Irina Yu. Grubova

Context. Understanding where up to 99% of the expected sulfur is hidden in dense molecular clouds remains one of the long-standing unresolved problems in astrochemistry. Binding energies (BEs) control desorption, diffusion, and residence times of molecules on amorphous-solid-water (ASW) mantles and hence also determine these input parameters in gas–grain models. The effect of excess negative charge in ASW on sulfur adsorption remains entirely unexplored. Aims. We aim to derive statistically robust and physically interpretable BE distributions for atomic S and the sulfur-bearing molecules H ₂ S, SO ₂ , and OCS on neutral and negatively charged ASW and assess how excess negative charge alters their retention and potential role in the sulfur reservoir of cold, dense molecular clouds. Methods. The basis set superposition error (BSSE) and zero-point energy (ZPE) corrected BEs of S, SO ₂ , OCS, and H ₂ S were calculated via density functional theory (DFT) using the ORCA software. Molecule-specific DFT levels of theory were first selected from benchmark calculations on neutral and charged small water complexes against coupled-cluster reference energies. The selected protocols were then applied to study adsorption on neutral and charged ASW clusters. A range of adsorption sites were sampled on five independent amorphous ice clusters, yielding BE distributions that account for the site heterogeneity of ASW. Results. Neutral ASW yields broad, site-dependent BE distributions consistent with previous water-ice estimates. On charged ASW, three general cases are identified: the BE always increases due to electron transfer (S-atom, SO ₂ ), increases slightly without any electron transfer (H ₂ S), or remains the same unless an electron transfer occurs under specific conditions (OCS). These findings are inherently linked to the molecular properties of these molecules. Conclusions. Excess negative charge does not uniformly increase sulfur binding on ASW. Instead, it produces molecule-specific adsorption regimes, showing that charged ASW must be treated explicitly in gas–grain models of sulfur chemistry.

New β-stabilized Ti-based alloys are highly promising for bone implants, thanks in part to their low elasticity. The nature of this elasticity, however, is as yet unknown. We here present combined first-principles DFT calculations and experiments on the microstructure, structural stability, mechanical characteristics, and electronic structure to elucidate this origin. Our results suggest that the studied β Ti-35Nb-7Zr-5Ta wt % (TNZT) alloy manufactured by the electron-beam powder bed fusion (E-PBF) method has homogeneous mechanical properties (H = 2.01 ± 0.22 GPa and E = 69.48 ± 0.03 GPa) along the building direction, which is dictated by the crystallographic texture and microstructure morphologies. The analysis of the structural and electronic properties, as the main factors dominating the chemical bonding mechanism, indicates that TNZT has a mixture of strong metallic and weak covalent bonding. Our calculations demonstrate that the softening in the Cauchy pressure (C' = 98.00 GPa) and elastic constant C̅₄₄ = 23.84 GPa is the origin of the low elasticity of TNZT. Moreover, the nature of this softening phenomenon can be related to the weakness of the second and third neighbor bonds in comparison with the first neighbor bonds in the TNZT. Thus, the obtained results indicate that a carefully designed TNZT alloy can be an excellent candidate for the manufacturing of orthopedic internal fixation devices. In addition, the current findings can be used as guidance not only for predicting the mechanical properties but also the nature of elastic characteristics of the newly developed alloys with yet unknown properties.

The β-type Ti–42Nb alloy has been successfully manufactured from pre-alloyed powder using the E-PBF method for the first time. This study presents thorough microstructural investigations employing diverse methodologies such as EDS, XRD, TEM, and EBSD, while mechanical properties are assessed using UPT, nanoindentation, and compression tests. Microstructural analysis reveals that Ti–42Nb alloy primarily consisted of the β phase with the presence of a small amount of nano-sized α″-martensite formed upon fast cooling. The bimodal-grained microstructure of Ti–42Nb alloy comprising epitaxially grown fine equiaxed and elongated equiaxed β-grains with an average grain size of 40 ± 28 µm exhibited a weak texture. The study shows that the obtained microstructure leads to improved mechanical properties. Young’s modulus of 78.69 GPa is significantly lower than that of cp-Ti and Ti–6Al–4V alloys. The yield strength (379 MPa) and hardness (3.2 ± 0.5 GPa) also meet the criteria and closely approximate the values typical of cortical bone. UPT offers a reliable opportunity to study the nature of the ductility of the Ti–42Nb alloy by calculating its elastic constants. XPS surface analysis and electrochemical experiments demonstrate that the better corrosion resistance of the alloy in SBF is maintained by the dominant presence of TiO₂ and Nb₂O₅. The results provide valuable insights into the development of novel low-modulus Ti–Nb alloys, which are interesting materials for additive-manufactured implants with the desired properties required for their biomedical applications.

