Simultaneously enhancing strength and ductility of metals and alloys has been a tremendous challenge. Here, we investigate a CoCuFeNiPd high-entropy alloy (HEA), using a combination of Monte Carlo method, molecular dynamic simulation, and density-functional theory calculation. Our results show that this HEA is energetically favorable to undergo short-range ordering (SRO), and the SRO leads to a pseudo-composite microstructure, which surprisingly enhances both the ultimate strength and ductility. The SRO-induced composite microstructure consists of three categories of clusters: face-center-cubic-preferred (FCCP) clusters, indifferent clusters, and body-center-cubic-preferred (BCCP) clusters, with the indifferent clusters playing the role of the matrix, the FCCP clusters serving as hard fillers to enhance the strength, while the BCCP clusters acting as soft fillers to increase the ductility. Our work highlights the importance of SRO in influencing the mechanical properties of HEAs and presents a fascinating route for designing HEAs to achieve superior mechanical properties.

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Nanostructures are technological devices constructed on a nanometer length scale more than a thousand times thinner than a human hair. Due to the unique properties of matter at this scale, such devices offer great potential for creating novel materials and behaviors that can be leveraged to benefit mankind. This paper addresses a particular challenge involved in the design of nanostructures-their stochastic or apparently random response to external loading. This is because fundamentally the function that relates the energy of a nanostructure to the arrangement of its atoms is extremely nonconvex, with each minimum corresponding to a possible equilibrium state that may be visited as the system responds to loading. Traditional atomistic simulation techniques are not capable of systematically addressing this complexity. Instead, we construct an equilibrium map (EM) for the nanostructure, analogous to a phase diagram for bulk materials, which fully characterizes its response. Using the EM, definitive predictions can be made in limiting cases and the spectrum of responses at any desired loading rate can be obtained. The latter is important because standard atomistic methods are fundamentally limited, by computational feasibility, to simulations of loading rates that are many orders of magnitude faster than reality. In contrast, the EMbased approach makes possible the direct simulation of nanostructure experiments. We demonstrate the method's capabilities and its surprisingly complex results for the case of a nanoslab of nickel under compression.nonconvexity | lattice statics | continuation | bifurcation | stability T he current revolution in nanotechnology promises to bring great benefits to mankind. Much of the excitement focuses on the creation of nanostructures and nanodevices that, due to their small size, have unique properties relative to macroscopic bulk materials. However, a challenge in harnessing such properties is that nanostructures can behave in a stochastic or apparently random manner when loaded. Thermal and mechanical fluctuations can cause nanostructures to morph from one deformed structure to another and nominally identical loadings can lead to many different response sequences. An example of this is the uniaxial compression of nanopillars that exhibit stochastic behavior when internal defects are nucleated (1). This indeterminacy in response stems from the fact that for a given state of external loading there are a great many ways for the atoms comprising the nanostructure to arrange internally. Mathematically, this can be understood by considering the dependence of the system's potential energy E on the positions of the atoms u and a scalar λ representing the applied loading, i.e., E = Eðu; λÞ. The position of every atom has three coordinates, therefore u has n = 3N components (or "degrees of freedom") less those fixed by imposed constraints. For a given value of λ, one can imagine plotting this function in an n + 1-dimensional space (this is of course not practically possible). [At most one can v...

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Body-centred-cubic (BCC) transition metals (TMs) tend to be brittle at low temperatures, posing significant challenges in their processing and major concerns for damage tolerance in critical load-carrying applications. The brittleness is largely dictated by the screw dislocation core structure; the nature and control of which has remained a puzzle for nearly a century. Here, we introduce a universal model and a physics-based material index χ that guides the manipulation of dislocation core structure in all pure BCC metals and alloys. We show that the core structure, commonly classified as degenerate (D) or non-degenerate (ND), is governed by the energy difference between BCC and face-centred cubic (FCC) structures and χ robustly captures this key quantity. For BCC TMs alloys, the core structure transition from ND to D occurs when χ drops below a threshold, as seen in atomistic simulations based on nearly all extant interatomic potentials and density functional theory (DFT) calculations of W-Re/Ta alloys. In binary W-TMs alloys, DFT calculations show that χ is related to the valence electron concentration at low to moderate solute concentrations, and can be controlled via alloying. χ can be quantitatively and efficiently predicted via rapid, low-cost DFT calculations for any BCC metal alloys, providing a robust, easily applied tool for the design of ductile and tough BCC alloys.