ABSTRACT
High-entropy ceramics are attracting significant interest due to their exceptional chemical stability and physical properties. While configurational entropy descriptors have been successfully implemented to predict their formation and even to discover new materials, the contribution of vibrations to their stability has been contentious. This work unravels the issue by computationally integrating disorder parameterization, phonon modeling, and thermodynamic characterization. Three recently synthesized carbides are used as a testbed: (HfNbTaTiV)C, (HfNbTaTiW)C, and (HfNbTaTiZr)C. It is found that vibrational contributions should not be neglected when precursors or decomposition products have different nearest-neighbor environments from the high-entropy carbide.
ABSTRACT
Rhenium is an important alloying agent in catalytic materials and superalloys, but the experimental and computational data on its binary alloys are sparse. Only 6 out of 28 Re transition-metal systems are reported as compound-forming. Fifteen are reported as phase-separating, and seven have high-temperature disordered σ or χ phases. Comprehensive high-throughput first-principles calculations predict stable ordered structures in 20 of those 28 systems. In the known compound-forming systems, they reproduce all the known compounds and predict a few unreported ones. These results indicate the need for an extensive revision of our current understanding of Re alloys through a combination of theoretical predictions and experimental validations. The following systems are investigated: AgRe(â ), AuRe(â ), CdRe(â ), CoRe, CrRe(â ), CuRe(â ), FeRe, HfRe, HgRe(â ), IrRe, MnRe, MoRe, NbRe, NiRe, OsRe, PdRe, PtRe, ReRh, ReRu, ReSc, ReTa, ReTc, ReTi, ReV, ReW(â ), ReY, ReZn(â ), and ReZr ((â ) = systems in which the ab initio method predicts that no compounds are stable).
ABSTRACT
On the basis of ab initio density-functional calculations we have analyzed the character of the interatomic bonding in the intermetallic compounds Al(3)(Sc,Ti,V) with the D0(22) and L1(2) structures. In all structures we found an enhanced charge density along the Al-transition-metal (TM) bonds, a characteristic feature of covalent bonding. The series Al3Sc-Al3V corresponds to gradual d-band filling which leads to a gradual increase of bond strength and covalent bond formation. For this series, the tensile anisotropy in the elastic limit has been investigated and a trend towards an increased anisotropy of the elastic constants and Young modulus has been observed. Additionally we performed a study of the response of trialuminides to uniaxial tensile deformation along the [110] direction. This direction is known to be the weak direction for face-centered cubic (fcc) materials under tensile strain, and it is generally accepted that their deformation path is characterized by a 'flip strain' instability which restores the fcc structure after full relaxation by interchanging the [110] and [100] directions. The structures of trialuminides have a close structural relationship with fcc metals. We found that L1(2)-type trialuminides respond to tension along the [110] direction differently to fcc metals, and the 'flip strain' mechanism is not active here. Their deformation path is strongly affected by TM-TM interaction acting along the [001] direction. In contrast, the D0(22)-type trialuminides react in the same way as the fcc metals and regenerate with the same 'flip strain' mechanism.
