ORCID Profile
0000-0002-5961-7422
Current Organisation
City University of Hong Kong
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Materials Engineering | Metals and Alloy Materials
Publisher: Wiley
Date: 31-10-2007
Publisher: Springer Science and Business Media LLC
Date: 02-2010
Publisher: American Physical Society (APS)
Date: 05-03-2008
Publisher: Springer Science and Business Media LLC
Date: 16-01-2013
Publisher: Springer Science and Business Media LLC
Date: 17-04-2018
Publisher: Springer Science and Business Media LLC
Date: 07-2014
Publisher: Elsevier BV
Date: 02-2011
Publisher: Elsevier BV
Date: 05-2018
Publisher: AIP Publishing
Date: 20-07-2009
DOI: 10.1063/1.3187539
Abstract: Most deformation twins in nanocrystalline face-centered cubic (fcc) metals have been observed to form from grain boundaries. The growth of such twins requires the emission of Shockley partials from the grain boundary on successive slip planes. However, it is statistically improbable for a partial to exist on every slip plane. Here we propose a dislocation reaction and cross-slip mechanism on the grain boundary that would supply a partial on every successive slip plane for twin growth. This mechanism can also produce a twin with macrostrain smaller than that caused by a conventional twin.
Publisher: Elsevier BV
Date: 06-2011
Publisher: AIP Publishing
Date: 25-02-2008
DOI: 10.1063/1.2870014
Abstract: Although there are a few isolated ex les of excellent strength and ductility in single-phase metals with ultrafine grained (UFG) structures, the precise role of different microstructural features responsible for these results is not fully understood. Here, we demonstrate that a large fraction of high-angle grain boundaries and a low dislocation density may significantly improve the toughness and uniform elongation of UFG Cu by increasing its strain-hardening rate without any concomitant sacrifice in its yield strength. Our study provides a strategy for synthesizing tough UFG materials.
Publisher: Trans Tech Publications, Ltd.
Date: 12-2011
DOI: 10.4028/WWW.SCIENTIFIC.NET/MSF.667-669.181
Abstract: The effect of grain size on the deformation twinning and de-twinning in a nanocrystalline Ni-Fe alloy was investigated using transmission electron microscopy. Specimens with different grain sizes were obtained by severely deforming an electrochemically deposited nanocrystalline Ni-20wt.% Fe alloy using high-pressure torsion, which resulted in continuous grain growth from an average grain size of ~ 21 nm in the as-deposited material to ~ 72 nm for the highest strain applied in this study. Results show that deformation de-twinning occurs at very small grain sizes while deformation twinning takes place when the grain size is larger than ~ 45 nm. The mechanism of the observed grain size effect on twinning and de-twinning is briefly discussed.
Publisher: AIP Publishing
Date: 04-03-2005
DOI: 10.1063/1.1881779
Abstract: By inserting a thin YBa2Cu3O7−δ (Y123) seed layer, high quality EuBa2Cu3O7−δ (Eu123) films can grow with epitaxial c-axis orientation on SrTiO3 (STO) substrate without increasing the growth temperature. The interfacial structures of Eu123∕STO and Eu123∕Y123∕STO were investigated using scanning transmission electron microscopy. Results show that the Eu mobility on the STO substrate is very low at the regular deposition temperature. This leads to nonuniform composition distribution at the Eu123∕STO interface and a mixture of c-axis and a-axis growth. A thin Y123 seed layer greatly improves the Eu mobility and therefore facilitates high quality c-axis growth.
Publisher: Elsevier BV
Date: 2009
Publisher: Elsevier BV
Date: 04-2011
Publisher: American Physical Society (APS)
Date: 12-2011
Publisher: Elsevier BV
Date: 03-2019
Publisher: Elsevier BV
Date: 09-2010
Publisher: Elsevier BV
Date: 08-2007
Publisher: Elsevier BV
Date: 11-2018
Publisher: CSIRO Publishing
Date: 1988
DOI: 10.1071/BT9880257
Abstract: Fossil roots with nodular and abbreviated lateral roots are described from the Devils Kitchen locality within the Lower Cretaceous Otway Group. The general morphology of these roots indicates a mycorrhizal association, the oldest such record from Australia. Based on the root morphology and associated megaflora it is considered that the roots are coniferous (Taxodiaceae or Podocarpaceae) an association of the roots with foliage of Geinitzia tetragona sp. nov., which is possibly taxodiaceous, is indicated. The major role of nodular mycorrhizal roots in extant plants is phosphate uptake which enhances minor nitrogen uptake. It is suggested that the fossil roots may have had a similar role. The Devils Kitchen locality is interpreted as a levee or near channel deposit with better drainage andlor a nutrient-poor soil relative to other soils in the sequence. The atypical plant associations of Cladophlebis australis, C. sp. b, and Geinitzia tetragona sp. nov. are a reflection of the above.
Publisher: Springer Science and Business Media LLC
Date: 07-09-2010
DOI: 10.1038/NCOMMS1062
Publisher: Walter de Gruyter GmbH
Date: 12-2009
DOI: 10.3139/146.110230
Abstract: Severe plastic deformation techniques have been widely used to produce bulk ultrafine-grained and nanocrystalline materials. Severe plastic deformation of materials is a complex process that includes both grain refinement via the activities of dislocations and twinning, and grain growth. In this paper, we present a short review on the mechanisms of grain refinement and grain growth induced by severe plastic deformation.
Publisher: Elsevier BV
Date: 06-2017
Publisher: Elsevier BV
Date: 02-2014
Publisher: Elsevier BV
Date: 07-2010
Publisher: Elsevier BV
Date: 05-2016
Publisher: Elsevier BV
Date: 12-2019
Publisher: AIP Publishing
Date: 20-04-2004
DOI: 10.1063/1.1734689
Abstract: A full dislocation often dissociates into two partial dislocations enclosing a stacking fault (SF) ribbon. The SF width significantly affects the mechanical behavior of metals. Al has very high stacking fault energy and, consequently, very narrow SF width in its coarse-grained state. We have found that some SFs in nanocrystalline Al are surprisingly 1.4–6.8 nm wide, which is 1.5–11 times higher than the reported experimental value in single crystal Al. Our analytical model shows that such wide SFs are formed due to the small grain size and possibly also to the interaction of SF ribbons with high density of dislocations.
Publisher: Elsevier BV
Date: 02-2010
Publisher: Walter de Gruyter GmbH
Date: 12-2009
DOI: 10.3139/146.110232
Abstract: In a previous study (R.Z. Valiev et al.: J. Mater. Res. 17 (2002) 5), unusual combinations of yield strength and ductility in ultrafine grained copper processed by equal-channel angular pressing of 360 MPa 23 % (by Bc route two passes) and 380 MPa 55 % (by Bc route 16 passes) were reported. However, results from recent work suggest that the tensile strain, when measured using a non-standard method and miniature dog-bone tensile specimens, is likely to contain significant errors. In this study, by implementing a standard strain measurement and tensile specimens with geometries that meet the ASTM requirements, we report yield strength and ductility combinations of 370 MPa 9 % (Bc 2 passes) and 370 MPa 16 % (Bc 16 passes) in the ultrafine grained copper. The higher ductility of the ultrafine grained copper processed by route Bc 16 passes was rationalized on the basis of several factors including the presence of equiaxed grains, lower dislocation density and higher fraction of high-angle grain boundaries (which result in larger strain rate sensitivity and strain hardening).
Publisher: Elsevier BV
Date: 2014
Publisher: Elsevier BV
Date: 2011
Publisher: AIP Publishing
Date: 03-12-2012
DOI: 10.1063/1.4769216
Abstract: We present evidence that the level of the applied stress plays a critical role in deformation twinning in face-centred cubic alloys. While conventional cold rolling of a face-centred cubic structure produces a microstructure with a high-density of extended dislocations, increasing the applied stress using high-pressure torsion gives a nano-twinned coarse-grained structure. This suggests the existence of a critical stress for deformation twinning which thereby delineates an approach for the production of nano-twinned microstructures in coarse-grained materials with superior mechanical properties.
Publisher: Elsevier BV
Date: 09-2011
Publisher: Elsevier BV
Date: 08-2009
Publisher: AIP Publishing
Date: 10-06-2013
DOI: 10.1063/1.4811157
Abstract: Nanocrystallization by high-energy severe plastic deformation has been reported to increase the solubility of alloy systems and even to mix immiscible elements to form non-equilibrium solid solutions. In this letter, we report an opposite phenomenon—nanocrystallization of a Cu-Al single-phase solid solution by high-pressure torsion separated Al from the Cu matrix when the grain sizes are refined to tens of nanometers. The Al phase was found to form at the grain boundaries of nanocrystalline Cu. The level of the separation increases with decreasing grain size, which suggests that the elemental separation was caused by the grain size effect.
Publisher: Wiley
Date: 05-09-2006
Publisher: Elsevier BV
Date: 10-2008
Publisher: Elsevier BV
Date: 04-2015
Publisher: Elsevier BV
Date: 06-2010
Publisher: Elsevier BV
Date: 07-2006
Publisher: Elsevier BV
Date: 09-2011
Publisher: Elsevier BV
Date: 05-2011
Publisher: AIP Publishing
Date: 02-03-2009
DOI: 10.1063/1.3095852
Abstract: High-pressure torsion (HPT) induced dislocation density evolution in a nanocrystalline Ni-20 wt %Fe alloy was investigated using x-ray diffraction and transmission electron microscopy. Results suggest that the dislocation density evolution is fundamentally different from that in coarse-grained materials. The HPT process initially reduces the dislocation density within nanocrystalline grains and produces a large number of dislocations located at small-angle subgrain boundaries that are formed via grain rotation and coalescence. Continuing the deformation process eliminates the subgrain boundaries but significantly increases the dislocation density in grains. This phenomenon provides an explanation of the mechanical behavior of some nanostructured materials.
Publisher: Elsevier BV
Date: 2012
Publisher: Springer Science and Business Media LLC
Date: 17-04-2010
Publisher: AIP Publishing
Date: 05-01-2009
DOI: 10.1063/1.3065025
Abstract: Deformation induced grain growth has been widely reported in nanocrystalline materials. However, the grain growth mechanism remains an open question. This study applies high-pressure torsion to severely deform bulk nanocrystalline Ni-20 wt % Fe disks and uses transmission electron microscopy to characterize the grain growth process. Our results provide solid evidence suggesting that high pressure torsion induced grain growth is achieved primarily via grain rotation for grains much smaller than 100 nm. Dislocations are mainly seen at small-angle subgrain boundaries during the grain growth process but are seen everywhere in grains after the grains have grown large.
Publisher: Elsevier BV
Date: 2020
Publisher: Elsevier BV
Date: 11-2005
Publisher: Wiley
Date: 17-11-2006
Publisher: Wiley
Date: 17-09-2007
Publisher: Elsevier BV
Date: 04-2012
Publisher: Elsevier BV
Date: 08-2013
Publisher: Wiley
Date: 10-09-2019
Abstract: A ductile duplex stainless steel consisting of a ferrite phase and an austenitic phase is chosen as a model material to investigate the development of shear strain patterns under high‐pressure torsion. Systematic analysis on the macroscopic and microscopic heterogeneities of local shear strain reveals that complex shear patterns can be developed only above a high strain level where the grain sizes are already refined to a steady state. It is concluded that grain boundary‐mediated deformation mechanisms provide sufficient freedom for reshaping the austenite phase domains, and thus the intrinsic factor required for the formation of these complicated shear patterns is well‐developed ultrafine grains and/or nanograins.
Publisher: Wiley
Date: 08-08-2019
Abstract: Commercial‐purity aluminum with 99.7% purity is processed by rotational accelerated shot peening (RASP) and cold‐rolling. RASP s les are rolled at room temperature to a thickness reduction of 20% and 30%, for the purpose of surface roughness reduction and strengthening. Detailed microstructural characterization and hardness tests reveal that cold‐rolling cause grain growth at the surfaces of RASP s les from ≈472 to ≈1000 nm. Moderate cold‐rolling is effective in smoothing the surface of RASP s les, while improving strength and maintaining ductility. However, cold‐rolling to more than 30% thickness reduction eventually diminishes the gradient nanostructure. During cold‐rolling of RASP s les, a transition zone with strong strain incompatibility is noticed by a sharp rise in hardness at some point of the hardness‐distribution curve from the surface to the core of the s le. This transition zone is a result of quick generation of geometrically necessary dislocations. Although the transition zone with strong strain incompatibility is short‐lived, it is anticipated to be common among gradient‐nanostructured materials under cold‐rolling condition.
Location: United States of America
Location: United States of America
Location: China
Start Date: 2019
End Date: 12-2022
Amount: $360,000.00
Funder: Australian Research Council
View Funded Activity