M. Haaks

High resolution positron-annihilation spectroscopy with a new positron microprobe

In cooperation with Zeiss/LEO GmbH, a monoenergetic positron source has been integrated in the electron optical system of a scanning electron microscope by help of a magnetic prism. The electron optics serves both to image the specimen with electrons and to form a positron microbeam that allows local positron-annihilation measurements with a resolution in the micron range. The fatigue damage profile along the cross section of a copper plate after a three-point bending test has been investigated. The obtained S-parameter profile coincides well with the expected fatigue damage distribution.

Measurements on cracktips in stainless steel AISI 321 by using a new positron microprobe

High resolution positron microscopy provides a new method for non-destructive investigations of plastic deformation with spatial resolution in the micron range. As positron annihilation is highly sensitive to lattice defects, low concentrations of dislocations are detectable, so that the plastic zone in front of a cracktip appears larger than in comparable metallographic methods. To demonstrate this, a plastic zone in the common stainless steel AISI 321 is imaged with the Bonn Positron Microprobe (BPM) with a spatial resolution of 20 μm.

Domain reversal properties and refractive index changes of magnesium doped lithium niobate upon ion exposure

Irradiation of optical damage resistant, magnesium doped lithium niobate crystals with fast, high-energy He2+3 ions changes important material properties. In the interaction region, where the ions transmit through the material, the ferroelectric coercive field EC is diminished from 6.0kVmm−1 down to 5.0–5.4kVmm−1 after transmission of 41MeV He2+3 particles. This enables easier domain reversal in irradiated crystals compared to untreated material. Besides, large changes of the refractive index of the crystals on the order of 6×10−3 are induced by the treatment. Moderate annealing treatments do not diminish Δn, but refresh the coercive field.

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.

Refractive index changes in lithium niobate crystals by high-energy particle radiation

Irradiation of lithium niobate crystals with 41 MeV 3He ions causes strong changes of the ordinary and extraordinary refractive indexes. We present a detailed study of this effect. Small fluence of irradiation already yields refractive index changes about 5×10−4; the highest values reach 3×10−3. These index modulations are stable up to 100°C and can be erased thermally, for which temperatures up to 500°C are required. A direct correlation between the refractive index changes and the produced lattice vacancies is found.

Defect production by the TEM beam-the first application of the positron microprobe

Abstract In cooperation with the ZEISS/LEO GmbH a positron microprobe has been constructed. Additionally, a conventional scanning electron microscope (SEM) is integrated in the setup. Measurements on radiation defects in Mo and Cu samples, made by 1 MeV electron irradiation in a transmission electron microscope (TEM), are presented. The results coincide with the expected damage profiles. Because of the small beam diameter—the great advantage of the positron microprobe—only very small areas (about 20 to 30 μm in diameter) have to be damaged. Therefore, the irradiation times required to produce samples for investigations of radiation defects caused by electrons are reduced enormously. This means that much higher defect concentrations can be produced in an acceptable time and the positron measurements can cover a wider range of defect concentrations.

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%

Material Science with Positrons: From Doppler Spectroscopy to Failure Prediction

We describe an alternative approach for a reliable lifetime prediction employing the local concentration of lattice defects as a precursor for fatigue failure. We present positron annihilation spectroscopy (PAS) as a non-destructive technique sensitive for defect concentrations in the range relevant to plasticity in metals.