Development of ultra- fine grained aluminium by severe plastic deformation

Abstract

Nanoscale science and technology is experiencing a rapid development and nano-materials have made profound impact on every field of materials research. Scientists believe that the nanoscale science and technology will bring revolutions in human history due to the unique properties of nano-materials. Nanomaterials are the fundamental basis of nanotechnology. Two basic and complementary approaches have been developed for the synthesis of nanomaterials. These are known as “bottom up” and “top down” approaches. In the bottam up approach, atoms, molecules and even nanosize particles themselves can be used as the building blocks for the creation of complex nanostructures. Whereas, in the “tops down” approach coarse-grained materials are refined into ultrafine -grained /nanostructured materials. Ultrafine grained (UFG) materials may be defined as polycrystalline materials having average grain size less than ~1μm. The grain sizes of UFG materials may lie within the sub-micrometer (100-1000 nm) range. In order to convert a coarse grained solid into a material with ultrafine grains, it is necessary to impose an exceptionally high strain in order to introduce a high density of dislocations which, in turn, re-arrange to form an array of grain boundaries with increase in strain. Conventional metal forming procedures, such as extrusion or rolling, are restricted in their ability to produce UFG structures for two important reasons. First, there is a limitation on the overall strains that may be imposed using these procedures because a very high strain requires corresponding reductions in the cross-sectional dimension of the work-pieces. Second, the strains imposed in conventional processing are limited and insufficient to give rise to UFG structures because of the generally low workability of metallic alloys at ambient temperatures. Another conventional method, such as rapid solidification and vapor condensation, are able to refine the materials up to nanoscale, but these processes are restricted to only thin layers/sections. As a consequence of these limitations, there is a paradigm shift in the synthesis routes for nanoscale materials and alternative processing techniques have evolved. The severe plastic deformation (SPD) technique is one of such processes, where extremely high strains are imposed at relatively low temperatures.During the last two decade SPD has emerged as a widely known procedure for the grain refinement of metals and alloys into sub-micrometer or even nanometer range. Synthesis of UFG materials by SPD refers to various experimental procedures of metal forming that may be used to impose very high strains on materials leading to exceptional grain refinement. For production of bulk UFG materials with equiaxed microstructure and high angle grain boundary misorientation, accumulative roll bonding (ARB) and equal channel angular pressing are the two well known SPD methods. Materials processed by SPD have shown superior mechanical properties such as high strength, excellent fatigue life, high toughness and low temperature superplasticity. As the specific material properties are becoming stringent in the current developmental scenario, there is a continuing increase in market requirements for high-strength metals and alloys. This finds applications in aerospace, automobile, transportation, food and chemical processing, electronics, and conventional defence industries. The market emphasis and exceptional material properties requirements have led to a considerable interest in the development of ultrafine-grained/nanomaterials by severe plastic deformation. Therefore, the present work has been undertaken to develop ultrafine-grained aluminum by severe plastic deformation method and to examine the microstructure and mechanical properties by different characterizing tools. An important objective of this work is to identify and quantify interrelationships between the deformation mechanisms active during grain refinement process and to determine their influence on the mechanical behavior of UFG aluminum processed by accumulative roll bonding technique, one of the variants of severe plastic deformation.