We report on recurrent shear localization by formation of strictly alternating shear and matrix bands during equal-channel angular pressing (ECAP) of a 6000 series aluminum alloy. The strain ...partitioning process is documented by analyzing the deformation of a grid of indents as well as reconstructing the corresponding flow lines. Interestingly, shear strains of ∼3.6 in the shear bands considerably exceed the conventional maximum shear strain achievable in a single ECAP pass, whereas much lower strains occur in the matrix bands, maintaining on average the macroscopic deformation that is expected for ECA-pressing with a 90° die. Microstructural analysis by electron back-scatter diffraction (EBSD) and scanning transmission electron microscopy documents the different stages of microstructural evolution in shear and matrix bands and confirms the pronounced differences associated with the novel strain partitioning process. Furthermore, an EBSD-based analysis of texture evolution for billets with different orientations with respect to the initial extrusion direction demonstrates the important role that texture softening plays in triggering shear localization in two characteristic orientations as opposed to homogeneous deformation in the third orientation. Shear banding during ECAP is often interpreted in the light of failure mechanisms and cracking; the present study demonstrates that stable strain partitioning facilitates the fabrication of bulk laminated materials by ECAP.
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•New methods of heat treatment were applied at low alloyed high strength steel.•High UTS of 1907MPa with ductility of 17% were obtained for low alloyed steel.•Test of deformation behavior of ...martensite–austenite microstructure in micro-volumes.•Plastic deformation higher than 17% was obtained for martensite microstructure RA.
By stabilising metastable austenite with a suitable morphology in a martensitic structure, it is possible to impart to multi-phase steels high ductility combined with tensile strengths exceeding 2000MPa. One way to achieve such mixed structures consisting of martensite and retained austenite (RA) is the Q&P (quenching and partitioning) process. The resulting structure contains metastable austenite in the form of thin foils located between martensite laths or plates. The stability of austenite under mechanical loading is the essential factor contributing to the extraordinary plasticity of such materials during cold deformation. A steel with 0.43% of carbon, alloyed with manganese, silicon and chromium was chosen for the experiment described in the present paper. Using the Q&P process, a martensitic structure with 20% of retained austenite was obtained. As cold plastic deformation causes the austenite to transform, 10% cold deformation was applied after the Q&P process. This deformation reduced the RA fraction to 11%. Materials prepared by this method were examined using micro-pillar compression experiments. Using the focused ion beam (FIB) method, pillars of 3×3μm cross-section and 8μm length were fabricated. These were afterwards mechanically tested in situ in an electron microscope in quasi-static compression at a true strain rate of 3×10−4s−1 to different amounts of plastic strain. The experiment showed that mechanical properties of the two conditions of material differ in terms of yield strength and the strain hardening exponent. An additional metallographic analysis of structures, including the exploration of the influence of decomposition of retained austenite, was performed.
Cold extrusion is an unconventional approach that can potentially combine the benefits of plastic shaping of metals and simultaneous strengthening by grain refinement and strain hardening. In this ...study, cast billets of the commercially pure AA1080 aluminum alloy were extruded at room temperature in a conventional extrusion press with various extrusion diameters, corresponding to extrusion ratios in the range of about 10 up to 54. (Micro-)hardness measurements as well as microstructural analyses by means of electron microscopy were carried out at several locations along the extrudates’ longitudinal directions in order to characterize axial and radial deformation gradients. Interestingly, the extruded profiles exhibit a radially graded microstructure with four distinct annular sections independent of their respective extrusion ratio, reported here for the first time: A double-fiber textured center followed by a single-fiber textured ring, a double-fiber textured region with grains arranged alternatingly in an iris-like shape, and an (ultra-)fine grained surface layer. The microstructural features of these sections can be directly related to hardness distribution, with pronounced gradients in the center and surface sections. The results highlight that cold extrusion of AA1080 results in a pronounced multi-gradient deformation structure characterized by distinct differences in terms of texture, grain size and strain hardening.
•Cold-extruded microstructure consists of four distinct annular sections.•Distinct hardness gradients in center and surface region.•Axial gradient especially affecting the hardness in the surface region.•Ultrafine-grained microstructure evolves in surface region.•Local texture: ⟨111⟩ single fiber (regular or tilted) or ⟨001⟩/⟨111⟩ double fiber.
