期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：14
DOI：10.1073/pnas.2122218119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Nacre exhibits outstanding mechanical performance, which results from coordinated deformation mechanisms synergistically working in characteristic microstructures at multiple length scales. A comprehensive understanding of crystal defects within aragonite is critical for discussing the deformation behavior of nacre on microstructure at the nanoscale through atomic scale. By integrating aberration-corrected transmission electron microscopy, crystallographic analysis, and theoretical calculations, we reveal various crystal defects within aragonite at atomic scale and discuss their potential effects on deformation. Our work will serve as cornerstones for modeling analysis and in-depth discussions on nanoscale deformation mechanisms within nacre. Additionally, these atomic-scale insights will benefit theoretical evaluation of the environmental effect on defect formation, enabling defect control in synthetic aragonite and designing of stronger and tougher bioengineering materials.
Knowledge of deformation mechanisms in aragonite, one of the three crystalline polymorphs of CaCO
3, 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.
