Twinning-induced plasticity plays a critical role in determining the deformation behaviors of hexagonal close-packed metals (e.g., Mg and Ti), due to the lack of five independent slip systems required for a general deformation. In particular, twinning modes that can accommodate c-axis strain (tensile or compressive) are especially important, which are necessary complementary deformation modes to <a>-type dislocations. However, only limited types of extension twins, e.g., $\left\{10\overline{1}2\right\}$ type twin, have been unambiguously identified in the literature, which is theoretically inadequate to accommodate a complex deformation with c-axis extension. Using topological defect theory, here we show that another extension twinning mode, $\left\{11\overline{2}6\right\}$ twin, can be formed through reactions between disclinations and $\left\{10\overline{1}2\right\}$ twins in Mg-alloys. Based on symmetry of the deformation space, we demonstrate that the $\left\{11\overline{2}6\right\}$ twin originates from the inter-connection of correlated deformation paths, which can provide a tensile strain of 5.2% along c-axis and a compressive strain of 4.5% along $〈11\overline{2}0〉$. $\left\{11\overline{2}6\right\}$ twins can be formed through three formation mechanisms, i.e., direct formation, twin-twin intersection, and double twinning, which are validated by a combination of experimental characterizations and phase field simulations. Our work not only suggests a symmetry-based method to analyze twinning-induced plasticity, but also provides a new insight into the deformation mechanisms and mechanical properties of Mg-alloys.

由于缺乏一般变形所需的五个独立滑移系统，孪生诱发的塑性在决定六方密排金属（例如 Mg 和 Ti）的变形行为中起着关键作用。特别地，可以适应 c 轴应变（拉伸或压缩）的孪晶模式尤为重要，这是<a>型位错的必要补充变形模式。然而，只有有限类型的扩展双胞胎，例如，$\left\{10\overline{\mathrm{1个}}\mathrm{2个}\right\}$双胞胎型，已在文献中明确确定，理论上不足以适应具有 c 轴延伸的复杂变形。使用拓扑缺陷理论，在这里我们展示了另一种扩展孪生模式，$\left\{11\overline{\mathrm{2个}}\mathrm{6个}\right\}$双胞胎，可以通过向错和之间的反应形成$\left\{10\overline{\mathrm{1个}}\mathrm{2个}\right\}$镁合金中的孪晶。基于变形空间的对称性，我们证明了$\left\{11\overline{\mathrm{2个}}\mathrm{6个}\right\}$twin 源于相关变形路径的​​相互连接，可提供沿 c 轴 5.2% 的拉伸应变和沿 c 轴 4.5% 的压应变$〈11\overline{\mathrm{2个}}0〉$.$\left\{11\overline{\mathrm{2个}}\mathrm{6个}\right\}$双胞胎可以通过三种形成机制形成，即直接形成、孪生孪生交叉和双孪生，这些机制通过实验表征和相场模拟的组合得到验证。我们的工作不仅提出了一种基于对称性的方法来分析孪生诱导的塑性，而且还为镁合金的变形机制和机械性能提供了新的见解。