This paper focuses on scaling of Equal-Channel Angular Pressing (ECAP) from conventional, laboratory scale (billet cross section 15×15mm2) to large scale (50×50mm2). We study pure copper billets ...produced by ECAP in two identical ECAP-dies (but with different cross-sections) that have been optimized to provide reduced contact friction. In order to characterize processing parameters and the resulting properties, the billets are processed by 4 and 8 passes on both scales. Mechanical and microstructural characterization is performed by hardness testing and EBSD measurements. The materials produced in the different scales show very similar properties. A slight top to bottom hardness gradient (<6%) is detected in the billets on both scales. After 4 passes, this gradient is also reflected in grain size distributions. The higher cumulated strain after 8 passes leads to a more homogenized microstructure, again with similar grain sizes for both scales. Our results show that there are no scaling effects regarding the mechanical properties and the microstructures when comparing laboratory and large scale ECAP. This study clearly highlights the potential for scaling ECAP (using a suitable die-design) for a commercial implementation of ultrafine-grained materials.
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•We scale up Equal-Channel Angular Pressing from laboratory scale to large-scale.•We present an up-scaled die-design which significantly reduces contact friction.•Hardness distributions as well as microstructures are very similar for both scales.•We found no scaling effects when comparing laboratory and large-scale ECAP.•Our study highlights the potential for scaling ECAP with a suitable die-design.
Severe plastic deformation (SPD) processes offer the possibility of improving the mechanical properties of metallic materials by grain refinement. However, this great potential has so far mostly been ...applied on a laboratory scale or on small series. Equal-Channel Angular Pressing (ECAP) also enables to integrate the advantages in industrial processes with large output—so far, mainly for bars or thick plates. In this paper, we investigate the ECAP process for sheet metal. Preliminary investigations have shown that cracks form on the surface when aluminum AA5083 sheets are processed. To solve this problem, we determined the Johnson–Cook fracture criterion for the material and modeled the process numerically. The simulation was carried out with the superposition of a backpressure and subsequently implemented and validated experimentally. The semi-finished sheet metal products from the ECAP investigation were then mechanically characterized with microhardness measurements and tensile tests. In addition, the microstructure was investigated with Electron Back Scatter Diffraction (EBSD). Even comparatively small amounts of backpressure (10 MPa) already result in a significant suppression of the crack formation in the numerical and experimental investigations. The microhardness measurements indicate a more homogeneous strain distribution for a sufficient level of applied backpressure which enables the processing of crack-free sheets in multiple ECAP passes. As with ECAP of bulk materials, tensile tests on the processed sheets show a reduced elongation to failure (− 73%) but a significantly increased yield strength (+ 157%) compared to the initial condition of the material. Distinct substructures are found in the EBSD measurements and explain this behavior. The findings provide the basis for using ECAP on an application-oriented scale and demonstrate an advanced manufacturing method for the production of high-strength aluminum sheets.
In this study, we investigate the piezoresistive properties of flexible, strain sensitive multi-walled carbon nanotubes (MWCNTs)/epoxy composites. The deformation over the sensor area was tested by ...digital image correlation (DIC) under quasi-static uniaxial tension. The piezoresistive characteristics of the films were investigated quantitatively by electrochemical impedance spectroscopy (EIS) over a wide range of frequencies from 40 Hz to 110 MHz. Scanning electron microscopy (SEM) images confirmed that MWCNTs/epoxy composites with different CNT concentrations have a good homogeneity and dispersion. Additionally, in order to tailor the piezoresistivity of the strain sensor, an RC equivalent circuit was derived based on the impedance responses and the corresponding parameters were extracted under tensile strain. Compared with traditional strain gauges, higher sensitivity is obtained in particular at the concentrations close to the percolation threshold (13.6 for 0.3 wt.%). Due to the tunneling effect, a non-linear piezoresistivity is observed at low concentrations. It was found that sensors with 1 wt.% shows the highest linearity with a correlation coefficient of 0.999. The standard deviation of the cyclic readings was found to be 0.05%, indicating a high repeatability.
