Dynamic Recrystallization in Sintered Cobalt during High-Temperature Deformation
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INTRODUCTION
THE hot deformation behavior of wrought cobalt in the fcc phase has been studied by the present authors in detail.[1,2] These studies clearly established dynamic recrystallization (DRX) as the primary restoration mechanism of this material in the temperature range of 973 to 1223 K (700 to 950 C) on the basis of the generated processing map, the nature of flow behavior, the kinetic analysis, and the development of microstructure. Of the various hot deformation mechanisms, DRX is the most desired for working, because it enhances hot ductility, reconstitutes the initial microstructure, and refines the grain structure by movement of high-angle boundaries, resulting in improved mechanical properties of the product. DRX has also been reported to be successfully used to consolidate powder products.[3,4] Powder metallurgy (PM) routes are being increasingly employed for fabrication of engineering components, because these offer several advantages such as a much finer microstructure with a uniform distribution of microconstituents, resulting in attractive mechanical properties. The PM route is more than an option where the primary source of matrix metal of an alloy is scarce or the matrix metal has to be recovered from secondary resources. Cobalt is an example of the latter case, and it is generally recovered in powder form, from various secondary resources (e.g., spent ammonia cracker catalyst and tool steel).[5–7] However, the formability of PM-processed products is greatly restricted because of the presence of prior particle boundaries (PPBs) and residual porosity in the bulk. Thus, for developing B. PAUL, A.C. BIDAYE, and I.G. SHARMA, Materials Processing Division, A. SARKAR, J.K. CHAKRAVARTTY, A. VERMA, and R. KAPOOR, Mechanical Metallurgy Section, and A.K. SURI, Materials Group, are with the Bhabha Atomic Research Centre, Mumbai 400085, India. Contact e-mail: [email protected] Manuscript submitted August 26, 2009. Article published online March 4, 2010 1474—VOLUME 41A, JUNE 2010
components from semifinished sintered products, it is important to identify the hot deformation conditions (strain, strain rate, and temperature) that will promote operation of DRX so that a final product with high bulk density, minimum pores, and without PPBs results. In this article, we report hot deformation behavior of sintered cobalt. A large volume of research has been carried out to understand the hot working behavior of wrought products of various metals and alloys. The technique of constructing processing maps has been found to be very useful to characterize the hot deformation behavior, to control the microstructure of the product, and to identify the hot deformation mechanisms.[8–12] However, it appears from the literature that not much effort has been directed to characterize hot deformation behavior of sintered PM products employing processing maps. The primary aim of the present investigation is to study the constitutive flow behavior of sintered Co powder compact, to generate a processing map for optimizing the hot workab
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