The Effect of Small Additions of Fe and Heavy Deformation on the Precipitation in an Al–1.1Mg–0.5Cu–0.3Si At. Pct Alloy

Metallurgical and Materials Transactions A - The effect of 0.03 and 0.08 at. pct Fe additions on the formation of secondary phases in an Al–1.1Mg–0.5Cu–0.3Si at. pct alloy was...     

Hierarchical nanostructures of PbTiO3 through mesocrystal formation

Novel hierarchical self-assembled structures; bur-like PbTiO3 nanostructures were made by self-assembly of PbTiO3 nanocrystals under hydrothermal conditions using sodium dodecylbenzene sulfonate surfactant. The bur-like nanostructures exhibit a unique geometrical shape with cores of agglomerated nanocrystals and outershells of nanorods. The nanorods were between 30 nm and 100 nm in diameter and from several hundred nm up to 2 microm in length. We demonstrate that these nanostructures are formed in a two step process where agglomeration of PbTiO3 nanoparticles into microspheres occurs in a first step, followed by assembly of cube-shaped nanoparticle building blocks into PbTiO3 mesocrystals in a second step. The mesocrystals continuously grow into nanorods from the surface of the microspheres acting as a substrate.     

Clustering and Vacancy Behavior in High- and Low-Solute Al-Mg-Si Alloys

The precipitate microstructure and vacancy distribution in Al-Mg-Si alloys with different amounts of solute and different heat treatments were investigated by transmission electron microscopy and muon spin relaxation measurements. A high amount of vacancies is normally present in Al-Mg-Si alloys as these bind to atomic clusters. We observe these vacancies to leave the material not before over-aging at very high temperatures such as 623 K (350 °C), meaning that vacancies do not bind to incoherent over-aged precipitates. For samples only stored at room temperature after solution heat treatment, a reduction of muon trapping was found at a temperature of 140 K (?133 °C) when reducing the amount of solute in the alloy. This might be connected to a lower number density of Cluster (1), which contrary to Cluster (2) do not nucleate precipitates upon further aging of the material.     

Thermal and mechanical properties of LaNbO4-based ceramics

Thermal and mechanical properties of polycrystalline La_1−xA_xNbO₄ (x = 0, 0.005, 0.02 and A = Ca, Sr and Ba) are reported. The materials possess a ferroelastic to paraelastic phase transition close to 500 °C, and the linear thermal expansion is significantly lower (8.6 ± 0.5 × 10⁻⁶ °C⁻¹) for the paraelastic phase compared to the ferroelastic phase (15 ± 3 × 10⁻⁶ °C⁻¹). The hardness was significantly higher for acceptor doped materials (6 GPa) compared to pure LaNbO₄ (3 GPa) due to a significantly smaller average grain size. The fracture toughness of La_0.98Sr_0.02NbO₄, measured by single edge V-notched beam method, was 1.7 ± 0.2 MPa m^1/2 independent of temperature up to 600 °C. The ferroelastic properties of the materials were confirmed by non-linear relationships between stress and strain during compression/decompression, a remnant strain after decompression and the presence of ferroelastic domains. The mechanical properties of LaNbO₄-based materials are discussed with focus on ferroelasticity, microcracking due to crystallographic anisotropy and pinning of ferroelastic domain boundaries.     

Atomic Structures of Precipitates in Al–Mg–Si Alloys with Small Additions of Other Elements

In Al–Mg–Si alloys, additions of only a few weight percent of Mg and Si enable formation of hardening precipitates during heat treatment. The precipitation is complex and is influenced by chemical compositions and thermo‐mechanical treatment. Structural analysis at the atomic scale has played an important role for understanding the Al–Mg–Si system. This review paper gives a summary of the influence of elements on the precipitate structures of Al–Mg–Si alloys at the atomic scale. The structures are modified by small additions of different elements, but all the encountered precipitates are structurally connected with the Si network, except for the main hardening phase which exhibit a partially discontinuous Si network. The influence of the selected elements (Li, Cu, Zn, Ge, Ag, Ni, Co, and Au) is discussed in detail.

