Diffusion in high-purity iron: Influence of magnetic transformation on diffusion
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Section I: Basic and Applied Research
Diffusion in High-Purity Iron: Influence of Magnetic Transformation on Diffusion Yoshiaki Iijima
(Submitted July 14, 2005) Influence of magnetic transformation on volume diffusion, dislocation diffusion, and grain boundary diffusion in high-purity iron (Fe) has been studied. The intensity of the influence on volume diffusion is found to be proportional to the change of magnetization of the Fe matrix around the first and second shells of the diffusing atom. The influence of magnetic transformation on the grain boundary diffusion and dislocation diffusion in Fe is found to be much larger than that on the volume diffusion.
1. Introduction The oldest measurement of tracer self-diffusion in ␣-iron (Fe) is that by Birchenall and Mehl in 1948, which is listed in a compilation by Askill.[1] During the 1950s, the influence of magnetic transformation on self-diffusion in Fe was recognized. Many experimental and theoretical works on this topic were carried out in the 1960s. However, the traditional mechanical sectioning techniques, which limit the measurable minimum diffusion coefficient to 10−16 m2/s, have mainly been used. Therefore, the self-diffusion coefficient of Fe could be determined down to only 70 K below the Curie temperature (Tc ⳱1043 K); thus, the diffusion behavior of ferromagnetic Fe was not definite.[2] However, in 1977 Hettich et al.[3] succeeded in measuring the diffusivity down to 259 K below Tc by a new technique, ionbeam sputter-microsectioning. Their work supplied a powerful tool for the study of the influence of magnetic transformation on diffusion in ␣Fe.[4] Below the Tc, an Arrhenius plot of diffusion coefficients in the ferromagnetic state of Fe deviates downward from the Arrhenius relationship extrapolated from the paramagnetic state. Equations describing such temperature dependence for the diffusion coefficients in the whole temperature range across the Tc have been proposed by several authors such as Ruch et al.,[5] Hettich et al.,[3] Kucˇera,[6] and Braun and Feller-Kniepmeier.[7] According to Ruch et al.,[5] the temperature dependence of the diffusion coefficient D can be expressed by an equation: D = D0p exp关−Qp 共1 + ␣s2兲 Ⲑ RT兴
(Eq 1)
Here, D0p and Qp are the preexponential factor and the activation energy, respectively, in the paramagnetic state. This article is a revised version of the paper printed in the Proceedings of the First International Conference on Diffusion in Solids and Liquids—DSL-2005, Aveiro, Portugal, July 6-8, 2005, Andreas Öchsner, José Grácio and Frédéric Barlat, eds., University of Aveiro, 2005. Yoshiaki Iijima, Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, Ueda 4-3-5, Morioka 0208551, Japan. Contact e-mail: [email protected].
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The value of s, the ratio of the spontaneous magnetization at temperature T K to that at 0 K, has been experimentally determined by Potter[8] and Crangle and Goodman.[9] The constant ␣, expressing the extent of the influence of the magnetic transformation
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