Based on the pioneering work of Erbel, we propose a simple modification of classical high-pressure torsion anvils that allows a strong reduction or even elimination of the inherent strain gradient in ...disk-shaped samples deformed by high-pressure torsion. This is realized by using an adapted anvil design, which entails a linearly varying depth of the anvil’s depression with the radius. In order to compare results of using the modified anvil design with classical flat anvils, a comparative study using high purity nickel was performed. The possible change in the strain gradient is assessed with hardness and microstructural investigations. The results prove that the strain gradient can be considerably reduced or even eliminated. The technical features of this technique, in the following termed as uniform strain high-pressure torsion (US-HPT), are presented with this feasibility study and practical limitations are discussed.
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In this review article we consider the crack growth resistance of micrometer and sub-micrometer sized samples from the fracture mechanics point of view. Standard fracture mechanics test procedures ...were developed for macro-scale samples, and reduction of the specimen dimensions by three to five orders of magnitude has severe consequences. This concerns the interpretation of results obtained by micro- and nano-mechanics, as well as the life time and failure prediction of micro- and nano-devices. We discuss the relevant fracture mechanics length scales and their relation to the material-specific structural lengths in order to conduct rigorous fracture mechanics experiments. To ensure general validity and applicability of evaluation concepts, these scaling considerations are detailed for ideally brittle, semi-brittle and micro ductile crack propagation, subject to both monotonic and cyclic loading. Special attention is devoted to the requirements for determining specimen size for various loading types to measure material characteristic crack propagation resistance at small scales. Finally, we discuss novel possibilities of micron and sub-micron fracture mechanics tests to improve the basic understanding of specific crack propagation processes.
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•Fundamental concepts of fracture mechanics are revisited with respect to their applicability in miniaturized experiments•Stress state and notch sharpness in miniaturized samples need to be considered•For size independent fracture toughness values, sample sizes must obey relations to the microstructure and fracture process zone•Different contributions to material toughness can be separated by appropriate experimental design
Micromechanical experiments with 3 × 3 × 6 μm3 sized micro pillars were used to examine orientation dependencies of the mechanical properties in a severely plastically deformed high strength steel ...and compared with the undeformed state. For the synthesis, an initially ultrafine-lamellar (UFL) fully pearlitic steel was subjected to high pressure torsion (HPT) transforming the steel into a nanolamellar (NL) composite. Both microstructural states were then tested in-situ inside a scanning electron microscope. Within the individual micro pillars, fabricated by focused ion beam milling, the ferrite and cementite lamellae were aligned parallel, normal or inclined to the loading direction. The main findings are: First, the strength and strain hardening capacity is more than doubled comparing the UFL with the NL composite. Second, an anisotropic mechanical response exists in terms of i) strain hardening capacity and ii) stress level at the onset of plateau formation. Third, deformation and localization mechanisms at large compressive strains vary with the lamellae orientation, however they are independent of the lamellae thickness.
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Ultra-fine-grained high-purity copper (99.99%) deformed by means of high-pressure torsion into the steady-state regime was subjected to additional rolling deformation. The microstructural changes as ...a function of the applied strain were analysed by means of orientation imaging microscopy. It was found that after a distinctive rolling strain a steady state with respect to microstructural features such as grain size, misorientation distribution and texture evolves again. A special spilt specimen technique was used to perform quasi in situ observations of the microstructure between additional strain increments. Profound insights into the local deformation and restoration processes within the steady-state regime were gained. The observations lead to the conclusion that grain boundary migration perpendicular to the rolling direction leads to the disappearance of certain grains, enabling the occurrence of a steady state.
The "wet pressure drop" of oil mist filters (i.e. the increase in differential pressure of the air flow due to loading of the filter with liquid) is presented as a function of two mechanisms by which ...coalesced oil is transported through the filter. These mechanisms operate in separate regions of the filter and make separate (and separately measurable) contributions to the overall wet pressure drop. This new concept, which was first formulated qualitatively in a phenomenological model by Kampa et al. (2014), leads to semi-quantitative predictions regarding the dependence of pressure drop Delta p and saturation S on filter operating conditions, filter properties and liquid properties. These predictions are first formulated and then validated for a range of wettable and non-wettable filter media in combination with 4 mineral oils of different viscosity. The key findings, summarized below, are consistent with the model and apply to both wettable and non-wettable media. Oil transport across media interfaces (i.e. transitions between regions of different porosity and/or wettability) was associated with a relatively sharp increase in pressure drop Delta p and oil saturation S over a very thin layer of the filter (a " Delta p jump"). The magnitude of this Delta p jump was determined by the media properties. It correlated well with the respective static break-through pressures for oil or air, but did not depend on the oil viscosity and loading rate of the filter (at constant air velocity). Oil transport through channel regions of the filter (i.e. the regions connecting interfaces) was associated with a linear increase in Delta p with channel length and liquid throughput. The corresponding saturation level S was relatively flat throughout the channel region and lower than at an interface. (Both quantities are media dependent, of course.) An increase in oil viscosity mu (at constant oil throughput) led to different responses depending on filter wettability.
