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Enrique J. Lavernia - One of the best experts on this subject based on the ideXlab platform.
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Stabilized plasticity in ultrahigh strength, submicron Al crystals
Acta Materialia, 2015Co-Authors: Lin Jiang, Hanry Yang, Troy D. Topping, Joshua K. Yee, Amiya K. Mukherjee, Julie M. Schoenung, Enrique J. LaverniaAbstract:Abstract It is well known that micrometer-sized and/or submicrometer-sized metallic crystals exhibit “smaller is stronger” size effect: the yield strength σ varies with sample dimension D roughly as a power-law σ ∼ D − m . For some materials, near-theoretical strength values can be attained by reducing the dimensions of crystals to sub-micrometric or nanometric values. At these size scales, however, plastic instabilities, such as Strain bursts, Strain softening or Low Strain Hardening rates, become operative due to the avalanche-like dislocation generation and escape; such instabilities contribute to disappointing fLow intermittency. From a scientific standpoint, the onset of plastic instabilities has hindered fundamental study of the deformation behavior of materials near theoretical strength values. From a technological standpoint, these instabilities limit potential applications in microelectromechanical devices, for example. In this study we demonstrate that by concurrently introducing grain boundaries and secondary phase particles, plastic instabilities can be dramatically suppressed in submicron Al pillars at large Strain and that this behavior is attributable to substantial dislocation storage and subsequent grain boundary (GB) mediated plasticity. Consequently the crystals possess superior strength with fLow stress larger than 1.0 GPa.
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Deformation Mechanisms of Nanostructured Al Alloys
Advanced Engineering Materials, 2005Co-Authors: Bing O. Han, Enrique J. LaverniaAbstract:Cryomilling has emerged in recent years as an effective approach to process large amounts of nanostructured Al alloys. In the present review, progress in our understanding of the microstructural characteristics and deformation behavior of nanostructured Al alloys processed via consolidation of cryomilled nanostructured powders is reviewed in an effort to elucidate salient findings and point out additional studies needed. A survey of published papers show that the microstructure of consolidated cryomilled Al alloys is comprised of equiaxed grains in the range of ∼ 50 nm to 200 nm, depending primarily on process parameters. Moreover, in many cases the microstructure consists of a supersaturated solid solution, despite the presence of significant amounts of alloying elements. As far as mechanical properties, the tensile behavior of these nanostructured Al systems is characterized by high strength and Low Strain Hardening. The high strength was primarily attributed to three types of strengthening: grain size effect, solid solution Hardening and Orowan strengthening. The Low Strain Hardening or work softening behavior was accompanied with the occurrence of Luders banding. Recent work suggests that there are two approaches that may be effectively used to address the lack of dislocation activity that typically accompany the presence of nanocrystalline grains < 10–50 nm. The first approach involves selective blending of powders to achieve a microstructure that contains multiple length scales (i.e., nanostructured, 50–200 nm, and submicron, 200–750 nm). The second approach involves thermal annealing in an effort to introduce small amounts of submicron grains for improvement of ductility. Recent related work is discussed herein.
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Tension and compression of bulk Al-7.5 wt% Mg alloy
Philosophical Magazine Letters, 2003Co-Authors: Bing Q. Han, Enrique J. Lavernia, Farghalli A. MohamedAbstract:The mechanical behaviour of bulk ultrafine-grained Al-7.5 wt% Mg alloy consolidated from cryomilled powders has been investigated. The experimental data show that the alloy exhibits high strength, Low Strain Hardening, serrated fLow and relatively high ductility. In addition, the data indicate that the yield strength in tension is essentially equal to that in compression. The yield and fLow strengths of the alloy are discussed in terms of strengthening processes that are related to grain size, the Orowan mechanism and solid-solution Hardening. The serrations in the stress-Strain curve are discussed in terms of dynamic Strain ageing and deformation twinning.
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Tensile behavior of bulk nanostructured and ultrafine grained aluminum alloys
Journal of Materials Science, 2003Co-Authors: Bing Q. Han, Farghalli A. Mohamed, Enrique J. LaverniaAbstract:In the present study, data on tensile behavior of bulk nanostructured aluminum alloys processed via consolidation of mechanically milled powders and severe plastic deformation are analyzed. High strength and Low Strain Hardening were observed in bulk nanostructured and ultrafine-grained Al alloys. The ductility of aluminum alloys decreases with decreasing grain size. The high amount of intercrystalline components may have an influence on tensile properties of bulk nanostructured materials when grain sizes are less than 100 nm. The high strength in bulk nanostructured Al-Mg alloy may be attributed to contributions arising from grain size strengthening, the presence of high dislocation densities, Orowan strengthening, precipitation Hardening and solid-solution Hardening. The large and sudden stress drops in the stress-Strain curves of cryomilled Al alloys are most probably indicative of the dislocation annihilation in the vicinity of or breakaway from the strong pinning role of dispersoids.
