Search

Friction stir welding is an innovative welding technology that is often employed to weld aluminum alloys of the 2xxx-series. These alloys are primarily used in the aerospace industry to join structural parts or tanks. For these applications, high quality requirements have to be met. In a previous study, it was shown that controlling the welding temperature is one key to ensuring the quality of the welds. It was observed that the tensile strength varies depending on the welding temperature and the welding speed. In this paper, Positron Annihilation Spectroscopy (PAS) with a scanning positron beam was applied to detect crystallographic defects and changes of the microstructure of friction stir welds. The results were compared to optical microscopy and to hardness maps. Hereby, vacancy formation in the cross section of friction stir welds could be observed. Furthermore, it could be demonstrated that the state of the precipitates is the primary determinant for the mechanical properties of the welds. Finally, the observations made through PAS were employed to explain the trends in the mechanical properties, which were observed in the previous study. In particular, the varying ultimate tensile strength (UTS) could be clearly attributed to dissolution and re-precipitation phenomena, which occur during and after welding.

We measured the temperature dependent equilibrium vacancy concentration using in-situ positron annihilation spectroscopy in order to determine the enthalpy $H_\text{f}$ and entropy $S_\text{f}$ of vacancy formation in elementary fcc-La. The Arrhenius law applied for the data analysis, however, is shown to fail in explaining the unexpected high values for both $S_\text{f}$ and $H_\text{f}$: in particular $S_\text{f}=17(2)~k_\text{B}$ is one order of magnitude larger compared to other elemental metals, and the experimental value of $H_\text{f}$ is found to be more than three standard deviations off the theoretical one $H_\text{f}=1.46~\text{eV}$ (our \acs{dft} calculation for La at $T=0~\text{K}$). A consistent explanation is given beyond the classical Arrhenius approach in terms of a temperature dependence of the vacancy formation entropy with $S_\text{f}^\prime=-0.0120(14)~k_\text{B}/\text{K}$ accounting for the anharmonic potential introduced by vacancies., Comment: 5 pages, 3 figures

The CDB-spectrometer at NEPOMUC enables spatially resolved Doppler Broadening Spectroscopy of the electron-positron annihilation line with a lateral resolution of up to 200~$\mu$m (FWHM). We have applied this technique for studying single particles of silica gel dependent on loading with an altering amount of the ionic liquid [C2MIM][NTf2]. Our results reveal a strong correlation between the observables of DBS and the loading factor which presumably arises from a smaller inner surface of voids in loaded silica gel.

In this study the structure and evolution of vacancy type defects in lithium ion batteries are investigated in respect of crystallographic properties. The relation between positron annihilation and electronic structure is discussed in terms of structural dynamics during the lithiation process. Samples of Li1−xNi1/3Mn1/3Co1/3O2 (NMC-111) electrodes with decreasing lithium content (x = 0–0.7) covering the whole range of state of charge were electrochemically prepared for the non-destructive analysis using positron coincidence Doppler broadening spectroscopy (CDBS). The positron measurements allowed us to observe the evolution of the defect structure caused by the delithiation process in the NMC-111 electrodes. The combination of CDBS with X-ray diffraction for the characterization of the lattice structures enabled the analysis of the well-known kinetic-hindrance-effect in the first charge-discharge cycle and possible implications of vacancy ordering. In particular, CDBS revealed the highest degree of relithiation after discharge to 3.0 V at 55 °C. For the first time, we report on the successful application of CDBS on NMC-111 electrodes yielding new insights in the important role of defects caused by the delithiation process and the kinetic hindrance effect.

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 ” precipitates and the role of vacancies in connection to this. Towards higher aging times, the transformation from coherent ” to semi-coherent ’ 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.

A brief report is given on recent studies of the atomistic processes during charging of battery cathode material LixCoO2 by means of magnetometry and positron annihilation. A set-up for operando magnetometry is implemented which, for the first time, allowed to continuously monitor the distinct variation of the magnetic susceptibility χ of LixCoO2 which occurs during consecutive charging and discharging cycles. The variation of χ with Li+ content in the concentration range 1>x≥0.77 arises from a variation of the electronic density of states and from electronic correlation effects. The χ (x)-behaviour for xx)-variations in this concentration regime. First measurements on LixCoO2 thin-films performed at the positron beam line NEPOMUC of FRM II at the Heinz Maier-Leibnitz neutron source will be presented.

