Enhanced Densification of Carbonyl Iron Powder Compacts by the Retardation of Exaggerated Grain Growth through the Use o
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ONYL iron powders are widely used in making soft magnets and powder injection molded parts due to the fine particle size, large surface area, and high driving force for sintering. The sinterability of this powder is, however, still unacceptable for applications in which high density is required. This drawback is mainly caused by the significant decrease in the densification rate of the powder when the a-c phase transformation occurs at 912 °C. This peculiar densification behavior has been studied by several authors.[1–3] Cizeron and Lacombe examined the dilatometer curves and microstructure changes and arrived at the conclusion that the dramatic decrease in the densification rate during the phase transformation was not caused by the reduction of the diffusion rate when the bcc structure transforms to an fcc structure, but is mainly caused by the exaggerated grain growth, which isolates the pores KUEN-SHYANG HWANG, Professor, and BOR-YUAN CHEN, Graduate Student, are with the Materials Science and Engineering Department, National Taiwan University, Taipei, Taiwan, 106, R.O.C. Contact e-mail: [email protected]. YUNG-CHUNG LU, formerly Graduate Student, Materials Science and Engineering Department, National Taiwan University, is General Manager, Taiwan Powder Technologies, Taoyuan, Taiwan, 335, R.O.C. GUO-JIUN SHU, formerly Graduate Student, Materials Science and Engineering Department, is Research Associate, Center for Condensed Matter Sciences, National Taiwan University. Manuscript submitted December 10, 2008. Article published online September 25, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
from the grain boundaries.[1] Once these pores are trapped inside the grain, they cannot be eliminated within a practical time frame, due to the long distance over which the vacancies flow from pores through volume diffusion to the nearest grain boundary. This exaggerated grain growth does not occur in every material, however. For most elemental powders, which have no allotropic phase transformation during heating, the normal grain growth prevails. It starts with the recombination of grain boundaries within the same particle. As the intraparticle grain growth proceeds, some grain boundaries meet the interparticle neck and become anchored, because any further movement will cause an increase in the grain boundary area and, consequently, its energy.[4] It is believed that as sintering continues and the neck size increases, the relative amount of increase in the grain boundary area for a pinned grain boundary to break away from the neck decreases. When the neck size reaches the point at which the chemical driving force of the grain growth overcomes the energy barrier, grain boundary breakaway occurs and the regular grain growth continues. In such cases, little drastic change occurs in the densification rate during heating and no exaggerated grain growth is observed. In the case of carbonyl iron powder, the period of the grain boundary pinning at the neck lasts a much shorter time, because the grain boundary breaks away and causes ex
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