Scanning transmission electron microscopy investigation of an Al–Mg–Si–Ge–Cu alloy

Precipitation in a Mg-rich Al–Mg–Si–Ge–Cu alloy was investigated using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The precipitates were needle or lath shaped with the longest dimension parallel to ?001?_Al. The precipitates had no repeating unit cell when viewed along this direction. However, the precipitate structure in projection consisted of a hexagonal network of mixed Si and Ge columns, with Mg, Al, and Cu columns occupying specific sites in between the network columns. The Cu columns appeared with the same local arrangement of atomic columns as in Al–Mg–Si–Cu precipitates, and the Cu-free regions consisted of structural units with Mg and Al at specific sites. These structural units were often arranged in a locally ordered fashion, and in some cases the projected structure possessed and overall point symmetry. The amount of strain on the surrounding matrix was found to vary depending on the width of the precipitate cross section.     

High-resolution transmission electron microscopy study on reversion of Al2CuMg precipitates in Al–Cu–Mg alloys under irradiation

Image deconvolution analyses showed that reversion of S-Al₂CuMg precipitates occurred in an Al–Cu–Mg alloy during high-resolution transmission electron microscopy observations. A fraction of Mg and Cu atoms in the precipitates diffused into Al matrix due to electron beam irradiation at 300 kV, resulting in structural/chemical reversion of the precipitates. The structural reversion of the S-Al₂CuMg precipitates is closely related with irradiation-induced displacement of atoms. The strong attraction between Cu and Mg atoms might assist the sub-threshold displacement of Cu atoms. One transitional structure is determined to be S′′-Al₁₀Cu₃Mg₃, a precursor of S-Al₂CuMg. Two other transitional structures, Al₃CuMg and Al₁₈Cu₅Mg₅ which have the same lattice parameters of a = c = 0.405 nm as that of S′′-Al₁₀Cu₃Mg₃, but different b values, are suggested.     

Microstructural characterisation of a commercial Al–Cu–Mg alloy combining transmission electron microscopy and positron annihilation spectroscopy

Transmission electron microscopy and positron annihilation spectroscopy techniques were used to characterise the microstructure of the 2024 Al–Cu–Mg commercial alloy artificially aged with and without pre-deformation. In non-deformed samples, a very dense dispersion of small, needle-shaped particles, with mean size in the order of the nanometres, was observed homogeneously distributed in the matrix, together with a coarse distribution of S-phase precipitates. In pre-deformed samples, the needle-shape particles were not seen, only a high density of S-phase precipitates nucleated on dislocations. The needle-shaped particles were identified as solute-rich Guinier–Preston–Bagaryatski (GPB) zones by combining coincidence Doppler broadening positron annihilation measurements with TEM imaging. The relationship between the microstructure and measurements of hardness and positron lifetime evolution during artificial ageing is also discussed.     

The structure of a long-period stacking phase in an Mg–Al–Y alloy studied by electron microscopy

A new phase with a 10H-type long-period stacking (LPS) structure was found in an Mg₇₅Al₁₀Y₁₅ alloy annealed at 823?K. The LPS structure in the Mg₇₅Al₁₀Y₁₅ alloy annealed at 823?K for 2?h has an ordered arrangement of L1₂-type structural Al₆Y₈ clusters on the two-dimensional plane parallel to the c-plane of hexagonal Mg lattice and a disordered arrangement along the c-axis, whereas a perfectly ordered structure along the c-axis, which has a period with two times of that of the 10H-type LPS structure, was established by annealing at 823?K for 24?h. The structural model of the ordered LPS phase is proposed by high-resolution images taken with a Cs-corrected scanning transmission electron microscope and also electron diffraction patterns.     

A study on the early-stage decomposition in the Al–Mg–Si–Cu alloy AA6111 by electrical resistivity and three-dimensional atom probe

Electrical resistivity measurements and three-dimensional atom probe (3DAP) analysis were employed to investigate early-stage decomposition of the Al alloy AA6111 in the temperature range 60–180°C where electrical resistivity initially increased with ageing time. 3DAP measurements provided information on the shape, number density and solute content of the precipitates, as well as the solute concentration of the matrix, for the ageing conditions corresponding to the resistivity maxima. Using the 3DAP results, the precipitate size distributions for these ageing conditions were determined in terms of the measured number of solute atoms per precipitate. The number density and the Cu content of the precipitates decreased with increasing temperature, whereas the Mg/Si ratio increased. The size distribution of precipitates at the higher ageing temperatures showed the addition of larger size precipitates to the precipitate population. A modification to Matthiessen's law was employed to describe the anomalous resistivity increase by considering the effect of solutes and precipitates on the resistivity evolution. Using the 3DAP results in analysing the resistivity anomaly, it was found that the decrease in the resistivity maxima with increasing temperature was associated with the decrease in the number density of precipitates and not the scattering power of precipitates. The 3DAP results were further used to provide information on the mechanisms of early-stage decomposition and the temperature dependence of the nucleation rate. From this, the nucleation rate appeared to be controlled by the migration of solute atoms, which was assisted by quenched-in vacancies.     

