M. Kaisermayr

Enhanced iron self-diffusion in the near-surface region investigated by nuclear resonant scattering

The access to X-rays of third generation synchrotron radiation sources enables studies of dynamics in metallic systems in grazing incidence geometry. Combining grazing incidence reflection of X-rays with nuclear resonant scattering of synchrotron radiation allows depth-selective investigations of hyperfine parameters and diffusion phenomena of iron and iron compounds. The unique feature of this method is its sensitivity to near-surface motions of atoms and not exclusively to the atoms on the surface. The depth sensitivity can be varied between about two and more than 10 nm. A 300 nm thick 57 Fe sample grown by molecular beam epitaxy on a cleaved MgO(001) substrate was investigated. The diffusion coefficient of iron in the near-surface layer (thickness about 2 nm) is almost two orders of magnitude larger than in bulk bcc iron at the same temperature.

Coherent QENS for the investigation of self-diffusion in intermetallic alloys

Up to now, self-diffusion in intermetallic alloys has been successfully studied in various compounds containing strong incoherent scatterers, namely Ni and Co, by means of QENS. The range of accessible compounds could be significantly enlarged by using coherent scattering. We, therefore, derive the coherent scattering function for self-diffusion via vacancies in intermetallic alloys and consider how coherent scattering can be used for diffusion studies as well as how coherent background in incoherent experiments on intermetallic alloys can be taken into account.

Quasielastic scattering of synchrotron radiation from non-resonant atoms

The beat pattern produced by nuclear resonant scattering of synchrotron radiation scattered from two stainless steel foils in constant relative motion has been measured at 3-ID beamline at the Advanced Photon Source. Contrary to theoretical prediction and different from the result in glassy material, the scattering in Bragg directions from single crystal of CoGa diffusional smoothening of the quantum beats was absent within the statistical errors.

Diffusion Studies in Ordered Alloys

1. Introduction Diffusion is an important and fundamental phenomenon for the properties and the behavior of metals, alloys, semiconductors, ceramics, glasses and polymers at higher temperatures. It plays an important role in the kinetics of microstructural changes of a material. It is a driving force for nucleation of new phases, recrystallization and phase transformations with a wide use in current technology, e.g. surface hardening, changing of deformation behavior by nucleation, diffusion doping or sintering. Especially the intermetallic alloys attracted attention as suitable materials for high-temperature applications due to their corrosion stability and strength. Many of the physical processes, developed for intermetallics are applicable for diffusion in all crystalline solids. The knowledge of the diffusion behavior of intermetallic alloys is, therefore, of interest for basic material science and for their use in technological applications. [1]

Self-diffusion in the B2-ordered alloy NiGa

Abstract Quasielastic neutron scattering (QNS) has been used to investigate the self-diffusion of Ni in the intermetallic B2-compound NiGa. Our studies have shown that the Ni-atoms diffuse via nearest-neighbour jumps in contrast to the next-nearest-neighbour jump mechanism proposed on the basis of tracer diffusion experiments. The residence time of the Ni-atoms on the antistructure sites has been found to be of the order of 1/10 of the residence time on the Ni-sublattice. From this proportion the vacancy concentration has been derived.