In situ monitoring of artificial aging and solution heat treatment of a commercial Al-Mg-Si alloy with a high intensity positron beam.

Microstructural changes of a commercial Al-Mg-Si alloy were studied during artificial aging by in situ Doppler broadening spectroscopy using a high-intensity positron beam. The in situ positron annihilation characteristics at high temperatures differ considerably from the conventionally applied ex situ measurements at low temperatures. Therefore, a more comprehensive view of precipitation processes in Al-Mg-Si alloys is obtained. Further, in situ positron-electron annihilation techniques allow for an investigation of aging processes with increased sensitivity. For the artificial aging temperatures of 180 °C and 210 °C pronounced variations of the Doppler broadening S-parameter reveal (i) the evolution of clusters into larger precipitates and (ii) the time of the formation of [Formula: see text]" precipitates and the role of vacancies in connection to this. Towards higher aging times, the transformation from coherent [Formula: see text]" to semi-coherent [Formula: see text]' precipitates could be verified. Additional insights are gained by in situ measurements of the S-parameter during the solution heat treatment of the previously overaged sample. Here, the S-parameter reveals both the dissolution of precipitates starting from temperatures of 364 °C and the thermal generation of vacancies.

Kinetics of vacancy annealing upon time-linear heating applied to dilatometry.

A kinetic model for the diffusion-controlled annealing of excess vacancies under the experimentally relevant, non-isothermal condition of time-linear heating is presented and applied to dilatometry. The evolution of the vacancy concentration with time is quantitatively analyzed, considering as ideal sinks either dislocations or grain boundaries of spherical- or cylindrical-shaped crystallites. The validity of the model is tested using dilatometry data that were obtained for ultrafine-grained Ni prepared by high-pressure torsion. The entire two-stage annealing curve of the dilatometric length change can be analyzed by combining the present kinetic model of vacancy annealing at grain boundaries with established non-isothermal kinetics of recrystallization.

Internal stress and defect-related free volume in submicrocrystalline Ni studied by neutron diffraction and difference dilatometry.

A combined study of neutron diffraction and difference dilatometry on submicrocrystalline Ni prepared by high pressure torsion aims at studying the anisotropic behaviour during dilatometry and its relation to internal stress and structural anisotropy. Macroscopic stresses were undetectable in the dilatometer samples. Along with specific tests such as post cold-rolling, this shows that an observed anisotropic length change upon annealing is not caused by internal stress, but can be explained by the inherent microstructure, i.e. the anisotropic annealing of relaxed vacancies at grain boundaries of shape-anisotropic crystallites.

Atomic structure and structural stability of Sc75Fe25 nanoglasses.

Nanoglasses are solids consisting of nanometer-sized glassy regions connected by interfaces having a reduced density. We studied the structure of Sc(75)Fe(25) nanoglasses by electron microscopy, positron annihilation spectroscopy, and small-/wide-angle X-ray scattering. The positron annihilation spectroscopy measurements showed that the as-prepared nanoglasses consisted of 65 vol% glassy and 35 vol% interfacial regions. By applying temperature annealing to the nanoglasses and measuring in situ small-angle X-ray scattering, we observed that the width of the interfacial regions increased exponentially as a function of the annealing temperature. A quantitative fit to the small-angle X-ray scattering data using a Debye-Bueche random phase model gave a correlation length that is related to the sizes of the interfacial regions in the nanoglass. The correlation length was found to increase exponentially from 1.3 to 1.7 nm when the sample temperature was increased from 25 to 230 °C. Using simple approximations, we correlate this to an increase in the width of interfacial regions from 0.8 to 1.2 nm, while the volume fraction of interfacial regions increased from 31 to 44%. Using micro-compression measurements, we investigated the deformation behavior of ribbon glass and the corresponding nanoglass. While the nanoglass exhibited a remarkable plasticity even in the annealed state owing to the glass-glass interfaces, the corresponding ribbon glass was brittle. As this difference seems not limited to Sc(75)Fe(25) glasses, the reported result suggest that nanoglasses open the way to glasses with high ductility resulting from the nanometer sized microstructure.

Structure and electrical properties of nanoparticulate tungsten oxide prepared by microwave plasma synthesis.

Nanoparticulate WO(3) films were prepared using microwave plasma synthesis and studied with respect to the electrical conductivity in dependence of ambient conditions. The WO(3) films with a monoclinic structure were made from cluster-assembled nanoparticles (diameter 3 nm) by means of dispersion and spin-coating. Above 100 °C a thermally activated decrease of the electrical resistance due to oxygen vacancy donors is found. A reversible increase of the electrical resistance R due to oxygen uptake is observed. The decrease of R in response to reducing H(2)S in the ppm range is studied in dependence of temperature and pre-annealing conditions.

Electrically tunable resistance of a metal.

Electric field-induced tuning of material properties is usually restricted to nonmetals such as semiconductors and piezoelectric ceramics. We show that variations of the electrical resistance of a metal (Pt) in the range of several percent can be reversibly induced at low charging voltages making use of a nanocrystallite-electrolyte composite. The charge-induced resistance variation is analyzed taking into account the modification of the charge carrier density and scattering rate by surface charging. The contribution arising from the charge-induced variation of the lattice constant is found to be small.

Study of an order-disorder phase transition on an atomic scale: the example of decagonal Al-Ni-Co quasicrystals.

In the present paper we succeeded in studying structural phase transitions from an atomistic point of view by positron annihilation Doppler broadening. This differs and is complementary to conventionally used diffraction experiments with large coherence lengths. In the exemplary case of the 1140 K order-disorder transition in decagonal Al71.5Ni14Co14.5 quasicrystals the importance of this atomistic approach and its wide scope of application is demonstrated.

Charge-induced reversible strain in a metal.

Dimension changes on the order of 0.1% or above in response to an applied voltage have been reported for many types of materials, including ceramics, polymers, and carbon nanostructures, but not, so far, for metals. We show that reversible strain amplitudes comparable to those of commercial piezoceramics can be induced in metals by introducing a continuous network of nanometer-sized pores with a high surface area and by controlling the surface electronic charge density through an applied potential relative to an electrolyte impregnating the pores.