The influence of alloy composition on precipitates of the Al-Mg-Si system

To study how changes in solute elements affect precipitation, six Al-Mg-Si alloys aged at 175 °C were investigated by transmission electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears after 3 hours that correlates with a high density of fine Guinier-Preston (GP) zones. A second, broader peak correlates with β″ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The β″-Mg₅Si₆ phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of β′, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of β″ and β′ develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior.     

Hydrogen absorption kinetics and structural features of NaAlH4 enhanced with transition-metal and Ti-based nanoparticles

The hydrogen cycled (H) planetary milled (PM) NaAlH₄ + xM (x < 0.1) system (M = 30 nm Ag, 80 nm Al, 2–3 nm C, 30 nm Cr, 25 nm Fe, 30 nm Ni, 25 nm Pd, 65 nm Ti) has been studied by high resolution synchrotron powder X-ray diffraction. Isothermal absorption kinetic isotherms have been measured over the first two H cycles. The PM NaAlH₄ + 0.1Ti system has also been studied by high resolution transmission electron microscopy (TEM). 80 nm Al and 2–3 nm C were inactive, and would not allow hydrogen (H) desorption from NaAlH₄. 30 nm Cr, 25 nm Fe, 30 nm Ni, and 25 nm Pd showed activity, but with weak kinetics of only ca. 1 wt.% H/hour. The NaAlH₄ + 0.1Ti system displays absorption kinetics of ca. 7 wt.% H/hour, comparable to TiCl₃ enhanced NaAlH₄ after five H cycles. After H cycling the PM NaAlH₄ + 0.1Ti system, we observe a body centred tetragonal (bct) χ-TiH₂ phase, which displays intense anisotropic peak broadening. The broadening is evident as a massive dislocation density of ca. 1.20 × 10¹⁷/m² in high resolution TEM images of the χ-TiH₂ phase. All originally added Ti can be accounted for in the bct χ-TiH₂ phase by quantitative phase analysis (QPA) after five H cycles. The PM NaH + Al + 0.02 (Ti-nano-alloy) system shows absorption kinetic rates in the order TiO₂ > TiN > TiC > Ti, with rapid hydrogenation kinetics of ca. 23 wt.% H/hour for TiO₂ enhanced NaAlH₄, equivalent to TiCl₃ enhanced NaAlH₄. The TiN and TiC are partially reduced by ca. 7 and 22% respectively, and the TiO₂ is completely reduced. The location of the reduced Ti cannot be discerned by X-ray diffraction at these minor proportions.     

PbTiO(3) nanorod arrays grown by self-assembly of nanocrystals

Arrays of ferroelectric lead titanate (PbTiO(3)) nanorods have been grown on a substrate by a novel template-free method. Hydrothermal treatment of an amorphous PbTiO(3) precursor in the presence of a surfactant and PbTiO(3) or SrTiO(3) substrates resulted in the growth of PbTiO(3) nanorod arrays aligned perpendicular to the substrate surface. Two steps in the growth mechanism were demonstrated: first an epitaxial layer was formed on the substrate; this was followed by self-assembly of nanocrystals forming a mesocrystal layer which matured into arrays of PbTiO(3) nanorods.     

Muon kinetics in heat treated Al (–Mg)(–Si) alloys

Al–Mg–Si alloys are heat-treatable and rely on precipitation hardening for their mechanical strength. We have employed the technique of muon spin relaxation to further our understanding of the complex precipitation sequence in this system. The muon trapping kinetics in a material reveals a presence of atom-sized defects, such as solute atoms (Mg and Si) and vacancies. By comparing the muon kinetics in pure Al, Al–Mg, Al–Si and Al–Mg–Si when held at different temperatures, we establish an interpretation of muon trapping peaks based on different types of defects. Al–Mg–Si samples have a unique muon trapping peak at temperatures around 200 K. This peak is highest for samples that have been annealed at 70–150 °C, which have microstructures dominated by a high density of clusters/Guinier–Preston zones. The muon trapping is explained by the presence in vacancies inside these structures. The vacancies disappear from the material when the clusters transform into more developed precipitates during aging.     

The influence of alloy composition on precipitates of the Al−Mg−Si system

To study how changes in solute elements affect precipitation, six Al−Mg−Si alloys aged at 175 °C were investigated by transmission electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears after 3 hours that correlates with a high density of fine Guinier-Preston (GP) zones. A second, broader peak correlates with β″ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The β″-Mg₅Si₆ phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of β′, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of β″ and β′ develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior.     

Comparison of TEM specimen preparation of perovskite thin films by tripod polishing and conventional ion milling

In this article, the effects of the transmission electron microscopy (TEM) specimen preparation techniques, such as ion milling and tripod polishing on perovskite oxides for high-resolution TEM investigation, are compared. Conventional and liquid nitrogen cooled ion milling induce a new domain orientation in thin films of SrRuO(3) and LaFeO(3) grown on (001)-oriented SrTiO(3) substrates. This is not observed in tripod-polished specimens. Different ion milling rates for thin films and substrates in cross-section specimens lead to artefacts in the interface region, degrading the specimen quality. This is illustrated by SrRuO(3) and PbTiO(3) thin films grown on (001)-oriented SrTiO(3). By applying tripod polishing and gentle low-angle, low-energy ion milling while cooling the sample, the effects from specimen preparation are reduced resulting in higher quality of the TEM study. In the process of making face-to-face cross-section specimens by tripod polishing, it is crucial that the glue layer attaching the slabs of material is very thin (<50 nm).