Hebei Key Laboratory of Optic-electronic Information and Materials, The College of Physics Science and Technology, Hebei University, Baoding 071002, China.
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.
Proc Natl Acad Sci U S A. 2022 Apr 5;119(14):e2122218119. doi: 10.1073/pnas.2122218119. Epub 2022 Mar 31.
Knowledge of deformation mechanisms in aragonite, one of the three crystalline polymorphs of CaCO3, is essential to understand the overall excellent mechanical performance of nacres. Dislocation slip and deformation twinning were claimed previously as plasticity carriers in aragonite, but crystallographic features of dislocations and twins have been poorly understood. Here, utilizing various transmission electron microscopy techniques, we reveal the atomic structures of twins, partial dislocations, and associated stacking faults. Combining a topological model and density functional theory calculations, we identify complete twin elements, characters of twinning disconnection, and the corresponding twin shear angle (∼8.8°) and rationalize unique partial dislocations as well. Additionally, we reveal an unreported potential energy dissipation mode within aragonite, namely, the formation of nanograins via the pile-up of partial dislocations. Based on the microstructural comparisons of biogenic and abiotic aragonite, we find that the crystallographic features of twins are the same. However, the twin density is much lower in abiotic aragonite due to the vastly different crystallization conditions, which in turn are likely due to the absence of organics, high temperature and pressure differences, the variation in inorganic impurities, or a combination thereof. Our findings enrich the knowledge of intrinsic crystal defects that accommodate plastic deformation in aragonite and provide insights into designing bioengineering materials with better strength and toughness.
了解方解石三种晶型之一的文石的变形机制对于理解珍珠母整体优异的机械性能至关重要。先前曾有人声称位错滑移和变形孪晶是文石的塑性载体，但位错和孪晶的晶体学特征却知之甚少。在这里，我们利用各种透射电子显微镜技术揭示了孪晶、部分位错及其相关堆垛层错的原子结构。结合拓扑模型和密度泛函理论计算，我们确定了完整的孪晶单元、孪晶断开的特征以及相应的孪晶切变角（约 8.8°），并合理地解释了独特的部分位错。此外，我们还揭示了文石中一种未被报道的潜在能量耗散模式，即通过部分位错的堆积形成纳米晶粒。基于生物成因和非生物成因文石的微观结构比较，我们发现孪晶的晶体学特征相同。然而，由于结晶条件的巨大差异，非生物成因文石中的孪晶密度要低得多，这可能是由于缺乏有机物、高温和压力差异、无机杂质的变化或它们的组合所致。我们的发现丰富了文石中容纳塑性变形的内在晶体缺陷的知识，并为设计具有更好强度和韧性的生物工程材料提供了思路。
