Hexagonal High-Entropy Alloys

We report on the discovery of a high-entropy alloy with a hexagonal crystal structure. Equiatomic samples in the alloy system Ho-Dy-Y-Gd-Tb were found to solidify as homogeneous single-phase high-entropy alloys. The results of our electron diffraction investigations and high-resolution scanning transmission electron microscopy are consistent with a Mg-type hexagonal structure. The possibility of hexagonal high-entropy alloys in other alloy systems is discussed.Comment: Changes upon replacement: inserted submission date of manuscript to journal. No other changes were mad

Compressibility of Al64Pd30.4Fe5.6

The stability of C2-Al64Pd30.4Fe5.6, an approximant phase to the icosahdral Al-Fe-Pd quasicrystal, has been investigated by high-pressure X-ray diffraction using a diamond-anvil cell and synchrotron radiation up to 24.30(1) GPa. The material is structurally stable within the framework of the experiment. The unit cell volume as a function of pressure follows a second-order Birch-Murnaghan equation of state with K0 = 169.5(2.5) GPa. This value is comparable to that of other quasicrystals and approximant phases containing Al and heavy transition metal

Compressibility of Al64Pd30.4Fe5.6

The stability of C2-Al64Pd30.4Fe5.6, an approximant phase to the icosahdral Al-Fe-Pd quasicrystal, has been investigated by high-pressure X-ray diffraction using a diamond-anvil cell and synchrotron radiation up to 24.30(1) GPa. The material is structurally stable within the framework of the experiment. The unit cell volume as a function of pressure follows a second-order Birch-Murnaghan equation of state with K0 = 169.5(2.5) GPa. This value is comparable to that of other quasicrystals and approximant phases containing Al and heavy transition metal

Hall Effect of the Triclinic Al73Mn27 and T-Al73Mn27–xPdx (0 ≤ x ≤ 6) Complex Metallic Alloys

The Hall coefficient, RH, of the triclinic Al73Mn27 and Taylor-phase Al73Mn27xPdx (x = 0, 2, 4 and 6) complex metallic alloys has been measured from 90 to 400 K. The Hall coefficients of all the samples are positive and they decrease strongly with the increase of temperature, T. For the separation of the normal, R0, and anomalous, RS, Hall coefficient the results for the paramagnetic susceptibility,χ(T), and electrical resistivity, ρ(T), have been used. The well defined linearity of the RH vs. χ(T)·ρ2(T) plots confirms the assumption that in these materials RH is dominated by spin-orbit interaction. The values deduced from the RH vs. χ and RH vs. χ·ρ2 plots in T­AlMnPd phases, fall between –2 × 10–10 m3 C–1 and 0 for R0, and are about 5 × 10–7 m3 C–1 for RS. The values deduced from the RH vs. χ·ρ2 plots in the triclinic Al73Mn27 alloy are about –15 × 10–10 m3 C–1 for R0, and about 1.5 × 10–5 m3 C–1 for RS.</p

The Generalization of the Kinetic Equations and the Spectral Conductivity Function to Anisotropic Systems: Case T-Al_72.5Mn_21.5Fe_6 Complex Metallic Alloy

Electrical conductivity, σ, and thermoelectric power, S, of the monocrystalline T-Al_72.5Mn_21.5Fe_6 complex metallic alloy have been investigated in the temperature range from 2 to 300 K. The crystallographic-direction-dependent measurements were performed along the [0 0 1], [0 1 0] and [1 0 0] directions of the orthorhombic unit cell, where the stacking direction is along the [0 1 0] direction. The electrical conductivity exhibits a very small anisotropy, and in all directions shows the non-metallic behaviour with square root, √T, temperature behaviour and finite value in the T = 0 limit. Spectral conductivity function, σS(E), constructed out of measurements, reflects anisotropy of the experimental data and indicate non-analytic square root like singularity at Fermi level. Asymmetry of the spectral conductivity function has been extracted from the thermoelectric power data

The Generalization of the Kinetic Equations and the Spectral Conductivity Function to Anisotropic Systems: Case T-Al72.5Mn21.5Fe6 Complex Metallic Alloy

