous determinations of the interdiffusivity in amorphous Ni-Zr and of the self-diffusivity of Ni in the same phase permit the determination of the thermodynamic enhancement factor for interdiffusion. Using thin compositionally graded films containing an initially thin layer enriched in 62Ni, the evolution on annealing of the composition profile was determined by Rutherford backscattering spectrometry and that of the 62Ni profile by secondary ion mass spectrometry. The effective thermodynamic factor in a-Ni57Zr43 at 623 K is found to be 33 ± 8 in the first hour of annealing of an as-deposited film, decreasing on prolonged annealing. This factor, larger than any previously reported, is somewhat in excess of the value expected from the Darken analysis, but does not deviate markedly as has been suggested by recent calculations questioning the validity of the mean-field approach in that analysis.

Study of the diffusion of Ni into amorphous Ni50Zr50 and the resulting strain formation

Abstract A set of multilayers of Ni/amorphous Ni50Zr50 of various periodicities was deposited using dc planar magnetron sputtering. The diffusion of Ni into the amorphous phase and the strain build-up in the Ni layers were measured in situ from the reduction and shift in Bragg peak intensity and angle respectively. For short periodicities, a lowered interdiffusivity was observed in agreement with other earlier experimental observations and theoretical predictions of stress effects in some amorphous multilayers with a high viscosity. Furthermore, direct evidence of stress formation owing to an unbalanced diffusional flow of the two species in Ni-Zr was obtained. These findings give further support for Stephensons theory for lowered interdiffusion due to restricted stress relaxation in amorphous alloys.

Using electrical resistivity to analyze the kinetics of irradiation induced amorphization of metals

Abstract The use of electrical resistivity as a tool for analyzing the transformation kinetics during ion-irradiation induced amorphization of metallic alloys is discussed. It is pointed out that the amorphized volume fraction can be estimated with some confidence, because absolute bounds for the maximal and minimal volume fraction can be derived from established two-phase models for the resistivity. The promising and elegant in-situ measurements of the electrical resistivity by Jaouen, Delafond and Riviere [J. Phys. 17 (1987) 335] are reanalyzed by taking into consideration an effective medium theory for the resistivity in a two-phase mixture of a crystalline and an amorphous phase. The procedure is exemplified using their data for the amorphization of AlNi with 360-keV Xe2+ ions. In this case, evidence is found for amorphization through a direct intra-cascade mechanism. Also, the need is stressed for correcting data for an inhomogeneous depth distribution of the irradiation damage.

Influence of deposition conditions and ion irradiation on thin films of amorphous CuZr superconductors

Abstract Amorphous films of CuZr have been prepared by electron-beam (e-beam) codeposition and ion beam mixing. Various treatments, such as deposition onto liquid nitrogen cooled substrates and ion irradiation with 1 MeV Xe 2+ ions or 500 keV Ar + ions at 100 K and at 300 K, were used to vary the level of disorder in the amorphous state. The residual resistance ratio, the relative resistance change after irradiation, the temperature and width of the superconducting transition, and the upper critical field were measured and used to compare qualitatively the level of disorder and homogeneity. The following observations could be made: (1) amorphous thin films, e-beam deposited at 300 K and below, are more disordered than amorphous ribbons; (2) ion irradiation with Xe and Ar ions performed at 100 K and 300 K only leads to a slightly increased disorder level even for doses corresponding to about one displacement per atom; (3) ion irradiation gives rise to a homogenization of the specimens as evidenced by a decreasing width of the superconducting transition; and (4) a significantly increased level of disorder is achieved by low-temperature deposition of very thin films.

Nanocrystalline ion-irradiated gold Electrical resistivity used to study defects and grain boundaries

Abstract Thin films of nanocrystalline gold have been prepared by electron-beam deposition on to liquid-nitrogen cooled substrates, giving a grain size of about 8 nm. Some of these films were annealed at 773 K for 1 h, giving a second set of films with a grain size of about 40 nm. These two sets of films, denoted nanocrystalline and polycrystalline, were ion irradiated using 1 MeV Xe2+ and 500 keV Ar+ with doses up to about 10 displacements per atom. Changes in the electrical resistivity and its temperature dependence were measured. A relatively small resistivity contribution due to grain-boundary scattering was observed, giving no evidence for a strongly disordered grain-boundary phase. The temperature-dependent part of the electrical resistivity was found to be very similar to that of pure bulk gold, suggesting the same characteristic Debye temperature for nanocrystalline thin films and bulk gold (185±10K). The saturation level for irradiation-induced defects at room temperature was found to be similar ...

Diffusion in thin-film amorphous metallic alloys

Abstract Due to their potential technological value and the interest from a basic scientific point of view, amorphous metallic alloys have been studied extensively in recent years. This includes investigations of atomic transport processes which play a key role for understanding, for example, crystallization processes and the solid-state amorphization reaction. The first measurements of diffusion in amorphous alloys were carried out about two decades ago, being difficult because of crystallization yielding very narrow temperature and time intervals in which the measurements can be carried out. Such measurements have been made possible only with the development of ion beam analysis and sputter sectioning techniques. A short summary of the experimental diffusion studies, mainly based on ion beam analysis, is given together with various theoretical models. These are based on defect-mediated diffusion and collective motion of many atoms. Nonconventional diffusion behavior caused, e.g., by buildup of stress during thermal annealing, relevant for solid-state amorphization reactions, will be discussed, and studies of the thermodynamic factor will be addressed.

Measurements of self-diffusion of Ni and interdiffusion in thin-film amorphous NiZr using RBS

Abstract An application of RBS, making possible simultaneous measurements of the self-diffusion and the interdiffusion coefficients, is demonstrated for amorphous NiZr. Thin films with composition gradients were prepared by magnetron sputtering, film thickness being about 150 nm and composition variations between the surface and the bottom about 11 at.%. In the middle of the films, a thin layer enriched in 62Ni was deposited without changing the constant composition gradient. By RBS, the interdiffusion coefficient was derived from the change of the slope of the concentration profile during annealing, utilizing a simple approximation to the solution to the diffusion equation. Due to the isotope enrichment, a dip and a peak appeared in the RBS Ni signal, and from their changes due to thermal annealing, the self-diffusion coefficient was evaluated by comparison with simulated RBS spectra employing the RUMP program. The data of independent measurements of the self-diffusion using SIMS agree with the RBS results. The simultaneous measurements of self-diffusion and interdiffusion also enables the determination of the thermodynamic factor.