Minor impurities can cause catastrophic fracture of normally ductile metals. Here, a classic example is represented by the sulfur embrittlement of nickel, whose atomic-level mechanism has puzzled researchers for nearly a century. In this study, coupled aberration-corrected electron microscopy and semi-grand-canonical-ensemble atomistic simulation reveal, unexpectedly, the universal formation of amorphous-like and bilayer-like facets at the same general grain boundaries. Challenging the traditional view, the orientation of the lower-Miller-index grain surface, instead of the misorientation, dictates the interfacial structure. We also find partial bipolar structural orders in both amorphous-like and bilayer-like complexions (a.k.a. thermodynamically two-dimensional interfacial phases), which cause brittle intergranular fracture. Such bipolar, yet largely disordered, complexions can exist in and affect the properties of various other materials. Beyond the embrittlement mechanism, this study provides deeper insight to better understand abnormal grain growth in sulfur-doped Ni, and generally enriches our fundamental understanding of performance-limiting and more disordered interfaces.
First-order interfacial phaselike transformations that break the mirror symmetry of the symmetric ∑5 (210) tilt grain boundary (GB) are discovered by combining a modified genetic algorithm with hybrid Monte Carlo and molecular dynamics simulations. Density functional theory calculations confirm this prediction. This first-order coupled structural and adsorption transformation, which produces two variants of asymmetric bilayers, vanishes at an interfacial critical point. A GB complexion (phase) diagram is constructed via semigrand canonical ensemble atomistic simulations for the first time.
Nacre has attracted widespread interest because its unique hierarchical structure, which is assembled by 95 wt% brittle aragonite and 5 wt% soft organic materials, leads to several orders of improvement in fracture toughness. Apart from the well proposed toughening mechanisms such as mineral bridges and tablets interlocks, the organic materials including biopolymers between tablets and proteins exist within a tablet can also potentially improve the toughness. In this work, we employ a novel approach combining steered molecular dynamics (SMD) and classical molecular dynamics (MD) to build a model of mineral-protein composite to mimic nacre tablet. The critical role of protein in improving the fracture toughness of nacre is investigated for the first time. MD simulations of single crystalline aragonite, polycrystalline aragonite and mineral-protein composite under uniaxial tensile loading are performed, and the obtained constitutive responses are compared with experimental measurements of nacre under tension. It is shown that the fracture toughness of mineral-protein composite is significantly larger than that of single crystalline or polycrystalline aragonite. Detailed atomic configuration analyses reveal that the fracture of individual computer model is governed by its unique failure mechanisms. Dislocation motion and phase transformation are observed during the failure of single crystalline aragonite. Polycrystalline aragonite fails by the inter-granular cleavage, as well as phase transformation within grain. It is surprisingly noted that other than the stretching of protein chains on grain boundaries, intra-granular fracture is triggered in mineral-protein composites. Proteins serve as strong glue between the inorganic nanograins. It is believed that the strong electrostatic interaction between protein and aragonite nanograins, combined with the remarkable plastic ductility of protein lead to the intra-granular failure, which consequently enhance the fracture toughness of the whole specimen.
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