Calin Daniel Marioara

Single-Walled and Multi-Walled Carbon Nanotubes Promote Allergic Immune Responses in Mice

The adjuvant effect of particles on allergic immune responses has been shown to increase with decreasing particle size and increasing particle surface area. Like ultrafine particles, carbon nanotubes (CNTs) have nano-sized dimensions and a large relative surface area and might thus increase allergic responses. Therefore, we examined whether single-walled (sw) and multi-walled (mw) CNTs have the capacity to promote allergic responses in mice, first in an sc injection model and thereafter in an intranasal model. Balb/cA mice were exposed to three doses of swCNT, mwCNT, as well as ultrafine carbon black particles (ufCBPs, Printex90) during sensitization with the allergen ovalbumin (OVA). Five days after an OVA booster, OVA-specific IgE, IgG1, and IgG2a antibodies in serum and the numbers of inflammatory cells and cytokine levels in bronchoalveolar lavage fluid (BALF) were determined. Furthermore, ex vivo OVA-induced cytokine release from mediastinal lymph node (MLN) cells was measured. In separate experiments, differential cell counts were determined in BALF 24 h after a single intranasal exposure to the particles in the absence of allergen. We demonstrate that both swCNT and mwCNT together with OVA strongly increased serum levels of OVA-specific IgE, the number of eosinophils in BALF, and the secretion of Th2-associated cytokines in the MLN. On the other hand, only mwCNT and ufCBP with OVA increased IgG2a levels, neutrophil cell numbers, and tumor necrosis factor-alpha and monocyte chemoattractant protein-1 levels in BALF, as well as the acute influx of neutrophils after exposure to the particles alone. This study demonstrates that CNTs promote allergic responses in mice.

Composition of β″ precipitates in Al–Mg–Si alloys by atom probe tomography and first principles calculations

The composition of β″ precipitates in an Al–Mg–Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at 175 °C for 36 h shows that the β″ phase contains ∼20 at. % Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published β″-Mg5Si6 structure. Ab initio calculations show that replacing the Si3-site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the β″-phase.

The influence of composition and natural aging on clustering during preaging in Al–Mg–Si alloys

This work provides a detailed atom probe tomography study of clustering in the Al–Mg–Si system. Focus is on separating and understanding the influence of natural aging, preaging, and alloy composition on the clustering behavior of solute atoms. Two dilute alloys with the same total solute content have been studied, one Mg-rich and one Si-rich. The detrimental effect of natural aging for these alloys is investigated by comparing directly preaged samples to samples stored at room temperature before the preaging treatment. Clusters were identified in the atom probe datasets by the maximum separation method employing heuristically determined input parameters. It was found that seven days of intermediate natural aging gave a five times lower number density of clusters as compared to direct preaging for both alloy types. The clusters were of comparable size but their compositions depended on heat treatment history. Preaging promoted the formation of clusters with an Mg:Si ratio close to 1 in both alloys, while ...

The effect of Zn on precipitation in Al–Mg–Si alloys

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185 °C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.

HRTEM study of the effect of deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy

The effect of 10% pre-ageing deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy aged 10 min at 190°C was investigated by high-resolution transmission electron microscopy (HRTEM) in ⟨100⟩Al projections. The precipitate nucleation was heterogeneous since all precipitates were found to grow on dislocation lines. The pre-ageing deformation suppresses growth of Gunier–Preston zones and β″ phase. The resulting precipitates are still largely coherent with the aluminium matrix. They appear with two main morphologies; one consists of independent, small cross-sections arising from needles with disordered β′ and B′ structures. The other morphology is a much more continuous decoration where precipitates have elongated and conjoined cross-sections and where a particular precipitate phase could not be determined. All precipitates in this work were found to contain a common near-hexagonal sub-cell (SC) with projected bases a = b ≈ 0.4 nm. This strongly indicates that they are built over the same Si network, which recently has been demonstrated to exist in all precipitates in the Al–Mg–Si(–Cu) system. For the discrete morphology type the network has one hexagonal base vector parallel to or very near a ⟨510⟩Al direction. For the continuous type, one base vector falls along a ⟨100⟩Al direction. This orientation of the network is different from previous studies of ternary Al–Mg–Si alloys and must be a direct consequence of the deformation.

Aberration-corrected HAADF-STEM investigations of precipitate structures in Al–Mg–Si alloys with low Cu additions

Precipitates in a lean Al–Mg–Si alloy with low Cu addition (~0.10 wt.%) were investigated by aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Most precipitates were found to be disordered on the generally ordered network of Si atomic columns which is common for the metastable precipitate structures. Fragments of known metastable precipitates in the Al–Mg–Si–(Cu) alloy system are found in the disordered precipitates. It was revealed that the disordered precipitates arise as a consequence of coexistence of the Si-network. Cu atomic columns are observed to either in-between the Si-network or replacing a Si-network column. In both cases, Cu is the center in a three-fold rotational symmetry on the Si-network. Parts of unit cells of Q′ phase were observed in the ends of a string-type precipitates known to extend along dislocation lines. It is suggested that the string-types form by a growth as extension of the B′/Q′ precipitates initially nucleated along dislocation lines. Alternating Mg and Si columns form a well-ordered interface structure in the disordered Q′ precipitate. It is identical to the interface of the Q′ parts in the string-type precipitate.

