Search

The interactions between several typical alloying elements (i.e., Al, Cr, Mo, Nb and Ti) and hydrogen at coherent twin boundaries (CTB) in Ni alloys were systematically studied through first-principles calculations. It was found that solute atoms generally prefer to segregate at CTBs, with the segregation tendency prescribed by the size mismatch between the solute and host Ni atom. Moreover, there exist attractive interaction between certain solute atoms, potentially promoting solute co-segregation at CTBs. Though the solute presence does not favor hydrogen accumulation, it can considerably reduce the formation energies of vacancies and vacancy-hydrogen clusters at CTBs, which, if augmented by large-scale plastic deformation, may facilitate nanovoid nucleation to promote eventual crack initiation at CTBs. The present study demonstrated the critical role of alloying in hydrogen-induced crack initiation at CTBs, providing a new perspective towards understanding HE in Ni alloys. [ABSTRACT FROM AUTHOR]

Carbide precipitates, particularly V/Nb carbides, play a vital role in modulating many properties, including strength, hardness, wear resistance and hydrogen embrittlement (HE) susceptibility of steels. Our work presented a comprehensive study employing a cluster expansion (CE) approach combined with first-principles calculations to investigate the sub-stoichiometry induced by C vacancies within V/Nb carbides, an important aspect lacking sufficient understanding. Ground-state sub-stoichiometric V/Nb carbide compounds as the C composition varies were predicted, consistent with experimental observations. The predictions were further supported by the good agreement between the calculated bulk moduli and experimental measurements. The elastic moduli of carbides were shown to monotonically decrease as the C composition decreases, attributed to pd bonds being replaced by dd bonds with the introduction of vacancies. We have demonstrated that C vacancies in the carbides would arrange into particular patterns, with notable preference to form 3NN vacancy pairs and triplets. Such vacancy pattern formation was elucidated to be mainly driven by the elastic interaction between vacancies. The present study provides critical new knowledge regarding sub-stoichiometry and vacancy structures in carbide precipitates in steels. The findings are critical for an accurate understanding of the structures and properties of carbides and offer essential inputs for engineering steels of enhanced performance. [Display omitted] [ABSTRACT FROM AUTHOR]

Rare-earth pyrochlores, commonly exhibiting remarkably low lattice thermal conductivities, are considered as promising topcoat materials for thermal barrier coatings. However the structural origin underlying their low thermal conductivities remains unclear. In the present study, we investigated the phonon properties of two groups of RE pyrochlores, Ln 2 Zr 2 O 7 (Ln = La, Nd, Sm, Gd) and Gd 2 T 2 O 7 (T = Zr, Hf, Sn, Pb) employing density functional theory and quasi harmonic approximation. Through the relaxation time approximation (RTA) with Debye model, the thermal conductivities of those RE pyrochlores were predicted, showing good agreement with experimental measurements. The low thermal conductivities of RE pyrochlores were shown to largely come from the interference between the low-lying optical branches and acoustic branches. The structural origin underlying the low-lying optical branches was then clarified and the competition between scattering processes in transverse and longitude acoustic branches was discussed. [ABSTRACT FROM AUTHOR]

