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Advances and Applications in High Strain Rate Superplasticity
Higashi, Kenji 대한금속재료학회(대한금속학회) 1998 METALS AND MATERIALS International Vol.4 No.3
High strain rate superplasticity (i.e., superplastic behavior at strain rates over 10^(-2) s^(-1)) has been observed in many metallic materials such as aluminum alloys and their matrix composites and it is associated with an ultra-fine grained structure of less than about 3 ㎛. Its deformation mechanism appears to be different from that in conventional superplastic materials. A new model was considered from the viewpoint of the accommodation mechanism by an accommodation helper such as a liquid or glassy phase. The new mechanism was proposed in which superplasticity was critically controlled by the accommodation helper both to relax the stress concentration resulting from the sliding at grain boundaries and/or interfaces and to limit the build up of internal cavitation and subsequent failure. The possibility of the industrial applications was demonstrated for high-strain-rate superplasticity.
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>