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Lee, S.; Matsunaga, H.; Sauvage, X.; Horita, Z. |
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Strengthening of Cu–Ni–Si alloy using high-pressure torsion and aging |
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Journal Article |
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2014 |
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Abstract |
Materials Characterization |
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90 |
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62-70 |
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High-pressure torsion; Ultrafine grain; Cu–Ni–Si; Electrical conductivity; Apt |
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Abstract An age-hardenable Cu–2.9%Ni–0.6%Si alloy was subjected to high-pressure torsion. Aging behavior was investigated in terms of hardness, electrical conductivity and microstructural features. Transmission electron microscopy showed that the grain size is refined to ~ 150 nm and the Vickers microhardness was significantly increased through the HPT processing. Aging treatment of the HPT-processed alloy led to a further increase in the hardness. Electrical conductivity is also improved with the aging treatment. It was confirmed that the simultaneous strengthening by grain refinement and fine precipitation is achieved while maintaining high electrical conductivity. Three dimensional atom probe analysis including high-resolution transmission electron microscopy revealed that nanosized precipitates having compositions of a metastable Cu3Ni5Si2 phase and a stable NiSi phase were formed in the Cu matrix by aging of the HPT-processed samples and these particles are responsible for the additional increase in strength after the HPT processing. |
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1044-5803 |
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NU @ karnesky @ |
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11492 |
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Lee, Thomas C. M. |
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ON ALGORITHMS FOR ORDINARY LEAST SQUARES REGRESSION SPLINE FITTING: A COMPARATIVE STUDY |
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Journal Article |
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2002 |
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Journal of Statistical Computation and Simulation |
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72 |
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8 |
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647-663 |
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Bivariate smoothing; Generalized cross-validation; Genetic algorithms; Regression spline; Stepwise selection |
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Regression spline smoothing is a popular approach for conducting nonparametric regression. An important issue
associated with it is the choice of a ‘‘theoretically best’’ set of knots. Different statistical model selection
methods, such as Akaike’s information criterion and generalized cross-validation, have been applied to derive
different ‘‘theoretically best’’ sets of knots. Typically these best knot sets are defined implicitly as the optimizers
of some objective functions. Hence another equally important issue concerning regression spline smoothing is
how to optimize such objective functions. In this article different numerical algorithms that are designed for
carrying out such optimization problems are compared by means of a simulation study. Both the univariate and
bivariate smoothing settings will be considered. Based on the simulation results, recommendations for choosing a
suitable optimization algorithm under various settings will be provided. |
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NU @ karnesky @ |
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860 |
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Lee, W.-B.; Hong, S.-G.; Park, C.-G.; Park, S.-H. |
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Carbide precipitation and high-temperature strength of hot-rolled high-strength, low-alloy steels containing Nb and Mo |
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Journal Article |
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2002 |
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Metallurgical and Materials Transactions A |
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33 |
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6 |
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1689-1698 |
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Abstract The effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo. These steels were fabricated by four-pass hot rolling and coiling at 650C, 600C, and 550C. Microstructural analysis of the carbides has been performed using field-emission gun transmission electron microscopy (TEM) employing energy-dispersive X-ray spectroscopy (EDS). The steels containing both Nb and Mo exhibited a higher strength at high temperatures (∼600 C) in comparison to the steel containing only Nb. The addition of Mo increased the hardenability and led to the refinement of the bainitic microstructure. The proportion of the bainitic phase increased with the increase of Mo content. The TEM observations revealed that the steels containing both Nb and Mo exhibited fine (<10 nm) and uniformly distributed metal carbide (MC)-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb only. The EDS analysis also indicated that the fine MC carbides contain both Nb and Mo, and the ratio of Mo/Nb was higher in the finer carbides. In addition, electron diffraction analysis revealed that most of the MC carbides had one variant of the B-N relationship ((100)MC//(100)ferrite, [011]MC//[010]ferrite) with the matrix, suggesting that they were formed in the ferrite region. That is, the addition of Mo increased the nucleation sites of MC carbides in addition to the bainitic transformation, which resulted in finer and denser MC carbides. It is, thus, believed that the enhanced high-temperature strength of the steels containing both Nb and Mo was attributed to both bainitic transformation hardening and the precipitation hardening caused by uniform distribution of fine MC particles. |
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NU @ p-kolli @ |
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9838 |
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Lee, W.-S.; Chen, T.-H. |
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Mechanical and microstructural response of aluminum-scandium (Al-Sc) alloy as a function of strain rate and temperature |
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Journal Article |
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2009 |
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Materials Chemistry and Physics |
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113 |
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2-3 |
