Compacts of tungsten powder with five different powder-particle sizes (from to ) are subjected to pressureless sintering. We investigate the change in microstructure during the sintering process by positron lifetime spectroscopy. So as to be able to distinguish between defects having the same positron lifetime, we investigate their kinetics when the sample is annealed. In particular, we consider the annealing out of vacancy clusters after low-temperature electron irradiation, as well as recovery and recrystallization of a tungsten sheet, in as-manufactured form. Making measurements on uncompacted powder, we find an increasing fraction of positrons annihilating in surface states with decreasing powder-particle size. The powder-particle and grain sizes (influencing the x-ray domain size) are monitored additionally by means of metallography and x-ray diffraction. We find that all of the methods give results in agreement with each other. The small grain sizes at lower temperature, about one fifth of the powder-particle size, cause positrons to annihilate at grain boundaries, leading to vacancy-cluster-like signals. At the intensive-shrinkage stage, there are certainly contributions from different shrinkage mechanisms. The observed shrinkage rates can be explained by Coble creep. It is possible that dislocations also play a role as vacancy sources and sinks, since the intensive-shrinkage stage occurs in a temperature region wherein recrystallization takes place.

We report on the identification of native vacancies in GaAs by positron annihilation with a special emphasis on As vacancy-related defects. In annealed highly Si-doped GaAs, we observe a neutral vacancy defect with a positron lifetime τ of 280–285 ps and a high intensity of the core annihilation, in contrast to Ga vacancies which exhibit a lifetime of ∼260 ps and a lower intensity of the core annihilation. This defect is identified by scanning tunneling microscopy measurements to be an As vacancy ${\mathrm{Si}}_{\mathrm{Ga}}$ donor complex. We find that the same defect is also present in low n-doped GaAs, where it was earlier assigned to a neutral As vacancy. The high positron lifetime is explained by a large outward lattice relaxation. Theoretical calculations of the momentum distribution employing free atomic wave functions are in good agreement with the experimental results, provided only relative changes are considered and an outward lattice relaxation is included which yields the experimental positron lifetime. These calculations also yield annihilation parameters for Ga vacancies, in good agreement with the experiment. Our results demonstrate that vacancies in both sublattices of GaAs can directly and unambiguously be identified by positron annihilation once the annihilation characteristics are calibrated by a complementary method. On the basis of this identification, the abundance of As vacancies in GaAs is discussed in terms of stoichiometry and formation energies.

We study Al-1.74 at.% Cu as a typical binary model alloy to obtain new information on the precipitation sequence. The alloy has been solution heat treated, rapidly quenched, and then isochronally annealed up to 540 °C. We reveal new effects on the evolution of the precipitation sequence by combining positron annihilation lifetime and coincidence Doppler broadening spectroscopies. Positron parameters do very sensitively respond to changes in the type of precipitates present. We find that in the as-quenched state after exposing the alloy just a few minutes to room temperature GP-I zones containing vacancies inside their copper platelet dominate. They represent about 80% of all detected GP-I zones. At 200 °C GP-II (Θ″) is the dominating type of precipitate. However, the GP-II zones have obviously expelled all structural vacancies on growing from larger GP-I zones. Also the transition from GP-II zones dissolving while precipitates of the Θ′ phase do form at about 250 °C can be clearly observed. The signals support that Θ′ precipitates contain vacancies on their Cu sublattice, since they have to grow in a copper-poor environment. Finally, our signals indicate coarsening of Θ′-precipitates for T > 400 °C and their dissolution until 450 °C. All our experimental results agree well with ab initio theoretical calculations of positron parameters. However, the formation of the equilibrium Θ-phase cannot be observed since these have a too low number density due to their large size.

Aluminium-copper alloys of the 2xxx type receive their excellent mechanical properties by the formation of copper-rich precipitates during hardening. Size, distribution and crystal structure of the formed precipitates determine the final strength of those alloys. Adding traces of certain elements, which bind to vacancies, significantly influences the decomposition behaviour, i.e. the diffusion of the copper atoms. For high-purity ternary alloys (Al-1.7 at.% Cu-X), we investigate the interaction of copper and trace element atoms (X=In, Sn, and Pb) with quenched-in vacancies by Positron Annihilation Lifetime Spectroscopy (PALS). By employing Vickers microhardness, Differential Scanning Calorimetry (DSC) and Small Angle X-Ray Scattering (SAXS) we obtain a comprehensive picture of the decomposition process: opposite to predicted binding energies to vacancies by ab-initio calculations we find during ageing at room and elevated temperature a more retarded clustering of copper in the presence of In rather than for Sn additions, while Pb, having the highest predicted binding to vacancies, shows nearly no retarding effect compared to pure Al-Cu. If the latter would be due to a limited solubility of lead, it had to be below 2 ppm. Transmission Electron Microscopy (TEM) as imaging method complements our findings. Annealing the quenched Al-1.7 at.% Cu-X-alloys containing 100 ppm In or Sn at $$150\,^\circ {\text {C}}$$150∘C leads to finely distributed $$\theta \, '$$θ′ -precipitates on the nanoscale, since due to the trace additions the formation temperature of $$\theta \, '$$θ′ is lowered by more than $$100\,^\circ {\text {C}}$$100∘C . According to TEM small agglomerates of trace elements (In, Sn) may support the early nucleation for the $$\theta \, '$$θ′ -precipitates.