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>

Microstructural homogeneity and superplastic behavior in an aluminum-copper eutectic alloy processed by high-pressure torsion

Kawasaki, Megumi,Lee, Han-Joo,Langdon, Terence G. Springer-Verlag 2015 JOURNAL OF MATERIALS SCIENCE - Vol.50 No.20

Evolution of hardness, microstructure, and strain rate sensitivity in a Zn-22% Al eutectoid alloy processed by high-pressure torsion

Kawasaki, Megumi,Lee, Han-Joo,Choi, In-Chul,Jang, Jae-il,Ahn, Byungmin,Langdon, Terence G IOP Publishing 2014 IOP conference series. Materials science and engin Vol.63 No.-

Micro-Mechanical Responses of Ultrafine-Grained Materials Processed through High-Pressure Torsion

Kawasaki, Megumi,Jang, Jae Il,Ahn, Byung Min,Langdon, Terence G. Trans Tech Publications, Ltd. 2016 Materials science forum Vol.879 No.-

<P>The processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk metal solids. These ultrafine grains in the bulk metals usually show superior mechanical and physical properties. Especially, the development of micro-mechanical behavior is observed after significant changes in microstructure through processing and it is of great importance for obtaining practical future applications of these ultrafine-grained metals. Accordingly, this presentation demonstrates the evolution of small-scale deformation behavior through nanoindentation experiments after HPT on various metallic alloys including a ZK60 magnesium alloy, a Zn-22% Al eutectoid alloy and a high entropy alloy. Special emphasis is placed on demonstrating the essential microstructural changes of these materials with increased straining by HPT and the evolution of the micro-mechanical responses in these materials by measuring the strain rate sensitivity.</P>

Fabrication of nanocomposites through diffusion bonding under high-pressure torsion

Kawasaki, Megumi,Han, Jae-Kyung,Lee, Dong-Hyun,Jang, Jae-il,Langdon, Terence G. Published for the Materials Research Society by th 2018 Journal of materials research Vol.33 No.18

<▼1><B>Abstract</B><P/></▼1><▼2><P>This report summarizes a recent study demonstrating simple and rapid synthesis of a new Al-Mg alloy system and ultimately synthesizing a metal matrix nanocomposite, which was achieved by processing stacked disks of the two dissimilar metals by conventional high-pressure torsion (HPT) processing. The synthesized Al-Mg alloy system exhibits exceptionally high hardness through rapid diffusion bonding and simultaneous nucleation of intermetallic phases with increased numbers of HPT turns through 20, and improved plasticity was demonstrated by increasing strain rate sensitivity in the alloy system after post-deformation annealing. An additional experiment demonstrated that the alternate stacking of high numbers of dissimilar metal disks may produce a faster metal mixture during HPT. Metal combinations of Al-Cu, Al-Fe, and Al-Ti were processed by the same HPT procedure from separate pure metals to examine the feasibility of the processing technique. The microstructural analysis confirmed the capability of HPT for the formation of heterostructures across the disk diameters in these processed alloy systems. The HPT processing demonstrates a considerable potential for the joining and bonding of dissimilar metals at room temperature and the expeditious fabrication of a wide range of new metal systems.</P></▼2>

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>

Using high-pressure torsion to process an aluminum-magnesium nanocomposite through diffusion bonding

Kawasaki, Megumi,Ahn, Byungmin,Lee, HanJoo,Zhilyaev, Alexander P.,Langdon, Terence G. Cambridge University Press (Materials Research Soc 2016 Journal of materials research Vol.31 No.1

<▼1><B>Abstract</B><P/></▼1><▼2><P>Disks of commercial Al-1050 and ZK60A alloys were stacked together and then processed by conventional high-pressure torsion (HPT) through 1 and 5 turns at room temperature to investigate the synthesis of an Al-Mg alloy system. Measurements of microhardness and observations of the microstructures and local compositions after processing through 5 turns revealed the formation of an ultrafine multi-layered structure in the central region of the disk but with an intermetallic β-Al3Mg2 phase in the form of nano-layers in the nanostructured Al matrix near the edge of the disk. The activation energy for diffusion bonding of the Al and Mg phases was estimated and it is shown that this value is low and consistent with surface diffusion due to the very high density of vacancy-type defects introduced by HPT processing. The results demonstrate a significant potential for making use of HPT processing in the preparation of new alloy systems.</P></▼2>

