No abstract

Oxides can experience structural transformations resulting from variations in cation oxidation states or coordination geometry upon thermal treatment. Whether such structural distortions can affect the stability of high‐entropy oxides has not been studied. In this research, a new, high‐entropy, lanthanide sesquioxide, Gd0.4Tb0.4Dy0.4Ho0.4Er0.4O3 solid solution having a single phase, cubic‐bixbyite structure was synthesized, with no phase transformation from room temperature to 1650°C. The phase stability was examined via both in situ and ex situ, high‐temperature, synchrotron, X‐ray powder diffraction. This high‐entropy oxide could inhibit the phase transformations occurring in constituent monocation sesquioxides, Tb2O3 and Gd2O3, via random arrangement of multications.

High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing together scientists and engineers from physics, chemistry, and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the critical importance of understanding a material’s crystallinity, composition, and mixing length scales in determining its true definition.

The previously unknown experimental HfO 2 -Ta 2 O 5 -temperature phase diagram has been elucidated up to 3000°C using a quadrupole lamp furnace and conical nozzle levitator system equipped with a CO 2 laser, in conjunction with synchrotron X-ray diffraction. These in-situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in-situ X-ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO 2 -Ta 2 O 5 -temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.

Electron back-scattering diffraction was used to track the microstructure evolution of a fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C TWinning Induced Plasticity (TWIP) steel during interrupted reverse (tensioncompression) loading. Direct observation of the same selected area revealed that all deformation twins formed during forward tension loading (0.128 true strain) were removed upon subsequent reverse compression loading (0.031 true strain). Consequently, the present study provides the first unambiguous experimental evidence of de-twinning during the reverse loading of a polycrystalline TWIP steel. The reverse loading behaviour was simulated by a dislocation-based hardening model embedded in the Visco-Plastic Self-Consistent (VPSC) polycrystal framework taking into account the accumulation and annihilation of dislocations and back-stress effects. The model has been extended to account for the processes of twinning and de-twinning, as well as the twin barrier effect under load reversal. A new formulation based on the changes in the dislocation mean free path is proposed to track twin lamellae generation/annihilation throughout deformation along with its associated effect on hardening.