Achieving Superplasticity and Superplastic Forming through Severe Plastic Deformation
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Achieving Superplasticity and Superplastic Forming through Severe Plastic Deformation Minoru Furukawa1, Zenji Horita2 and Terence G. Langdon3 1 Department of Technology, Fukuoka University of Education, Munakata, Fukuoka 811-4192, Japan 2 Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan 3 Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, U.S.A. ABSTRACT The application of severe plastic deformation to metals provides a convenient procedure for achieving nanometer and submicrometer microstructures. Several different processing methods are available but Equal-Channel Angular Pressing (ECAP) is especially attractive because it provides an opportunity for preparing relatively large bulk samples. This paper describes the use of ECAP in preparing materials with ultrafine grain sizes and the subsequent properties of these materials at elevated temperatures. It is demonstrated that, provided precipitates are present to retain these small grain sizes at the high temperatures where diffusion is reasonably rapid, it is possible to achieve remarkably high superplastic elongations in the as-pressed materials and there is a potential for making use of this processing procedure to develop a superplastic forming capability at very rapid strain rates.
INTRODUCTION Superplasticity refers to the ability of some crystalline materials to exhibit very high strains when pulled in tension [1]. In general, two significant conditions must be fulfilled in order to achieve superplastic elongations. First, since flow occurs through a diffusion-controlled mechanism, it is necessary that the testing temperature is sufficiently high that diffusion occurs reasonably rapidly. This generally means that the temperature must be of the order of at least ~0.5 Tm, where Tm is the melting temperature of the material in degrees Kelvin. Second, the dominant flow mechanism in superplasticity is grain boundary sliding [2] and this requires that the grain size is very small and, typically, not larger than ~10 µm. When both of these conditions are met, there is a potential for making use of these materials in developing a superplastic forming capability. The superplastic forming of aluminum sheet metals is now well established for the fabrication of complex parts for a wide range of applications including in the aerospace, automotive, electronic and architectural industries [3,4]. Nevertheless, any further development and expansion of the superplastic forming technology has been limited by the relatively slow strain rates associated with the forming process. For example, the production forming rates of conventional aluminum sheet metals are typically within the range of ~10-3 - 10-2 s-1 so that the forming times are generally of the order of ~20 - 30 minutes for the fabrication of each individual component. Since grain boundary sliding is the dominant flow mechanism in superplasticity, and theory suggests the r
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