Different ODS RAF steels with nominal composition Fe–14Cr–2W–0.3-Ti–0.3Y2O3 (wt%) and a model Fe–14Cr (wt%) alloy have been subjected to single Fe+ or He+ ion irradiations to simulate the effect of a fusion environment at different doses and temperatures. The irradiation induced open volume defects have been characterised by positron annihilation spectroscopy and transmission electron microscopy. For low temperature Fe+ irradiations, positron annihilation spectroscopy results show a high concentration of irradiation induced vacancy clusters, much more evident in the model alloy as compared to the ODS steels. The extent of damage due to He+ irradiations at RT and high temperatures is considerably lower than for the Fe+ irradiations. Regardless of the temperature He+ irradiations introduced changes in the chemical environment of open-volume positron traps possibly due to the creation of He-vacancy complexes. Positron annihilation spectroscopy results also suggest that irradiation induced vacancy-type defects in ODS steels are not associated with nanoparticles but are mainly in the ferritic matrix. The presence of irradiation induced bubbles was studied after He+ irradiation at high temperatures. Small bubbles were observed, being smaller in the ODS steels as compared to the model alloy.

The oxygen deficiency $\ensuremath{\delta}$ in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) plays a crucial role for affecting high-temperature superconductivity. We apply (coincident) Doppler broadening spectroscopy of the electron-positron annihilation line to study in situ the temperature dependence of the oxygen concentration and its depth profile in single crystalline YBCO film grown on ${\mathrm{SrTiO}}_{3}$ (STO) substrates. The oxygen diffusion during tempering is found to lead to a distinct depth dependence of $\ensuremath{\delta}$, which is not accessible using x-ray diffraction. A steady state reached within a few minutes is defined by both, the oxygen exchange at the surface and at the interface to the STO substrate. Moreover, we reveal the depth-dependent critical temperature ${T}_{\mathrm{c}}$ in the as prepared and tempered YBCO film.

We present studies on the free volume of silica gel dependent on water absorption and on loading with a different amount of the ionic liquid (IL) [C2 MIM][NTf2] using positron lifetime spectroscopy (PLS). The found two long lifetime components (2 and 10–20 ns) are attributed to the annihilation of ortho-positronium (o-Ps) in micropores of different size. The application of a semi-empirical quantum mechanical model allowed us to correlate the measured o-Ps lifetimes to the absolute mean size of the micropores by approximation with a spherical shape. The mean diameter of the pores in the pure silica gel could, hence, be determined to 1.72(6) nm. IL loading with up to 70 vol% lead to the formation of smaller voids with a mean diameter of 1.28(4) nm according to a reduction of the mean pore volume by about 60%. In addition, the accumulation of H2 O molecules from the surrounding air inside the pores could be observed in-situ by time dependent PLS.

Outstanding crystalline perfection is a key requirement for the formation of new forms of electronic order in a vast number of widely different materials. Whereas excellent sample quality represents a standard claim in the literature, there are, quite generally, no reliable microscopic probes to establish the nature and concentration of lattice defects such as voids, dislocations and different species of point defects on the level relevant to the length and energy scales inherent to these new forms of order. Here we report an experimental study of the archetypical skyrmion-lattice compound MnSi, where we relate the characteristic types of point defects and their concentration to the magnetic properties by combining different types of positron spectroscopy with ab-initio calculations and bulk measurements. We find that Mn antisite disorder broadens the magnetic phase transitions and lowers their critical temperatures, whereas the skyrmion lattice phase forms for all samples studied underlining the robustness of this topologically non-trivial state. Taken together, this demonstrates the unprecedented sensitivity of positron spectroscopy in studies of new forms of electronic order., 8 pages, 4 figures

The damage profile of oxide dispersion strengthened steels after single-, or simultaneous triple-ion irradiation at different conditions has been characterized using a low energy positron beam in order to provide information on microstructural changes induced by irradiation. Doppler broadening and coincident Doppler broadening measurements of the positron annihilation line have been performed on different Fe-Cr-(W,Ti) alloys reinforced with Y2O3, to identify the nature and stability of irradiation-induced open-volume defects and their possible association with the oxide nanoparticles. It was found that irradiation induced vacancy clusters are associated with Cr atoms. The results are of highest interest for modeling the damage induced by 14 MeV neutrons in reduced activation Fe-Cr alloys relevant for fusion devices. This investigation was supported by the Ministerio de Ciencia e Innovación (Contract ENE2010-17462), and the European Commission through the European Fusion Development Agreement (Contract No. 09-240) and 7th FP through the “Research infrastructures” action of the “Capacities” Program, NMI3-II (Grant 283883). The authors acknowledge the JANNUS-Saclay team for their scientific and technological advice.