Xiao Jianwei, Li Songwei, Ma Xiaoxiao, Gao Junjie, Deng Chuang, Wu Zhaoxuan, Zhu Yuntian
Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong, China.
Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB R3T 5V6, Canada.
Phys Rev Lett. 2023 Sep 29;131(13):136101. doi: 10.1103/PhysRevLett.131.136101.
Twinning is profuse in bcc transition metals (TMs) except bulk W and Mo. However, W and Mo nanocrystals surprisingly exhibit twinning during room temperature compression, which is completely unexpected as established nucleation mechanisms are not viable in them. Here, we reveal the physical origin of deformation twinning in W and Mo. We employ density functional theory (DFT) and a reduced-constraint slip method to compute the stress-dependent generalized stacking fault enthalpy (GSFH), the thermodynamic quantity to be minimized under constant loading. The simple slipped structures and GSFH lines show that compressive stresses stabilize a two-layer twin embryo, which can grow rapidly via twinning disconnections with negligible energy barriers. Direct atomistic simulations unveil the explicit twinning path in agreement with the DFT GSFH lines. Twinning is thus the preferred deformation mechanism in W and Mo when shear stresses are coupled with high compressive stresses. Furthermore, twinnability can be related to the elastic constants of a stacking fault phase (SFP). The hcp phase may serve as a candidate SFP for the {112}⟨1[over ¯]1[over ¯]1⟩ twinning system in bcc TMs and alloys, which is coincident with the {111}⟨112[over ¯]⟩ twinning in fcc structures.
除了块状钨（W）和钼（Mo）之外，孪晶在体心立方（bcc）过渡金属（TMs）中很常见。然而，令人惊讶的是，W和Mo纳米晶体在室温压缩过程中会出现孪晶，这完全出乎意料，因为既定的成核机制在它们身上并不适用。在这里，我们揭示了W和Mo中变形孪晶的物理起源。我们采用密度泛函理论（DFT）和一种简化约束滑移方法来计算应力相关的广义堆垛层错能（GSFH），这是在恒定载荷下要最小化的热力学量。简单的滑移结构和GSFH线表明，压缩应力使两层孪晶胚胎稳定，该胚胎可以通过具有可忽略能量势垒的孪晶解离快速生长。直接的原子模拟揭示了与DFT GSFH线一致的明确孪晶路径。因此，当剪应力与高压缩应力耦合时，孪晶是W和Mo中首选的变形机制。此外，孪晶能力可能与堆垛层错相（SFP）的弹性常数有关。六方密排（hcp）相可能是bcc TMs和合金中{112}⟨1[反]1[反]1⟩孪晶系统的候选SFP，这与面心立方（fcc）结构中的{111}⟨112[反]⟩孪晶一致。