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734-745 |
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Al-Sc alloy; Strain rate sensitivity; Activation energy; Shearing; Dislocation; Precipitates |
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This study applies a compressive split Hopkinson bar to investigate the mechanical response, microstructural evolution and fracture characteristics of aluminum-scandium (Al-Sc) alloy at temperatures ranging from -100 °C to 300 °C and strain rates of 1.2 × 103 s-1, 3.2 × 103 s-1 and 5.8 × 103 s-1. The relationship between the dynamic mechanical behaviour of the Al-Sc alloy and its microstructural characteristics is explored. The fracture features and microstructural evolution are observed using scanning and transmission electron microscopy techniques. The stress-strain relationships indicate that the flow stress, work hardening rate and strain rate sensitivity increase with strain rate, but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. However, at room temperature under a low strain rate of 1.2 × 103 s-1 and at a high experimental temperature of 300 °C under all three tested strain rates, the specimens do not fracture, even under large strain deformations. The Zerilli-Armstrong fcc constitutive model is used to describe the plastic deformation behaviour of the Al-Sc alloy. Comparing the predicted flow stress values with the experimental values over all the considered strain rate and temperature conditions, the maximum error between the two sets of results is found to be less than 4%. SEM observations show that the specimens fracture predominantly as a result of a shearing mechanism. Moreover, the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of a ductile fracture mode. Fine Al3Sc precipitates are found to be distributed in the matrix and at the grain boundaries. Finally, the TEM analysis results reveal that the dislocation density increases, but the dislocation cell size decreases, with increasing strain rate. However, a higher temperature causes the dislocation density to decrease, thereby increasing the dislocation cell size. |
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NU @ karnesky @ |
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10512 |
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Lee, W.S.; Chen, T.H. |
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Effects of strain rate and temperature on dynamic mechanical behaviour and microstructural evolution in aluminium-scandium (Al-Sc) alloy |
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Journal Article |
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2008 |
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Materials Science and Technology |
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24 |
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10 |
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1271-1282 |
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AL-SC ALLOY; STRAIN RATE SENSITIVITY; ACTIVATION ENERGY; ADIABATIC SHEARING; DISLOCATION; AL3SC PRECIPITATES |
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The present study applies a compressive split Hopkinson bar to investigate the mechanical response, microstructural evolution and fracture characteristics of an aluminium-scandium (Al-Sc) alloy at temperatures ranging from − 100 to 300°C and strain rates of 1·2 × 103, 3·2×103 and 5·8 × 103 s−1. The relationship between the dynamic mechanical behaviour of the Al-Sc alloy and its microstructural characteristics is explored. The fracture features and microstructural evolution are observed using scanning and transmission electron microscopy techniques. The stress-strain relationships indicate that the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. The Zerilli-Armstrong fcc constitutive model is used to describe the plastic deformation behaviour of the Al-Sc alloy, and the error between the predicted flow stress and the measured stress is found to be less than 5%. The fracture analysis results reveal that cracks initiate and propagate in the shear bands of the Al-Sc alloy specimens and are responsible for their ultimate failure. However, at room temperature, under a low strain rate of 1·2 × 103 s−1 and at a high experimental temperature of 300°C under all three tested strain rates, the specimens do not fracture, even under large strain deformations. Scanning electron microscopy observations show that the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of ductile fracture. The depth and density of these dimples are significantly influenced by the strain rate and temperature. The transmission electron microscopy structural observations show the precipitation of Al3Sc particles in the matrix and at the grain boundaries. These particles suppress dislocation motion and result in a strengthening effect. The transmission electron microscopy analysis also reveals that the dislocation density increases, but the dislocation cell size decreases, with increasing strain rate for a constant level of strain. However, a higher temperature causes the dislocation density to decrease, thereby increasing the dislocation cell size. |
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no |
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NU @ m-krug @ |
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10531 |
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Lee, Woei-Shyan; Chen, Tao-Hsing |
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Rate-dependent deformation and dislocation substructure of Al–Sc alloy |
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Journal Article |
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2006 |
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Scripta Materialia |
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Scripta Mater. |
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54 |
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8 |
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1463-1468 |
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Al–Sc alloy; Strain rate effect; Activation volume; Dislocation |
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High strength aluminum–scandium (Al–Sc) alloy is impacted at strain rates ranging from 1.2 × 103 s−1 to 5.8 × 103 s−1 at room temperature. The deformation behaviour of Al–Sc alloy is highly dependent on the strain rate. A higher strain rate increases the dislocation density, thereby reducing the size of the dislocation cells. |
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NU @ karnesky @ |
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653 |
| Permanent link to this record |
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Lee, Woei-Shyan; Chen, Tao-Hsing; Lin, Chi-Feng; Chen, Ming-Shiang |
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Impact deformation behaviour and dislocation substructure of Al-Sc alloy |
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Journal Article |
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2010 |
