Atomistic Origin of Deformation Twinning in Biomineral Aragonite

Deformation twinning rarely occurs in mineral materials which typically show brittle fracture. Surprisingly, it has recently been observed in the biomineral aragonite phase in nacre under high rate impact loading. In this Letter, the twinning tendency and the competition between fracture and deformation twinning were revealed by first principles calculations. The ratio of the unstable stacking fault energy and the stacking fault energy in orthorhombic aragonite is hitherto the highest in a broad range of metallic and oxide materials. The underlining physics for this high ratio is the multineighbor shared ionic bonds and the unique relaxation process during sliding in the aragonite structure. Overall, the unique deformation twining along with other highly coordinated deformation mechanisms synergistically work in the hierarchical structure of nacre, leading to the remarkable strengthening and toughening of nacre upon dynamic loading, and thus protecting the mother-of-pearl from predatory attacks.

Figure 1

Nacre’s extraordinary strengthening and toughening and its multifunctional deformation mechanisms. (a) The normalized toughness performance of different materials under high strain rate ($\sim {10}^3$) and low strain rate ($\sim {10}^{-3}$). Nacre show dramatic toughness increase over metals, polymers, and even traditional composites under high deformation rate. (b) The deformation mechanisms that can be activated in the atomic level. When deformation rate is high, only the mechanisms illustrated in gray platelets can be activated.

Figure 2

The atomic structure of deformation twinning (a) upon impact the deformation twinning or stacking fault takes over the deformation process, rather than the full-dislocation slipping [10]. (b) The atomic-scale measurement of the white box area in (a) clearly demonstrates the twinning relationship between the (110) and $(1\overline{1}0)$ planes. (c) The atomic eight-layer slab model is built according to (b) and is shown in projections along y and x directions, respectively.

Figure 3

The computed generalized stacking fault energy (GSFE) landscape and the fracture energy of aragonite. The USFE is higher than the fracture energy during rigid sliding, but drops below the fracture energy when layer expansion is allowed. Moreover, the USFE/SFE ratio is the highest in a broad range of metallic and oxide materials (as shown in Table 1).

Figure 4

Structural origin of different USFE/SFE ratio. The Perfect slab, USFE and SFE structures of (a) aragonite, (c) Cu, and (d) ${\mathrm{SrTiO}}_3$ show the lattice structure and Burger’s vector combination leads to a high USFE/SFE ratio. The orange line indicates the slipping plane. (b) Detailed analysis of one diamond shaped crystal cells that indicate ${\mathrm{Ca}}^{2+}$ prefers to bond with ${\mathrm{CO}}_3^{2-}$ in the short diagonal. In the fault plane, the equal tendency of jumping to two sites (crosses) leads to equalization of the ${\mathrm{CO}}_3\text{−}\mathrm{Ca}$ bond. The triangles are ${\mathrm{CO}}_3^{2-}$ groups, and dots in the square and round are ${\mathrm{Ca}}^{2+}$. Different transparency indicates different depth.