Evolution of submicrocrystalline iron containing dispersed oxides under mechanical milling followed by consolidation
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NTRODUCTION
THE mechanical properties of metallic materials are strongly affected by the structural state, i.e., grain size distribution, grain-boundary conditions, dislocation substructures, etc. The structural analysis of metals and alloys produced by various thermomechanical technological treatments are of great practical importance. Submicron and nanostructured materials have aroused great interest among the materials scientists.[1–4] These materials are expected to have a beneficial combination of mechanical properties, specifically a high strength at ambient temperature and an improved workability during processing.[4,5,6] There has been a sharp increase in various publications dealing with the ultra fine-grained materials that are processed by severe plastic deformation in recent years. Several methods based on the large strain deformation have been proposed to produce bulk submicrocrystalline materials, including mechanical milling with consolidation, equal channel angular extrusion, multiple multiaxial forging, etc.[3,7–13] Because the mechanical milling is applicable to a large variety of materials, this method may be considered as one of the most powerful processes for the production of improved structural alloys using specific materials. The process including mechanical milling followed by warm consolidation has been applied recently to develop ultra fine-grained high-strength steels containing dispersed oxide particles.[14,15] In addition to the dispersion strengthening, the fine oxides, which are homogeneously distributed throughout the matrix, play an important role in the structure evolution.[15,16,17] The final microstructure of the oxide-bearing mechanically milled steels that evolved after consolidation depends A. BELYAKOV, Postdoctoral Fellow, Y. SAKAI, T. HARA and Y. KIMURA Senior Researchers, and K. TSUZAKI, Deputy Director– General, are with the Steel Research Center, National Institute for Materials Science, Ibaraki 305-0047, Japan. Contact e-mail: andrey.belyakov@ nims.go.jp Manuscript submitted January 22, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
significantly on the oxygen content and the oxide distribution. The average grain size can be decreased to a submicron scale by increasing the oxygen content.[15,16] Increasing the milling time leading to the dispersion of oxides also results in the finer grains that evolve after consolidation.[17] The formation of the submicrocrystalline structures is accompanied by the evolution of grain boundaries with nearly random misorientations. On the other hand, this is a very complex processing method consisting of two major sequential stages: mechanical milling and consolidating working. Both of them, along with preheating before consolidation, may be very important for the final structure evolution. The aim of the present work is to study the general sequence of the structural changes taking place in the Fe0.6 pct O alloy under mechanical milling followed by consolidating working. The present study focuses mainly on the evolution of (sub)grain
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