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

Impact of temperature and hydrogen on the nanomechanical properties of a highly deformed high entropy alloy

Due to their quite attractive properties, high-entropy alloys have emerged to an intensely studied class of alloys within the past years. Besides their high strength and maintained ductility, literature reports modest sensitivity to hydrogen embrittlement for conventional microstructures. Utilizing severe plastic deformation methods, for example high-pressure torsion, it is possible to further tailor the mechanical properties by microstructure refinement to the nanometer regime, which in turn increases the hydrogen storage capability at internal defects and boundaries. Furthermore, the nanocrystalline grain size provides markedly enhanced strength values, while the high fraction of grain boundaries influences the hydrogen diffusion and storage kinetics. Within this study, the micromechanical characteristics of pure Ni and a single phase face-centered cubic CrMnFeCoNi alloy in fine and ultra-fine grained microstructural conditions, fabricated by high pressure torsion, will be investigated in detail. Moreover, electrochemical in-situ nanoindentation will be employed to determine the impact of hydrogen charging on the mechanical performance of this high-entropy alloy class and will be set into context to result found for pure Ni

Phase Transformation Induced by High Pressure Torsion in the High-Entropy Alloy CrMnFeCoNi

The forward and reverse phase transformation from face-centered cubic (fcc) to hexagonal close-packed (hcp) in the equiatomic high-entropy alloy (HEA) CrMnFeCoNi has been investigated with diffraction of high-energy synchrotron radiation. The forward transformation has been induced by high pressure torsion at room and liquid nitrogen temperature by applying different hydrostatic pressures and large shear strains. The volume fraction of hcp phase has been determined by Rietveld analysis after pressure release and heating-up to room temperature as a function of hydrostatic pressure. It increases with pressure and decreasing temperature. Depending on temperature, a certain pressure is necessary to induce the phase transformation. In addition, the onset pressure depends on hydrostaticity; it is lowered by shear stresses. The reverse transformation evolves over a long period of time at ambient conditions due to the destabilization of the hcp phase. The effect of the phase transformation on the microstructure and texture development and corresponding microhardness of the HEA at room temperature is demonstrated. The phase transformation leads to an inhomogeneous microstructure, weakening of the shear texture, and a surprising hardness anomaly. Reasons for the hardness anomaly are discussed in detail

Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures

High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centered cubic (fcc) solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature the alloy shows tensile strengths of almost 1 GPa, failure strains of ~70%, and KJIc fracture-toughness values above 200 MPa.m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa.m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.Comment: 7 pages, 4 figure

Enhanced fatigue endurance of metallic glasses through a staircase-like fracture mechanism

We believe this article is of broad interest to the materials science and engineering community. Bulk-metallic glasses (BMGs) are currently considered candidate materials for numerous structural applications. A major limitation in their use as engineering material is the often poor and inconsistent fatigue behavior. Although recently developed BMG composites provide one solution to this problem, fatigue remains a main issue for monolithic metallic glasses. The authors report unexpectedly high fatigue resistance in a monolithic Pd-based glass arising from extensive shear-band plasticity, resulting in a very rough and periodic “staircase” crack trajectory. The research both reveals a unique mechanism in fatigue of a monolithic metallic glass and demonstrates that this mechanism mitigates previous limitations on its use as an engineering material

Nanomaterials by severe plastic deformation: review of historical developments and recent advances

International audienceSevere plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity

Vliv mikrostruktury a krystalografie na trajektorie trhlin a intrinzitní odolnost vůči růstu únavových trhlin ve smykových módech

The paper focuses on the effective resistance and the near-threshold growth mechanisms in the ferritic-pearlitic and the pure pearlitic steel. The influence of microstructure on the shear-mode fatigue crack growth is divided here into two factors: the crystal lattice type and the presence of different phases. Experiments were done on ferritic-pearlitic steel and pearlitic steel using three different specimens, for which the effective mode II and mode III threshold values were measured and fracture surfaces were reconstructed in three dimensions using stereophotogrammetry in scanning electron microscope. The ferritic-pearlitic and pearlitic steels showed a much different behaviour of modes II and III cracks than that of the ARMCO iron. Both the deflection angle and the mode II threshold were much higher and comparable to the austenitic steel. Mechanism of shear-mode crack behaviour in the ARMCO iron, titanium and nickel were described by the model of emission of dislocations from the crack tip under a dominant mode II loading. In other tested materials the cracks propagated under a dominance of the local mode I. In the ferritic-pearlitic and pearlitic steels, the reason for such behaviour was the presence of the secondary-phase particles (cementite lamellas), unlike in the previously austenitic steel, where the fcc structure and the low stacking fault energy were the main factors. A criterion for mode I deflection from the mode II crack-tip loading, which uses values of the effective mode I and mode II thresholds, was in agreement with fractographical observations.Článek se zaměřuje na efektivní odpor vůči šíření trhlin a mechanismus jejich růstu ve feriticko-perlitické a perlitické oceli. Vliv mikrostruktury na šíření smykových trhlin je rozdělen na dva faktory: krystalová mřížka a přítomnost různých fází. Byly naměřeny efektivní prahové hodnoty pro módy II a III a lomové plochy byly pozorovány ve 3D s použitím stereofotogrammetrie ve skenovacím elektronovém mikroskopu. Chování smykových trhlin ve feriticko-perlitické oceli se velmi lišilo od chování ARMCO železe (čistě feritická ocel). Mechanismus šíření trhlin v ARMCO železe, titanu a niklu byl popsán modelem emise dislokací ze špice trhliny s převahou módu II. V ostatních materiálech se smykové trhliny šířily s převahou lokálního módu I. Kritérium pro odklon do módu I, které započítává efektivní prahové hodnoty pro mód I a II, bylo v souladu s fraktografickým pozorováním