Sample Preparation Methods for Scanning Electron Microscopy (original) (raw)
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Acta Materialia, 2011
The precipitates present in an Al-0.59Mg-0.71Ge (at.%) alloy have been studied using aberration-corrected high-angle annular darkfield scanning transmission electron microscopy. Two types of needle-shaped precipitates growing along h0 0 1i Al were found: a phase isostructural to the trigonal U1 phase found in Al-Mg-Si alloys, and finer precipitates with a hexagonal arrangement of Ge columns. The study revealed the presence of a complex interface structure surrounding the U1-like precipitates, and an explanation based on interatomic distances is proposed.
The influence of alloy composition on precipitates of the Al-Mg-Si system
Metallurgical and Materials Transactions A, 2005
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 Љ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The Љ-Mg 5 Si 6 phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of Ј, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of Љ and Ј 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.
The Effects of Low Cu Additions and Predeformation on the Precipitation in a 6060 Al-Mg-Si Alloy
Metallurgical and Materials Transactions A, 2013
Effects of low Cu additions (≤0.10 wt%) and 10% pre-deformation before aging on precipitate microstructures and types in a 6060 Al-Mg-Si alloy have been investigated using transmission electron microscopy (TEM). It was found that pre-deformation enhances precipitation kinetics and leads to formation of heterogeneous precipitate distributions along dislocation lines. These precipitates were often disordered. Cu additions caused finer microstructures, which resulted in highest materials hardness, in both the un-deformed and the pre-deformed conditions. The introduced pre-deformation led to microstructure coarsening. This effect was less pronounced in the presence of Cu. The precipitate structure was studied in detail by high resolution TEM (HRTEM) and high angle annular dark field scanning TEM (HAADF-STEM). The Cu additions did not alter the respective precipitation sequence in either the un-deformed or the pre-deformed conditions, but caused a large fraction of β'' precipitates to be partially disordered in the undeformed conditions. Cu atomic columns were found in all the investigated precipitates, except for perfect β''. Although no unit cell was observed in the disordered precipitates, the presence of a periodicity having hexagonal symmetry along the precipitate length was inferred from the fast Fourier transforms (FFT) of HRTEM images, and sometimes directly observed in filtered HAADF-STEM images.
Journal of Applied Physics, 2009
The composition of Љ 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 Љ phase contains ϳ20 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 Љ-Mg 5 Si 6 structure. Ab initio calculations show that replacing the Si 3 -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 Љ-phase.
Journal of Alloys and Compounds, 2013
The effect of high-intensity ultrasonic treatment (HIUST) on microstructure, hardness and wear behavior in Mg-5wt.%Si hypereutectic alloy has been investigated. The results showed clearly that without HIUST, most of primary Mg 2 Si appeared as coarse dendritic morphology with average size of about 200 μm. With HIUST, the average size of primary Mg 2 Si decreased significantly to about 33 μm and their morphologies changed to polyhedral shape. The modification mechanism is mainly attributed conjugation of two mechanisms: cavitation-enhanced heterogeneous nucleation and cavitation-induced dendrite fragmentation. The alloy treated with HIUST has higher hardness and wear resistance than that untreated with HIUST. The wear mechanism of investigated alloys at low applied load (10 N) and low sliding speed (0.3 m/s) is a mild abrasive oxidative wear with little adhesion. However, the wear mechanism due to the applied high loads (30, 50 N) at low sliding speed (0.3 m/s) and/or to the applied high sliding speeds (0.6, 0.9 m/s) under low load (10 N), could be described as delamination mechanism. The microstructures of the specimens were analyzed by optical microscope (OM) (model OPTIKA M-790, Italy). Energy dispersion spectrum (EDS) affiliated to field emission scanning electron microscopy (FESEM) (model Quanta FEG, The Netherlands) were performed to reveal the concentration of alloying elements in selected areas of the microstructure.
Structure Determination of Mg5Si6 Particles in Al by Dynamic Electron Diffraction Studies
Science, 1997
Precipitation hardening, in which small particles inhibit the movement of dislocations to strengthen a metal, has long been used to improve mechanical strength, especially of aluminum alloys. The small size of precipitates and the many possible variants of the orientation relation have made their structural determination difficult. Small precipitates in commercial aluminum-magnesium-silicon alloys play a crucial role in Increasing the mechanical strength of these alloys. The composition and structure of the B' phase in an aluminum-magnesium-silicon alloy, which occur as precipitates (typically 4 nanometers by 4 nanometers by 50 nanometers) and are associated with a particularly strong increase in mechanical strength, were determined. Element analysis indicates that the composition is Mg"Siu. A rough structure model was obtained from exit waves reconstructed from high-resolution electron microscopy images. The structure was refined with electron nanodiffraction data (overallR value of 3.1 percent) with the use of a recently developed least squares refinement procedure in which dynamic diffraction is fully taken into account. Prre metals are in general rather soft. For example, Al and Fe have a strengrh of 40 to 70 Mla, depending on grain size, srress level (which reflecrs, among other things, the dislocation density), and solutes in the matrix, whereas tool steel has a strength of 8OO to 1400 MPa. For common applications, most pure metals are too soft, and for many centuries research has been focused on methods to incfease the strength of metals. The methods used to obtain this result can be divided into a few groups, inciuding cold working, solid solution hardening, and precipitation hardening. Cold working and solid solution hardening have been used since ancient times. Research on precipitation hardening started at the beginning of this cenrury with the discovery of Al-Cu alloys and \ff/ilm's research (I) on the strength of Al-Mg alloys. \7ith precipitation hardening, small amounts of other elements are added that form precipitates during a specified temperature cycle. Commonly used precipitationhardened Al alloys are the,binary systems
Microstructural evolution during semisolid processing of Al–Si–Cu alloy with different Mg contents
Transactions of Nonferrous Metals Society of China, 2017
A series of Al−6Si−3Cu−(0.3−2)Mg alloys were produced by a conventional casting process. Cooling slope technique was employed to produce feedstocks before they were thixoformed at 50% liquid fraction. The effect of Mg on the microstructure of Al−Si−Cu aluminium alloys under as-cast and semisolid conditions was investigated. It was found that by adding Mg to Al−Si−Cu alloy, some of the Al 2 Cu phase and silicon were consumed to form Al 5 Cu 2 Mg 3 Si 5 and Mg 2 Si phases. The needle-like β-Al 5 FeSi phase transformed to Chinese-script-like π-Al 8 Mg 3 FeSi 6 with the addition of Mg. In the as-cast alloys, the primary α(Al) was dendritic, but as the Mg content increased, the phase became less dendritic. Moreover, the Mg addition considerably modified the size of the α(Al) phase, but it had no significant effect on the silicon morphology. In the thixoformed alloys, the microstructure showed a fine globular primary phase surrounded by uniformly distributed silicon and fragmented intermetallic phases. The eutectic silicon was modified from a flaky and acicular shape to fine fibrous particles. The effect of Mg on eutectic silicon during semisolid processing was evident. The primary Mg 2 Si particles were modified from big polygonal particles to become smaller and more globular, whereas the morphology of the Chinese-script-like π-Al 8 Mg 3 FeSi 6 changed to a compact shape. The results also exhibit that as the Mg content in the A319 alloy increased, the hardness, yield strength and ultimate tensile strength of the thixoformed alloys significantly improved, but the elongation to fracture dropped.
Possibilities and limitations of scanning electrochemical microscopy of Mg and Mg alloys
Magnesium (Mg) and its alloys undergo corrosion at high rates in aqueous electrolytes of essentially any concentration, releasing copious amounts of hydrogen (H-2) into the electrolyte, thereby making conventional scanning electrochemical microscopy (SECM) measurements difficult upon Mg. Examples of the issues associated with Mg SECM, and strategies to counter these, are discussed in this review. SECM has been employed in Sample Generation-Tip Collection (SG-TC) mode to image cathodic sites on the Mg surface, wherein, the platinum (Pt) tip oxidizes H-2 emanating from the specimen surface. This mode of SECM does not clearly reveal a close correspondence between the microstructure and its associated reactivity; the H-2 currents measured by the Pt tip found to vary by over an order of magnitude with time. Convective fluxes due to H-2 bubbles, local acidification from protons released during H oxidation on the Pt tip, and the negative difference effect of Mg may contribute to these high tip currents. SECM has also been performed on Mg using Mg2+ ion selective electrodes (ISEs), where an Mg2+ ionophore cocktail serves as a Mg2+ ion permeable membrane. The Mg2+ ions diffuse/migrate through the membrane which eventually settles at the equilibrium membrane potential (Donnan potential). This potential varies with the logarithm of Mg2+ ion concentration in the electrolyte, and thus, could be used to ascertain the local Mg2+ ion concentrations (or anodic sites) on the surface. This technique needs to be complemented with other modes of SECM to obtain holistic electrochemical micrographs of a corroding Mg surface.
Acta Materialia, 2006
AlMgSi alloys are commonly used in the automotive industry. In the bake-hardening process of AA6xxx alloys, nanometer-size Mg x Si y Al z -type precipitates play a crucial role. A large number of structures and compositions occur between the initial supersaturated solid solution and the stable Mg 2 Si phase. The transition from pre-b 00 to b 00 phase marks the transition from face-centered cubic (fcc)-type structures to non-fcc-type structures. A new pre-b 00 phase is identified with the composition Mg 4 Si 7 that is energetically very favorable. Although the late precipitate phases (b 00 , b 0 , U1, U2 and U3) are very different in composition and crystallography, their mutual correspondence is made clear by an analysis of the orientation relationships and substructures. It is shown that they share the same substructures consisting of Mg hexagons which enclose a parallelogram of four atoms. Phase transitions are characterized by columnar and planar shifts of these substructures, and not by a total rearrangement of all the atoms.