Unconventional {101¯2} twinning assisted by pyramidal II stacking faults

Twinning significantly affects the deformation behavior of hexagonal close-packed Mg, so a thorough understanding of twin nucleation and growth mechanisms is required for enhancing the properties of Mg-based materials. The commonly observed {101¯2} tension twins have been traditionally linked to 〈c + a〉 dislocation dissociation, which results in zonal dislocations with large Burgers vectors several times that of a single twinning dislocation and some residual dislocations. Contrarily, our molecular dynamics simulations reveal {101¯2} twin nucleation from pyramidal II stacking faults through atomic shuffling without shear displacements. This introduces an alternative twin nucleation mechanism, different from the classically accepted mechanism of dislocation dissociation. Unconventional {101¯2} twin nucleation from pyramidal II stacking faults via atomic shuffling was observed, contradicting the proposed nucleation from pyramidal dislocation dissociation and providing design solutions for hcp metals.

孪晶（Twinning）对六方密排（hexagonal close-packed, hcp）镁的变形行为具有显著影响，因此若要优化镁基材料的性能，需全面阐明孪晶形核与生长机制。学界普遍观测到的{10-12}拉伸孪晶（{101¯2} tension twins）传统上被认为与〈c+a〉位错（〈c+a〉 dislocation）分解相关，该过程会产生伯格斯矢量（Burgers vector）为数倍单根孪晶位错的带状位错与部分残余位错。与之相反，本研究的分子动力学（molecular dynamics）模拟结果显示，{10-12}拉伸孪晶可通过原子洗牌（atomic shuffling）过程从锥面Ⅱ型层错（pyramidal II stacking faults）形核，且该过程不存在剪切位移。这一发现提出了一种区别于经典公认位错分解机制的全新孪晶形核路径。本研究观测到的基于锥面Ⅱ型层错、经由原子洗牌过程形成的非常规{10-12}拉伸孪晶形核现象，与此前提出的锥面位错分解形核理论相悖，同时为六方密排金属的性能优化提供了新的设计思路。