Abstract The transitional structures of 7075 aluminum alloy ingot after switching off a low-frequency electromagnetic field (LFEF) in the horizontal direct chill casting was investigated. The results showed that the equiaxed grains with the LFEF are firstly transformed to columnar grains and then to feathery grains without the LFEF. Also, there exists a transitional region consisting of relatively fine equiaxed grains between the equiaxed grains and the columnar grains. Crystallographic analysis indicated that only the relatively fine equiaxed grains with ideal orientations can grow into columnar grains. Moreover, there exist multiple twin relationships between some adjacent feathery grains, which have a near-common or common 〈1 1 0〉 direction. The equiaxed grains with the LFEF result from the forced convection. The relatively fine equiaxed grains and the columnar grains are attributed to the weakening of the forced convection when the LFEF is switched off. For the feathery grains, it is of interest to find that the quasicrystal-enhanced nucleation mechanism is responsible for their formation under high thermal gradient although it has been reported for twinned equiaxed grains under nearly nil thermal gradient in the literature.

中文翻译：

---

7075铝合金锭水平直冷连铸关闭低频电磁场后的过渡组织研究

摘要 研究了7075铝合金锭在水平直冷铸造中关闭低频电磁场(LFEF)后的过渡组织。结果表明，有LFEF的等轴晶粒先转变为柱状晶粒，再转变为没有LFEF的羽状晶粒。此外，在等轴晶和柱状晶之间存在由较细的等轴晶组成的过渡区。晶体学分析表明，只有具有理想取向的较细的等轴晶粒才能生长成柱状晶粒。此外，一些相邻的羽状颗粒之间存在多重孪生关系，具有近乎共同或共同的〈1 1 0〉方向。具有 LFEF 的等轴晶粒来自强制对流。较细的等轴晶粒和柱状晶粒归因于关闭 LFEF 时强制对流减弱。对于羽毛状晶粒，有趣的是发现准晶增强的成核机制是它们在高热梯度下形成的原因，尽管文献中已经报道了在几乎为零的热梯度下孪晶等轴晶粒。