透射-电子背散射衍射技术表征双相钢中马氏体-奥氏体岛的相结构及晶粒取向

为研究高强度低屈强比双相钢中马氏体-奥氏体岛(M-A岛)的精细结构及晶粒取向差关系,利用自制的透射-电子背散射衍射(T-EBSD)样品台和透射电镜样品,在扫描电镜上完成了纳米级高分辨电子背散射衍射(EBSD)表征。通过优化工作电压、倾转角度、工作距离、探头距离、步长等参数,确定了最佳实验条件:工作电压为30kV,倾转角度为30°~38°,步长为15nm,最终获得了高质量的透射菊池花样,实现了微观组织的精细表征,将常规EBSD的分辨率提升了一个数量级。同时,利用透射电镜的高分辨透射电子显微术(HRTEM)进行了验证。EBSD分析结果表明,LYR690钢中M-A岛内部存在超细铁素体、铁素体与奥氏体混合相、残余奥氏体和马氏体等4种结构。残余奥氏体主要以两种形态存在:一种为数百纳米级别的独立晶粒,其与周边铁素体存在超过15°的大角度取向差;另一种为铁素体晶粒中分布的细微奥氏体结构,其与铁素体晶粒的取向差小于10°。

Characterization of phase structure and grain orientation of martensite-austenite island in duplex steel by transmission-electron backscatter diffraction

In order to investigate the relationship between fine structure and grain misorientation of martensite-austenite island (M-A island) in high-strength low-yield ratio dual-phase steel, the self-made transmission-electron backscatter diffraction (T-EBSD) sample stage and transmission electron microscope samples were used for the nanoscale high-resolution electron back scattering diffraction (EBSD) characterization on scanning electron microscope (SEM). By optimizing the parameters including working voltage, tilting angle, working distance, probe distance and step size, the optimal experimental conditions were determined as follows: the working voltage was 30kV, the tilting angle was 30°~38° and the step size was 15nm. Finally, the high quality transmission Kikuchi pattern was obtained, which achieved a fine characterization of microstructure to increase the resolution of conventional EBSD by one order of magnitude. Meanwhile, it was verified by high resolution microscopy of transmission electron microscope (HRTEM). The results of EBSD analysis showed that there were four structures in M-A island of LYR690 steel, including ultrafine ferrite, ferrite and austenite mixed phase, retained austenite and martensite. The retained austenite mainly existed in two forms: one was an individual crystal grain at level of several hundred nanometers, which had a high angle misorientation of more than 15° with the surrounding ferrite, and another was the fine austenitic structure distributed in ferrite grains, which had a misorientation of less than 10° with ferrite grains.