thermomechanical processing routes which introduce high densities of dislocations and interfaces into materials. These defect populations introduce long-range stress fields, enabling substantial strengthening of materials. Thus, processing with the aim to introduce high densities of lattice defects into materials is a wellestablished and efficient method to enhance their strength. [12] Yet, when single-phase metallic materials have few defects such as in the recrystallized state and at the beginning of plastic yielding, they often have insufficient intrinsic lattice friction and thus low flow stress. [13,14] The lattice friction, quantified by the Peierls stress, is a measure of the resistance that an infinite straight dislocation has to overcome when moving from one potential valley to the next. The height of this energy barrier scales with the intrinsic atomic-scale lattice distortions and thus differs profoundly in nature from the long-range stress fields imposed by dense defect populations. In other words, the friction stress describes how severely dislocations are dragged as they move through the distorted Peierls potential landscape of massive solid solutions. In that respect, multiprincipal element solid mixtures, often termed high-or medium-entropy alloys (HEAs or MEAs), provide a very promising material design basis because each individual atom experiences a set of different neighbor atoms creating high and ubiquitous local lattice distortions and stresses. [15,16] In that context, multiprincipal element face-centered cubic (fcc) alloys have the potential for achieving an outstanding yield strength-ductility ratio as they naturally carry high inherent lattice distortions. [15,17,18] However, two key challenges have not been addressed in this context so far. First, the yield strength is low due to the close packed fcc lattice structure. [19][20][21] Second, although the friction stress of these alloys is often higher than in pure metals or binary alloys, it is still usually too close to conventional structural alloys. This means that the level of lattice distortion currently exploited in most substitutional solid solution alloys does not contribute significantly to the yield strength. [20,[22][23][24][25][26][27][28] Here, based on a combined theoretical and experimental approach, we show that the degree of lattice distortion is indeed a key parameter in controlling strengthening mechanisms for the design of hitherto unexplored ultrastrong medium-entropy single-phase alloys. For this purpose, we exploit vanadium (V) as a very efficient element in a Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength values of medium-and high-entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium-entropy alloy exhibits a near 1 GP...

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High-entropy alloys are solid solutions of multiple principal elements that are capable of reaching composition and property regimes inaccessible for dilute materials. Discovering those with valuable properties, however, too often relies on serendipity, because thermodynamic alloy design rules alone often fail in high-dimensional composition spaces. We propose an active learning strategy to accelerate the design of high-entropy Invar alloys in a practically infinite compositional space based on very sparse data. Our approach works as a closed-loop, integrating machine learning with density-functional theory, thermodynamic calculations, and experiments. After processing and characterizing 17 new alloys out of millions of possible compositions, we identified two high-entropy Invar alloys with extremely low thermal expansion coefficients around 2 × 10 −6 per degree kelvin at 300 kelvin. We believe this to be a suitable pathway for the fast and automated discovery of high-entropy alloys with optimal thermal, magnetic, and electrical properties.

Understanding the composition evolution of grain boundaries and grain boundary precipitation at near-atomic scale in aluminum alloys is crucial to tailor mechanical properties and to increase resistance to corrosion and stress corrosion cracking. Here, we elucidate the sequence of precipitation on grain boundaries in comparison to the bulk in a model Al-Zn-Mg-Cu alloy. We investigate the material from the solution heat treated state (475°C), through the very early stages of aging to the peak aged state at 120°C and further into the overaged regime at 180°C. The process starts with solute enrichment on grain boundaries due to equilibrium segregation accompanied by solute depletion in their vicinity, the formation of Guinier-Preston (GP) zones in the solute-enriched grain boundary regions, and GP zones growth and transformation. The equilibrium segregation of solutes to grain boundaries during aging accelerates this sequence compared to the bulk. Analysis of the ~10 nm wide precipitate-free zones (PFZs) adjacent to the solute-enriched grain boundaries shows that the depletion zones are determined by (i) interface equilibrium segregation; (ii) formation and coarsening of the grain boundary precipitates and (iii) the diffusion range of solutes in the matrix. In addition, we quantify the difference in kinetics between grain boundary and bulk precipitation. The precipitation kinetics, as observed in terms of volume fraction, average radius, and number density, is almost identical next to the depletion zone in the bulk and far inside the bulk grain remote from any grain boundary influence. This observation shows that the region influenced by the grain boundaries does not extend beyond the PFZs.