Fracture behavior of high strength pearlitic steel wires

Steel wires are widely used in various industrial applications. Hence, the drawing process and the resulting mechanical properties are of significant scientific and industrial importance. In this investigation the focus is on pearlitic steel wires with relatively high drawing strains and resulting high ultimate tensile strengths up to several GPa. The fracture behavior was investigated for two different wires with a drawing strain of 3.10 and 6.52 with a diameter of about 100 µm and 20 µm, respectively. The resulting ultimate tensile strength varies between 4 and 7 GPa [1]. The fracture toughness was measured with crack propagation direction in drawing direction and perpendicular to it. To test the fracture toughness of the samples in drawing direction micro-bending beams were fabricated utilizing a focused ion beam (FIB). For investigating the direction perpendicular to the drawing direction, the wires were notched with a FIB and tested under tensile and bending loading. The fracture toughness experiments for both directions were performed in-situ in the SEM. In addition, some samples of the perpendicular direction were tested ex-situ as well. The results of the fracture experiments show a strong anisotropy of the fracture behavior. It was revealed that in drawing direction the wires show a significantly lower fracture toughness than perpendicular to it. This is further supported by the in-situ and ex-situ bending experiments of the second testing direction, where the crack kinks into the drawing direction

Femtosecond laser and FIB: A revolutionary approach in rapid micro- mechanical sample preparation

The established Focused Ion Beam (FIB) technique usually poses a bottleneck in the preparation of samples for micro-mechanical experiments. This is due to its limited material removal rate. Especially for tungsten, the sputter yield of the Ga+ ion beam is very low. Therefore the practical sample size is restricted to dimensions of a few micrometers. On the contrary a femtosecond laser offers ablation rates 4-6 orders of magnitude higher compared to a Ga+ FIB [1] and therefore allows a rapid fabrication of specimens on the meso-scale. A prototype, which combines both methods, has been developed on the basis of the Zeiss Auriga Laser platform [2]. This system consists of the main chamber, where the FIB milling is conducted, and a separated airlock chamber for the femtosecond laser ablation. This setup prevents the contamination of the main chamber with laser ablated material and allows laser processing under atmospheric, inert gas or vacuum conditions. Fracture toughness experiments on single crystal tungsten in the micro-regime [3] exhibit a different behavior compared to specimens on the macro-scale [4]. The rapid processing of specimens with the novel laser system allows to sample the transition region from a discrete flow behavior of the micro-sized cantilevers to macroscopic plasticity. An analysis of fracture experiments for sample sizes ranging from 20 x 20 x 100 µm^3 to 200 x 200 x 1000 µm^3 is conducted. In addition to that, the quality of the laser processed samples is analyzed regarding the influence of processing parameters. Please click Additional Files below to see the full abstract

Anisotropy of ultrafine-lamellar and nanolamellar pearlitic structures revealed by in-situ micro compression testing

Applying severe plastic deformation to pearlitic steels leads to a transformation of the random ultrafine-lamellar (ufl) colony structure to a nanolamellar (nl) composite. The distinct reduction of the interlamellar spacing generates a high-strength steel with a strength of up to 3.7 GPa, which can readily be produced. Additionally, the ferrite and cementite lamellae align in a preferential direction giving rise to an anisotropic mechanical response. The aim of this work is to determine the material anisotropy in terms of flow stress and deformation behavior in the nl state and to compare it with its ufl state. Thus, micron sized samples are fundamental in order to fit inside a single colony of a distinct lamellae alignment. In addition, these small dimensions ensure a homogeneous structure within the nl samples. The compression setup on the other side enables to characterize the deformation behavior up to large strains, since early failure as a consequence of necking is prevented. Hence, micro compression experiments are an established tool for characterizing deformation mechanisms of fine-scaled materials and they further allow to link the deformation characteristics observed in the scanning electron microscope (SEM) to the mechanical data. In this work anisotropic mechanical properties could be successfully measured by micromechanical testing of pillars consisting of single ufl pearlite colonies and nl pearlitic structures for the first time. For both lamellae spacings three different types of micro pillars were focused ion beam milled, with the lamellae being aligned parallel, normal and inclined with respect to the loading direction. Comparing the stress-strain curves and the deformation characterstics of the ufl and nl micro pillars, it could be revealed that not only the interlamellar spacing but also the loading direction of the lamellae have a significant influence on the materials behavior. Especially, it was found that the yield point of the material is mainly controlled by the interlamellar spacing, whereas the lamellae orientation governs the hardening capability, the critical stress for the onset of localized deformation and thereby also the strength in a subsequently arising plateau regime. Furthermore, it could be shown that the global failure and deformation characteristics vary depending on the lamellae alignment. Finally it should be noted, that the anisotropy of the hardening behavior in the ufl and nl pearlite is different

Metastable nanomaterials and nanocomposites obtained by high-pressure torsion powder consolidation

Nanostructuring can dramatically improve the mechanical and functional properties of metallic materials and composites and to achieve the goal of a nanostructured bulk material innovative techniques have to be used. High Pressure Torsion (HPT), a method of Severe Plastic Deformation (SPD) is a novel processing route for powder consolidation, featuring a complete absence of a sintering treatment – bulk samples are the direct result of the SPD process. HPT further shows two advantages: First, the starting material (powder mixtures) can be processed at any concentration. Even immiscible compositions were successfully processed for different systems (e.g. Cu-Fe, Cu-Co). Second, the severe deformation gives rise to supersaturated solid solutions (“far from equilibrium”) yielding interesting material properties, different from the pure or alloyed elements’ ones. These supersaturated solid solutions, upon adequate annealing, show phase separations yielding nanoscaled composites. This gives a tool in one’s hands to systematically tune certain material properties. The nanostructures that were formed in such a way show high strength and ductility but also interesting functional properties. To give an example, annealing of above mentioned binary systems changes their magnetic properties regarding coercivity, remanence and magnetoresistance. Thus, combining the right choice of processed powders (composition, size, shape), HPT-processing parameters (applied strain, processing temperatures) and subsequent annealing treatment (time, temperature) results in desired, tailored microstructures of optimized properties. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 757333