Commercially pure vanadium was processed by high pressure torsion at room temperature into the saturation regime and investigated with respect to the microstructure and resulting mechanical ...properties. Besides hardness, strength and ductility special emphasis was put towards the fracture toughness after processing. The results demonstrate, so far unusual for pure ultrafine-grained body-centered cubic metals, remarkable ductility and fracture toughness while also having high strength.
Plasma-facing materials and components in a fusion reactor are the interface between the plasma and the material part. The operational conditions in this environment are probably the most challenging ...parameters for any material: high power loads and large particle and neutron fluxes are simultaneously impinging at their surfaces. To realize fusion in a tokamak or stellarator reactor, given the proven geometries and technological solutions, requires an improvement of the thermo-mechanical capabilities of currently available materials. In its first part this article describes the requirements and needs for new, advanced materials for the plasma-facing components. Starting points are capabilities and limitations of tungsten-based alloys and structurally stabilized materials. Furthermore, material requirements from the fusion-specific loading scenarios of a divertor in a water-cooled configuration are described, defining directions for the material development. Finally, safety requirements for a fusion reactor with its specific accident scenarios and their potential environmental impact lead to the definition of inherently passive materials, avoiding release of radioactive material through intrinsic material properties. The second part of this article demonstrates current material development lines answering the fusion-specific requirements for high heat flux materials. New composite materials, in particular fiber-reinforced and laminated structures, as well as mechanically alloyed tungsten materials, allow the extension of the thermo-mechanical operation space towards regions of extreme steady-state and transient loads. Self-passivating tungsten alloys, demonstrating favorable tungsten-like plasma-wall interaction behavior under normal operation conditions, are an intrinsic solution to otherwise catastrophic consequences of loss-of-coolant and air ingress events in a fusion reactor. Permeation barrier layers avoid the escape of tritium into structural and cooling materials, thereby minimizing the release of tritium under normal operation conditions. Finally, solutions for the unique bonding requirements of dissimilar material used in a fusion reactor are demonstrated by describing the current status and prospects of functionally graded materials.
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•Two distinct transport mechanisms for coalesced liquid govern the “wet” pressure drop.•Transport across capillary interfaces causes Δp jumps associated with film formation.•Internal ...transport occurs via multiple parallel channels with constant Δp/length.
A phenomenological model is presented to explain the increase in pressure drop (Δp) of air filters during steady operation with oil mist. It is based on (currently) semi-quantitative conclusions obtained from measurements of liquid distribution patterns in the media associated with the transport of coalesced liquid by the flowing air. Correlation of these patterns in space and time with the evolution of the pressure drop suggests that the over-all increase in Δp (the “wet” pressure drop) is governed by two distinctly different liquid transport mechanisms:
A steep Δp jump is required to overcome the capillary exit (or entry) pressure and pump liquid into non-wettable, or out of wettable fibrous matrices. It is associated with the formation of a thin liquid film covering almost the entire front (or rear) face of the respective media. With the help of a polymerization technique to “freeze” the liquid distribution, the film is shown to be confined to the outermost surface without entering the media while the aerosol flow is on.
Liquid transport inside the media is shown to occur in multiple parallel channels spanning almost the entire thickness of a filter. The channel Δp associated with this transport mechanism increases linearly with media thickness. Wettable media form numerous fine channels which feed a liquid film on the rear face by which drainage takes place. Non-wettable media form fewer, relatively wide channels ending in large drops on the rear face, through which drainage takes place during steady operation.
Sandwiched combinations of wettable and non-wettable media show the same combination of features in their respective Δp curves. There are separate Δp jumps and channel regions for each media type. In case of a transition from wettable to non-wettable media, the combined exit and entry Δp jump takes place at the internal interface.
Tungsten and tungsten alloys show the typical change in fracture behavior from brittle at low temperatures to ductile at high temperatures. In order to improve the understanding of the effect of ...microstructure the fracture toughness of pure tungsten, potassium doped tungsten, tungsten with 1
wt.% La
2O
3 and tungsten rhenium alloys were investigated by means of 3-point bending, double cantilever beam and compact tension specimens. All these materials show the expected increase in fracture toughness with increasing temperature. The experiments demonstrate that grain size, texture, chemical composition, grain boundary segregation and dislocation density seem to have a large effect on fracture toughness below the DBTT. These influences can be seen in the fracture behavior and morphology, where two kinds of fracture occur: on the one hand transgranular and on the other hand intergranular fracture. Therefore, techniques like electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES) and X-ray line profile analysis were used to improve the understanding of the parameters influencing fracture toughness.
•CP-Ti was deformed by high pressure torsion, results in nc microstructure.•Tensile tests showed increased strength while maintaining ductility.•An Orientation dependent fracture toughness was found.
...The mechanical properties with emphasis on the fracture behaviour of ultrafine-grained Ti processed by high pressure torsion were evaluated. Through this process a substantial refinement of the microstructure was induced. As a consequence, a pronounced increase of tensile strength was achieved compared to conventional Ti while the ductility exhibited only a minor reduction after processing. The fracture toughness in terms of the critical stress intensity factor and crack tip opening displacement was evaluated in different crack growth directions and showed a pronounced orientation dependency, which can be related to the typically elongated microstructure induced by this deformation method.
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