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Niels Hansen - One of the best experts on this subject based on the ideXlab platform.
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Fine scale Structures from Deformation of aluminium containing small alumina particles
Acta Materialia, 2002Co-Authors: Claire Y. Barlow, Niels Hansen, Y. L. LiuAbstract:Abstract MicroStructures and mechanical properties have been studied in aluminium containing a fine dispersion of alumina particles, deformed by cold-rolling to strains between 1.4 and 3.5. The microStructure was characterised by TEM. The Deformation Structures evolved very rapidly, forming a nanoStructured material, with fine subgrains about 0.2 μm in diameter and a fraction of high-angle boundaries which was already high at a strain of 1.4, but continued to increase with rolling strain. The yield stress and ductility of the rolled materials were measured in tension, and properties were similar for all materials. Yield stress measurements were correlated with estimates made using microstructural models. The role of small particles in forming and stabilising the Deformation Structure is discussed. This nanoStructured cold-deformed alloy has mechanical properties which are usefully enhanced at comparatively low cost. This gives it, and similar particle-strengthened alloys, good potential for commercial exploitation.
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effect of grain orientation on Deformation Structure in cold rolled polycrystalline aluminium
Acta Materialia, 1998Co-Authors: Juul D Jensen, Niels HansenAbstract:Abstract Pure (99.996%) polycrystalline aluminium (grain size about 300 μm) has been cold-rolled to reductions in the range from 5 to 50% ( e vm =0.06–0.80) and the Deformation microStructure has been analysed by TEM in a large number of grains. The Deformation microStructure is subdivided by dislocation boundaries having different characteristics depending on the orientation of the deformed grain. An analysis of the dislocation boundaries based on Frank's formula shows that the majority of the dislocations in the boundaries originate from active slip systems predicted by a Schmid factor analysis. Thereby a link is created between the microstructural evolution and the macroscopic plastic behaviour. The boundary parameters have also been used in a calculation of the dislocation density and the stored energy for grains of different orientations. These calculations together with measurements of the angular misorientation distribution shows that microstructural differences caused by differences in grain orientation are enhanced as the strain is increased.
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grain orientation dependence of microStructure in aluminium deformed in tension
Scripta Materialia, 1997Co-Authors: Xiaoxu Huang, Niels HansenAbstract:The evolution of Deformation microStructure in medium to high stacking fault energy fcc metals has been described in terms of a general framework of grain subdivision involving the formation of rotates volume elements for all Deformation modes. The rotated volume elements are ordinary dislocation cells and cell blocks, where a cell block contains a group of cells. The cell blocks are bounded by dislocation boundaries accommodating the lattice misorientations between neighboring cell blocks that deform with different slip system combinations and/or by different strains or strain amplitudes. An important aspect of the Deformation Structure development is the pronounced dependence on the crystal orientation. This has been shown by the formation of different types of Deformation microStructures, for which the dislocation boundaries formed during Deformation are characterized by their different morphology, spacing, crystallographic orientation and misorientation angle, in single crystals of different orientations deformed in tension and in rolling. Studies of polycrystal behavior are much less extensive but an orientation effect has been observed on a macroscopic scale in specimen deformed in rolling and on a microscopic scale in specimens deformed in tension and in rolling. The previous studies have shown a need for a more precise characterization of the effectmore » of grain orientation on the microstructural evolution of polycrystalline metals. Such a characterization, if successful, could lead to prediction of the evolution of the Deformation microStructure in order to establish quantitative correlation between microStructure, crystallographic texture and properties of polycrystals. The present study only concerns the microStructure, which has been characterized in pure polycrystalline aluminium deformed in tension at room temperature.« less
I M Robertson - One of the best experts on this subject based on the ideXlab platform.
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the effect of nanosized ti mo c precipitates on hydrogen embrittlement of tempered lath martensitic steel
Acta Materialia, 2014Co-Authors: Akihide Nagao, May L Martin, I M Robertson, Mohsen Dadfarnia, Petros Athanasios SofronisAbstract:Abstract Nanosized (Ti,Mo)C precipitates in a high-strength tempered lath martensitic steel are shown to increase resistance to hydrogen embrittlement. The hydrogen-induced failure mode transitions from failure along lath and prior austenite boundaries in the absence of the (Ti,Mo)C precipitates to a mixed failure mode of microvoid coalescence and lath boundary failure in their presence. In the absence of hydrogen and regardless of the presence or absence of the (Ti,Mo)C precipitates, failure occurs via ductile microvoid coalescence. By correlating the macroscale mechanical properties, the fractography of the resulting failure surfaces and observation of the evolved Deformation Structure immediately beneath the fracture surfaces, a hydrogen-enhanced and plasticity-mediated failure mechanism is proposed in which the role of the nanosized (Ti,Mo)C precipitates is to serve as effective traps for hydrogen.