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Mechanical properties of an ultrafine-grained Al-7.5 Pct Mg alloy
Metallurgical and Materials Transactions A, 2003Co-Authors: B. O. Han, Farghalli A. Mohamed, Zonghoon Lee, Steven Nutt, Enrique J. LaverniaAbstract:In the present study, the relationships between the structure and properties of a cryomilled Al-7.5 pct Mg alloy were investigated. The microstructure of the cryomilled Al-7.5 pct Mg alloy consisted of equiaxed grains with an approximate size of 300 nm. Thermal treatment had only a minor effect on microstructure, as evidenced by X-ray diffraction (XRD) and transmission electron microscopy (TEM) results. The tensile behavior was characterized by high strength, high ductility, and Low-Strain-Hardening. The tensile deformation was relatively uniform, with limited necking deformation, and fracture surfaces were characterized by microdimples. The variation of Strain rates from 4 · 10−4 to 4 · 10−2 s−1 had an insignificant effect on tensile behavior. Comparison of compressive and tensile behavior revealed similar moduli and yield strengths, although the postyield behavior was markedly asymmetric. The present results indicate that grain-size effects, solid-solution strengthening, Orowan strengthening, and dislocation strengthening contribute significantly to the properties of a cryomilled Al-7.5 pct Mg alloy.
Gregory J. Hancock - One of the best experts on this subject based on the ideXlab platform.
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Compression Tests of Cold-Reduced High Strength Steel Sections. I: Stub Columns
Journal of Structural Engineering, 2004Co-Authors: Demao Yang, Gregory J. HancockAbstract:This paper describes a series of compression tests performed on stub columns fabricated from cold-formed high strength steel plates with nominal yield stress of 550MPa. The steel is classified as G550 to Australia Standard AS1397. The test results presented in this paper are the first stage of an Australian Research Council research project entitled “Compression stability of high strength steel sections with Low Strain-Hardening.” The tests include lipped-square and hexagonal sections, including 94 box-shaped fix-ended stub columns. The purpose of these tests was to determine the influence of Low Strain Hardening of G550 steel on the compressive section capacities of the column members. The results of the successful stub column tests have been compared with the design procedures in the Australian/New Zealand Standard for Cold-Formed Steel Structures and recent (1999) amendments to the American Iron and Steel Institute Specification. As expected, the greatest effect of Low Strain Hardening was for the stoc...
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Compression Tests of High Strength Steel Channel Columns with Interaction between Local and Distortional Buckling
Journal of Structural Engineering, 2004Co-Authors: Demao Yang, Gregory J. HancockAbstract:This paper describes a series of compression tests performed on lipped channel section columns fabricated from cold-reduced high strength steel of thickness 0.42mm with nominal yield stress 550MPa. The test results presented in the paper are the third stage of an Australia Research Council research project entitled “Compression Stability of High Strength Steel Sections with Low Strain-Hardening” A range of lengths of lipped channels with intermediate stiffeners in the web and the flanges were tested between fixed ends to determine the strength of the sections. For the lipped channel sections, failure resulted from local and distortional buckling with interaction between these modes. The tests indicated that distortional buckling and the interaction of local and distortional buckling may have a significant effect on the strength of the sections formed from such thin high strength steel. The paper presents the results obtained experimentally and theoretically using the effective width method and the direct ...
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COMPRESSION TESTS OF COLD-REDUCED HIGH STRENGTH STEEL SECTIONS. II: LONG COLUMNS
Journal of Structural Engineering, 2004Co-Authors: Demao Yang, Gregory J. Hancock, Kim J.r. RasmussenAbstract:This paper describes a series of compression tests performed on long columns fabricated from cold-formed high strength steel plates with nominal yield stress of 550 MPa. The steel is classified as G550 to Australia Standard AS1397. The test results presented in this paper are the second stage of an Australian Research Council research project entitled “Compression stability of high strength steel sections with Low Strain-Hardening.” A total of 28 long columns, which were made from two thicknesses of sheet steel (0.42 and 0.6 mm), were tested. A box shaped section was tested between pinned ends over a range of lengths. This paper shows the comparison of loads obtained experimentally with those predicted on the basis of AS/NZS 4600 and the AISI specification including Supplement No. 1, 1999. The finite element program, \IABAQUS\N, was also used to simulate the column behavior. For sections which undergo local instability at loads significantly less than the ultimate loads, the column design rules in AS/NZS 4600 and the AISI Specification are unconservative if used in their current form for G550 steel. Proposals for improved column design of high strength slender sections are proposed in this paper. A first companion paper (Part I) describes a series of compassion tests on stub columns.
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Stability of high strength G550 steel compression members
Advances in Steel Structures (ICASS '02), 2002Co-Authors: D. Yang, Gregory J. HancockAbstract:Publisher Summary High strength cold-reduced steel is typically of stress grade G550 (550 MPa nominal yield and tensile strength) and is less than 1 mm thick. The steel has been used for many years for sheeting and decking, but it is now used for structural members such as roof trusses and stud walls of steel framed houses. The steel has Low Strain Hardening with a tensile to yield stress ratio of 1.0. This chapter summarizes a major research program on the stability of this steel at the University of Sydney. Short and long column compression members are tested and compared with design standards and finite element analyses (FEA). A range of stub columns in G550 sheet steel to AS 1397 has been tested in compression to obtain the strength characteristics of the steel with Low Strain-Hardening. Pin-ended column tests with box-sections and constructed from high strength G550 steel are successfully performed. The plate slenderness (b/t) ranges from 33 to 119 and the column slenderness (L/rx) ranges from 27 to 148. A load eccentricity, which produces a column response equivalent to L/1000, is used for all tests.
