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[Display omitted] Carbide nano-precipitates are commonly used to improve mechanical properties of steel. It has been experimentally observed that TiC, NbC, and VC carbide precipitates initially form as 'plate-like' particles oriented in the {1 0 0} planes of the ferrite lattice. These platelets share similarities with Guinier-Preston zones in Al-Cu alloys. The clustering of group IV and V transition metal atoms (M = Ti, Zr, Hf, V, Nb, Ta) in ferrite is studied using density functional theory. It is deduced that the transition metal carbides all form in a similar way. Furthermore, the transition from an initial M–C cluster to a NaCl-structured platelet to a NaCl-structured precipitate is examined through atomistic simulations using Modified Embedded Atom Method potentials. A route is established along which transition metal carbides form and transform into precipitates that possess the Baker-Nutting orientation relation with the ferrite matrix. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Interstitial transition metal nitrides and carbides based on simple crystal structures with partial occupation of certain sites by N and C frequently show long-range ordering. In this work the focus is on the important group of superstructures based on a face-centered cubic (fcc)-based arrangement of the metal atoms and octahedral site occupation by N or C. The ordering tendencies are explored for various Ni 4 N and Ni 4 C superstructures using density-functional-theory. The ordering tendencies differ strikingly as revealed by the low-energy superstructures encountered for Ni 4 N and for Ni 4 C: in the case of Ni 4 N 2nd-next-neighbor N-N pairs appear favorable whereas in Ni 4 C 1st-next neighbor C-C pairs are preferred. This can be explained by an interplay between elastic interactions favoring 1st-next neighbor pairs of N and C atoms in an fcc-based arrangement of metal atoms and chemical/Coulomb interactions favoring 2nd-next neighbor (i.e. more distant) pairs. While the elastic interactions are expected to be similar for N and C, the chemical/Coulomb interactions are stronger for N-N as compared to C-C and are thus able to explain the different ordering tendencies. [ABSTRACT FROM AUTHOR]

Experimental results have shown that two-dimensional early transition metal carbides such as Nb 2 C also can be terminated by S atoms. In this study, 20 types of sulfur terminated MXenes (M 2 XS 2 , where M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W; X = C, N) are investigated using the density functional theory method. Phonon frequency analysis and molecule dynamic simulations upon 1000 K results show that V 2 CS 2 , Cr 2 NS 2 , Nb 2 CS 2 , Hf 2 CS 2 , and Ta 2 CS 2 are dynamically stable. Formation enthalpy calculations indicate that two-dimensional V 2 CS 2 , Nb 2 CS 2 , Ta 2 CS 2 , Hf 2 CS 2 , and Cr 2 NS 2 structures have the highest stability and may be easily synthesized. It indicates that MXenes can also be terminated by S atoms, which is influenced by the synthesis process. The binding energies for V 2 CS 2 , Nb 2 CS 2 , and Cr 2 NS 2 bilayer systems are smaller than 0.18 eV/atom, which indicates they can be easily separated into single layers. In contrast, the binding energies for Ta 2 CS 2 and Hf 2 CS 2 bilayers systems are up to 0.326 and 0.36 eV/atom, respectively. This indicates that Ta 2 CS 2 and Hf 2 CS 2 are relatively difficult to separate into single layers, which can be attributed to the localized p-orbitals of S atoms. Ta 2 CS 2 is a direct band gap semiconductor with band gap of 0.36 eV. Cr 2 NS 2 system is ferromagnetic material among MXenes, with the average magnetic moment of Cr atoms of 1.60 μ B . This research provides new viewpoints for the synthesis and surface termination of MXenes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Group IVB and VB transition metal carbides have different mass diffusion mechanisms. • Vacancy clustering is much more favorable in group IVB than VB metal carbides. • Off-lattice diffusion mechanisms have lower activation energies than on-lattice. • Off-lattice diffusion might be the most dominant mechanism in the group IVB TMCs. Using ab initio simulations, we report potential metal atom diffusion mechanisms in the group IVB and VB transition metal carbides. By computing the metal vacancy formation energies of vacancy clusters, we find that a metal vacancy surrounded by six carbon vacancies is the lowest metal vacancy formation energy structure for the group IVB carbides while the lowest energy for the group VB carbides is a metal vacancy surrounded by two carbon vacancies. The vacancy cluster mechanisms reveal activation energies that are consistent with experiments in both TiC (group IVB) and TaC (group VB). We also report that an off-lattice diffusion mechanism, that is only energetically favorable in the group IVB transition metal carbides, has a lower formation energy than the regular vacancy cluster mechanism. This new mechanism shows lower formation energies for a given carbon vacancy concentration which indicates this off-lattice mechanism might be the most dominant metal-atom diffusion mechanism among the existing mechanisms proposed for the group IVB metal carbides. [ABSTRACT FROM AUTHOR]