Ralf Schlesiger

Design of a laser-assisted tomographic atom probe at Münster University

To benefit from the latest technical improvements in atom probe analysis, a new tomographic atom probe has been built at the University of Münster, Germany. The instrument utilizes a femtosecond laser system with a high repetition rate combined with the ability of using a micrometer-sized extraction electrode and a wide angle configuration. Since field evaporation is triggered by laser pulses instead of high-voltage pulses, the instrument offers the ability to expand the range of analyzed materials to poorly conducting or insulating materials such as oxides, glasses, ceramics, and polymeric materials. The article describes the design of the instrument and presents characterizing measurements on metals, semiconductors, and oxide ceramic.

Mechanism of Intermediate Temperature Embrittlement of Ni and Ni-based Superalloys

Ni-based superalloys play an important role in aircraft engine propulsion. However, many experiments demonstrated that these alloys as well as Ni always show ductility loss at intermediate temperature, which constrains further development. A comparison of published papers by various authors reveals considerable differences in understanding the mechanism of intermediate temperature embrittlement. To clarify this situation, the present article first confirms the generality of intermediate temperature embrittlement of Ni and Ni-based alloys by the experimental results reported in the literature. The existing interpretations of the mechanism are then outlined. Based on the generality, these interpretations are discussed through the representative investigations on intermediate temperature embrittlement. It is shown that the mechanism of intermediate temperature embrittlement has not been satisfactorily explained yet and “nonequilibrium interface segregation” of impurities taking into account the effect of strain rate may be the origin of intermediate temperature embrittlement of Ni and Ni-based superalloys. Future research directions aiming at the reason of abnormal fracture mode, the effect of the state of applied stress, the influence of strain rate, and the development of the theory nonequilibrium grain boundary segregation, are suggested to provide a complete understanding of intermediate temperature embrittlement.

Triple junction transport and the impact of grain boundary width in nanocrystalline Cu.

Triple junctions (TJ), singular topological defects of the grain boundary (GB) structure, get a dominant role for grain growth and atomic transport in nanocrystalline matter. Here, we present detailed measurements by atom probe tomography, even of the temperature dependence of TJ transport of Ni in nanocrystalline Cu in the chemical regime of interdiffusion. An unexpected variation of the effective width of merging GBs with temperature is detected. It is demonstrated that proper measurement of TJ transport requires taking into account this remarkable effect. TJ diffusion is found to be a factor of about 200 faster than GB diffusion. Its activation energy amounts to only two-thirds of that of the GB.

A quantitative assessment of microelectrodes

In order to improve the performance and applicability of atom probe tomography, the application of microelectrodes has been suggested and is realized in modern commercial instruments. In contrast to the original proposition by Nishikawa, in practical realization the down scaling of the microelectrode is limited to about 10microm due to the requirements of a stable measurement. In this work, the field enhancement by electrodes of this size was measured in the FIM mode and compared to finite element calculations of the electric field. The experimental data reveal considerable scattering between individual microelectrodes and specimen tips, but on the average the predictions by the finite element calculation are confirmed. Even a microelectrode of 50microm diameter yields a reasonable field enhancement close to a factor of two.

Interfacial reaction and phase growth for various metal/amorphous silicon system

The practical importance of the reactions between a semiconductor and a metal system cannot be overstated. Fundamental physics itself offers also a wide variety of interesting phenomena. Unlike the “clean” metal-metal reactions nucleation or interface control is observed for some metal-silicon reactions.

Physics on the Top of the Tip: Atomic Transport and Reaction in Nano-Structured Materials

Nanoscale systems show a wide variety of physical properties that cannot be observed in the bulk. Using atom probe tomography, it is possible to study nanostructured materials with almost atomic resolution in all three dimensions. In this article, we will present a short review of the latest atom-probe measurements carried out at University of Münster with particular focus on diffusion and segregation measurements in triple junctions and interface analysis.