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观点，即在贝氏体相变的成核阶段和生长早期，存在非塑化-无扩散机制。本研究通过配备场发射电子枪的高分辨率扫描电子显微镜（SEM）进行断口分析，以验证贝氏体板及其基体沿相界面的扩散或破坏和原子组成这两种机制中的哪一种。用于验证的材料是碳钢（JIS S25C和S%%C）和Cu-28.3Xn-6.0Al(at。%)合金，后一种合金的结果将在下面报道，因为碳钢的结果已在前面报道过。为了准确地观察到在拉伸断裂表面上的贝氏体板材及其基体，贝氏体板材与基体的体积比必须合适。首先通过改变贝氏体转变的等温保温温度和保温时间来考察该比值，发现473 K和28.8 K是合适的。热处理后的贝氏体试样呈拉伸断裂，断口形貌由高分辨断口形貌观察。搜索贝氏体板及其基体可能面对的区域，并对面对的贝氏体板和基体进行EDX分析。结果表明，板和基体的原子组成完全相同，因此，无消塑扩散机制应该是真正可行的。然而，需要进行更详细的检查，因为对于识别断裂表面上的贝氏体板和基体仍然有一点模糊。

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.