密排六方金属中的孪生及孪晶位错机制

密排六方晶体结构金属中可同时启动的滑移系少，孪生成为密排六方金属中重要的塑性变形形式．由于密排六方金属复杂的晶体结构，均匀切变不能保证所有晶格点都能与基体形成对称的晶体结构，因此密排六方金属的孪生通常为滑移和原子重组（shuffle）机制相结合．本文以密排六方金属中常见的{101 ?2}、{101 ?1}、{112 ?2}及{102 ?1}孪生为例，阐述不同类型孪生过程中的孪晶位错机制．分析表明，由于原子重组机制的参与，密排六方金属的孪生可以通过不同形式的孪晶位错实现．以上四种密排六方金属孪晶中，只有{112 ?2}孪生中的一层孪晶位错是纯剪切机制，其余的孪生机制都需要原子重组的参与．孪生机制可以大致分为滑移主导、原子重组主导以及滑移-重组相结合的机制．当孪生类型确定时，即第一不畸变面（孪晶面）k_1（和孪晶剪切方向η_1）确定时，不同孪晶位错机制对应的孪晶剪切大小和方向均不同，第二不畸变面k_2和共轭剪切方向η_2也不相同，所导致孪晶的拉压性质也不同．不同剪切方向和大小的孪晶位错机制有可能在不同应力和温度条件下被激活，从而作为密排六方金属塑性的重要来源．

Mechanisms of Twinning and Twinning Dislocations in hcp Metals

For metals with the hexagonal close-packed (hcp) crystal structure, deformation twinning plays an important role in their plastic deformation. Due to the complex lattice structures of hcp metals, the atomic motions in twinning are very complex because not all of the atoms move in the direction of twinning shear. Therefore, the glide-shuffle mechanisms associated with the motion of twinning dislocations (TDs) are responsible for the twinning in hcp metals. In this paper, we focus on the mechanisms of TDs for different types of twinning, such as {101 ?2}, {101 ?1},{112 ?2}, and {102 ?1}. It is revealed that the lattice shuffles are needed for almost all hcp twinning mechanisms, except for the one-layer TD in {112 ?2} twinning. This one-layer TD can be achieved by pure shear via the glide of partial dislocations along the twinning plane. Therefore, the twinning mechanisms in hcp metals can be classified as the glide-dominated, shuffle-dominated and glide-shuffle mechanisms. For a twinning with a determined invariant plane k_1 (and the shear direction η_1), different types of TDs with different shears and shuffles can be involved, while the second undistorted plane k_2 and the conjugate shear direction η_2 are altered accordingly, resulting in the corresponding tensile or compressive deformation of twinning. The prediction of possible twinning modes is based on the hypothesis that both the twinning shear and the shuffle magnitudes should be small. The {101 ?2} twinning is the most easily activated twinning mode in hcp metals due to its small twinning shear and atomic shuffles. Furthermore, the activation of various twinning mechanisms under specific temperature and stress conditions can be responsible for the improvement of plasticity in hcp metals and alloys. The comprehensive understanding of twinning mechanisms will be helpful for further experimental and theoretical exploration on designing alternative and more innovative hcp-type structural materials with superior mechanical properties.