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Kawasaki, Megumi,Langdon, Terence G. Trans Tech Publications, Ltd. 2016 Materials science forum Vol.838 No.-
<P>The synthesis of ultrafine-grained (UFG) materials is very attractive because small grains lead to excellent creep properties including superplastic ductility at elevated temperatures. Severe plastic deformation (SPD) is an attractive processing technique for refining microstructures of metallic materials to have ultrafine grain sizes within the submicrometer to even the nanometer level. Among the SPD techniques, most effective processing is conducted through equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) and there are numerous reports demonstrating the improved tensile properties at elevated temperature. This report demonstrates recent results on superplasticity in metals after ECAP and HPT. Moreover, superplastic flow of the UFG materials is evaluated by using flow mechanisms developed earlier for coarse-grained materials and depicted by plotting deformation mechanism maps which provide excellent visual representations of flow properties over a wide range of testing conditions.</P>
Review: achieving superplastic properties in ultrafine-grained materials at high temperatures
Kawasaki, Megumi,Langdon, Terence G. Springer-Verlag 2016 JOURNAL OF MATERIALS SCIENCE - Vol.51 No.1
<P>The mechanisms of superplasticity occurring in conventional materials, having grains sizes of the order of a few microns, are now understood reasonably well. However, very recent advances in the processing of ultrafine-grained (UFG) metals have provided an opportunity to extend the understanding of flow behavior to include UFG materials with submicrometer grain sizes. In practice, processing through the application of severe plastic deformation (SPD), as in equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), has permitted the fabrication of relatively large samples having UFG microstructures. Since the occurrence of superplastic flow generally requires a grain size smaller than similar to 10 mu m, it is reasonable to anticipate that materials processed by SPD will exhibit superplastic ductilities when pulled in tension at elevated temperatures. This review examines recent results that demonstrate the occurrence of exceptional superplastic flow in a series of UFG aluminum and magnesium alloys after ECAP and HPT. The results are analyzed to evaluate the superplastic flow mechanism and to compare with materials processed using different techniques. The critical issue of microstructural inhomogeneity is examined in two-phase UFG materials after SPD processing and the influence of microstructural homogeneity on the superplastic properties is also demonstrated.</P>
Kumar, Praveen,Kawasaki, Megumi,Langdon, Terence G. Springer-Verlag 2016 JOURNAL OF MATERIALS SCIENCE - Vol.51 No.1
<P>Ultrafine-grained (UFG) materials with grain sizes in the submicrometer or nanometer range may be prepared through the application of severe plastic deformation (SPD) to bulk coarse-grained solids. These materials generally exhibit high strength but only very limited ductility in low-temperature testing, thereby giving rise to the so-called paradox of strength and ductility. This paradox is examined and a new quantitative diagram is presented which permits the easy insertion of experimental data. It is shown that relatively simple procedures are available for achieving both high strength and high ductility in UFG materials including processing the material to a very high strain and/or applying a very short-term anneal immediately after the SPD processing. Significant evidence is now available demonstrating the occurrence of grain boundary sliding in these materials at low temperatures, where this is attributed to the presence of non-equilibrium grain boundaries and the occurrence of enhanced diffusion along these boundaries.</P>
Factors Influencing Microstructural Development in Equal-Channel Angular Pressing
Furukawa, Minoru,Horita, Zenji,Langdon, Terence G. 대한금속재료학회 2003 METALS AND MATERIALS International Vol.9 No.2
Processing through the imposition of severe plastic deformation (SPD) provides an opportunity for achieving very significant grain refinement in bulk materials. Although different SPD procedures are available, the process of equal-channel angular pressing (ECAP) is especially attractive because it can be scaled easily to produce relatively large samples. This paper describes the principles of ECAP processing and demonstrates the potential for achieving unusual mechanical properties in the samples subjected to ECAP. Special emphasis is placed on the possibility of attaining a high strain rate superplastic forming capability in the as-pressed materials: examples are presented for an Al-Mg-Sc alloy prepared in the laboratory by casting and for a commercial Al-2024 alloy.