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Compression Tests of Cold-reduced High Strength Steel Stub Columns
2002Co-Authors: Demao Yang, Gregory J. HancockAbstract:This paper describes a series of compression tests performed on long columns fabricated from cold-formed high strength steel plates with nominal yield stress of 550 MPa (80 ksi). The steel is classified as G550 to Australia Standard AS 1397. The test results presented in this paper are the second stage of an Australian Research Council research project entitled "Compression Stability of High Strength Steel Sections with Low Strain-Hardening". A total of 28 long columns, which were made from two thicknesses of sheet steel (0.42 mm and 0.6 mm) (0.017 in. and 0.024 in.), were tested. A box shaped section was tested between pinned ends over a range of lengths. This paper shows the comparison of loads obtained experimentally with those predicted on the basis of ASINZS4600 and the AISI specification including Supplement No.1, 1999. The finite element program, ABAQUS, was also used to simulate the column behaviour. For sections which undergo local instability at loads significantly less than the ultimate loads, the column design rules in ASINZS 4600 and the AISI Specification are unconservative if used in their current form for G550 steel. Proposals for improved column design of high strength slender sections are proposed in this paper.
J. Degrieck - One of the best experts on this subject based on the ideXlab platform.
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Determining the stress-Strain behaviour at large Strains from high Strain rate tensile and shear experiments
International Journal of Impact Engineering, 2011Co-Authors: J. Peirs, P. Verleysen, W. Van Paepegem, J. DegrieckAbstract:To characterise the high Strain rate mechanical behaviour of metals, split Hopkinson bar experiments are frequently used. These experiments basically yield the force and elongation history of the specimen, reflecting not only the specimen material behaviour but also the specimen structural behaviour. Calculation of the real material behaviour from this global response is not straightforward, certainly for materials such as Ti6Al4V where due to Low Strain Hardening, the specimen deformation is very inhomogeneous. However, for fundamental material research and constitutive material modelling, knowledge of the true effective stress versus plastic Strain, Strain rate and temperature is essential.
D A Koss - One of the best experts on this subject based on the ideXlab platform.
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on the influence of void clusters on void growth and coalescence during ductile fracture
Acta Materialia, 2008Co-Authors: J P Bandstra, D A KossAbstract:Abstract Based on the behavior of a three-void cluster embedded within a representative volume element, this study utilizes three-dimensional finite element analyses to examine the sensitivity of void growth and coalescence to Strain Hardening, multiaxial stress state and inter-void spacing. The Strain-induced growth of voids within the cluster is accelerated when the voids are closely spaced in a Low Strain-Hardening material subject to high levels of stress triaxiality. Far-field deformation causes Strain to concentrate within the inter-void ligament, and the resulting behavior induces a load–loss response of the inter-void region. Based on the load–loss criterion for the onset of void coalescence, the results show that coalescence is accelerated by increasing stress triaxiality and decreasing Strain Hardening and inter-void spacing. A straightforward analysis is then presented that relates void coalescence to (a) the Strain-Hardening exponent and (b) the dependence of the plastic conStraint factor within the inter-void ligament on Strain, the latter being sensitive to far-field stress triaxiality and void geometry.
Yan Li - One of the best experts on this subject based on the ideXlab platform.
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In situ synchrotron X-ray diffraction investigations of the physical mechanism of ultra-Low Strain Hardening in Ti-30Zr-10Nb alloy
Acta Materialia, 2018Co-Authors: C.y. Xiong, Runguang Li, Y. D. Wang, Yan LiAbstract:Abstract A warm-rolled, metastable β-type Ti-30Zr-10Nb alloy exhibited a peculiar two-stage yielding behavior under uniaxial tensile loading, showing a first plastic stage with obvious Strain Hardening at 0.4%–10.4% Strain and a second plastic stage with ultra-Low Strain Hardening at 10.4%–23.5% Strain. In situ high-energy X-ray diffraction (HE-XRD) was used to reveal the stress-induced martensitic transformation scenarios and physical mechanism of the different Strain Hardening rates. It was found that the deformation-induced phase transformation dominated the onset of the first plastic stage corresponding to the selection of favorable martensitic variants, and their elastic interaction contributed to the obvious Strain Hardening. HE-XRD experiments further verified that the ultra-Low Strain Hardening rate in the second plastic stage was related to an interesting superelasticity of the martensite, which was characterized by the reversible, stress-induced reorientation of the martensite variants. This reorientation of the martensite variants was primarily due to the rigid lattice rotation of ∼23° about the [110]α″ axis toward the tensile direction. Our investigations provide in-depth understanding of the mechanism of the excellent plasticity with ultra-Low Strain Hardening in β-type titanium alloys.
