A grand challenge in material science is to understand the correlation between intrinsic properties and defect dynamics. Radiation tolerant materials are in great demand for safe operation and advancement of nuclear and aerospace systems. Unlike traditional approaches that rely on microstructural and nanoscale features to mitigate radiation damage, this study demonstrates enhancement of radiation tolerance with the suppression of void formation by two orders magnitude at elevated temperatures in equiatomic single-phase concentrated solid solution alloys, and more importantly, reveals its controlling mechanism through a detailed analysis of the depth distribution of defect clusters and an atomistic computer simulation. The enhanced swelling resistance is attributed to the tailored interstitial defect cluster motion in the alloys from a long-range one-dimensional mode to a short-range three-dimensional mode, which leads to enhanced point defect recombination. The results suggest design criteria for next generation radiation tolerant structural alloys.

Interactions of impurity interstitial atoms with dislocations may play an important role in strengthening of structural materials as well as in other phenomena like hydrogen embrittlement. Impurity interstitial atoms occupy octahedral (typical for carbon) or tetrahedral (typical for hydrogen) sites in bcc metals, both showing solely tetragonal symmetry of three types distinguished by orientation with respect to the main crystallographic (1, 2, 3) directions. Placing an atom at one of such sites one of the three types of local multiaxial eigenstrain states is provoked. This eigenstrain state interacts with the stress field of the dislocation, and the corresponding interaction term provides a mechanical part of the generalized chemical potential of the interstitial component. For a proper description of the distribution of the interstitial component in the stressed lattice three site fractions X-1, X-2 and X-3, corresponding to the types of sites, are necessary. Interstitials like hydrogen and carbon are mobile even at room temperature and diffuse spontaneously to places with the lowest interaction energy. As the individual types of interstitial sites are interconnected by fast diffusion paths, one can assume local equilibrium among atoms occupying the three types of interstitial sites. This fact is expressed by a unique value of their generalized chemical potential. The gradient of this potential drives diffusion of interstitial atoms. At any time the atoms redistribute locally to the three types of sites according to the condition of local equilibrium. Thus, generally in the stressed bcc lattice the three site fractions X-1, X-2 and X-3 have different values and provoke a multiaxial eigenstrain state resulting in an eigenstress state. Hence, the stress field of the dislocation is superposed by the eigenstress state. This may cause a significant relaxation (i.e. reduction) of the total stress state depending on the amount of the interstitial atoms in the system and their diffusion kinetics. Very recently, the individual diffusion paths of interstitial atoms occupying octahedral or tetrahedral positions in a stressed bcc lattice have been identified. The respective model predicts a significant anisotropy of diffusion caused by the differences in local values of X-1, X-2 and X-3 in addition to the standard elastodiffusion due to deformation of the lattice. This leads to a significant modification of the standard diffusion equation, because the introduced "diffusion occupancy factors" are directly dependent on the values of X-1, X-2 and X-3. Assuming a straight dislocation (with its dislocation line coinciding with the z-coordinate axis and the slip plane as the x-z-plane of the local coordinate system) and its interaction with the interstitial component, it is advantageous to evaluate the eigenstrain tensor in the local coordinate system (x, y, z) and solve the problem in that system. Then the space around the dislocation can be divided into cylindrical representative volume elements with axes parallel to z-axis and small cross-sections in x- and y- directions. In each of the representative volume elements spatially constant values of Xi, X-2 and X-3 and of the eigenstrain tensor are assumed. As one can calculate the eigenstress state inside and outside of each cylindrical representative volume element, embedded in an eigenstrain-free infinite matrix, the total stress state is then given by linear superposition of the eigenstress states of all representative volume elements and the stress state induced by the dislocation. The goal of the proposed review paper is to comprise a collection of the knowledge of the last seven decades dealing with the following topics: stress fields induced by edge and screw dislocation; a presentation of the most recent solution technique and discussion of previous solutions; eigenstrains of carbon and hydrogen atoms placed in octahedral and tetrahedral sites; a selection of the values of reliable sources; assembling of the interstitial atoms, acting as inclusions with a misfit eigenstrain state, in cylindrical representative volume elements; eigenstrain states in cylindrical representative volume elements due to occupancy of interstitial atoms described by X-1, X-2 and X-3; eigenstress fields generated by eigenstrains in cylindrical representative volume elements; formulation of the generalized chemical potential of interstitial atoms in the stressed bcc lattice; anisotropic diffusion equations for interstitials in a stressed bcc lattice, accounting for elastodiffusion and the effect of occupancy of different types of interstitial sites. A collection of equations provides a basis for acquiring a complex model of kinetics of interactions of interstitial atoms with (especially prioritizing) dislocations in the bcc lattice. This represents the "research part" of the review paper. Thus the paper focuses on kinetics of interactions of interstitials occupying tetrahedral and octahedral positions with edge and screw dislocations. As results of simulations the kinetics of distribution of the site fractions X-1, X-2 and X-3 around the dislocation as well as the relaxation (i.e. reduction) of the dislocation stress due to eigenstress are provided.

Back stress hardening is usually not obvious in single-phase homogeneous grains, but can be made unusually large and sustained to large tensile strains by creating an unusually heterogeneous grain structure in single-phase alloys with low stacking fault energy (SFE), as demonstrated here for the face-centered cubic CrCoNi medium-entropy alloy. The low SFE facilitates the generation of twinned nanograins and stacking faults during tensile straining, dynamically reinforcing the heterogeneity. Large uniform tensile strain can be achieved after yielding even at gigapascal stress, in the absence of heterogeneities from any second phase.

Interstitial solid strengthening is an effective strategy to harden metallic materials, however, it usually deteriorates the ductility. Here, we report that addition of carbon into the medium-entropy NiCoCr alloy successfully enhances the strength at no expense of ductility. It was found that up to 0.75 at.% carbon was completely solid-solutionized in (NiCoCr)(100-x)C-x (x = 0, 0.10, 0.25, 0.50 and 0.75 at.%) without formation of any carbides. With the increase of carbon content from 0 to 0.75 at.%, the yield and fracture strength were increased from 242 to 347 MPa to 727 and 862 MPa, respectively, whilst the ductility kept as high as about 75%. It is noteworthy that the integral of the stress over strain for the alloy with 0.75 at.% carbon reaches a value of 59 GPa %, surmounting the level of many reported multi-principal elements alloys. Our analysis indicates that carbon addition increases stacking fault energy, thus delaying occurrence of twinning and decreasing the thickness of twin lamellas. At the early deformation stage, carbon decreases the stress localization and stimulates dislocation multiplication. After occurrence of deformation twinning, finer twinning structure in the alloys added with carbon not only can obstacle and trigger more dislocations, but also transfer plastic deformation more efficiently, thus promoting the twinning process, postponing the plastic instability and eventually giving rise to a more pronounced work-hardening. Our results not only have important implications for understanding the solid solution strengthening mechanism in medium-entropy alloys, but also shed lights on developing advanced metallic alloys with a unique combination of strength and ductility.