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Journal of Alloys and Compounds |
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493 |
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1-2 |
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580-589 |
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Al-Sc alloy; Strain rate sensitivity; Activation volume; Adiabatic shear band; Dislocation; Precipitates |
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This paper employs a compressive split-Hopkinson pressure bar to investigate the impact deformation behaviour of Al-Sc alloy under high strain rates of 1.2103s-1, 3.2103s-1 and 5.8103s-1, respectively, and temperatures of -100C, 25C and 300C. It is shown that for a constant temperature, the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, while the activation volume decreases. Conversely, for a constant strain rate, the flow stress, work hardening rate and strain rate sensitivity decrease with increasing temperature, while the activation volume increases. It is found that the impact deformation behaviour of Al-Sc alloy can be accurately described using the Zerilli-Armstrong constitutive equation. Optical microscopy (OM) observations reveal that the specimens fail principally as the result of an adiabatic shearing mechanism. Furthermore, scanning electron microscopy (SEM) observations show that the fracture surfaces are characterised by a dimple-like structure, which indicates a ductile failure mode. Transmission electron microscopy (TEM) observations indicate that the dislocation density and cell size are related to the strain rate, flow stress and temperature. Finally, the TEM observations suggest that the strengthening effect observed in the deformed Al-Sc alloy is the result of Al3Sc precipitates within the matrix and at the grain boundaries, which suppress dislocation motion and prompt an increase in the work hardening stress. |
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0925-8388 |
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NU @ karnesky @ |
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10784 |
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Lee, Woei-Shyan; Chen, Tao-Hsing; Lin, Chi-Feng; Lu, Ging-Ting |
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Adiabatic Shearing Localisation in High Strain Rate Deformation of Al-Sc Alloy |
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Journal Article |
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2010 |
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Materials Transactions |
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51 |
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7 |
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1216-1221 |
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aluminium-scandium alloy, strain rate sensitivity, adiabatic shearing, precipitates |
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Aluminium-scandium (Al-Sc) alloy is subjected to shear deformation at high strain rates ranging from 3.0×105 s−1 to 6.2×105 s−1 using a compressive-type split-Hopkinson pressure bar (SHPB). The effects of the strain rate on the shear stress, adiabatic shear band characteristics, and fracture features of the Al-Sc alloy are systematically examined. The results show that both the shear stress and the strain rate sensitivity increase with an increasing strain rate. In addition, it is shown that an adiabatic shear band is formed within the deformed specimens for all values of the strain rate. As the strain rate is increased, the width of the shear band decreases, but the microhardness increases. Moreover, the distortion angle and the magnitude of the local shear strain near the shear band both increase with an increasing strain rate. At a strain rate of 3.0×105 s−1, the fracture surface is characterised by multiple transgranular clearage fractures. However, for strain rates greater than 4.4×105 s−1, the fracture surface has a transgranular dimple-like characteristic, and thus it is inferred that the ductility of the Al-Sc alloy improves with an increasing strain rate. |
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NU @ karnesky @ |
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10917 |
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Lee, Y. H.; Song, M. H.; Ju, B. K.; Shin, D. K.; Oh, M. H. |
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Thin film phosphor prepared by physical vapor deposition for field emission display application |
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Journal Article |
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1997 |
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Journal of Vacuum Science & Technology A |
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J. Vac. Sci. Technol. A |
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B15 |
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512-515 |
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Field Emission |
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2483 |
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Lee, Z.; Witkin, D.B.; Radmilovic, V.; Lavernia, E.J.; Nutt, S.R. |
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Bimodal microstructure and deformation of cryomilled bulk nanocrystalline Al-7.5Mg alloy |
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Journal Article |
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2005 |
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Materials Science and Engineering: A |
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The Langdon Symposium: Flow and forming of Crystalline Materials |
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410-411 |
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462-467 |
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Nanocrystalline; Bimodal; Cryomilling; Aluminum; Deformation |
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The microstructure, mechanical properties and deformation response of bimodal structured nanocrystalline Al-7.5Mg alloy were investigated. Grain refinement was achieved by cryomilling of atomized Al-7.5Mg powders, and then cryomilled nanocrystalline powders blended with 15 and 30% unmilled coarse-grained powders were consolidated by hot isostatic pressing followed by extrusion to produce bulk nanocrystalline alloys. Bimodal bulk nanocrystalline Al-7.5Mg alloys, which were comprised of nanocrystalline grains separated by coarse-grain regions, show balanced mechanical properties of enhanced yield and ultimate strength and reasonable ductility and toughness compared to comparable conventional alloys and nanocrystalline metals. The investigation of tensile and hardness test suggests unusual deformation mechanisms and interactions between ductile coarse-grain bands and nanocrystalline regions. |
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NU @ karnesky @ |
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10321 |
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