Additive manufacturing of high-strength multiphase nanostructures in the binary Ni–Nb system for the discovery of new types of superalloys

Ni-based superalloys have been studied extensively due to their impressive mechanical properties, including strength and creep resistance at high temperatures. Growing interest surrounding additive manufacturing (AM) methods has led to recent investigations of alloys that are traditionally difficult to process, including Ni-based superalloys. Recent work has shown that AM methods enable high-throughput materials discovery and optimization of difficult- or impractical-to-process alloys, including those with high or even majority refractory element compositions. This work focuses on AM-enabled investigations of composition-dependent mechanical and microstructural properties for Ni–Nb binary alloys. Specifically, we report on the mechanical behavior of compositionally-graded NixNb1−x and uniform composition Ni59.5Nb40.5 specimens made with AM. The AM fabrication process resulted in extraordinarily high strength, attributed to the formation of a dual-phase microstructure consisting of δ-Ni3Nb and µ-Ni6Nb7 intermetallic compounds with nanostructured and multimodal grain size and eutectic lamellar spacing. Graphic Abstract

Dust and minerals in the HR 8799 atmospheres: JWST MIRI prediction

<p>We are learning rapidly about the gas composition of exoplanet atmospheres, but know almost nothing about their solid composition. The upcoming James Webb Space Telescope will radically change this! The HR8799 exoplanetary system is a perfect candidate because it provides us with a unique opportunity of simultaneously measuring mineral clouds and refractory element composition of its four gas giant atmospheres. The HR8799 system is very young and additionally contains two particle belts. The giant planets are predicted to be bombarded with material from the belts, analogous to the Late Heavy Bombardment. Signatures of this bombardment, such as mineral clouds and refractory element composition, might be observable in their atmospheres. JWST MIRI will allow to characterise these exoplanets in the mid-infrared thermal regime (5-28 μm) which is not possible from the ground. We use the ARtful modeling Code for exoplanet Science (ARCiS) to calculate the mid-infrared spectra of planets HR 8799 b, c, d and e, and we simulate MIRI spectroscopic observations. Besides the dust features, we also expect to identify narrower gas features from molecular species such as CO<sub>2</sub>, H<sub>2</sub>O, HCN, C<sub>2</sub>H<sub>2</sub> etc.</p>

CHAPTER 4 High Temperature Oxidation of Stainless Steels

This chapter is dedicated to the description of high temperature oxidation of both chromia and alumina forming alloys. The defect structures of iron and chromium are firstly reviewed. The effects of elements on stainless steel oxidation behaviour are further addressed. For the chromia-forming stainless steel, the oxidation rate is reduced with the increased silicon content but not in a monotonic manner. Titanium and niobium can reduce breakaway oxidation of Fe–18Cr–10Ni austenitic stainless steel. Titanium can enhance the adhesion of scale to the Fe–18Cr by mechanical keying effect of TiO2 formed at the steel/scale interface. For the alumina-forming stainless steel, the formation of alumina and its transformation during oxidation are reviewed. Chromium can be added to reduce the critical aluminium content in the steels in order to form alumina at high temperatures. The addition of reactive elements with appropriate level can improve scale adhesion and reduce the steel oxidation rate. Refractory element like molybdenum can increase strength of material but also accelerate the oxidation rate of the steels containing reactive elements. The development of new alumina-forming austenitic alloy grades is finally described.

Effects of Mo, Nb, Ta, Ti, and Zr on Mechanical Properties of Equiatomic Hf-Mo-Nb-Ta-Ti-Zr Alloys

Nowadays refractory high-entropy alloys (RHEAs) are regarded as great candidates for the replacement of superalloys at high temperature. To design a RHEA, one must understand the pros and cons of every refractory element. However, the elemental effect on mechanical properties remains unclear. In this study, the subtraction method was applied on equiatomic HfMoNbTaTiZr alloys to discover the role of each element, and, thus, HfMoNbTaTiZr, HfNbTaTiZr, HfMoTaTiZr, HfMoNbTiZr, HfMoNbTaZr, and HfMoNbTaTi were fabricated and analyzed. The microstructure and mechanical properties of each alloy at the as-cast state were examined. The solid solution phase formation rule and the solution strengthening effect are also discussed. Finally, the mechanism of how Mo, Nb, Ta, Ti, and Zr affect the HfMoNbTaTiZr alloys was established after comparing the properties of these alloys.

