Annealing behavior of submicrocrystalline oxide-bearing iron produced by mechanical alloying
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I. INTRODUCTION
MICROSTRUCTURE has a profound effect on the mechanical properties of metallic materials. Fine-grained metals and alloys provide a unique combination of high service strength at ambient temperature and improved workability during processing. Therefore, the production of materials with submicron-scaled microstructures has aroused great interest among materials scientists.[1–6] Several methods have been proposed to produce the submicron-grained structures, including rapid solidification, vapor condensation, severe plastic deformation, and powder metallurgy. One of the promising methods for the processing of ultrafinegrained materials is mechanical milling followed by consolidating working. This method combines the advantages of two techniques: severe plastic deformation and powder metallurgy. A special feature of this method is that it allows the manufacture of sizeable products, and it is applicable to various materials. The method makes it possible to develop specific alloys; therefore, it can be referred to as mechanical alloying.[3,6] Recently, mechanical milling followed by consolidating working has been applied to the production of high-strength steels with submicrocrystalline structures, which contain dispersed oxide particles that are homogeneously distributed throughout the ferrite matrix.[7,8] In addition to dispersion strengthening, the dispersed oxides play an important role in grain refinement, which can be achieved down to 0.2 m.[9] Wide industrial production and engineering applications of the ultrafine-grained materials are limited by their relatively poor thermal stability, which is dictated by the incredibly small grain size itself. The rapid grain coarsening at elevated temperatures can be suppressed by any secondary-phase precipitation. From this standpoint, a uniform dispersion of fine oxide particles is quite effective. Ultrafinegrained steels containing dispersed oxides have been shown to be essentially stable against dramatic coarsening in the 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, Tsukuba, Ibaraki 305-0047, Japan. Contact e-mail: andrey. [email protected] Manuscript submitted November 15, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
ferrite region.[10] The main mechanism for the structural changes that operates during annealing is slow grain growth accompanied by recovery. However, further systematic investigations are required for a more-comprehensive analysis of the annealing behavior of ultrafine-grained materials and the stabilizing effect of dispersed particles. The present work continues the studies of structure evolution in iron oxide alloys processed by mechanical milling followed by consolidating working.[8,9,10] The aim of the present work is to study experimentally the structural changes during annealing of ultrafine-grained ferrite containing dispersed oxides. The softening and grain-growth kin
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