Ti˗Nb-based alloys are attractive biomaterials due to their excellent biocompatibility and suitable mechanical properties for preventing stress-shielding problems and consecutive implant failure. In this work, the bulk specimens of pre-alloyed β-phase Ti˗42Nb alloy were successfully produced for the first time by electron beam melting. The present investigation focuses on the determination of processing parameter window by varying the beam power, scan velocity, line offset, and layer thickness to obtain a material with good properties. Overall, 49 different regimes were studied. The microstructures of the raw and sintered or heated Ti˗42Nb powder were characterized by means of the DSC/TG, XRD, and SEM/EDX analyses. Raw powder exhibits a dendrite-like β-Ti structure, while the ‘sintered’ powder subjected to intense preheating revealed the presence of the peaks corresponding to the martensitic α′′ phase. Based on DSC results, the powder consists of the metastable β-phase with a small amount of α'' martensite phase formed upon cooling to 190 °C. The presence of the martensite phase also was revealed by the visualization of the delocalization of electrons using 2D Fourier maps. Mechanical tests, EBSD, and TEM analysis were performed for the specimens produced with a manufacturing regime with a beam current of 4 mA and a beam velocity of 700 mm/s and 800 mm/s as they had the lowest porosity. Coarse microstructures and strong texture in the direction, as well as grains of about 300 µm, were observed for both regimes. The specimens produced with a beam current of 4 mA and a beam velocity of 800 mm/s have about 50% of equiaxed grains, while the second regime has only 35%. This contributed to the mechanical properties of alloy expressed in hardness of 3.0 GPa and elastic modulus of 68 GPa in comparison with hardness of 3.2 GPa and elastic modulus of 85 GPa for the specimen from the second regime. Due to the manufacturing of low-porous and low-modulus titanium alloys was an initial aim of the current study, the first regime was selected for further corrosion resistance investigations and manufacturing of implants with complex geometries for biological testing.

High-current pulsed electron-beam (PEB) treatment was applied as a surface finishing procedure for Ti–35Nb–7Zr–5Ta (TNZT) alloy produced by electron beam melting (EBM). According to the XRD results the TNZT alloy samples before and after the PEB treatment have shown mainly the single body-centered cubic (bcc) β-phase microstructures. The crystallite size, dislocation density, and microstrain remain unchanged after the PEB treatment. The investigation of the texture coefficient at the different grazing angle revealed the evolution of the crystallite orientations at the re-melted zone formed at the top of the bulk samples after the PEB treatment. The top-view SEM micrographs of the TNZT samples treated by PEB exhibited the bcc β-phase grains with an average size of ~85 μm. TEM analysis of as-manufactured TNZT alloy revealed the presence of the equiaxed β-grains with the fine dispersion of nanocrystalline α and NbTi4 phases together with β-Ti twins. Meanwhile, the β phase regions free of α phase precipitation are observed in the microstructure after the PEB irradiation. Nanoindentation tests revealed that the surface mechanical properties of the melted zone were slightly improved. However, the elastic modulus and microhardness in the heat-affected zone and the deeper regions of the sample were not changed after the treatment. Moreover, the TNZT alloy in the bulk region manufactured by EBM displayed no significant change in the corrosion resistance after the PEB treatment. Hence, it can be concluded that the PEB irradiation is a viable approach to improve the surface topography of EBM-manufactured TNZT alloy, while the most important mechanical parameters remain unchanged.

Hydroxyapatite (HAP) is frequently used as biocompatible coating on Ti-based implants. In this context, the HAP-Ti adhesion is of crucial importance. Here, we report ab initio calculations to investigate the influence of Si incorporation into the amorphous calcium-phosphate (a-HAP) structure on the interfacial bonding mechanism between the a-HAP coating and an amorphous titanium dioxide (a-TiO₂) substrate, contrasting two different density functionals: PBE-GGA, and DFT-D3, which are capable of describing the influence of the van der Waals (vdW) interactions. In particular, we discuss the effect of dispersion on the work of adhesion (W_ad), equilibrium geometries, and charge density difference (CDD). We find that replacement of P by Si in a-HAP (a-Si-HAP) with the creation of OH vacancies as charge compensation results in a significant increase in the bond strength between the coating and substrate in the case of using the PBE-GGA functional. However, including the vdW interactions shows that these forces considerably contribute to the W_ad. We show that the difference (W_ad − W_ad(vdW)) is on average more than 1.1 J m⁻² and 0.5 J m⁻² for a-HAP/a-TiO₂ and a-Si-HAP/a-TiO₂, respectively. These results reveal that including vdW interactions is essential for accurately describing the chemical bonding at the a-HAP/a-TiO₂ interface.