The Structure, Phase Composition and Rheological Properties of Powders Obtained by Chemical Dispersion of Al Alloys with Mo, V and Zr

Introduction of refractory elements into alumina ceramics to improve its properties, is usually carried out by mixing the alumina with oxides of refractory metals. In this work this problem has been solved by pre-alloying refractory element with aluminum and subsequent dispersion of alloy in aqueous alkaline solutions. Characteristics of microstructure, phase composition and rheological properties of powders obtained by chemical dispersion of alloys Al-Mo, Al-V and Al-Zr with 10 wt.% refractory element in 20% aqueous sodium hydroxide solution, as well as the impact of heat treatment at 1250 oC on these properties, have been discussed. On the basis of X-Ray analysis (XRA) and electron microscopy the conclusion was adopted that heat treatment of powder leads to significant phase and structural transformations of such powders and is a necessary stage of preparation for sintering.

Outgassing on stagnant-lid super-Earths

Aims. We explore volcanic CO2-outgassing on purely rocky, stagnant-lid exoplanets of different interior structures, compositions, thermal states, and age. We focus on planets in the mass range of 1–8 M⊕ (Earth masses). We derive scaling laws to quantify first- and second-order influences of these parameters on volcanic outgassing after 4.5 Gyr of evolution. Methods. Given commonly observed astrophysical data of super-Earths, we identify a range of possible interior structures and compositions by employing Bayesian inference modeling. The astrophysical data comprise mass, radius, and bulk compositional constraints; ratios of refractory element abundances are assumed to be similar to stellar ratios. The identified interiors are subsequently used as input for two-dimensional (2D) convection models to study partial melting, depletion, and outgassing rates of CO2. Results. In total, we model depletion and outgassing for an extensive set of more than 2300 different super-Earth cases. We find that there is a mass range for which outgassing is most efficient (~2–3 M⊕, depending on thermal state) and an upper mass where outgassing becomes very inefficient (~5–7 M⊕, depending on thermal state). At small masses (below 2–3 M⊕) outgassing positively correlates with planet mass, since it is controlled by mantle volume. At higher masses (above 2–3 M⊕), outgassing decreases with planet mass, which is due to the increasing pressure gradient that limits melting to shallower depths. In summary, depletion and outgassing are mainly influenced by planet mass and thermal state. Interior structure and composition only moderately affect outgassing rates. The majority of outgassing occurs before 4.5 Gyr, especially for planets below 3 M⊕. Conclusions. We conclude that for stagnant-lid planets, (1) compositional and structural properties have secondary influence on outgassing compared to planet mass and thermal state, and (2) confirm that there is a mass range for which outgassing is most efficient and an upper mass limit, above which no significant outgassing can occur. Our predicted trend of CO2-atmospheric masses can be observationally tested for exoplanets. These findings and our provided scaling laws are an important step in order to provide interpretative means for upcoming missions such as JWST and E-ELT, that aim at characterizing exoplanet atmospheres.

Adaptation of TiC Hard Particles Properties and Morphology in Metal Matrix Composites by Refractory Elements

High mechanical loads, corrosion, and abrasion decrease the lifetime of tools. One way to increase the wear resistance of tool materials can be achieved by adding hard particles to the metal matrix such as titanium carbide, which protect the softer metal matrix against abrasive particles. This material concept is designated as metal matrix composite (MMC). Ferro-Titanit® is such MMC material, possessing high wear and a simultaneously high corrosion resistance, for which reason this material is used in the polymers industry. The material concept is based on a corrosion-resistant Fe-base matrix with up to 45 vol% titanium carbide (TiC) as a hard particle addition to improve the wear resistance against abrasion. These TiC hard particles must be adapted to the present tribological system in terms of hardness, size and morphology. This study shows how the size and morphology of TiC hard particles can be influenced by the refractory element niobium (Nb). Therefore, the element Nb was added with 2 and 4 mass% to the soft-martensitic Ferro-Titanit® Grade Nikro128. The investigated materials were compacted by sintering, and the densified microstructure was further characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), and optical image analyses. Furthermore, microstructure and properties of the compacted Nb-alloyed samples were compared to the reference material Nikro128. The results show that the addition of Nb influences the morphology, size and chemical composition of the TiC hard particle. These changes in the hard phase characteristics also influence the materials properties. It was shown that the phase niobium carbide (NbC) is formed around the TiC during the densification process, leading to a change in morphology and size of the TiC.