Sigrun Hirsekorn has studied physics in Braunschweig and Münster, Germany, and finished with the Ph.D. in physics. She is working as a scientist at the Fraunhofer Institute for Nondestructive Testing (IZFP) in Saarbrücken, Germany, her present position is head of the department "NDT Fundamentals". Her main fields of research activity are:

• theoretical developments and calculations in ultrasonic scattering and propagation for nondestructive defect detection and materials characterization especially in microscopically inhomogeneous (e.g. polycrystalline and/or fibre reinforced) materials,
• the development of evaluation methods in acoustic microscopy for defect detection and the determination of local elastic constants with a lateral resolution within the µm range,
• theoretical investigations and calculations of high-frequency dynamic modes of an atomic force microscope (AFM) in order to determine quantitatively local material surface properties with a lateral resolution within the nm range,
• modelling of the mechanical and piezoelectrical and the resulting vibration properties of piezoceramic composites and piezoelectric bimorph plates for the construction of ultrasonic transducers, and
• theoretical developments and calculations of the nonlinear transfer of ultrasound through bonded interfaces in order to evaluate binding forces and bond strengths.

S.H. is also a lecturer at the University of the Saarland, Saarbrücken, within the Department of Materials Science and Production Engineering. In 1988, S.H. got the Berthold-Award of the German Society of Nondestructive Testing (DGZfP) for her theory of ultrasonic scattering and propagation in polycristalline materials. She is an active member of the DGZfP, the German Acoustical Society (DEGA), and the German Society of Physics (DPG). S.H. was a member of the DEGA board from 2007 to 2010, is the head of the association Acoustics within the German Society of Physics (DPG) and as such involved in the organization of the annual workshop "Physical Acoustics" in Bad Honnef, Germany, is associate editor of the journal Ultrasonics, and member of the Board of the International Congress on Ultrasonics (ICU). S.H. has authored and co-authored more than 100 publications in peer-reviewed journals, about 150 contributions to conferences, symposia, and workshops, and holds two patents.

Abstract

Atomic force microscopy - what is it all about, and what does it tell us about the microstructure of metals?

Atomic force microscopy (AFM) is a near-field technique to generate high-resolution images of surfaces while they are scanned with a sharp sensor tip integrated at the end of a microfabricated elastic beam. In conventional AFM, the deflection or torsion angle of the cantilever beam is kept constant while scanning in order to image topography or friction, respectively. A variety of dynamic AFM operation modes such as e.g. force modulation microscopy, ultrasonic force microscopy, scanning local acceleration microscopy or pulsed force microscopy allow to generate images the contrast of which depend on the local elasticity.

Conventional AFM cantilevers are fabricated in one piece with the chip by etchingout of silicon single crystals. The small elastic beams have a length of a few 100 µm, a width of a few 10 µm, and a thickness of a few µm. These properties entail vibration resonances in the ultrasonic frequency range, i.e. AFM cantilevers are usable as near field ultrasonic probes. The integrated sensor tip is about 10 to 15 µm long, the radius at its end is a few nm up to of a few 100 nm and defines the accessible spatial resolution. Dynamic AFM operation modes with ultrasonic frequencies exploit flexural and torsional vibration resonances of the cantilevers. In tapping mode, free resonances are used for topography imaging. Contact resonance AFM such as e.g. atomic force acoustic microscopy (AFAM), ultrasonic friction force microscopy (UFFM), magnetic force microscopy (MFM), and ultrasonic piezo-mode (UPM) exploit the change in resonance frequencies by contact forces between the tip and a sample surface. Some of the methods require special sensor tips, e.g. electrically conductive ones for UPM and with a magnetic coating for MFM. Convenient contact models are used for quantitative evaluation of the frequency shifts to determine local sample surface properties. In short, AFM with all its different operation modes is a powerful tool for microstructure imaging and local materials characterization of surfaces with a spatial resolution down to the nm range. Thus it can be used to reveal correlations between the micro- and nanostructure and macroscopic materials behaviour paving the way for materials design.

New design concepts for the construction of advanced light-weight and crash resistant transportation systems require the development of high strength and supra-ductile steels with enhanced energy absorption and reduced specific weight. TWIP (Twinning Induced Plasticity) steels have excellent mechanical properties combining high strength levels with a large uniform elongation. This is caused by intensive mechanical twinning resulting in a high sustained degree of strain-hardening. Investigations of the mechanisms and the related microstructures are shown. Cementite (Fe₃C) is a very important phase in steels because its morphology directly controls the macroscopic mechanical properties. The cementite phase embedded in a ferrite matrix is characterized by Atomic Force Acoustic Microscopy (AFAM) and nanoindentation studies. Magnetic force microscopy (MFM) coupled with an external coil providing an in-plane controlled magnetic field is employed to image the dynamic behaviour of the magnetic domains in the cementite precipitates as well as the ferrite matrix of unalloyed steels. Furthermore results on nanostructured nickel samples are presented.

Keywords: atomic force microscopy (AFM), atomic force acoustic microscopy (AFAM), contact resonance, dynamic operation modes, imaging, magnetic force microscopy (MFM), microstructure, nanostructure, near-field, piezo-mode, quantitative evaluation, ultrasound, metals