Studies on hot deformation of sintered cobalt
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INTRODUCTION
COBALT is an important ingredient in materials such as cemented carbides, tool steels, magnetic materials, paint pigments, catalysts, and artificial g-ray sources. One of the key uses of pure Co is for radiation therapy in medical applications, where the Co is primarily used as pellets of about 1-mm diameter. These pellets are fabricated through the powder processing route involving appropriate combination of hot working techniques, such as hot extrusion, hot swaging under reducing atmosphere. Due to the low ductility of cobalt in the low-temperature hcp phase, it is expected that hot working be carried out in the hightemperature fcc phase (.417 °C).[1] The coexistence of both hcp and fcc phases in cobalt powder at room temperature has also been reported,[2] and it was suggested that the volume fraction of phases can be selectively controlled by optimizing the powder process parameters. A variation in microhardness with crystal orientation was observed in an earlier study reported on the zone refining of cobalt.[3] Apart from these studies, there appear to be a limited amount of data available on the tensile and compressive strength of sintered cobalt at either ambient or at higher temperatures. No attempt has been made so far to evaluate strain rate sensitivity (SRS) and temperature sensitivity, essential parameters for understanding hot working, of cobalt prepared by powder metallurgy. The properties of sintered metallic products depend on the size, morphology, and purity of the powders. Various routes such as (1) aqueous cobaltous hydroxide slurry under hydrogen reduction conditions using palladium chloride catalyst;[4] (2) reduction of cobalt oxide, chloride, or sulfate;[5] and (3) electrowinning[5] have been explored for the preparation of cobalt powder. S. MAJUMDAR, I.G. SHARMA, R. KAPOOR, J.K. CHAKRAVARTTY, Scientific Officers, Materials Group, and A.K. SURI, Associate Director (P), Materials Group, are with the Materials Processing Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400085, India. Contact e-mail: sanjib@ apsara.barc.ernet.in Manuscript submitted March 17, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
The objective of the current study was to produce Co rods of about 6-mm diameter and study its hot deformation behavior. These data could then be used to optimize the hot working conditions of cobalt for preparing long wires, for subsequent conversion to pellets of required dimensions for use as a radiation source in medical applications. In the present work, cobalt powder was produced by thermal decomposition of cobalt oxalate in hydrogen atmosphere, which was then characterized for size and shape by scanning electron microscopy. Cobalt powder was subsequently consolidated by cold isostatic pressing (CIP) and sintering. Hot deformation studies of the sintered cobalt rods were carried out by strain rate change as well as continuous compression tests at temperatures ranging from 900 °C to 1300 °C. Optical microstructures for both undeformed and deformed samples were observed.
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