The aim of this work is to investigate the formation of an anomalously high fraction of twin boundaries (TBs) during laser directed energy deposition (DED) of a Mg-3Al-1 Zn (wt.%) alloy. With that goal, single tracks are deposited using different combinations of scan speed and powder feed rate. The melt pool dimensions are related to the DED parameters. Irrespective of the processing conditions, the resulting microstructures are always formed by fine grains with a very weak texture. However, some DED parameter combinations give rise to a fraction of TBs that is significantly higher than that corresponding to a random texture. These TBs are characterized by an irregular morphology and by the presence of segregated atoms, solute clusters, and nanoparticles. Additionally, the lattice in the vicinity of the TBs is highly distorted and contains a high population of stacking faults. All these characteristics set them apart from conventional tensile TBs, which exhibit a large degree of coherency and limited segregation. Rapid solidification is thus presented here as a novel avenue to design TBs with unique properties in Mg alloys. This work paves the way for future studies on grain boundary design via additive manufacturing methods, a field that is still in its infancy.

中文翻译：

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镁合金快速凝固过程中异常孪晶界的形成

这项工作的目的是研究 Mg-3Al-1 Zn (wt.%) 合金激光定向能量沉积 (DED) 过程中异常高比例孪晶界 (TB) 的形成。为了实现这一目标，使用扫描速度和送粉速率的不同组合来沉积单个轨道。熔池尺寸与DED参数有关。无论加工条件如何，所得的微观结构总是由织构非常弱的细晶粒形成。然而，一些 DED 参数组合产生的 TB 分数明显高于对应于随机纹理的 TB 分数。这些结核病的特征是不规则的形态以及分离的原子、溶质簇和纳米颗粒的存在。此外，TB 附近的晶格高度扭曲，并且包含大量堆垛层错。所有这些特性使它们与传统的拉伸 TB 区分开来，传统的拉伸 TB 表现出很大程度的相干性和有限的偏析。因此，本文提出快速凝固作为一种设计具有独特性能的镁合金TB的新途径。这项工作为未来通过增材制造方法进行晶界设计的研究铺平了道路，这一领域仍处于起步阶段。