T.E.M. Staab

Self-interstitials in 3C-SiC

We report results from density-functional plane-wave pseudopotential calculations for carbon and silicon self-interstitials in cubic silicon carbide (3C-SiC). Several initial ionic configurations are used in the search for the global total-energy minimum including tetragonal, split [100] and split [110] geometries. Neutral carbon interstitials are found to have several nearly degenerate total-energy minima configurations in split-interstitial geometries, with formation energies ranging?besides higher metastable ones?from 6.3 to 6.7?eV in stoichiometric SiC. By contrast, the neutral silicon interstitials have a clear single minimum total-energy configuration at the tetrahedral configuration with carbon nearest neighbours, exhibiting a formation energy of 6.0?eV. The split interstitial in the [110] direction at the silicon site and the tetrahedral configuration with silicon nearest neighbours are metastable and have significantly higher formation energies. The present calculations indicate that the carbon interstitial introduces deep levels in the band gap while the silicon interstitial at the tetrahedral site behaves like a shallow donor.

Vacancy clusters in plastically deformed semiconductors

Experimental investigations of plastically deformed elemental and III-V semiconductors prove that a high number of vacancies and vacancy clusters are formed. The formation of point defects by the motion of jogged dislocations is analysed. Vacancies formed behind the jog are not stable as a simple chain of vacancies. Instead, they are transformed immediately to stable three-dimensional agglomerates. The stability of various vacancy clusters has been investigated by means of density-functional calculations. The positron lifetime in such clusters was calculated and compared to experimental results. The association of open-volume defects with the dislocation can be derived from positron lifetime measurements. The analysis in a positron-trapping model characterizes the dislocation as a combined defect. The undisturbed dislocation line is a precursor trap for the positron capture in a deep trap related to the vacancies bound to the dislocation.

Positron annihilation spectroscopy: a non-destructive method for lifetime prediction in the field of dynamical material testing

The fatigue behavior of iron-based materials has been investigated by rotating bending testing, employing positron annihilation spectroscopy to probe defects on the atomic level. Positron annihilation spectra have been recorded at various stages of material fatigue. The defect density has been determined by analysing the line shape of the Doppler broadening of the annihilation radiation in the photo peak. The line shape parameter (S parameter), a measure of the defect density, showed a linear relation to the logarithm of the number of loadings, thus from only a small number of loadings it is possible to determine the remaining useful life of the sample. Furthermore, along the longitudinal sample axis spatially resolved line-scans are taken using the Bonn Positron Microprobe. Due to the special sample geometry, the stress gradient allows to obtain the S parameter for different values of the applied load using the very same sample. This leads to a way to determine a complete Wohler diagram using a non-destructive method for just one sample.

Atomic structure of pre-Guinier-Preston and Guinier-Preston-Bagaryatsky zones in Al-alloys

We present results on the structure of nano-sized particles (Guinier-Preston (GP) and Guinier-Preston-Bagaryatsky (GPB) zones) in Aluminum alloys. Precipitates of alloying elements like Cu, Mg, or Si hinder the motion of dislocations and, thus, are responsible for the strength of AlCuMg- and AlMgSi-alloys - used e.g. as AA2024 (old aircrafts) and AA6013 for the fuselage of the new Airbus A380, respectively. We will discuss the role of quenched-in vacancies for diffusive motion at room temperature (RT) enabling the growth of the precipitates. Using positron annihilation spectroscopy (PAS) – both lifetime and Doppler broadening – gives information on the local atomic environment in the vicinity of vacancies. On the other hand X-ray absorption fine structure (XAFS) spectroscopy is capable of characterizing the local atomic environment around selected elements (Cu, Mg). We will interpret the measured data by comparing them to numerical calculations of PAS and XAFS spectra. However, reliable numerical calculations of spectroscopic quantities are only possible provided that relaxed atomic positions are used as an input. We calculate those employing the ab-initio code SIESTA. Thus, considering decomposition of Al-alloys, we obtain extremely valuable information on the earliest stages, forming immediately after solution heat treatment and quenching, i.e. during the first few minutes of storage at RT.

Defect investigations of micron sized precipitates in Al alloys

A lot of light aluminium alloys achieve their favourable mechanical properties, especially their high strength, due to precipitation of alloying elements. This class of age hardenable Al alloys includes technologically important systems such as e.g. Al-Mg-Si or Al-Cu. During ageing different precipitates are formed according to a specific precipitation sequence, which is always directed onto the corresponding intermetallic equilibrium phase. Probing the defect state of individual precipitates requires high spatial resolution as well as high chemical sensitivity. Both can be achieved using the finely focused positron beam provided by the Bonn Positron Microprobe (BPM) [1] in combination with the High Momentum Analysis (HMA) [2]. Employing the BPM, structures in the micron range can be probed by means of the spectroscopy of the Doppler broadening of annihilation radiation (DBAR). On the basis of these prerequisites single precipitates of intermetallic phases in Al-Mg-Si and Al-Cu, i.e. Mg2Si and Al2Cu, were probed. A detailed interpretation of these measurements necessarily relies on theoretical calculations of the DBAR of possible annihilation sites. These were performed employing the DOPPLER program. However, previous to the DBAR calculation the structures, which partly contain vacancies, were relaxed using the ab-initio code SIESTA, i.e. the atomic positions in presence of a vacancy were recalculated.

Non-Destructive Lifetime Prediction in Material Testing with the Bonn Positron Microprobe

For the construction of heavy duty mechanical components it is essential to know the lifetime of the sample under dynamical load conditions. Material fatigue tests in rail vehicle axles were first made in the year 1858 by August Wöhler [1]. Up to now the measurement of a Wöhler diagram remains extraordinarily time and money consuming. Since iron based materials fail at a critical defect density independent of cycle number and load, we are able to extrapolate the remaining lifetime of the sample from a few cycles. With positron annihilation measurements employing the Bonn Positron Microprobe different stress states in standard samples can be observed as a function of position [2,3,4]. This leads to a novel non-destructive way to determine a complete Wöhler diagram (SN-diagram) with just one sample. Introduction To assess the useful life of complex components before their first implementation engineers rely heavily on input parameters for finite element method (FEM) calculations. One of this necessary input parameters is contained in a Wöhler diagram (WD) (fig.1). A WD gives statistical information about the lifetime of a sample under specific cyclic load. Figure 1.: Example of a Wöhler diagram. Every point in the graph represent the failure of one sample in a fatigue experiment. For the specified applied tension ( 17) the number of cycles is given where a sample with specific load failed. The three lines represent the different probabilities for the failure of a sample. But up to the present one sample has to be destroyed to get only one point for a WD. And for a complete WD with good statistic many samples have to be destroyed at different loads in a large number of cycles (up to 10 8 ). Employing positron annihilation spectroscopy (PAS) it is now possible to reduce the number of samples for iron-based materials from hundreds to just a few. 10 4 10 5 10 6 10 7 number of cycles P= 50% P= 100%