Electrical conductivity, σ, and thermoelectric power, S, of the monocrystalline T-Al72.5Mn21.5Fe6 complex metallic alloy have been investigated in the temperature range from 2 to 300 K. The crystallographic-direction-dependent measurements were performed along the [0 0 1], [0 1 0] and [1 0 0] directions of the orthorhombic unit cell, where the stacking direction is along the [0 1 0] direction. The electrical conductivity exhibits a very small anisotropy, and in all directions shows the non-metallic behaviour with square root, &radic;T, temperature behaviour and finite value in the T = 0 limit. Spectral conductivity function, σS(E), constructed out of measurements, reflects anisotropy of the experimental data and indicate non-analytic square root like singularity at Fermi level. Asymmetry of the spectral conductivity function has been extracted from the thermoelectric power data.</p

The Generalization of the Kinetic Equations and the Spectral Conductivity Function to Anisotropic Systems: Case T-Al_72.5Mn_21.5Fe_6 Complex Metallic Alloy

Electrical conductivity, σ, and thermoelectric power, S, of the monocrystalline T-Al_72.5Mn_21.5Fe_6 complex metallic alloy have been investigated in the temperature range from 2 to 300 K. The crystallographic-direction-dependent measurements were performed along the [0 0 1], [0 1 0] and [1 0 0] directions of the orthorhombic unit cell, where the stacking direction is along the [0 1 0] direction. The electrical conductivity exhibits a very small anisotropy, and in all directions shows the non-metallic behaviour with square root, √T, temperature behaviour and finite value in the T = 0 limit. Spectral conductivity function, σS(E), constructed out of measurements, reflects anisotropy of the experimental data and indicate non-analytic square root like singularity at Fermi level. Asymmetry of the spectral conductivity function has been extracted from the thermoelectric power data

Direct observation of dislocation plasticity in FeCrCoMnNi high-entropy alloys

In the past decade, high-entropy alloys (HEAs) have been intensively investigated not only because of fundamental scientific interests, but also their outstanding mechanical properties, for example, high ductility and fracture toughness. Among hundreds of different combinations of principal elements, the equiatomic FeCrCoMnNi alloy, the so-called Cantor alloy, has been studied as a model system, which is a single phase material with face-centered cubic (FCC) structure at room temperature and shows outstanding ductility and strain hardening especially at cryogenic temperatures. However, dislocation-based deformation mechanisms of HEAs remain elusive and require a fundamental understanding in order to tailor their mechanical properties. Several models have been suggested possible strengthening mechanisms of HEAs, for instance, the high entropy effect and the lattice distortion effect. In the case of the Cantor alloy, the main strengthening mechanism was identified as deformation twinning with critical twinning stress of 720 MPa. At room temperature, dislocation slip by full dislocations is dominant, however, at strains exceeding 20 % and high flow stresses, deformation twinning was also observed. To reveal the hardening mechanism in more detail, direct observation of dislocation plasticity and deformation dynamics is required. Here, we present a study correlating the microstructure and dislocation plasticity of a single crystalline FeCrCoMnNi FCC single phase HEA by employing in-situ transmission electron microscope (TEM) compression and tensile deformation. Moreover, an atomic-scale chemical analysis is conducted by aberration-corrected scanning TEM energy dispersive X-ray spectroscopy (STEM-EDS) and atom probe tomography to investigate chemical inhomogeneity, for example, precipitate formation or local inhomogeneity. Compression tests with sub-micron pillars with 250 and 120 nm diameter show less pronounced mechanical size effects in the alloy compared to other FCC metals as the size exponent is measured as 0.53. It suggests that relatively strong inherent hardening processes are present which attenuate the FCC reported size scaling exponent, which is typically 0.6 to 1.0 for pure FCC metals. The elemental distribution and lattice strains at the atomic scale are rather uniform without long-range ordering analyzed by high-resolution scanning TEM (STEM) and atom probe tomography. Finally, dislocation glide motion was directly observed during in situ TEM tensile tests. The local shear stress measured from gliding of individual dislocations is exceeding 400 MPa. Kink-pair-like glide behavior and periodic fluctuation in the stacking fault width suggest that local pinning points, severe lattice distortion or short-range ordering hinder dislocation motion in HEAs

Entropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy