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Secondary phases, either introduced by alloying or heat treatment, are commonly present in most high-entropy alloys (HEAs). Understanding the formation of secondary phases at high temperatures, and their effect on mechanical properties, is a critical issue that is undertaken in the present study, using the Al x CoCrFeNi (x = 0.3, 0.5, and 0.7) as a model alloy. The in-situ transmission-electron-microscopy (TEM) heating observation, an atom-probe-tomography (APT) study for the reference starting materials (Al 0.3 and Al 0.5 alloys), and thermodynamic calculations for all three alloys, are performed to investigate (1) the aluminum effect on the secondary-phase fractions, (2) the annealing-twinning formation in the face-centered-cubic (FCC) matrix, (3) the strengthening effect of the secondary ordered body-centered-cubic (B2) phase, and (4) the nucleation path of the σ secondary phase thoroughly. The present work will substantially optimize the alloy design of HEAs and facilitate applications of HEAs to a wide temperature range. [ABSTRACT FROM AUTHOR]

Joining NiTi to Ti6Al4V is of great interest for applications in the biomedical and aerospace fields. Despite the importance, no joining techniques have been developed that avoid the formation of brittle intermetallics to produce high strength joints. In this work, Niobium was used as an interlayer to prevent the formation of these brittle phases when joining NiTi to Ti6Al4V. The presence of this interlayer ensured that crack free welds were obtained and no brittle intermetallic compounds were observed. The Niobium interlayer was of a much higher melting temperature than the base materials so the bulk Niobium did not melt during the joining process, acting as a diffusion barrier between the NiTi and Ti6Al4V. The laser was focused on the Ti6Al4V side of the joint, which joined the Ti6Al4V and Niobium by fusion welding. At this interface a (Ti, Nb) region was formed due to dilution of the Niobium and mixing with the Ti6Al4V. At the NiTi–Nb interface a eutectic reaction was responsible for joining. Mechanical testing of the joints revealed that the minimum tensile strength matched the ultimate tensile strength of the weakest material, Niobium. These results highlight new possibilities for the use of high melting point filler materials when joining NiTi to dissimilar materials, so that the formation of undesired phases can be avoided. [ABSTRACT FROM AUTHOR]

Ternary zinc-aluminum-magnesium (ZnAlMg) alloy coatings are the focus of significant attention in the automotive and steel industries due to several advantages over traditional Zn-based coatings. Currently, the literature on this type of coating is limited and focuses mainly on their corrosion resistance and room temperature tensile properties. To assess the relevance of ZnAlMg coatings in current manufacturing processes such as hot stamping and welding of advanced high strength steels (AHSS), it is essential to understand their high-temperature performance, particularly their resistance to liquid metal embrittlement (LME) cracking. This study showed that the ZnAlMg coating had complete resistance to LME cracking at a temperature of about 900 °C, which is traditionally recognized as the temperature at which the highest levels of LME susceptibility are observed in the different families of AHSS. Elemental distribution analysis confirmed that due to an increase in the testing temperature, the lamellar eutectic microstructure of the coating dissolved into the Zn-matrix, with the constituent elements, Al and Mg, segregating towards the steel substrate and the coating surface, respectively. This led to the in-situ formation of a uniform α-Fe(Zn, Al) layer at the steel/coating interface which prevented the direct contact of liquid metal with the steel substrate, resulting in complete suppression of LME at high temperature. Numerical calculations of interdiffusion flux were used to investigate the diffusion behavior of the elements of interest at the interface which indicated that the α-Fe(Zn, Al) layer formed due to the high diffusion rate of Al towards the Fe substrate at 900 °C. The effectiveness of the α-Fe(Zn, Al) layer in mitigating LME was evaluated by calculating the work of adhesion, which showed that this layer preserved its integrity under an applied tensile load, successfully mitigating the initiation of LME. The findings of this study offer valuable insights into developing new avenues for advancing LME resistant coatings by utilizing ternary Zn-based alloy systems. [Display omitted] [ABSTRACT FROM AUTHOR]