Verification of the Displasive-Diffusion-less Mechanism in Bainitic Phase Transformations by a High Resolution Fractography

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The bainitic transformation is believed to finally proceed by a diffusion mechanism of atoms. However, there has also been proposed another idea where a displasive-diffusionless mechanism is operated in the nucleation stage and early stage of growth of bainitic transformations. The present investigation was carried out to verify which of the two mechanisms, diffusion or displeasures and atomic compositions of the bainitic plate and its matrix confronted along their interphase boundary on tensile-fractured surfaces, that is, by doing fractography with a high resolution scanning electron microscope (SEM) equipped with a field emission electron gun.The materials used for the verification are carbon steels (JIS S25C and S%%C) and Cu-28.3Xn-6.0Al(at .%) alloy, and the results obtained for the latter alloy will be reported below, because those for carbon steels were previously reported. In order to observe certainly a bainite plate and its matrix confronted on tensile-fractured surfaces, the volume ration of bainite plates to the matrix must be appropriate. The ratio was first examined by changing both isothermal holding temperature and time for the bainitic transformation, and 473 K and 28.8 Ks were found to be appropriate. The bainitic specimens subjected to this heat-treatment were tensile-fractured, and the fractured surfaces were observed by the high resolution fractography. Areas, where a bainite plate and its matrix seem to be confronted, were search, and EDX analyses were performed for the confronted bainite plates and matrices. As a result, it was found that atomic compositions of the plate and matrix were the same, and therefore the displasive-diffusionless mechanism is supposed to be really operated. However, more detailed examination is needed, because a little ambiguity is remained for the identification of bainite plates and matrices confronted on fractured surfaces.