     

Improving Thermal Stability in Cu-Containing Al-Mg-Si Alloys by Precipitate Optimization

It is demonstrated that good thermal stability in Al-Mg-Si-Cu aluminum alloys correlates with a high density of fine lath-shaped, Cu-containing, disordered L-precipitates. Alloys optimized for L retained hardness above 90 HV after 3 weeks over-aging at 473 K (200 °C). Further improvement was achieved by substituting Si by Ge in one alloy. High-angle annular dark-field scanning transmission electron microscopy showed that at peak-hardness conditions, L coexists with more common needle-shaped precipitates, often with Cu-enriched interfaces.     

Orientation of silicon particles in a binary Al–Si alloy

The orientations Si-crystals take in aluminium, in an alloy with composition Al–1.3at%Si, were investigated by transmission electron microscopy. Hardness was measured for isothermal heat-treatments at 175 °C and 260 °C. Conditions analysed by TEM were 17 h at 175 °C and an additional 3 h at 260 °C, both containing a high density of small Si-crystals, the finest corresponding to 175 °C. Two main orientation relationships were found: The first accounted for approximately 60% of Si precipitates in condition 17 h_175 °C. Despite a high number density and well-aligned interfaces, the Si precipitates have negligible influence on hardness. Findings are consistent with Ge particles in Al–Ge alloys.     

Microstructural studies of self-supported (1.5–10 μm) Pd/23 wt%Ag hydrogen separation membranes subjected to different heat treatments

The microstructure of self-supported 1.5–10-μm thick Pd/23 wt%Ag membranes grown by magnetron sputtering have been studied after heat treatment and hydrogen permeation tests using electron microscopy and synchrotron X-ray diffraction. After hydrogen flux stabilization and permeance measurements at 300 °C, the membranes were annealed in air at 300 °C or in N₂/Ar at 300/400/450 °C for 4 days and then tested for hydrogen permeation. The permeation results show that changes in permeability depend on the treatment atmosphere and temperature, as well as membrane thickness. Air treatment at ~300 °C generally induced a positive effect on permeation in the thickness range of 1.5–10 μm. Significant microstructural changes, including grain growth, strain relief, void formation, and growth of nodules occurred in the membranes. The changes in microstructure are more severe for the thinner membranes, and may be attributed mainly to the oxidation processes at or near the surface. For samples annealed in N₂/Ar, enhanced permeation was only obtained with treatment at ~450 °C for 5 and 10 μm. The changes in the microstructure generally increased with heat-treatment temperature, and decreased with membrane's thickness. The membrane with enhanced permeation was accompanied by significant grain growth, strain relief, and surface roughening. For all the membranes, the relative changes in the microstructure were substantially more prominent on the permeate surface than on the feed surface. Details of the analysis are presented and discussed.     

Aberration-corrected scanning transmission electron microscopy study of β′-like precipitates in an Al–Mg–Ge alloy

Precipitates in an Al–Mg–Ge alloy similar to the β′ phase in Al–Mg–Si alloys were investigated using qualitative and quantitative aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). The needle-shaped β′-Ge precipitates are coherent with the Al matrix along the needle direction which is parallel to a 〈0 0 1〉_Al direction, as well as one direction in the cross-section plane that is parallel to a 〈1 0 0〉_Al direction. This is linked to a smaller lattice parameter in the needle cross-section plane than that of coarser, less coherent β′ precipitates in Al–Mg–Si alloys, despite Ge having a larger atomic radius than Si. Quantitative HAADF STEM results show that the nominal Ge columns of the β′-Ge precipitates are not fully occupied by Ge, the intensity of these columns being consistent with an Al concentration of 30 ± 10%, or a vacancy concentration of 20 ± 10%. The coherent interface is atomically smooth, but with matrix columns containing Ge spaced periodically along the interface.