On the Precipitation of Carbon in Super Purity α-Iron

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  • 帯域純化した鉄中の過飽和Cの析出
  • タイイキ ジュンカ シタ テツ チュウ ノ カホウワ C ノ セキシュツ
  • On the Precipitation of Carbon in Super Purity α-Iron
  • On the Precipitation of Carbon in Super Purity <i>α</i>-Iron

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In order to separate the effect of impurities from the inherent process in the precipitation of super saturated carbon in α-iron, super-purity iron specimens were made by zone refining, carburized to 0.009 wt% carbon, and used in the quench aging experiment. The results were compared with the specimens from the same sources (Puron 99.9% and Johnson-Matthey) but not zone refined.<BR>The precipitation process was followed by measuring the electrical resistivity and internal friction and by the direct observation with the transmission electron microscope. All of the measurements showed appreciable differences between the super-purity specimens and lower purity specimens, as are summarized in the following.<BR>(1) The precipitation process consisted of two steps, as previously reported(1), showing the formation of a meta-stable phase before that of the cementite phase. For the fact that clear and explainable differences were observed in the first step while they were not always well-interpreted in the second step, only the former step was dealt with in this report.<BR>(2) The internal friction measurement revealed that for purer specimens the time necessary for the precipitation is shorter. As most of the impurity atoms were considered to be oxygen and no difference in the density of precipitation nuclei was found by electron microscopy, it was quite conceivable that the oxygen atoms do not give rise to the decrease of the nucleation rate but temporarily trap the migrating carbon atoms to make the process delayed.<BR>(3) In the lower purity iron the aging curves obtained by the electrical resistivity measurement were in fairly good accordance with those by the internal friction measurement, while in the zone melted super-purity iron the resistivity decreased in the order of ten times slower than the C-Snoek peak decay in the first step. It was suggested that in super-purity iron were formed a special configuration of precipitates with a large capacity of scattering the conduction electrons which brings about a considerably large resistivity increase.<BR>It was concluded that the precipitation phenomena of interstitial elements so far reported in pure iron are not characteristic of the pure state but mostly arise from the impurity effect.

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