Transition of deformation mechanisms in nanotwinned single crystalline SiC

The ability to experimentally synthesise ceramic materials to incorporate nanotwinned microstructures can drastically affect the underlying deformation mechanisms and mechanics through the complex interaction between stress state, crystallographic orientation, and twin orientation. In this study, molecular dynamics simulations are used to examine the transition in deformation mechanisms and mechanical responses of nanotwinned zinc-blende SiC ceramics subjected to different stress states (uniaxial compressive, uniaxial tensile, and shear deformation) by employing various twin spacings and loading/crystallographic orientations in nanotwinned structures, as compared to their single crystal counterparts. The simulation results show that different combinations of stress states and crystal/twin orientation, and twin spacing trigger different deformation mechanisms: (i) shear localised deformation and shear-induced fracture, preceded by point defect formation and dislocation slip, in the vicinity of the twin lamellae, shear band formation, and dislocation (emission) avalanche; (ii) cleavage and fracture without dislocation plasticity, weakening the nanotwinned ceramics compared to their twin-free counterpart; (iii) severe localised deformation, generating a unique zigzag microstructure between twins without any structural phase transformations or amorphisation, and (iv) atomic disordering localised in the vicinity of coherent twin boundaries, triggering dislocation nucleation and low shearability compared to twin-free systems.

通过实验合成含纳米孪晶微观结构的陶瓷材料，可借助应力状态、晶体学取向与孪晶取向间的复杂相互作用，显著调控材料内在的变形机制与力学行为。本研究借助分子动力学模拟（molecular dynamics simulations）方法，针对闪锌矿型（zinc-blende）纳米孪晶碳化硅陶瓷，通过调控其纳米孪晶结构中的孪晶间距、加载方式与晶体学取向，考察其在不同应力状态（单轴压缩、单轴拉伸与剪切变形）下的变形机制转变与力学响应，并与对应单晶体系进行对比。

模拟结果表明，应力状态、晶体/孪晶取向与孪晶间距的不同组合会触发不同的变形机制：
(i) 孪晶片层附近先发生点缺陷形成与位错滑移，随后出现剪切局域化变形与剪切诱导断裂，伴随剪切带形成与位错（发射）雪崩现象；
(ii) 未经历位错塑性即发生解理与断裂，相较于无孪晶体系，该变形模式会削弱纳米孪晶陶瓷的力学性能；
(iii) 发生剧烈局域化变形，在孪晶之间形成独特的锯齿状微观结构，且未发生任何结构相变或非晶化；
(iv) 原子无序化仅局限于共格孪晶界附近，相较于无孪晶体系，该情况会触发位错形核并降低材料的剪切可变形性。