Ductile and brittle crack-tip response in equimolar refractory high-entropy alloys

Abstract

Understanding the strengthening and deformation mechanisms in refractory high-entropy alloys (HEAs), proposed as new high-temperature material, is required for improving their typically insufficient room-temperature ductility. Here, density-functional theory simulations and a continuum mechanics analysis were conducted to systematically investigate the competition between cleavage decohesion and dislocation emission from a crack tip in the body-centered cubic refractory HEAs HfNbTiZr, MoNbTaVW, MoNbTaW, MoNbTiV, and NbTiVZr. This crack-tip competition is evaluated for tensile loading and a totality of 15 crack configurations and slip systems. Our results predict that dislocation plasticity at the crack tip is generally unfavorable -although the competition is close for some crack orientations, suggesting intrinsic brittleness and low crack-tip fracture toughness in these five HEAs at zero temperature. Fluctuations in local alloy composition, investigated for HfNbTiZr, can locally reduce the resistance to dislocation emission for a slip system relative to the configuration average of that slip system, but do not change the dominant cracktip response. In the case of single-crystal MoNbTaW, where an experimental, room-temperature fracture-toughness value is available for a crack on a {100} plane, theoretical and experimental results agree favorably. Factors that may limit the agreement are discussed. We survey the effect of material anisotropy on preferred crack tip orientations, which are found to be alloy specific. Mixedmode loadings are found to shift the competition in favor of cleavage or dislocation nucleation, depending on crack configuration and amplified by the effect of material anisotropy on crack tip stresses.

Progress and challenges in the theory and modeling of complex concentrated alloys

“…One approach uses extensive DFT calculations of a defect energy over many compositions and realizations followed by learning of the property trends, as demonstrated 37 for the USFE in bcc HEAs that is important for intrinsic ductility. [36][37][38] Another approach is to develop a similar DFT database to create a machine-learning interatomic potential, from which defect properties can be studied. Potentials are often aimed at the study one or a few properties, such as thermodynamics in the Cantor alloys, 39 the USFE in bcc alloys, 40 or dislocations in bcc MoNbTaW.…”

AI-accelerated materials informatics method for the discovery of ductile alloys

“…In this work, we present one possible application of our method: an efficient parameterization of material models that predict mechanical properties of a random alloy. More precisely, we parameterize the model for room temperature ductility of bcc random alloys from [28,34], which is based on the ratio of the K-factors for dislocation emission and cleavage, for the Mo-Nb-Ta medium-entropy alloy. This model was shown to be in good qualitative agreement with real experiments, and depends on the stacking fault energies on {112} planes, the {110} surface energies, and the elastic constants.…”

A First-Principles Study of Hydrogen Desorption from High Entropy Alloy TiZrVMoNb Hydride Surface

“…However, developing hydrogen storage materials with a low decomposition temperature and pressure (which are well suited for practical purposes), high recyclability, fast kinetics and high hydrogen storage capacity is one of the most crucial difficulties restricting the utilization of hydrogen energy for real applications [5][6][7]. Recently, high entropy alloys (HEAs), which are composed of five or more elements with atomic concentrations of 5-35% and tend to form single-phase solid solutions [8][9][10][11][12][13][14][15], have received much attention in the hydrogen storage field [16][17][18][19]. A number of investigations have suggested that hydrogen storage properties of HEAs can be enhanced by carefully tuning their constituents [17] and an excellent hydrogen storage performance can be achieved, like high hydrogen storage capacity [19,20], reversibly phase transformation [21], rapid hydrogen absorption [22] and moderate absorption/desorption temperature and pressure [23].…”