Compositionally Complex Alloys: Some Insights from Photoemission Spectroscopy

Photoemission spectroscopy (PES) is an underrepresented part of current and past studies of compositionally complex alloys (CCA) such as high-entropy alloys (HEA) and their derivatives. PES studies are very important for understanding the electronic structure of materials, and are therefore essential in some cases for a correct description of the intrinsic properties of CCAs. Here, we present several examples showing the importance of PES. First, we show how the difference between the split-band structure and the common-band structure of the valence band (VB), observed by PES, can explain a range of properties of CCAs and alloys in general. A simple description of the band crossing in CCAs composed from the early and late transition metals showing a split band is discussed. We also demonstrate how a high-accuracy PES study can determine the variation in the density of states at the Fermi level as a function of Cu content in Ti-Zr-Nb-Ni-Cu metallic glasses. Finally, the first results of an attempt to single out the contributions of particular constituents in Cantor-type alloys to their VBs are presented. The basic principles of PES, the techniques employed in studies presented, and some issues associated with PES measurements are also described.

Transition from High-Entropy to Conventional Alloys: Which Are Better?

The study of the transition from high-entropy alloys (HEAs) to conventional alloys (CAs) composed of the same alloying components is apparently important, both for understanding the formation of HEAs and for proper evaluation of their potential with respect to that of the corresponding CAs. However, this transition has thus far been studied in only two types of alloy systems: crystalline alloys of iron group metals (such as the Cantor alloy and its derivatives) and both amorphous (a-) and crystalline alloys, TE-TL, of early (TE = Ti, Zr, Nb, Hf) and late (TL = Co, Ni, Cu) transition metals. Here, we briefly overview the main results for the transition from HEAs to CAs in these alloy systems and then present new results for the electronic structure (ES), studied with photoemission spectroscopy and specific heat, atomic structure, thermal, magnetic and mechanical properties of a-TE-TL and Cantor-type alloys. A change in the properties of the alloys studied on crossing from the HEA to the CA concentration range mirrors that in the ES. The compositions of the alloys having the best properties depend on the alloy system and the property selected. This emphasizes the importance of knowing the ES for the design of new compositional complex alloys with the desired properties.

The effect of oxygen pickup during selective laser melting on the microstructure and mechanical properties of Ti-6Al-4V lattices

Additive manufacturing techniques such as Selective Laser Melting (SLM) can produce complex shapes with relatively thin sections and fine detail. However, common materials for the process, such as Ti–6Al–4V, have microstructure and properties that are sensitive to the pickup of interstitial impurities, such as oxygen, which the material will be exposed to during the process. This problem would be especially severe for parts with thin sections, where surface effects can be more significant, and where poor properties may coincide with locally-elevated stress. Here we explore the effects of oxygen level in thin sections with the use of lattice materials (materials which can be considered to consist exclusively of near-surface material). Oxygen levels are artificially raised using repeated melting passes to result in more pickup, leading to significantly reduced ductility and hence reduced strength measured in compression. A ductile to brittle transition in strut failure mechanism is found with increasing number of melting passes, with significant modification in chemistry and crystallographic structure, despite the presence of a similar fine plate-like microstructure throughout.

Microstructural Evolution of Rapid Solidified Al-Ni Alloys

<p>In this study, Al-Ni alloy ribbons in hypoeutectic, euctectic and hypereutectic compositions were obtained by melt spinning. The microstructural changes of the samples, from the as-cast to rapid solid-ification processes at different Cu wheel tangential velocities were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The rapid solidification caused <br />morphological transformation of the Al-Al3Ni microstructure from acicular in the as-cast condition to globular in the melt spun ribbons and the growth of the metastable Al9Ni2 intermetallic compound, which acted as reinforcing particles. Due to the increment of the Ni content and the presence of these intermetallic compounds, with size of ~5nm, distributed in the aluminum matrix, the hardness of the alloys significantly increased from the hypoeutectic (90 VHN) to the hypereutectic (205VHN) composition.</p>

Magnetic Properties of Metal - Polymer Composites for Ac Applications at High Frequencies

<p>This paper presents the soft magnetic properties of the Fe-, Ni-, Co- and Fe-Co-Ni polymer composites. The composites were ob-tained with Fe, Ni and Co powders (99.9% purity) with particle sizes of 150, 10 and 2 mm, respectively. Ring-shaped samples were ob-tained using uniaxial compaction at 91 MPa. The Fe-Co-Ni-polymer composite has the highest value of the magnetic permeability (μ=54) <br />at 107 Hz and saturation magnetization (Ms=10000 Am2Kg-1) higher than the Ni-polymer composite (4100 Am2Kg-1).</p>

Microbial Phosphite Oxidation and Its Potential Role in the Global Phosphorus and Carbon Cycles

Phosphite [Formula: see text] is a highly soluble, reduced phosphorus compound that is often overlooked in biogeochemical analyses. Although the oxidation of phosphite to phosphate is a highly exergonic process (E^o'=-650mV), phosphite is kinetically stable and can account for 10-30% of the total dissolved P in various environments. There is also evidence that phosphite was more prevalent under the reducing conditions of the Archean period and may have been involved in the development of early life. Its role as a phosphorus source for a variety of extant microorganisms has been known since the 1950s, and the pathways involved in assimilatory phosphite oxidation have been well characterized. More recently, it was demonstrated that phosphite could also act as an electron donor for energy metabolism in a process known as dissimilatory phosphite oxidation (DPO). The bacterium described in this study, Desulfotignum phosphitoxidans strain FiPS-3, was isolated from brackish sediments and is capable of growing by coupling phosphite oxidation to the reduction of either sulfate or carbon dioxide. FiPS-3 remains the only isolated organism capable of DPO, and the prevalence of this metabolism in the environment is still unclear. Nonetheless, given the widespread presence of phosphite in the environment and the thermodynamic favorability of its oxidation, microbial phosphite oxidation may play an important and hitherto unrecognized role in the global phosphorus and carbon cycles.