熔态镁合金在空气/SF_6保护气氛中形成的膜特征

Characterization of Protective Surface Films on Molten AZ91D Magnesium Alloy Formed in Air and SF_6 Protective Gas

镁及其合金的铸造、熔炼需要在SF6的保护气氛中进行,目前对其研究还不完善。为此,用SEM,TEM和XRD研究了熔态AZ91D镁合金在空气/SF6保护气氛中形成的表面膜特征。结果表明:保护性表面膜分为两层,最外层主要为疏松多晶的MgO及含有少量Al的氧化物和微量的氟化物;内层为结晶MgF2;随着AZ91D在保护气氛中暴露时间的增加,表面膜中F/O原子比显著增加,表面粗糙度增加;表面膜超过一定厚度后,主要为F和Mg发生反应;表面膜表面弥散着大小不一的白色片状物,其主要成分为MgO,内含少量MgF2。部分近表面区域断面中有不连续的白亮富F(原子分数约为70%)小半圆区域,富F的小半圆亮区之间F含量显著降低;部分为一条厚度不均匀的白亮带,其中F原子分数也很不均匀。富F区中F/Mg原子比显著大于MgF2中的2/1,F可能通过富F区域不断从保护气氛向AZ91D镁合金内部扩散形成MgF2。

With a view to the fact that the casting and melting of magnesium and its alloy are generally carried out in the protective gas SF_6, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction were used to analyze the characteristics of the surface film on AZ91D magnesium alloy molten in air/SF_6 protective gas. It was found that the protective surface film was composed of two layers. Namely, the upper layer was composed of a large amount of loose polycrystalline MgO, a little amount of aluminum oxide, and trace fluoride; and the inner layer was composed of crystalline MgF_2. The atomic ratio of F/O in the surface film increased evidently and so did the surface roughness with increasing exposure time. The reaction between magnesium and fluorine was dominant as the film thickness exceeded a certain value. Moreover, some white flakes composed of MgO and a little amount of MgF_2, with varied size and disperse distribution, existed on the film surface. There existed semicircular discontinuous F-rich white spots composed of about 70% fluorine in some near surface zones, and the F content in the area excluding the white spots was greatly reduced. Since the atomic ratio of F/Mg in the F-rich area was higher than 2/1, that of MgF_2, it could be supposed that the fluorine might diffuse from the protective atmosphere to the bulk melt of AZ91D magnesium alloy through the F-rich zone.