Applying precipitate–host lattice coherency for compositional determination of precipitates in Al–Mg–Si–Cu alloys

The present paper reports experimental and theoretical studies on the structure of the metastable C precipitate forming during precipitation hardening in Al–Mg–Si–Cu (6xxx) alloys. We describe the procedure of deriving an initial unit cell model based on experimental data and how this is further refined by quantitative use of nanobeam electron diffraction patterns. A reliable 3D refinement was prevented by the small precipitate thickness and its disorder/intergrowth with other phases, necessitating the development of a more theoretically based methodology for precipitate composition determination. We find that for experimental results to be acceptably reproduced in density functional theory-based calculations on bulk candidate structures, these would have to not only minimise precipitate formation enthalpy, but also reproduce the experimentally reported negligible lattice mismatch with the Al matrix along the precipitate main growth direction. We argue, through comparison of the isostructural Q′ and Q precipitate phases of Al–Mg–Si–Cu alloys, why the experimentally reported (semi-)coherency of metastable precipitates must be included as an optimisation criterion in a general theoretical analysis. The C phase structure determined has a monoclinic unit cell with space group P21/m, cell dimensions a C = 10.32 Å, b C = 4.05 Å, c C = 8.10 Å, β C = 100.9°, coherency relations with the matrix:  ∥ ⟨150⟩Al,  ∥ ⟨001⟩Al,  ∥ ⟨100⟩Al and composition Mg4AlSi3+ x Cu1− x , with x ∼ 0.3. The non-integer number of atoms in the unit cell emphasises that the concept of a unit cell for this phase is weakly defined.

Atomic structure of hardening precipitates in an Al–Mg–Zn–Cu alloy determined by HAADF-STEM and first-principles calculations: relation to η-MgZn2

The structures of two nanoscale plate precipitates prevalent at maximum strength and over-aged conditions in a 7449 Al–Mg–Zn–Cu alloy were investigated. Models derived from images of high angle annular dark field scanning transmission electron microscopy were supported by first-principles calculations. Both structures are closely linked to the η-MgZn2 Laves phase through similar layers of a rhombohedral atomic subunit. The finest plate contains one such layer together with a layer of an orthorhombic unit. The second plate contains rhombohedral layers only, normally four, but rotated relatively to form different stacking variants, one of which may be likened to η. For both structures, the same atomic planes describe the main interface with Al. Both plates could be described in space group P3. The unit cells comprise interface and arbitrary numbers of {111}Al (habit) planes. Eight Al-planes were included in the first-principles calculations. The enthalpy indicates high layer/unit stability. The plate thickness can be understood by a simple mismatch formulation.

Phase stabilization principle and precipitate-host lattice influences for Al–Mg–Si–Cu alloy precipitates

In this work, we seek to elucidate a common stabilization principle for the metastable and equilibrium phases of the Al–Mg–Si–Cu alloy system, through combined experimental and theoretical studies. We examine the structurally known well-ordered Al–Mg–Si–Cu alloy metastable precipitates along with experimentally observed disordered phases, using high angle annular dark field scanning transmission electron microscopy. A small set of local geometries is found to fully explain all structures. Density functional theory based calculations have been carried out on a larger set of structures, all fully constructed by the same local geometries. The results reveal that experimentally reported and hypothetical Cu-free phases from the set are practically indistinguishable with regard to formation enthalpy and composition. This strongly supports a connection of the geometries with a bulk phase stabilization principle. We relate our findings to the Si network substructure commonly observed in all Mg–Al–Si(–Cu) metastable precipitates, showing how this structure can be regarded as a direct consequence of the local geometries. Further, our proposed phase stabilization principle clearly rests on the importance of metal-Si interactions. Close links to the Al–Mg–Si precipitation sequence are proposed.

Effects of Germanium, Copper, and Silver Substitutions on Hardness and Microstructure in Lean Al-Mg-Si Alloys

It is shown that strength loss in a 6060 Al-Mg-Si alloy caused by reduction in solute can be compensated by adding back smaller quantities of Ag, Ge, and Cu. Nine alloys were investigated. Ge was found to be the most effective addition, strongly refining the precipitation. The hardness is discussed in terms of statistics of the precipitates near a T6 condition, as acquired by transmission electron microscopy (TEM). Precipitates in some conditions were also investigated by high-angle annular dark-field scanning TEM. The added elements have strong influence on the main hardening precipitate, β″, changing its structure and promoting disorder.