Though xenogeneic acellular scaffolds are frequently used for surgical reconstruction, knowledge of their mechanical properties is lacking. This study compared the mechanical, histological and ...ultrastructural properties of various native and acellular specimens.
Porcine esophagi, ureters and skin were tested mechanically in a native or acellular condition, focusing on the elastic modulus, ultimate tensile stress and maximum strain. The testing protocol for soft tissues was standardized, including the adaption of the tissue's water content and partial plastination to minimize material slippage as well as templates for normed sample dimensions and precise cross-section measurements. The native and acellular tissues were compared at the microscopic and ultrastructural level with a focus on type I collagens.
Increased elastic modulus and ultimate tensile stress values were quantified in acellular esophagi and ureters compared to the native condition. In contrast, these values were strongly decreased in the skin after acellularization. Acellularization-related decreases in maximum strain were found in all tissues. Type I collagens were well-preserved in these samples; however, clotting and a loss of cross-linking type I collagens was observed ultrastructurally. Elastins and fibronectins were preserved in the esophagi and ureters. A loss of the epidermal layer and decreased fibronectin content was present in the skin.
Acellularization induces changes in the tensile properties of soft tissues. Some of these changes appear to be organ specific. Loss of cross-linking type I collagen may indicate increased mechanical strength due to decreasing transverse forces acting upon the scaffolds, whereas fibronectin loss may be related to decreased load-bearing capacity. Potentially, the alterations in tissue mechanics are linked to organ function and to the interplay of cells and the extracellular matrix, which is different in hollow organs when compared to skin.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
► AZ31 processed by ECAP and/or bi-directional rolling at 523 and 573K. ► ECAP increases texture-induced softening and ductility. ► Bi-directional rolling and water quenching enhances strength. ► ...Interrelation of microstructures, crystallographic textures and mechanical properties. ► Novel route leads to yield strength of 345MPa and elongation to failure of 13.9%.
In this study the hexagonal close packed magnesium alloy AZ31 is deformed plastically by equal-channel angular pressing (ECAP) at 523K, bi-directional rolling (BR) at 573K, and by combinations of these processes. The evolution of microstructures, crystallographic textures and of the mechanical properties is investigated systematically as a function of the processing method, the amount of accumulated strain or the ECAP processing route (strain path). Both BR and ECAP enforce a crystallographic reorientation, with an alignment of (0002) basal planes parallel to the direction of shear deformation. This results in significant changes of the strongly texture-dependent mechanical properties. In combined processing, ECAP is applied as a method to increase texture-induced softening and ductility, whereas BR followed by water quenching is used as a method to induce strengthening by work hardening. The aim of tailoring materials and microstructures that are strong, but still ductile, and that can consume a moderate amount of plastic work during deformation, is best met by a combination of ECAP and subsequent BR. The corresponding yield strength of 345MPa and elongation to failure of 13.9% demonstrate the considerable potential of the novel processing routes presented here for the first time.
Simple shear tests are widely used for material characterization especially for sheet metals to achieve large deformations without plastic instability. This work describes three different shear tests ...for sheet metals in order to enhance the knowledge of the material behavior under shear conditions. The test setups are different in terms of the specimen geometry and the fixtures. A shear test setup as proposed by Miyauchi, according to the ASTM standard sample, as well as an in-plane torsion test are compared in this study. A detailed analysis of the experimental strain distribution measured by digital image correlation is discussed for each test. Finite element simulations are carried out to evaluate the effect of specimen geometries on the stress distributions in the shear zones. The experimental macroscopic flow stress vs. strain behavior shows no significant influence of the specimen geometry when similar strain measurements and evaluation schemes are used. Minor differences in terms of the stress distribution in the shear zone can be detected in the numerical results. This work attempts to give a unique overview and a detailed study of the most commonly used shear tests for sheet metal characterization. It also provides information on the applicability of each test for the observation of the material behavior under shear stress with a view to material modeling for finite element simulations.
We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning ...electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈001〉, 〈101〉 and 〈111〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈001〉 direction and lowest along the 〈111〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important.
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