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Twin boundary (TB) strengthening in nanotwinned metals experiences a breakdown below a critical spacing at which softening takes over. Here, we survey a range of nanotwinned materials that possess different stacking fault energies (SFEs) and understand the TB strengthening limit using atomistic simulations. Distinct from Cu and Al, the nanotwinned, ultralow SFE materials (Co, NiCoCr, and NiCoCrFeMn) intriguingly exhibit a continuous strengthening down to a twin thickness of 0.63 nm. Examining dislocation slip mode and deformation microstructure, we find the hard dislocation modes persist even when reducing the twin boundary spacing to a nanometer regime. Meanwhile, the soft dislocation mode, which causes detwinning in Cu and Al, results in phase transformation and lamellar structure formation in Co, NiCoCr, and NiCoCrFeMn. This study, providing an enhanced understanding of dislocation mechanism in nanotwinned materials, demonstrates the potential for controlling mechanical behavior and ultimate strength with broadly tunable composition and SFE, especially in multi-principal element alloys.

Mechanical properties of glassy nanowires have been intensively investigated recently by both nanomechanical experiments and atomic-level simulations. Unfortunately, there exists a huge gap in the strain rate of the nanomechanical tests between experiments and simulations, which makes it difficult to compare results even for the same material system. Using accelerated atomistic simulations based on a self-learning metabasin escape algorithm, here, we report the tensile mechanical properties of amorphous Stillinger–Weber silicon nanowires with different intrinsic ductility under strain rates ranging from 1010 to 10−1 s−1. It is found that both brittle and ductile glassy silicon nanowires display weakened strength with a decreasing strain rate, in agreement with the cooperative shear model. Moreover, as the strain rate decreases, the amount of plasticity remains unchanged for the brittle nanowires, yet it decreases for the ductile ones. Such deteriorated plasticity in ductile glassy nanowires is caused by enhanced strain localization at low strain rates. Lastly, we show that via the distance matrix of nonaffine displacement, a more hierarchical potential energy landscape is responsible for the higher strain localization propensity in ductile silicon glassy nanowires.

Single molecule experiments and simulations have been widely used to characterize the unfolding and folding pathways of different proteins. However, with few exceptions, these tools have not been applied to study prion protein, PrP(C), whose misfolded form PrP(Sc) can induce a group of fatal neurodegenerative diseases. Here, we apply novel atomistic modeling based on potential energy surface exploration to study the constant force unfolding of human PrP at time scales inaccessible with standard molecular dynamics. We demonstrate for forces around 100 pN, prion forms a stable, three-stranded β-sheet-like intermediate configuration containing residues 155-214 with a lifetime exceeding hundreds of nanoseconds. A mutant without the disulfide bridge shows lower stability during the unfolding process but still forms the three-stranded structure. The simulations thus not only show the atomistic details of the mechanically induced structural conversion from the native α-helical structure to the β-rich-like form but also lend support to the structural theory that there is a core of the recombinant PrP amyloid, a misfolded form reported to induce transmissible disease, mapping to C-terminal residues ≈160-220.

We utilize density functional theory to calculate the edge energy and edge stress for monolayer MoS$_{2}$ nanoribbons. In contrast to previous reports for graphene, for both armchair and zigzag chiralities, the edge stresses for MoS$_{2}$ nanoribbons are found to be tensile, indicating that their lowest energy configuration is one of compression in which Mo-S bond lengths are shorter than those in a bulk, periodic MoS$_{2}$ monolayer. The edge energy and edge stress is found to converge for both chiralities for nanoribbon widths larger than about 1 nm., Comment: 10 pages, 4 figures