Akihide Nagao - One of the best experts on this subject based on the ideXlab platform.
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the effect of nanosized ti mo c precipitates on hydrogen embrittlement of tempered lath martensitic steel
Acta Materialia, 2014Co-Authors: Akihide Nagao, May L Martin, I M Robertson, Mohsen Dadfarnia, Petros Athanasios SofronisAbstract:Abstract Nanosized (Ti,Mo)C precipitates in a high-strength tempered lath martensitic steel are shown to increase resistance to hydrogen embrittlement. The hydrogen-induced failure mode transitions from failure along lath and prior austenite boundaries in the absence of the (Ti,Mo)C precipitates to a mixed failure mode of microvoid coalescence and lath boundary failure in their presence. In the absence of hydrogen and regardless of the presence or absence of the (Ti,Mo)C precipitates, failure occurs via ductile microvoid coalescence. By correlating the macroscale mechanical properties, the fractography of the resulting failure surfaces and observation of the evolved Deformation Structure immediately beneath the fracture surfaces, a hydrogen-enhanced and plasticity-mediated failure mechanism is proposed in which the role of the nanosized (Ti,Mo)C precipitates is to serve as effective traps for hydrogen.
A. Sato - One of the best experts on this subject based on the ideXlab platform.
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training effect in fe mn si shape memory alloys
Journal of Materials Science, 1993Co-Authors: Yoshimi Watanabe, Yoshihiro Mori, A. SatoAbstract:The training effect in Fe-Mn-Si shape-memory alloys has been examined by length change and electrical resistivity measurements. After 13 Deformation-heating cycles, it was found that the major recovery took place at a temperature lower by 30 K than the first cycle. Simple thermal cycling also lowered the starting temperature of the reverse transformation and increased the finishing temperature. At the same time, the martensitic transformation temperature was found to increase significantly, for example by 35 K, at the 14th thermal cycle. The characteristics of shape-memory effect affected by development of the homogeneous and fine Deformation Structure by the thermal cycling are discussed in the light of the training effect.
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In situ observation of superdislocation motion and high-temperature strengthening of Ni3Al single crystals
Philosophical Magazine A, 1993Co-Authors: K. Jumonji, T. Shinohara, Masaharu Kato, A. SatoAbstract:Abstract The motion of superdislocations and the development of a Deformation Structure in a Ni3Al single crystal have been examined by in situ Deformation in a high-voltage electron microscope in the temperature range 300–1000 K. It has been revealed that the segments of superdislocations are locked locally because of the cross-slip from {111} to {010}, resulting in the exhaustion of longer mobile dislocations and the creation of immobile screw dislocations. A decrease in the pinning distance of the dislocation thus increases the strength, which is of an athermal nature, at high temperatures.
Zhiping Luo - One of the best experts on this subject based on the ideXlab platform.
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mechanical twinning and texture evolution in severely deformed ti 6al 4v at high temperatures
Acta Materialia, 2006Co-Authors: Guney Guven Yapici, I Karaman, Zhiping LuoAbstract:Abstract We have investigated the Deformation behavior and texture evolution of two-phase Ti–6Al–4V subjected to severe plastic Deformation using equal channel angular extrusion (ECAE) at a high temperature (∼0.55Tm). Significant Deformation twinning activity was observed after one and two ECAE passes in a 90° die at 800 °C. Twinning activity at such a high temperature is a first-time observation in this material and is attributed to the high strain and stress levels imposed during ECAE. High stress levels and the stress state can affect the separation of twinning partials considerably. Resolved shear stress magnitudes on twin partials were found to be high during the ECAE process that helps the nucleation of mechanical twinning. The twinning mode was identified as the { 1 0 1 ¯ 1 } type using electron diffraction patterns which is one of the twinning modes observed in Ti at temperatures above 350 °C. Although only one twinning variant was mainly evident after one pass, multiple twin variants of the same mode were observed after the second pass with a significant increase in twin volume fraction. ECAE processing aligned the basal planes of the hexagonal close-packed α phase, initially having a random texture, with the ECAE shear plane. Texture evolution during ECAE was successfully predicted using a viscoplastic self-consistent crystal plasticity framework capturing the effect of the observed twinning mode on texture. Mechanical twins formed during ECAE and grain refinement led to a noteworthy improvement in flow stresses under tension and compression at room temperature. A strong directional anisotropy in yield strengths was also evident which cannot be explained only by crystallographic texture. It was speculated that the asymmetry of critical resolved shear stresses of Deformation modes and the processing-induced Deformation Structure should play a role. With the supporting evidence from our previous works on the severe plastic Deformation of other difficult-to-work alloys, it was concluded that mechanical twinning can be one of the main modes of Deformation in difficult-to-work alloys in a wide range of temperatures when high strength levels are reached, irrespective of the way in which they are achieved.