Zhiliang Pan

Uncovering high-strain rate protection mechanism in nacre

Under high-strain-rate compression (strain rate ∼103 s−1), nacre (mother-of-pearl) exhibits surprisingly high fracture strength vis-à-vis under quasi-static loading (strain rate 10−3 s−1). Nevertheless, the underlying mechanism responsible for such sharply different behaviors in these two loading modes remains completely unknown. Here we report a new deformation mechanism, adopted by nacre, the best-ever natural armor material, to protect itself against predatory penetrating impacts. It involves the emission of partial dislocations and the onset of deformation twinning that operate in a well-concerted manner to contribute to the increased high-strain-rate fracture strength of nacre. Our findings unveil that Mother Nature delicately uses an ingenious strain-rate-dependent stiffening mechanism with a purpose to fight against foreign attacks. These findings should serve as critical design guidelines for developing engineered body armor materials.

Manipulating the interfacial structure of nanomaterials to achieve a unique combination of strength and ductility

The control of interfaces in engineered nanostructured materials has met limited success compared with that which has evolved in natural materials, where hierarchical structures with distinct interfacial states are often found. Such interface control could mitigate common limitations of engineering nanomaterials. For example, nanostructured metals exhibit extremely high strength, but this benefit comes at the expense of other important properties like ductility. Here, we report a technique for combining nanostructuring with recent advances capable of tuning interface structure, a complementary materials design strategy that allows for unprecedented property combinations. Copper-based alloys with both grain sizes in the nanometre range and distinct grain boundary structural features are created, using segregating dopants and a processing route that favours the formation of amorphous intergranular films. The mechanical behaviour of these alloys shows that the trade-off between strength and ductility typically observed for metallic materials is successfully avoided here.

Effect of grain boundary character on segregation-induced structural transitions

Segregation-induced structural transitions in metallic grain boundaries are studied with hybrid atomistic Monte Carlo/molecular dynamics simulations using Cu-Zr as a model system, with a specific emphasis on understanding the effect of grain boundary character. With increasing global composition, the six grain boundary types chosen for this study first form ordered complexions, with the local segregation pattern depending on the grain boundary core structure, then transform into disordered complexions when the grain boundary composition reaches a critical value that is temperature dependent. The tendency for this transition to a disordered interfacial structure consistently depends on the relative solute excess, instead of the grain boundary energy or misorientation angle. Grain boundaries with high relative solute excess go through gradual disordering transitions, whereas those with low relative solute excess remain ordered until high global Zr concentrations but then abruptly transform into thick disordered films. The results presented here provide a clear picture of the effect of interface character on both dopant segregation patterns and disordered intergranular film formation, showing that all grain boundaries are not equal when discussing complexion transitions.

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.

Kohn–Sham density functional theory prediction of fracture in silicon carbide under mixed mode loading

The utility of silicon carbide (SiC) for high temperature structural application has been limited by its brittleness. To improve its ductility, it is paramount to develop a sound understanding of the mechanisms controlling crack propagation. In this manuscript, we present direct ab initio predictions of fracture in SiC under pure mode I and mixed mode loading, utilizing a Kohn–Sham Density Functional Theory (KSDFT) framework. Our results show that in both loading cases, cleavage occurs at a stress intensity factor (SIF) only slightly higher than the Griffith toughness, focusing on a (1 1 1) crack in the 3C-SiC crystal structure. This lattice trapping effect is shown to decrease with mode mixity, due to the formation of a temporary surface bond that forms during decohesion under shear. Comparing the critical mode I SIF to the value obtained in experiments suggests that some plasticity may occur near a crack tip in SiC even at low temperatures. Ultimately, these findings provide a solid foundation upon which to study the influence of impurities on brittleness, and upon which to develop empirical potentials capable of realistically simulating fracture in SiC.

Mechanisms of near-surface structural evolution in nanocrystalline materials during sliding contact

The wear-driven structural evolution of nanocrystalline Cu was simulated with molecular dynamics under constant normal loads, followed by a quantitative analysis. While the microstructure far away from the sliding contact remains unchanged, grain growth accompanied by partial dislocations and twin formation was observed near the contact surface, with more rapid coarsening promoted by higher applied normal loads. The structural evolution continues with increasing number of sliding cycles and eventually saturates to a stable distinct layer of coarsened grains, separated from the finer matrix by a steep gradient in grain size. The coarsening process is balanced by the rate of material removal when the normal load is high enough. The observed structural evolution leads to an increase in hardness and decrease in friction coefficient, which also saturate after a number of sliding cycles. This work provides important mechanistic understanding of nanocrystalline wear, while also introducing a methodology for atomistic simulations of cyclic wear damage under constant applied normal loads.