The impact of alloying on defect-free nanoparticles exhibiting softer but tougher behavior.

The classic paradigm of physical metallurgy is that the addition of alloying elements to metals increases their strength. It is less known if the solution-hardening can occur in nano-scale objects, and it is totally unknown how alloying can impact the strength of defect-free faceted nanoparticles. Purely metallic defect-free nanoparticles exhibit an ultra-high strength approaching the theoretical limit. Tested in compression, they deform elastically until the nucleation of the first dislocation, after which they collapse into a pancake shape. Here, we show by experiments and atomistic simulations that the alloying of Ni nanoparticles with Co reduces their ultimate strength. This counter-intuitive solution-softening effect is explained by solute-induced local spatial variations of the resolved shear stress, causing premature dislocation nucleation. The subsequent particle deformation requires more work, making it tougher. The emerging compromise between strength and toughness makes alloy nanoparticles promising candidates for applications.

Structural phase transformations in metallic grain boundaries.

Structural transformations at interfaces are of profound fundamental interest as complex examples of phase transitions in low-dimensional systems. Despite decades of extensive research, no compelling evidence exists for structural transformations in high-angle grain boundaries in elemental systems. Here we show that the critical impediment to observations of such phase transformations in atomistic modelling has been rooted in inadequate simulation methodology. The proposed new methodology allows variations in atomic density inside the grain boundary and reveals multiple grain boundary phases with different atomic structures. Reversible first-order transformations between such phases are observed by varying temperature or injecting point defects into the boundary region. Owing to the presence of multiple metastable phases, grain boundaries can absorb significant amounts of point defects created inside the material by processes such as irradiation. We propose a novel mechanism of radiation damage healing in metals, which may guide further improvements in radiation resistance of metallic materials through grain boundary engineering.

Relationship between equilibrium fluctuations and shear-coupled motion of grain boundaries.

We derive general analytical expressions relating the equilibrium fluctuations of a grain boundary to key parameters governing its motion coupled to shear deformation. We validate these expressions by molecular dynamics simulations for symmetrical tilt boundaries and demonstrate how they can be used to extract the misorientation dependence of the grain-boundary mobility. The results shed light on fundamental relationships between equilibrium and nonequilibrium grain-boundary properties and provide new means to predict those properties.