R. Würschum

High-temperature atomic defect properties and diffusion processes in intermetallic compounds

Abstract Data on thermal vacancy formation in intermetallic compounds obtained from positron lifetime spectroscopy yield high effective formation enthalpies H F V in close-packed structures and low values in bcc-type structures which can be well understood theoretically. The vacancy migration enthalpy H M V could be determined at high temperatures for B2-FeAl by studying the equilibration process after temperature changes. As demonstrated here in a comparative study on B2-FeAl the thermal formation and migration of defects can also be sensitively investigated by time-differential length-change studies after temperature changes in the vicinity of the equilibration temperatures. The present vacancy data can explain the wide variation of the transition metal self-diffusivities in intermetallic compounds. For B2-FeAl it is shown that the high-temperature mechanical properties are closely linked to the formation of thermal defects as evidenced by the temperature variation of the yield stress anomaly and its time dependence after fast heating.

Nanometre-sized solids, their structure and properties☆

Abstract Nanometre-sized or nanocrystalline materials are solids with very small grain size (5–10 nm) and, therefore, a considerable fraction of atoms located in the disordered interfacial structure, giving rise to novel physical properties and potential technological applications. X-ray diffraction, Mossbauer spectroscopy and positron lifetime measurements indicate a wide interatomic distance distribution in the interfacial structure. The thermal, elastic and magnetic properties, the internal friction behaviour, as well as the high diffusivities and reactivities are discussed in terms of the present results on nanocrystalline metals, semiconductors and metal oxides. Some future prospects are outlined.

Self-diffusion in high-density nanocrystalline Fe

Abstract In order to gain insight into the characteristic behavior of grain boundaries in nanocrystalline (n-) materials, high-density n-Fe specimens are prepared by compaction of gas-condensed nanocrystallites at elevated temperatures and the self-diffusion coefficients are measured by radiotracer techniques. The self-diffusion coefficients of n-Fe (relative density higher than 91 %) determined by assuming a type-C kinetics are similar to those extrapolated from high temperature data of conventional grain boundaries, suggesting that the grain boundaries in the high-density n-Fe are similar to those in conventional polycrystalline Fe.

Magnetic properties of high purity nanocrystalline nickel

Abstract High purity nanocrystalline Ni has been prepared by means of the inert gas condensation technique with oxygen contents of less than 0.5 at% compared to 6 at% after exposure to air. In-situ magnetic measurements with the specimens sealed in vials under vacuum indicate that the saturation magnetization of high purity nanocrystalline Ni is unchanged compared to the bulk value but reduced to ∼ 80% of the bulk value in specimens which were exposed to air. The magnetization process in nanocrystalline Ni changes from a homogeneous rotation in powder specimens to domain wall movement in specimens after compaction where magnetic domains are observed by means of magneto-optic Kerr techniques.

Interface structure studies by atomic resolution electron microscopy, order–disorder phenomena and atomic diffusion in gas-phase synthesized nanocrystalline solids

Abstract The paper summarizes recent studies of the structure and atomic diffusion properties of gas-phase synthesized nanocrystalline solids. The atomic structure of interfaces in nanocrystalline solids with vacancy-like free volumes and nanovoids of triple junctions is specifically studied by positron lifetime spectroscopy. The recently studied temperature variation of the positron lifetime indicates a strong temperature dependence of the positron trapping rate of these free volumes. From the investigation of the orientation correlationship of pairs of two adjacent crystallites in n-Pd by atomic resolution microscopy it can be concluded that predominantly high-energy interfaces are present in nanocrystalline metals after gasphase synthesis. Tracer substitutional-diffusion and self-diffusion studied in highly dense nanocrystalline metals demonstrate that the atomic diffusion is similar to that in conventional grain boundaries. The 18O diffusion in the interfaces of n-ZrO2 is by 3 to 4 orders of magnitude faster than volume diffusion which gives prospects for an increase of oxygen conductivity in nanocrystalline ion conductors. Nanocrystalline ordered intermetallics as, e.g. n-FeAl and n-NiAl can be prepared by gasphase condensation in a partially disordered state. The ordering in n-FeAl occurs at lower temperatures than in n-NiAl which is correlated to the different vacancy migration enthalpies in the two intermetallic alloys.

