Scientific Events

Portevin Le-Chatelier (PLC) effect is a type of plastic instability that results in severe strain localization, reduction in ductility and formation of surface striations during forming operations. Understanding the underlying microscopic mechanism(s) that govern it requires detailed experimental investigations of the relationships between the phenomenon and local microstructural constituents. Most current models of PLC, both phenomenological and theoretical, are based on descriptions of mesoscopic observations and global responses observed in stress-strain curves. More predictive (or physically based) models will require investigations at the microstructural length-scales. In this talk, it will be shown that the gap in understanding of the microscopic origins and macroscopic manifestations of PLC can be bridged by nanoindentation testing. Specifically, it will be shown that by exploiting the high resolution of force and displacement measurements and the site-specific capabilities of the nanoindenter, coupled with complimentary microstructural characterization techniques, we are able to gain new insight into critical aspects of the PLC effect, including its anisotropy, underlying governing mechanisms and associated activation parameters. [more]

Materials with more non-metallic bonding are brittle, but are widely used, for instance as protective coatings. These often fail by cracking, so if their fracture resistance were increased, by making plastic flow easier, their lifetime could be extended. Some non-metallic materials deform readily, on a limited number of crystal planes, such as the ternary carbide Ti3SiC2 as well as Nb2Co7, W2B5 and Ta4C3. However, at present the understanding of how to design crystal structures for easy plastic flow is guided only by desirable ratios of elastic constants. Here, it is shown that flow is predicted to become very much easier if there are electronegativity differences within a crystal's unit cell, which cause non-uniform elastic deformation. Very substantial changes in flow behavior appear possible, suggesting this is a first step in developing a simple way of controlling plastic flow in non-metallic crystals. [more]

The analysis of deformation and failure mechanisms in small-scale devices and thin films is a critical issue, not yet solved. In this presentation, we describe recent advances and developments for the measurement of fracture toughness at small scales by the use of nanoindentation-based methods including techniques based on micro-cantilever, beam bending and micro-pillar splitting. A critical comparison of the techniques is made by testing a selected group of bulk and thin film materials. For pillar splitting, cohesive finite element simulations are used for analysis and development of a simple relationship between the critical load at failure, pillar radius, and fracture toughness for a given material. The minimum pillar diameter required for nucleation and growth of a crack during indentation is also estimated. An analysis of pillar splitting for a film on a dissimilar substrate material shows that the critical load for splitting is relatively insensitive to the substrate compliance for a large range of material properties. Micro-pillars are then produced by Focused Ion Beam (FIB) ring milling, being the pillar diameter approximately equal to its length; this ensures full relaxation of pre-existing residual stress in the upper portion of the specimen. Nanoindentation splitting tests are performed in-situ and the deformation mechanisms corresponding to each class of materials have been investigated. Experimental results from a selected group of materials show good agreement between single cantilever and pillar splitting methods, while a discrepancy of ~25% is found between the pillar splitting technique and double-cantilever testing. The limitations of the method are finally discussed. In particular, a minimum pillar’s diameter for the nucleation and growth of a crack during indentation is identified and quantified for a wide range of materials properties. It is concluded that both the micro-cantilever and pillar splitting techniques are valuable methods for micro-scale assessment of fracture toughness of brittle ceramics, provided the underlying assumptions can be validated. Although the pillar splitting method has some advantages because of the simplicity of sample preparation and testing, it is not applicable to most metals because their higher toughness prevents splitting, and in this case, micro-cantilever bend testing is preferred. [more]

III-V semiconductor nanowires (NWs) grown onto silicon substrate may become new building blocks of modern optoelectronic and electronic devices. For success in technical application it is necessary to explore their physical properties on the nanoscale. In MBE grown semiconductor NWs axial stacking faults separating zinc-blende and wurtzite entities are the major structural defects influencing the physical properties. Structural composition, phase arrangement and residual strain of individual GaAs NWs grown on Si(111) can be investigated X-ray nano-diffraction employing a focused synchrotron. It is found that even neighbouring NWs grown on the same sample under the same growth conditions differ significantly in their phase structure Moreover, the misfit strain at the substrate to NW interface releases within few monolayers due to relaxation towards the NW side planes [1]. The evolution of stacking faults is no constant but depends on growth time and the growth mode. In case of InAs NWs grown catalyst-free along the [111] we explored the dynamic relation between the growth conditions and the structural composition of the NWs using time-resolved X-ray scattering and diffraction measurements during the MBE growth. The spontaneous buildup of liquid indium droplet in the beginning of the growth process is accompanied by the simultaneous nucleation of InAs NWs predominantly grown in the wurtzite phase with low number of stacking faults. After nucleation the In droplets become consumed resulting in structural degradation of NWs due to the formation of densely spaced stacking faults [2]. For the first time the particular phase structure of single GaAs NWs could be correlated with their electrical properties. Here the V-I characteristics was measured in a dual Focused Ion Beam chamber the resistance and their effective charge carrier mobility was modeled in terms of thermo-ionic emission theory and space charge limited current model, respectively. Both resistance and inverse mobility show a qualitatively similar electric behavior comparing the inspected NWs. The same single NWs electrically measured have been inspected by X-ray nano-diffraction. The NWs were found to be composed by zinc-blende and twinned zinc-blende units separated by axial interfaces and a small plastic displacement. It turns out that the measured value of the extracted resistance and the inverse of effective mobility increases with the number of intrinsic axial interfaces, whereas the small plastic displacement has less influence on electrical properties [3]. We acknowledge support by BMBF and DFG. [more]