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The stability of high-entropy alloys (HEAs) is a key issue before their selection for industrial applications. In this study, in-situ high-pressure and high-temperature synchrotron radiation X-ray diffraction experiments have been performed on three typical HEAs Ni20Co20Fe20Mn20Cr20, Hf25Nb25Zr25Ti25, and Re25Ru25Co25Fe25 (at. %), having face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp) crystal structures, respectively, up to the pressure of ~80 GPa and temperature of ~1262 K. Under the extreme conditions of the pressure and temperature, all three studied HEAs remain stable up to the maximum pressure and temperatures achieved. For these three types of studied HEAs, the pressure-dependence of the volume can be well described with the third order Birch-Murnaghan equation of state. The bulk modulus and its pressure derivative are found to be 88.3 GPa and 4 for bcc-Hf25Nb25Zr25Ti25, 193.9 GPa and 5.9 for fcc-Ni20Co20Fe20Mn20Cr20, and 304.6 GPa and 3.8 for hcp-Re25Ru25Co25Fe25 HEAs, respectively. The thermal expansion coefficient for the three studied HEAs is found to be in the order as follows: fcc-Ni20Co20Fe20Mn20Cr20>bcc-Hf25Nb25Zr25Ti25 ≈hcp-Re25Ru25Co25Fe25. [ABSTRACT FROM AUTHOR]

Based on mechanical instability of individual shear transformation zones (STZs), a quantitative link between the microplastic instability and macroscopic deformation behavior of metallic glasses was proposed. Our analysis confirms that macroscopic metallic glasses comprise a statistical distribution of STZ embryos with distributed values of activation energy, and the microplastic instability of all the individual STZs dictates the macroscopic deformation behavior of amorphous solids. The statistical model presented in this paper can successfully reproduce the macroscopic stress-strain curves determined experimentally and readily be used to predict strain-rate effects on the macroscopic responses with the availability of the material parameters at a certain strain rate, which offer new insights into understanding the actual deformation mechanism in amorphous solids. [ABSTRACT FROM AUTHOR]

Both experimental studies and molecular dynamics (MD) simulations reveal a unique atomic-scale growth mechanism during crystallization of an amorphous alloy Zr65Ni25Ti10. By using a high-resolution transmission electron microscope incorporated with nanobeam diffraction technique, we have clearly seen imperfect ordered packing of nano-ordered clusters (NOCs) with a size of 1–2 nm embedded in this amorphous material. Under a very large undercooling condition, NOCs essentially act as preexisting nuclei that can grow directly into nanocrystals during annealing treatments. The growth mechanism includes three distinct steps in succession: formation of quasi-ordered structure with one-dimensional (1D) periodicity, and then 2D periodicity, and finally forming 3D nanocrystals. These three growth steps are cross-linked, and atomic movements are also accommodated by a rotation of atomic planes. This growth mechanism is unambiguously verified by MD simulation. [ABSTRACT FROM AUTHOR]

The dependences of phonons on the spin structures of BaFe2As2 were studied using first-principles supercell approach. We considered the nonmagnetic, checkerboard, stripe, and spin-density-wave structures as well as a spin structure labeled as STR17. We find that STR17 is lower in energy than the stripe. It is seen the ∼20 meV missing peak at the generalized phonon density-of-states and the measured specific heat are well reproduced when the magnetic effects are accounted. We see phonon energy gaps for all the four magnetic structures but not for the nonmagnetic structure. [ABSTRACT FROM AUTHOR]

The dependences of phonons on the spin structures of BaFe2As2 were studied using first-principles supercell approach. We considered the nonmagnetic, checkerboard, stripe, and spin-density-wave structures as well as a spin structure labeled as STR17. We find that STR17 is lower in energy than the stripe. It is seen the ∼20 meV missing peak at the generalized phonon density-of-states and the measured specific heat are well reproduced when the magnetic effects are accounted. We see phonon energy gaps for all the four magnetic structures but not for the nonmagnetic structure. [ABSTRACT FROM AUTHOR]

TiZrNbMoxVy high-entropy alloys (HEAs) with x = 0-2, y = 1 and y = 0.3, respectively, were designed and prepared by copper mold casting technology. The phase composition and stability of these HEAs were investigated. It is shown that the HEAs with low content of V are composed of only one type of bcc solid solution phase (SSP), and demonstrate excellent phase stability at 1273 K. The high content of V and Mo results in the formation of two types of bcc SSPs and the decrease of phase stability in the HEAs. Based on the previously proposed criteria, the formation ability of solid solution phase for this kind of HEAs was comprehensively evaluated. The compressive mechanical properties of the as-cast and annealed HEAs were measured. It has been found that Mo plays a strong solid solution strengthening effect on this kind of HEAs. Especially, TiZrNbMo0.3V0.3 has the yield strength and plastic strain of 1312 MPa and >50%, respectively, and still maintains the excellent plastic deformation ability even after annealed at 1273 K for 72 h. The strengthening effect in this kind of HEAs is considered to be due to the shear modulus mismatch. The solubility limit of HEAs is correspondent to shear modulus mismatch of 29. [ABSTRACT FROM AUTHOR]

The glass-forming abilities of Fe78B22, Fe70Y6B24, Fe72Y6B22 and Fe72.5Y3.5B24 alloys were characterized comprehensively using ab initio molecular dynamics simulations. The calculated results were correlated with the properties and atomic structures. It was found that the Fe72Y6B22 alloy consists of both the most stable and the least deformed body centered cubic atomic packing structures in the supercooled liquid and glassy states. It was observed that the local compositions in the Fe72Y6B22 alloy significantly deviate from the compositions of stable crystalline phases, indicating that the Fe72Y6B22 alloy has the best glass-forming ability among the alloys studied. However, Fe72Y6B22 alloy has two flaws in terms of glass-forming ability, i.e. relatively large atomic diffusivity and insufficiently close atomic packing. The best performance in these two aspects is observed in the Fe72.5Y3.5B24 alloy. Thus, the theoretical study predicts that the best glass former for the Fe-Y-B system is within the compositional range of 22-24at.% B and 3.5-6at.% Y. [ABSTRACT FROM AUTHOR]

A novel n-body potential for an Zr-Nb system was developed in the framework of the embedded-atom method. All the parameters of the constructed potential have been systematically evaluated by fitting to the ground state properties obtained from experimental measurements and first-principles calculations for pure elements and some alloys. It is shown that most of the static thermodynamics properties for Zr and Nb can be well reproduced by using the present potential. Some calculation results based on the present model are even closer to the experimental data than those based on previous potential models. The ground state properties of hypothetical Zr-Nb alloys were also calculated and found to be in agreement with first-principles calculations. Furthermore, the formation energies of random solid solutions of Zr-Nb with lattices of body centered cubic (bcc) and hexagonal close packed (hcp) type were calculated by fitting the energy-volume relations to Rose's equation of state. These values were compared with those obtained by first-principles calculations based on special quasirandom structure models and the Miedema-ZSL-07 model (the improved Miedema model proposed by Zhang, Sheng and Liu in 2007). It is indicated that our n-body constructed potential for a Zr-Nb alloy provides an effective description for the interaction between the dissimilar ion interactions for hcp-bcc systems.