热浸镀低合金高强钢Zn-Al-Mg镀层的组织结构表征

 热浸镀锌铝镁镀层因其具有更优异的切口保护性能和耐腐蚀性能等而被广泛应用于汽车和家电领域,而锌铝镁镀层钢板的耐蚀性与其物相组成密切相关。因此,为了更好地研究热浸镀低合金高强钢Zn-Al-Mg镀层表面与截面的微观组织与物相结构组成,利用聚焦离子束(FIB)对镀层截面进行了微区加工和精抛,制备出了平整光亮无应力的待观察面。首先利用扫描电镜(SEM)和能谱(EDS)初步分析了锌铝镁镀层表面与截面的微观组织形貌与成分,再利用电子背散射衍射技术(EBSD)菊池花样标定准确判定了锌铝镁镀层的物相结构与组成。研究表明,锌铝镁镀层表面与截面物相构成相同,均是由粗大枝晶状的初生富Zn相、相对细小鱼骨状或枝晶状的Zn-Mg二元共晶相和更为细小颗粒状或条状Zn-Mg-Al三元共晶相构成。另外还发现,在块状的初生富Zn相中均匀弥散分布着大量的纳米析出颗粒富Al相。经EBSD菊池花样标定准确判定了热浸镀锌铝镁镀层的物相结构,其中粗大枝晶状的初生富Zn相为具有密排六方结构的Zn相,相对细小鱼骨状或枝晶状的Zn-Mg二元共晶相为具有相同密排六方结构的Zn-MgZn₂,更为细小颗粒状或条状Zn-Mg-Al三元共晶相为Zn-MgZn₂-Al。另外,初生Zn相中析出的纳米级富Al相颗粒有效地提升了镀层的表面硬度。同时,在初生Zn相和共晶相的边缘均覆盖着一层MgZn₂相,该相可以有效地延缓腐蚀进程。

Microstructure characterization of hot-dip Zn-Al-Mg coating of low alloy high strength steel

Hot-dip Zn-Al-Mg coating is widely used in automobiles and home appliances because of its better notch protection performance and corrosion resistance,and the corrosion resistance of Zn-Al-Mg coated steel sheet is closely related to its phase composition. Therefore,in order to better study the microstructure and phase structure composition of the surface and cross-section of the Zn-Al-Mg coating on the hot-dip low alloy high strength steel. Micro-machining and fine polishing of the coating section by focused ion beam (FIB),a flat,bright and stress-free surface to be observed is prepared. Firstly,the microstructure,morphology and composition of the surface and cross-section of the Zn-Al-Mg coating were preliminarily analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS),secondly,the phase structure and composition of the Zn-Al-Mg coating were accurately determined by means of electron backscatter diffraction (EBSD) Kikuchi pattern calibration. The research shows that the phase composition of the surface of the Zn-Al-Mg coating is the same as that of the cross-section,which is composed of a coarse dendritic primary Zn-rich phase,a relatively fine fishbone-like or dendritic Zn-Mg binary eutectic phase,and more fine grained or striped Zn-Mg-Al ternary eutectic phase. In addition,a large number of nano-precipitated Al-rich phases are uniformly dispersed in the bulk primary Zn-rich phase. The phase structure of the hot-dip Zn-Al-Mg coating was accurately determined by the EBSD Kikuchi pattern calibration. The Zn-Mg binary eutectic phase is Zn-MgZn₂ with the same close-packed hexagonal structure,and the finer granular or strip-shaped Zn-Mg-Al ternary eutectic phase is Zn-MgZn₂-Al. In addition,the nano-scale Al-rich phase particles precipitated in the primary Zn phase can effectively improve the surface hardness of the coating. At the same time,the edges of the primary Zn phase and the eutectic phase are covered with a layer of MgZn₂ phase,which can effectively delay the corrosion process and improve the corrosion resistance of the coating.