Jürgen Schreuer

The VCoNi alloy is a face-centered cubic medium-entropy alloy with exceptionally high yield strength, serving as a prototypical system for investigating short-range order and phase stability in compositionally complex alloys. However, density functional theory calculations underestimate the stability of the random solid solution by several hundred Kelvin. To resolve this discrepancy, we present accurate Gibbs energy calculations for both the random solid solution and a prototypical L 1 2 ordered phase. Our findings reveal that vibrational and electronic excitations account for nearly half of the entropy difference between the ordered phase and disordered solid solution. These factors reduce the energy difference between the two phases by about one-third and help stabilize the solid solution. Our thermodynamic analysis is validated through direct comparison with experimental thermodynamic data.

The elastic behavior of Sr1.03Ga10.81Mg0.58Zr0.58O19 and Pr-doped SrAl12O19 single crystals grown by the top-seeded solution growth technique was investigated by resonant ultrasound spectroscopy at temperatures between 103 and 1573 K. The coefficients of thermal expansion were obtained by high-resolution dilatometry in the same temperature range. Values for bulk moduli at ambient temperatures are 206.5 and 181.0 GPa for Pr-doped SrAl12O19 and Sr1.03Ga10.81Mg0.58Zr0.58O19, respectively. The aluminate possesses higher values of the mean elastic stiffness compared to the gallate, which can be related to different oxygen packing densities. Apart from these differences, their elastic anisotropy is very similar, as expressed by the ratio of the maximum and minimum of the longitudinal elastic stiffness of 1.34 and 1.35, respectively. In the case of Pr-doped SrAl12O19, an unexpected, fully reversible softening of the stiffness coefficient c33 is noteworthy below 500 K, which is accompanied by a negative thermal expansion. In addition, Pr-doped SrAl12O19 possesses a well-reproducible strong ultrasound attenuation between 700 and 900 K, which can be interpreted as anelastic relaxation of point defects with an activation energy of 0.65 eV.

The tensors of thermal expansion of monoclinic RCa₄O(BO₃)₃ with R = Er, Y, Dy, Gd, Sm, Nd, La were studied in the temperature range from 100 K to 1373 K using high-resolution dilatometry. Reproducible anomalies, characterized by an excess strain at high temperatures, occur at different temperatures depending on the type of the R³⁺ cation. Additional single-crystal diffraction experiments on quenched samples and heat capacity measurements indicate that non-convergent cation ordering processes involving Ca²⁺ and R³⁺ play an essential role here. The cation distribution on the specific structural sites and the evolution of disorder with temperature are mainly influenced by the size of the trivalent cation, with the minimization of internal stresses being the driving force. The onset temperatures and the specific anisotropy of the anomalies in the thermal expansion are directly related to these processes.

The present work shows that thermal expansion experiments can be used to measure the γʼ-solvus temperatures of four Ni-base single-crystal superalloys (SX), one with Re and three Re-free variants. In the case of CMSX-4, experimental results are in good agreement with numerical thermodynamic results obtained using ThermoCalc. For three experimental Re-free alloys, the experimental and calculated results are close. Transmission electron microscopy shows that the chemical compositions of the γ- and the γʼ-phases can be reasonably well predicted. We also use resonant ultrasound spectroscopy (RUS) to show how elastic coefficients depend on chemical composition and temperature. The results are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. Graphical

Physical properties of ten single-phase FCC Cr_xMn₂₀Fe₂₀Co₂₀Ni_40-x high-entropy alloys (HEAs) were investigated for 0 ≤ x ≤ 26 at%. The lattice parameters of these alloys were nearly independent of composition while solidus temperatures increased linearly by ~30 K with increasing x. For x ≥ 10 at.%, the alloys are not ferromagnetic between 100 and 673 K and the temperature dependencies of their coefficients of thermal expansion and elastic moduli are independent of composition. Magnetic transitions and associated magnetostriction were detected below ~200 K and ~440 K in Cr₅Mn₂₀Fe₂₀Co₂₀Ni₃₅ and Mn₂₀Fe₂₀Co₂₀Ni₄₀, respectively. These composition and temperature dependencies could be qualitatively reproduced by ab initio simulations that took into account a ferrimagnetic ↔ paramagnetic transition. Transmission electron microscopy revealed that plastic deformation occurs initially by the glide of perfect dislocations dissociated into Shockley partials on {111} planes. From their separations, the stacking fault energy (SFE) was determined, which decreases linearly from 69 to 23 mJ·m^-2 as x increases from 14 to 26 at.%. Ab initio simulations were performed to calculate stable and unstable SFEs and estimate the partial separation distances using the Peierls-Nabarro model. While the compositional trends were reasonably well reproduced, the calculated intrinsic SFEs were systematically lower than the experimental ones. Our ab initio simulations show that, individually, atomic relaxations, finite temperatures, and magnetism, strongly increase the intrinsic SFE. If these factors can be simultaneously included in future computations, calculated SFEs will likely better match experimentally determined SFEs.