Superplastic Flow and Micro-Mechanical Response of Ultrafine-Grained Materials
Kawasaki, Megumi,Jang, Jae Il,Langdon, Terence G. Trans Tech Publications, Ltd. 2018 Diffusion and defect data. DDF. Pt. A, Defect and Vol.385 No.-
<P>The bulk ultrafine-grained (UFG) materials usually show superior mechanical properties. Since the occurrence of superplastic flow generally requires a grain size smaller than ~10 μm, it is anticipated that materials processed by severe plastic deformation (SPD) will exhibit superplastic ductilities when pulled in tension at elevated temperatures. Recent advances in the processing of UFG metals have provided an opportunity to extend the understanding of superplastic flow behavior to include UFG materials with submicrometer grain sizes. Recent studies showed the UFG materials demonstrated the development of plasticity in micro-mechanical response at room temperature by the significant changes in microstructure attributed to high-pressure torsion (HPT). Accordingly, this study summarizes recent results on excellent ductility and plasticity in a UFG Zn-22% Al alloy. Specifically, the alloy demonstrated the occurrence of exceptional superplastic flow at high temperature after equal-channel angular pressing and HPT and excellent room temperature plasticity of the alloy after HPT where the plasticity was evaluated by the nanoindentation technique. The significance of purity of the alloy is also considered for enhancing the ductility at room temperature.</P>
Equal - Channel Angular Pressing : A Novel Tool for Microstructural Control
NEMOTO, Minoru,Horita, Zenji,Furukawa, Minoru,Langdon, Terence G . 대한금속재료학회(대한금속학회) 1998 METALS AND MATERIALS International Vol.4 No.6
The principles of equal-channel angular (ECA) pressing are reviewed. It is demonstrated that this processing procedure is capable of introducing an ultrafine grain size into bulk materials and the precise nature of the microstructure is dependent upon the total strain introduced by the pressing. When the strain is sufficiently high, it is possible to achieve very high tensile ductilities at high strain rates, thereby providing the potential for substantially increasing the viability of superplastic forming in the metal forming industry.
Choi, In-Chul,Lee, Dong-Hyun,Ahn, Byungmin,Durst, Karsten,Kawasaki, Megumi,Langdon, Terence G.,Jang, Jae-il Elsevier 2015 Scripta materialia Vol.94 No.-
<P>A Mg–Zn–Zr alloy was processed by high-pressure torsion for up to 2 turns at room temperature to produce significant grain refinement together with enhanced plasticity and strength. Measurements were performed to determine the strain-rate sensitivity, shear yield strength and activation volume as a function of the processing conditions. The results suggest there is a significant contribution from grain boundary sliding to the flow process and the onset of plasticity is associated with heterogeneous dislocation nucleation.</P>
Han, Jae-Kyung,Lee, Han-Joo,Jang, Jae-il,Kawasaki, Megumi,Langdon, Terence G. Elsevier 2017 Materials science & engineering. properties, micro Vol.684 No.-
<P><B>Abstract</B></P> <P>High-pressure torsion (HPT) is one of the major severe plastic deformation (SPD) procedures where bulk metals, in the shape of a disk, achieve exceptional grain refinement at ambient temperatures. HPT has been applied for the consolidation of metallic powders and the bonding of machining chips whereas there are very limited reports examining the application of HPT for the production of new metal systems and the formation of nanocomposites. Accordingly, this investigation was initiated to evaluate the potential for the formation of a metal matrix nanocomposite (MMNC) by processing two commercial metal disks of an Al-1050 alloy and a ZK60 magnesium alloy through HPT under 6.0GPa for 20 turns at room temperature. Evolutions in microstructure, mechanical properties including hardness and plasticity and the tribological properties were examined in the MMNC region of the processed Al-Mg system. The significance of post-deformation annealing (PDA) at 573K for 1h was investigated by the change in microstructure and the enhancement in mechanical properties and wear resistance of the HPT-processed MMNC. This study demonstrates the promising feasibility of using HPT to fabricate a wide range of hybrid MMNCs from simple metals and for applying PDA for further improvement of the essential mechanical and tribological properties in the synthesized alloy systems.</P>
Shahmir, Hamed,Mousavi, Tayebeh,He, Junyang,Lu, Zhaoping,Kawasaki, Megumi,Langdon, Terence G. Elsevier 2017 Materials science & engineering. properties, micro Vol.705 No.-
<P><B>Abstract</B></P> <P>A CoCrFeNiMn high-entropy alloy (HEA) was processed by equal-channel angular pressing (ECAP) for up to four passes at 673K and the results show that the strength increases gradually with increasing straining up to ~ 1GPa with an elongation to failure of ~ 35% after four passes of ECAP. In this condition, the microstructure is a single-phase ultrafine-grained (UFG) CoCrFeNiMn HEA with an average grain size of ~ 100nm and a high dislocation density. This UFG HEA was subjected to post-deformation annealing (PDA) at temperatures of 673–1073K for 60min and it is shown that the hardness increases slightly due to precipitation to 773K and then decreases to 1073K due to a combination of recrystallization, grain growth and a dissolution of precipitates. The formation of brittle <I>σ</I>-phase precipitates improves the strength significantly but with a minor decrease in ductility. Annealing at the peak temperature of 773K produces a very high yield strength of ~ 1015MPa and an ultimate strength of ~ 1080MPa together with an excellent elongation to failure of ~ 30%. An analysis of the data shows that grain boundary strengthening is the most important strengthening mechanism in these ECAP samples both before and after PDA.</P>