Structure and glass transition of amorphous Zr65Cu17.5Ni10Al7.5 studied by positron lifetime

Abstract Structural studies below and above the glass transition of bulk-amorphous Zr65Cu17.5Ni10Al7.5 were performed by means of high-temperature positron-lifetime measurements using a 58Co-positron source. A dense atomic packing is concluded from the observation of thermal detrapping of positrons from free volumes at elevated temperatures below the glass-transition temperature Tg. At Tg a structural relaxation is detected and indirect indications of the thermal formation of structural free volumes are found.

18O Diffusion in nano crystalline ZrO2

The diffusion of oxygen in nanocrystalline, undoped monoclinic ZrO2 was studied using 18O tracer and SIMS profiling. Samples with a mass density of 97 to 99 % and average crystallite sizes of 80 nm and 300 nm were prepared from Zr-metal by DC sputtering, crystallite condensation in an inert-gas atmosphere, oxidation, in-situ consolidation of the n-ZrO2 powder and subsequent pressureless sintering at 950 or 1050 ° C in vacuum. Both volume (V) and interface (B) diffusivities were determined from the depth profiles in the type B regime. The activation energies QV = 2.29 eV and QB =1.95 eV are considerably higher than the values found in Ca- or Y-stabilized ZrO2. No influence of the crystallize size was observed.

Fe diffusion in Nanocrystalline alloys and the influence of Amorphous Intergranular Layers

Abstract 59Fe tracer diffusion is used as a highly structure-sensitive tool for probing the interface structure and residual amorphous intergranular phases in nanocrystalline (n-) Fe90Zr10 produced by crystallization of melt-spun amorphous ribbons. Analyses of 59Fe-diffusion profiles show the existence of two types of interfaces. In the one type the diffusivities are low; the other type acts as fast diffusion paths like conventional grain boundaries in α-Fe. In accordance with positron-annihilation and hydrogen-site spectroscopy of the interfacial structure on n-Fe90Zr10, the slow diffusion process may indicate the presence of amorphous intergranular layers similar as recently found for n-Fe73.5Si13.5B9Nb3Cu1.

Free volumes and thermal stability of electro-deposited nanocrystalline Pd

Abstract The structure and thermal stability of electro-deposited nanocrystalline Pd was studied by means of positron lifetime spectroscopy, x-ray diffraction, and transmission electron microscopy. In comparsion to nanocrystalline Pd prepared by gas-phase synthesis or severe plastic deformation, larger free volumes occur and the crystallite growth is retarded. The enhanced structural stability is attributed to solute atom drag and to nanovoids which are decorated with light impurities and which may act as pinning centers for the crystallite interfaces.

Self-diffusion in nanocrystalline Fe and Fe-rich alloys

The present work aims at a comparison of the self-diffusion behaviour of nanocrystalline (n-)Fe produced by cluster condensation and compaction with that of Fe-rich n-alloys made by crystallization of melt-spun amorphous ribbons. In cluster-synthesized Fe (relative density higher than 91 %), a decrease of the (59)Fe tracer diffusivity upon annealing indicates interface relaxation. The diffusion coefficients in the relaxed grain boundaries are similar to those extrapolated from high-temperature data of conventional grain boundaries. Substantially lower interface diffusivities in crystallized n-Fe(90)Zr(10) and n-Fe(90)Zr(7)B(3) presumably arise from residual intergranular amorphous layers. Due to the reduced amorphous fraction, in n-Fe(90)Zr(10) additional fast diffusion paths exist like in conventional grain boundaries.