Al−Si−Fe−Mn合金熔体保温过程中富铁相的三维形态特征和生长行为

3D characteristics and growth behavior of Fe-rich phases in Al−Si−Fe−Mn alloys during melt holding

采用扫描电子显微镜、透射电子显微镜、热力学计算和同步辐射X射线成像技术研究Al−7.0Si−1.0Fe−1.2Mn合金熔体保温过程中富铁相的形态演变和生长行为。结果表明,随着保温温度的降低,富铁相的形态由粗大的星形和多边形状转变成细小的汉字状以及汉字和多边形的耦合结构。富铁相的三维形貌显示,星形富铁相由多个多分支的初生相和衍生的不规则次生相组成;耦合结构由多面体和枝晶组成;单一汉字相则由枝晶组成。随着保温温度的降低,富铁相的数量、体积分数以及最大富铁相的体积和连通性显著降低。原位同步辐射X射线结果显示,在凝固过程中形成了两类初生富铁相,其中尺寸较小的初生富铁相保留在熔体中,并成为耦合共晶结构富铁相的形核质点。

The morphological evolution and growth behavior of Fe-rich phases in the Al−7.0Si−1.0Fe−1.2Mn alloy during melt holding were studied using scanning/transmission electron microscopy,thermodynamic calculations and synchrotron X-ray imaging.The results show that with a decrease in holding temperature,the Fe-rich phase underwent a morphological transformation from coarse star-like and polygonal structures to fine Chinese-script and coupled structures consisting of Chinese-script and polygonal shapes.The 3D structure of the star-like phase was composed of multibranched primary phases and irregular secondary phases;the coupled structure was composed of polyhedrons and dendrites;the Chinese-script phase was composed of dendrites.When the holding temperature was decreased,the number and volume fraction of Fe-rich phases were significantly reduced.A similar trend was also obtained for the largest volume particles and their connectivity.In situ synchrotron X-ray radiography results indicated that two types of primary Fe-rich phases were formed during the solidification process,and the smaller phase that remained in the melt could act as nucleation sites for the formation of the coupled eutectic structures.