Intermetallic compounds (IMCs) formation during friction stir welding (FSW) of dissimilar metals such as aluminum (Al) and steel (St) is known to cause a reduction in the joint strength. IMCs are brittle and act as the preferential sites for fracture propagation. In the present study, a modified FSW was used to join a commercially pure Al to a mild carbon St. Several characterization techniques were performed to analyze the joint interface and the results revealed that a nanostructured IMC layer made of Fe4Al13 (θ) and a micrometer IMC layer made of Fe2Al5 (η) were formed adjacent to the Al side and St side, respectively. During the tensile test, the crack propagated through the interface of η-St and not through the IMCs layer. It was shown that the interface of η-St was prone to brittle debonding under normal load, but it showed a slight plastic deformation at the triple junctions with the η grain boundaries. At the triple junctions of boundaries, the loading state was shear and no debonding occurred during the tensile test. This promoted a pseudo-brittle fracture surface. The results of the present study showed that the grain refinement of the IMCs can improve the joint strength of the dissimilar materials through increasing the fracture toughness of the IMC and increasing the number of triple junctions at the interface of metal-IMC.

A comprehensive analysis of a pseudo-brittle fracture at the interface of intermetallic of η and steel in aluminum/steel joints made by FSW: Microstructure and fracture behavior

Oxidation behaviors of compositionally complex MAX phases in air

Recently, a novel medium-entropy transition-metal (TM) carbide (TiZrHf)C has been synthesized with high flexural strength. However, the underlying mechanisms responsible for its outstanding properties are not well understood. Herein, the crystal, mechanical, electronic and thermodynamic properties of (TiZrHf)C are systematically investigated by the first-principles density functional theory (DFT) calculations. The obtained results reveal that the (TiZrHf)C structure is stable in the pressure range of 0 GPa–100 GPa. The elemental random distribution and atomic size difference give rise to local lattice distortion. The lattice distortion, elastic moduli, anisotropy, sound velocity and Debye temperature of (TiZrHf)C increase with the increase of pressure. Electron structure analysis shows that (TiZrHf)C has metallic, ionic and covalent bonding features. The thermodynamic calculation reveals that the thermal expansion and heat capacity of (TiZrHf)C are consistent with that of binary carbides, implying (TiZrHf)C can be used for high temperature applications like traditional carbides. 

A DFT insight into the mechanical, electronic and thermodynamic properties of (TiZrHf)C medium-entropy carbide ceramic

Abstract The preparation of high-entropy (HE) ceramics with designed composition is essential for verifying the formability models and evaluating the properties of the ceramics. However, inevitable oxygen contamination in non-oxide ceramics will result in the formation of metal oxide impurity phases remaining in the specimen or even escaping from the specimen during the sintering process, making the elemental compositions of the HE phase deviated from the designed ones. In this work, the preparation and thermodynamic analysis during the processing of equiatomic 9-cation HE carbide (HEC9) ceramics of the IVB, VB, and VIB groups were studied focusing on the removing of the inevitable oxygen impurity existed in the starting carbide powders and the oxygen contamination during the powder mixing processing. The results demonstrate that densification by spark plasma sintering (SPS) by directly using the mixed powders of the corresponding single-component carbides will inhibit the oxygen-removing carbothermal reduction reactions, and most of the oxide impurities will remain in the sample as (Zr,Hf)O 2 phase. Pretreatment of the mixed powders at high temperatures in vacuum will remove most part of the oxygen impurity but result in a remarkable escape of gaseous Cr owing to the oxygen-removing reaction between Cr 3 C 2 and various oxide impurities. It is found that graphite addition enhances the oxygen-removing effect and simultaneously prevents the escape of gaseous Cr. On the other hand, although WC, VC, and Mo 2 C can also act as oxygen-removing agents, there is no metal-containing gaseous substance formation in the temperature range of this study. By using the heat-treated powders with added graphite, equiatomic HEC9 ceramics were successfully prepared by SPS. 

Equiatomic 9-cation high-entropy carbide ceramics of the IVB, VB, and VIB groups and thermodynamic analysis of the sintering process

http://manuscript.elsevier.com/S0927025622002373/pdf/S0927025622002373.pdf

The role of entropy and enthalpy in high entropy carbides

In the last decade, single-phase high-entropy materials have attracted considerable research interest owing to their unexpected existence and unique combinations of properties. Recent development of 5-cation high-entropy carbides (HECs) has demonstrated alluring properties compared to the rule of mixtures and binary carbides. Proposed here is the development of ultrahigh-entropy carbides (UHECs) containing 6+ principal elements with greater combinatorial possibilities. The use of 6+ multi-cation compositions allows for the design of ceramics with further tunable properties, while likely possessing higher orders of entropic stabilization. There are 133 possible carbide compositions containing 6, 7, 8, or 9 refractory metal cations in equiatomic ratios. Candidate selection for fabrication and material testing was accelerated using a machine learning model that was originally trained to predict the synthesizability of five cation disordered metal carbides. Two compositions from each category of six through eight cations, one containing Cr and one without, plus the one possible nine cation carbide were fabricated and characterized. The potential for these 6+ cation UHECs as improved materials for oxidative environments is demonstrated by comparing the oxidation performance of a 5- and 7-cation system after 10 min at 1973 K in air. The oxidation behavior is correlated with Ellingham diagrams, and it is demonstrated that the 7-cation carbide has the ability to form a transitional stable 5+ cation HEC layer as elements preferentially form oxides, which results in significantly improved oxidation resistance.

Samuel D. Figueroa

Papers

Development of ultrahigh-entropy ceramics with tailored oxidation behavior

Mesoscale hetero-deformation induced (HDI) stress in FeAl-based metallic-intermetallic laminate (MIL) composites