Hydrogen-dislocation interactions have always been a crucial problem in understanding and predicting the long-standing hydrogen embrittlement (HE) phenomenon in structural metals. Particularly with respect to hydrogen diffusion, dislocations have often been assumed with the ability to facilitate hydrogen transport via dislocation pipe diffusion (DPD). Yet the experimental and theoretical studies supporting DPD remain elusive and even controversial. In this work, hydrogen kinetics at dislocations in two representative metal systems, fcc Ni and bcc Fe, were systematically investigated combining comprehensive atomistic and kinetic activation relaxation technique simulations to reveal atomic details while enabling long-time (i.e., microsecond to second) hydrogen diffusion analysis. The dislocations were found to favor hydrogen trapping and provide regions of low migration barriers for hydrogen. However, these regions of low barriers only lead to localized fast short-circuit H diffusion, but do not translate into DPD over long time, which is attributed to fast hydrogen diffusion pathways along the dislocation line direction being periodically bound by high hydrogen migration barriers. Using the polyhedral structural units (PSUs) as an effective tool, we further quantitatively analyzed the correlation between hydrogen diffusion behaviors and local dislocation structures, illustrating the structural origin leading to local short-circuit diffusion and inhibition of DPD at dislocations. The new mechanistic insights gained not only are critical for understanding hydrogen-dislocation interaction, but also advances the general knowledge on hydrogen-microstructure interaction in structural metals. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, vacancy Frank loop unfaulting via Shockley partial loop(s) in two model material systems (Al and Ni) has been investigated through comprehensive atomistic simulations combining continuum modeling. A general approach has been outlined and has been demonstrated to enable reliable construction of different dislocation loop configurations. The unfaulting of vacancy Frank loop is shown to occur when the enclosed Shockley loop exceeds a threshold size, independent of the Frank loop radius. A continuum model is then established to quantitatively predict the geometrical parameters of the Shockley loop and threshold condition for unfaulting. The continuum model prediction yields excellent agreement with simulation results, and has been shown to apply in various different unfaulting scenarios involving either single or multiple Shockley loops. Experimentally observed Butterfly and Mercedes hexagon morphologies have been reproduced from our simulations, with the underlying dislocation reactions fully clarified, and a mechanism of competitive growth identified as the cause for their subsequent transformation into normal prismatic dislocation loops. The present study offers generic and flexible computational tools towards understanding defect reactions and evolution mechanism of Frank loops, and more generally secondary defects in quenched and irradiated materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Hydrogen (H) induced damage in metals has been a longstanding woe for many industrial applications. One form of such damage is linked to H clustering, for which the atomic origin remains contended, particularly for non-hydride forming metals. In this work, we systematically studied H clustering behavior in bcc metals represented by W, Fe, Mo, and Cr, combining first-principles calculations, atomistic and Monte Carlo simulations. H clustering has been shown to be energetically favorable, and can be strongly facilitated by anisotropic stress field, dominated by the tensile component along one of the 〈001〉 crystalline directions. We showed that the stress effect can be well predicted by the continuum model based on H formation volume tensor, and that H clustering is thermodynamically possible at edge dislocations, evidenced by nanohydride formation at rather low levels of H concentration. Moreover, anisotropy in the stress effect is well reflected in nanohydride morphology around dislocations, with nanohydride growth occurring in the form of thin platelet structures that maximize one 〈001〉 tension. In particular, the 〈001〉 type edge dislocation, with the 〈001〉 tensile component maximized, has been shown to be highly effective in facilitating H aggregation, thus expected to play an important role in H clustering in bcc metals, in close agreement with recent experimental observations. This work explicitly and quantitatively clarifies the anisotropic nature of stress effect on H energetics and H clustering behaviors, offering mechanistic insights critical towards understanding H-induced damages in metals. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The effects of hydrogen charging on the mechanical response of FCC Ni and Pd under nanoindentation are systematically investigated by molecular dynamics simulations. Simulations consider a random H distribution and time scales prevent any diffusion of H during the simulations. Hydrogen charging is then found to reduce the threshold load for dislocation nucleation, i.e., the pop-in load, but only to a limited extent. After pop-in , the indentation response is largely independent of the presence of H. Furthermore, the influence of hydrogen charging on the pop-in load originates only from the hydrogen-induced swelling of the lattice. That is, H does not directly influence dislocation nucleation, either in terms of facilitating initial slip or interacting with the nascent dislocation(s). These findings suggest that rate-dependent processes, either associated with fluctuating nucleation or H transport, are necessary to interpret experimental observations of hydrogen-influenced reductions in the pop-in load. [ABSTRACT FROM AUTHOR]

Knowledge on structures and energetics of vacancy clusters is fundamental to understand defect evolution in metals. Yet there remain no reliable methods able to determine essential structural details or to provide accurate assessment of energetics for general vacancy clusters. Here, we performed systematic first-principles investigations to examine stable structures and energetics of vacancy clusters in bcc metals, explicitly demonstrated the stable structures can be precisely determined by minimizing their Wigner-Seitz area, and revealed a linear relationship between formation energy and Wigner-Seitz area of vacancy clusters. We further developed a new physics-based model to accurately predict stable structures and energetics for arbitrary-sized vacancy clusters. This model was well validated by first-principles calculations and recent vacancy cluster annealing experiments, and showed distinct advantages over the widely used spherical approximation. The present work offers mechanistic insights that crucial for understanding vacancy cluster formation and evolution, provides crucial benchmarks for assessing empirical interatomic potentials, and enables a critical step towards predictive control and prevention of vacancy cluster related damage processes in structural metals. [Display omitted] [ABSTRACT FROM AUTHOR]