Abstract:By means of density-functional calculations, we systematically investigated 24 transition metals for possible metastable phases in body-centered tetragonal structure (bct), including face-centered cubic (fcc) and body-centered cubic (bcc) geometries. A total of 36 structures not coinciding with equilibrium phases were found to minimize the total energy for the bct degrees of freedom. Among these, the fcc structures of Sc, Ti, Co, Y, Zr, Tc, Ru, Hf, Re, and Os, and bct Zr with c/a = 0.82 were found to be metast… Show more
“…At the same time, the fcc phase is always less stable with respect to the hcp phase for all compositions considered here. These theoretical predictions for TiZrHfTa x are in line with the experimental observation 19 , and are consistent with the previous theoretical results for the alloy components 23 . Figure 2 ( a ) The equilibrium total energy of TiZrHfTa x as a function of composition for the bcc and fcc structures relative to the hcp structure.…”
Recent developments of high-entropy alloys with high strength and high ductility draw attention to the metastability-engineering strategy. Using first-principle theory, here we demonstrate that reducing the Ta level in the refractory TiZrHfTax system destabilizes the body-centered cubic (bcc) phase and leads to the appearance of the hexagonal close-packed (hcp) phase embedded in the bcc matrix. The alloying-induced features of the elastic parameters for the cubic and hexagonal structures are mapped out in details, and strong sensitivity to the crystal lattice and chemistry is revealed. Results show softening of the bcc matrix with decreasing Ta concentration which ensures ductile behavior. However, the elastically nearly isotropic hcp precipitates possess enhanced resistance against shear which promotes strengthening of the TiZrHfTax dual-phase system. The present atomic-level insight provides strong evidence to the experimental observation, and emphasizes the significance of quantum-design for advanced multi-phase high-entropy alloys with excellent strength-ductility combinations.
“…At the same time, the fcc phase is always less stable with respect to the hcp phase for all compositions considered here. These theoretical predictions for TiZrHfTa x are in line with the experimental observation 19 , and are consistent with the previous theoretical results for the alloy components 23 . Figure 2 ( a ) The equilibrium total energy of TiZrHfTa x as a function of composition for the bcc and fcc structures relative to the hcp structure.…”
Recent developments of high-entropy alloys with high strength and high ductility draw attention to the metastability-engineering strategy. Using first-principle theory, here we demonstrate that reducing the Ta level in the refractory TiZrHfTax system destabilizes the body-centered cubic (bcc) phase and leads to the appearance of the hexagonal close-packed (hcp) phase embedded in the bcc matrix. The alloying-induced features of the elastic parameters for the cubic and hexagonal structures are mapped out in details, and strong sensitivity to the crystal lattice and chemistry is revealed. Results show softening of the bcc matrix with decreasing Ta concentration which ensures ductile behavior. However, the elastically nearly isotropic hcp precipitates possess enhanced resistance against shear which promotes strengthening of the TiZrHfTax dual-phase system. The present atomic-level insight provides strong evidence to the experimental observation, and emphasizes the significance of quantum-design for advanced multi-phase high-entropy alloys with excellent strength-ductility combinations.
“…The lattice parameter of metastable fcc Re is 3.89 Å. The formation of metastable Re phase might have been due to phase transformation from hcp to fcc structure following Bain transformation [29]. With increasing deposition temperature to 250 °C, the branching start dominating near the interface [ Fig.…”
We present results on growth of large area epitaxial ReS2 thin film both on c plane sapphire substrate and MoS2 template by pulsed laser deposition (PLD). Films tend to grow with (0001) 2 ⊥ (0001) 2 3 and (0001) 2 ⊥ (0001) 2 ∥ (0001) 2 3 at deposition temperature below 300°C. Films are polycrystalline grown at temperature above 300°C. The smoothness and quality of the films are significantly improved when grown onMoS2 template compared to sapphire substrate. The results show that PLD is suitable to grow ReS2 epitaxial thin film over large area for practical device application.
“…It is possible that the relationship between fcc and hcp is not explained by kinetics alone; i.e., they could be affected by thermodynamic factors as well. In particular, it is known that many metastable phases cannot exist above a certain critical particle size − due to a lower surface energy than that of thermodynamically favored (bulk) phases. Hence, metastability occurs when the particle becomes sufficiently small.…”
Section: Resultsmentioning
confidence: 99%
“…Similar kinds of staking faults have been reported for other types of compounds. − The observed stacking faults indicate that only a small energy difference separates the two crystal structures, incurring easy transformation from one structure to the other. Indeed, it has been shown by DFT calculations that the energy difference between the hcp and fcc structures of Ru is small . This explains why it is possible for the two phases to coexist across such a wide composition range.…”
Section: Resultsmentioning
confidence: 99%
“…Indeed, it has been shown by DFT calculations that the energy difference between the hcp and fcc structures of Ru is small. 52 This explains why it is possible for the two phases to co-exist across such a wide composition range.…”
The thermodynamic stability of nanocrystals is different from bulk systems, and nanoscale phase diagrams are to a large degree unknown. Here we present a systematic investigation of the Pt1-xRux phase diagram through supercritical flow-synthesis of Pt1-xRux nanoparticles across the entire compositional range. The synthesis was done in stoichiometric steps of 0.1 using an ethanol-toluene mixture as solvent at 450°C and 200 bar. The products were characterized by high resolution synchrotron powder X-ray diffraction, transmission electron microscopy and elemental mapping of individual particles using energy-dispersive X-ray spectroscopy. The diffraction data revealed a single phase fcc alloy for x ≤ 0.2, while an additional hcp phase emerges as x approaches 1. This behavior deviates significantly from the bulk phase diagram where a biphasic region is only observed 2 for 0.62 < x < 0.8. Thus, compositional design of Pt-Ru alloys is more flexible on the nanoscale opening up significant possibilities for catalyst optimization. Rietveld refinements and microstructural line profile analysis show that the fcc unit cell dimensions follow Vegard's law within a good approximation. On the other hand, crystallite size, microstrain, phase content and hcp c/a ratio depend non-linearly on x but shows some correlation to the bulk phase diagram. Elemental mapping show the nanoparticles to be homogeneous, but in some cases fcc-hcp phase boundaries and modulations in the elemental distribution were observed. All samples below x < 0.3 exhibit a spherical morphology. At higher ruthenium content, x ≥ 0.3, another morphology emerges with elongated particles together with the dominating spherical mophology.The TEM average particle sizes ranging from 5.0(8) to 10.4(7) nm.
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