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

贝氏体相变最终是通过原子扩散机制进行的。然而，也有人提出了另一种想法，即在贝氏体相变的形核阶段和生长早期运行一种无扩散机制。本研究是通过配备场发射电子枪的高分辨率扫描电子显微镜进行断口分析，以验证贝氏体板及其基体在拉伸-断裂表面上沿其相界面上的两种机制中的扩散或不愉快和原子组成中的哪一种。用于验证的材料是碳钢(JISS25C和S%%C)和铜-28.3Xn-6.0Al(at.%)合金，所获得的结果将在下文中报告，因为碳钢的结果已被报道。为了在拉伸断口上观察到贝氏体片及其母体，贝氏体片与母体的体积比必须适当。通过改变贝氏体相变的等温保温温度和时间，确定了合适的等温保温温度为473K和等温保温时间为28.8Ks。贝氏体热处理后拉伸断口，用高分辨断口观察断口。对贝氏体片与贝氏体片的对接区域进行了搜索，并对对接的贝氏体片和贝氏体片进行了能谱分析。结果发现，板材和基体的原子组成相同，因此认为无扩散扩散机制是真正可行的。然而，还需要更详细的检查，因为对于断口上的贝氏体板和贝氏体基的识别仍然存在一些模糊之处。

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.