Effect of dispersed particles on microstructure evolved in iron under mechanical milling followed by consolidating rolli
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INTRODUCTION
PRODUCTION of metallic materials with ultra-finegrained microstructures is of particular interest to metallurgical engineers. Such kinds of materials are believed to have a beneficial combination of mechanical properties, i.e., a high strength at ambient temperature and an improved workability during hot working.[1,2] Several methods have been proposed to produce materials with submicron grains, including rapid solidification, vapor condensation, severe plastic deformation, powder metallurgy, etc. The powder metallurgy method has some advantages over the other ones. A special feature of this method is that it allows us to make sizeable stocks and that it is applicable to a larger variety of materials. The latter makes it possible to use the special alloys, which are difficult if not impossible to produce by any other processing. One type of powder metallurgy processing is mechanical milling followed by consolidating plastic working. Recently, this method has been applied to the production of highstrength steels with fine-grained structures, which contain dispersed oxide particles that are homogeneously distributed throughout the matrix.[3,4] In addition to the conventional dispersion strengthening, the fine particles play an important role in the structure formation, which can also affect the mechanical properties of the steels. The uniform dispersion of the fine oxide particles in the matrix is effective for grain refinement and increasing the strength of steels produced by the mechanical milling.[4] However, the microstructural characteristics of ultra-fine-grained materials containing dispersed particles have not been studied in sufficient detail for the mechanical milling followed by hot working. The effect of fine particles (their size and distribution) on some structural parameters such as texture, (sub)grain-boundary A. BELYAKOV, STA Fellow, Y. SAKAI and T. HARA, Senior Researchers, Y. KIMURA, Researcher, and K. TSUZAKI, Unit Leader, are with the Frontier Research Center for Structural Materials, National Research Institute of Metals, Ibaraki 305-0047, Japan. Manuscript submitted October 9, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
misorientations, dislocation arrangements, etc. is still unclear. The aim of the present work is to study the peculiarities of the fine-grained microstructures developed under mechanical milling followed by consolidating hot rolling of iron powder with various amounts of oxide particles. The present study is focused mainly on the effect of dispersed oxide particles on the (sub)grain-boundary characteristics, such as the misorientation distribution of grain boundaries, their mutual alignment, etc. II. EXPERIMENTAL PROCEDURE Three kinds of iron-oxide powders with different amounts of oxygen, i.e., 0.2, 0.6, and 1.5 mass pct (Table I), were used as the starting materials. Each powder was mechanically milled by a planetary ball mill in an argon atmosphere for about 100 hours using a stainless steel pot (SUS403) and steel balls (SUJ2). After mechanical milling, the
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