330 research outputs found
Solid-liquid interfacial premelting
We report the observation of a premelting transition at chemically sharp
solid-liquid interfaces using molecular-dynamics simulations. The transition is
observed in the solid-Al/liquid-Pb system and involves the formation of a
liquid interfacial film of Al with a width that grows logarithmically as the
bulk melting temperature is approached from below, consistent with current
theories of premelting. The premelting behavior leads to a sharp change in the
temperature dependence of the diffusion coefficient in the interfacial region,
and could have important consequences for phenomena such as particle
coalescence and shape equilibration, which are governed by interfacial kinetic
processes.Comment: 6 pages, 4 figure
First-principles computational study of defect clustering in solid solutions of ThO with trivalent oxides
The energetics of mixing and defect ordering in solid solutions of
fluorite-structured ThO with oxides of trivalent cations (Sc, In, Y, Nd,
La) are investigated by electronic density-functional-theory (DFT). Through DFT
calculations of structures enumerated by lattice-algebra techniques, we
identify the lowest-energy patterns for defect clustering for four separate
dopant concentrations. The most stable structures are characterized by a
repulsive interaction between nearest-neighbor vacancies on the oxygen
sublattice. The enthalpies of formation with respect to constituent oxides are
positive for all dopants considered, and show a tendency to decrease in
magnitude as the size and electronegativity of the trivalent dopant decrease.
Due to the small positive formation enthalpies and low oxygen-vacancy binding
energy with La dopants, LaO-ThO solid solutions are predicted
to have relatively high ionic conductivities relative to those for the other
aliovalent dopants considered. Our results are compared with those for the more
widely studied ZrO and CeO fluorite-structured solid solutions with
trivalent cations.Comment: 9 pages, 8 figure
Tunable stacking fault energies by tailoring local chemical order in CrCoNi medium-entropy alloys
High-entropy alloys (HEAs) are an intriguing new class of metallic materials
due to their unique mechanical behavior. Achieving a detailed understanding of
structure-property relationships in these materials has been challenged by the
compositional disorder that underlies their unique mechanical behavior.
Accordingly, in this work, we employ first-principles calculations to
investigate the nature of local chemical order and establish its relationship
to the intrinsic and extrinsic stacking fault energy (SFE) in CrCoNi
medium-entropy solid-solution alloys, whose combination of strength, ductility
and toughness properties approach the best on record. We find that the average
intrinsic and extrinsic SFE are both highly tunable, with values ranging from
-43 mJ.m-2 to 30 mJ.m-2 and from -28 mJ.m-2 to 66 mJ.m-2, respectively, as the
degree of local chemical order increases. The state of local ordering also
strongly correlates with the energy difference between the face-centered cubic
(fcc) and hexagonal-close packed (hcp) phases, which affects the occurrence of
transformation-induced plasticity. This theoretical study demonstrates that
chemical short-range order is thermodynamically favored in HEAs and can be
tuned to affect the mechanical behavior of these alloys. It thus addresses the
pressing need to establish robust processing-structure-property relationships
to guide the science-based design of new HEAs with targeted mechanical
behavior.Comment: 23 pages, 5 figure
Ab initio modeling of the energy landscape for screw dislocations in body-centered cubic high-entropy alloys
In traditional body-centered cubic (bcc) metals, the core properties of screw
dislocations play a critical role in plastic deformation at low temperatures.
Recently, much attention has been focused on refractory high-entropy alloys
(RHEAs), which also possess bcc crystal structures. However, unlike
face-centered cubic high-entropy alloys (HEAs), there have been far fewer
investigations on bcc HEAs, specifically on the possible effects of chemical
short-range order (SRO) in these multiple principal element alloys on
dislocation mobility. Here, using density functional theory, we investigate the
distribution of dislocation core properties in MoNbTaW RHEAs alloys, and how
they are influenced by SRO. The average values of the core energies in the RHEA
are found to be larger than those in the corresponding pure constituent bcc
metals, and are relatively insensitive to the degree of SRO. However, the
presence of SRO is shown to have a large effect on narrowing the distribution
of dislocation core energies and decreasing the spatial heterogeneity of
dislocation core energies in the RHEA. It is argued that the consequences for
the mechanical behavior of HEAs is a change in the energy landscape of the
dislocations which would likely heterogeneously inhibit their motion
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