Description of the Superplastic Flow Process by Deformation Mechanism Maps in Ultrafine-Grained Materials

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>

Significance of Si impurities on exceptional room-temperature superplasticity in a high-purity Zn-22%Al alloy

Uesugi, Tokuteru,Kawasaki, Megumi,Ninomiya, Masaki,Kamiya, Yuhei,Takigawa, Yorinobu,Higashi, Kenji Elsevier 2015 Materials science & engineering. properties, micro Vol.645 No.-

<P><B>Abstract</B></P> <P>Recent numerous studies demonstrated the advantages of producing bulk metals with submicrometer grain sizes which provide the opportunity to demonstrate improved mechanical characteristics including superplastic properties. Besides the effort, although the impurity may cause low ductility due to grain boundary segregation, there are limited studies to date on the influence of general impurities upon flow behavior of conventional superplastic materials. Accordingly, the present report demonstrates the significance of Si impurity on superplastic properties in an ultrafine-grained high-purity Zn-22%Al eutectoid alloy at room temperature. The alloy was prepared to include different levels of Si contents up to 1500ppm in the high-purity alloy and the consistent fine grain sizes of ~0.60µm were introduced through a series of solutionizing followed by cold rolling. Tensile testing showed an occurrence of excellent room-temperature superplasticity and the maximum elongation of 500% was recorded at an optimal superplastic strain rate of 1.0×10<SUP>−3</SUP> s<SUP>−1</SUP> in the alloy with less Si. Increasing Si contents reduced ductility without changing the strain rate sensitivity, thereby implying the consistency in the deformation mechanism for superplastic flow but the difference in the fracture mode. The present analysis estimates a threshold stress and demonstrate the validity of applying the conventional superplastic relationship for depicting the room-temperature superplastic flow in the high-purity Zn-22%Al alloy. Moreover, the separate fracture modes are proposed for the alloy with increasing Si impurity contents by taking fractographs after superplastic elongations.</P>

Defect structure and hardness in nanocrystalline CoCrFeMnNi High-Entropy Alloy processed by High-Pressure Torsion

Heczel, Anita,Kawasaki, Megumi,Lá,bá,r, Já,nos L.,Jang, Jae-il,Langdon, Terence G.,Gubicza, Jenő Elsevier 2017 Journal of Alloys and Compounds Vol.711 No.-

<P><B>Abstract</B></P> <P>An equiatomic CoCrFeMnNi High-Entropy Alloy (HEA) produced by arc melting was processed by High-Pressure Torsion (HPT). The evolution of the microstructure during HPT was investigated after ¼, ½, 1 and 2 turns using electron backscatter diffraction and transmission electron microscopy. The spatial distribution of constituents was studied by energy-dispersive X-ray spectroscopy. The dislocation density and the twin-fault probability in the HPT-processed samples were determined by X-ray line profiles analysis. It was found that the grain size was gradually refined from ∼60 μm to ∼30 nm while the dislocation density and the twin-fault probability increased to very high values of about 194 × 10<SUP>14</SUP> m<SUP>−2</SUP> and 2.7%, respectively, at the periphery of the disk processed for 2 turns. The hardness evolution was measured as a function of the distance from the center of the HPT-processed disks. After 2 turns of HPT, the microhardness increased from ∼1440 MPa to ∼5380 MPa at the disk periphery where the highest straining is achieved. The yield strength was estimated as one-third of the hardness and correlated to the microstructure.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The microstructure and the hardness in HPT processed CoCrFeMnNi HEA were studied. </LI> <LI> The dislocation density increased up to 194 × 10<SUP>14</SUP> m<SUP>−2</SUP> after 2 turns of HPT. </LI> <LI> The grain size decreased to 27 nm at the periphery of the disk processed by 2 turns. </LI> <LI> After 2 turns the twin fault probability increased to 2.7% at the disk periphery. </LI> <LI> The hardness increased from 1440 MPa to 5380 MPa due to 2 turns of HPT. </LI> </UL> </P>