Microstructural design for large superplastic elongations in aluminum-base materials containing particles
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THE refinement of microstructures is a universal theme in materials science and engineering because of the relationship between the microstructure and the strength of materials. Developments in processing have now allowed nanostructure-scale materials to be studied in the laboratory. Nanocrystalline materials are a focus of academic interest because of their unique properties, such as extreme strength at the critical grain size,[1] ductility,[2] compatibility between high strength and high ductility,[3] and superplasticity.[4,5] On the other hand, from a practical application standpoint, materials having a grain size not less than that of ultrafine-grained materials (defined as grain sizes ranging from several tens of nanometers to several microns in size) are of special interest due to their potential for bulk production and mass production. Many structural products have already been made from materials having a grain size not less than that of ultrafine-grained materials.[6,7] Weight saving and complex shapes are challenges encountered in the development of structural components. The use of superplastic phenomena is one way to overcome these issues. However, the forming rate for superplasticity is slower than the conventional industrial forming rate. Refinement of the grain size resolves this drawback, because the logarithmic superplastic strain rate is proportional to the logarithmic inverse of the grain size.[8] There have been many reports on high strain rate superplasticity, especially for aluminum-base materials.[8–48] Materials attain large elongations in the superplastic region based on high strain rate sensitivity (m) values, which is the slope in the relationship between the logarithmic flow stress and the logarithmic strain rate. The region of high m values is affected by the microstructures of materials. In many of the constitutive equations for superplasticity, grain size is the only parameter related to the H. HOSOKAWA, Researcher, is with the Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, 463-8560, Japan. K. HIGASHI, Professor, is with the Department of Metallurgy and Materials Science, Osaka Prefecture University, Osaka, 599-8531, Japan. Contact e-mail: [email protected] Manuscript submitted October 28, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructural features. However, superplastic materials, especially high strain rate superplastic materials, contain large numbers of particles in order to achieve refinement of the grain size, and a great deal of experimental data have suggested that particle features also depend on the superplastic flow.[49] Therefore, the microstructural features for superplastic behavior in aluminum-base materials containing particles should be derived using the constitutive equations for the deformation behavior that takes particle features into consideration. A high m value is not sufficient in permitting a high degree of elongation, because excessive cavitation occurs duri
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