Quantitative description of the T1 formation kinetics in an Al–Cu–Li alloy using differential scanning calorimetry,small-angle X-ray scattering and transmission electron microscopy

The object of the present study is to design a methodology to follow the kinetics of T₁ precipitation, in an AA2198 alloy, in terms of precipitate size, morphology (thickness, diameter) and volume fraction, during a two-temperature isothermal heat treatment. We used in situ small-angle X-ray scattering (SAXS) as a way to measure the evolution of the T₁ mean thickness and diameter during the heat treatment. Transmission electron microscopy (TEM) was then performed in order to calibrate these evolutions. Furthermore, we demonstrate that the volume fraction evolution can be described successfully using a simple analysis of the differential scanning calorimetry (DSC) thermograms. The latter was calibrated by selected observations in high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). Microstructure evolution during DSC heating ramps was analysed using in situ SAXS: the T₁ phase transformation is found to consist in a two-step thickening process explained by two consecutive diffusion stages. The enthalpy of formation of the T₁ phase is deduced from the DSC measurements.     

Precipitation and solute distribution in an interrupted-aged Al–Mg–Si–Cu alloy

Quantitative analysis of the precipitate species and solute distribution was carried out on Al–Mg–Si–Cu alloy 6061 aged to peak hardness using a conventional T6 heat treatment and the so-called T6I6 heat treatments. In this latter, a dwell period at reduced temperature (65°C) is introduced into the T6 ageing cycle (at 177°C or 150°C) which modifies the microstructure and results in the simultaneous improvement of both tensile properties and fracture toughness. Analysis of three-dimensional atom probe data reveals that the superior mechanical properties of the T6I6/177 temper are achieved by a combined effect of a greater consumption of solute atoms by precipitates, an increased number density of fine precipitates and the presence of greater fractions of the effective strengthening precipitates in the final microstructure. Three types of precipitates were found to be characteristic of the peak aged conditions: β′′ precipitates, Guinier–Preston zones and Mg–Si(–Cu) co-clusters. The composition of the strengthening precipitates was found to vary over a wide range for the different heat treatment schedules, corresponding to a variation in the number density of stable nuclei, without any accompanying change in their morphology. All precipitates were found to contain substantial quantities of aluminium. The results also indicate that the strengthening precipitates are preferentially formed from Si-rich nuclei that contain Cu atoms, as opposed to Cu-free nuclei.     

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.     

Direct measurement of precipitate induced strain in an Al-Zn–Mg-Cu alloy with aberration corrected transmission electron microscopy

Precipitates and their associated strain fields significantly influence mechanical properties and, consequently, the industrial performance of aluminum alloys. In this work, we present a direct measurement of strains induced by η′ and η precipitates in an Al-Zn-Mg-Cu alloy using aberration-corrected high-resolution transmission electron microscopy and quantitative strain analysis. The results demonstrate that the strain induced by precipitates in the Al-Zn-Mg-Cu alloy shows significant tensile strains perpendicular to the longitudinal direction of the precipitate discs on the side of the discs and along the longitudinal direction at both ends of the η′ and η precipitates. This strain field can be described by an equivalent dislocation model, in which the lattice mismatch between the precipitate and the matrix is equivalent to a series of dislocation pairs along the precipitate/matrix interfaces.     

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

TEM investigations of two alloys isothermally heat treated at 175°C and 260°C show how Cu additions to the Al–Mg–Si system affect precipitation. Both alloys had a solute content Mg?+?Si?=?1.3 at.%, 0.127 at.% Cu, but with Mg/Si 0.8 and 1.25. Cu-containing Guinier-Preston (GP) zones and three types of Q′ precursors are identified as most common phases at peak-hardness conditions, whereas β″ accounts for maximum 30% of the total number of precipitates. The precursors have needle (L and S precipitates) or plate (C precipitate) morphologies. They consist of different arrangements of Al, Mg and Cu atoms on a grid defined by triangularly arranged Si planes parallel with and having the same period as {100} Al planes. The Si grid is composed of nearly hexagonal sub-cells of a?=?b?=?4.05?Å, c?=?4.05?Å. The Cu arrangement on the grid is often disordered in the needle precursors. The plate precursor is ordered, with a monoclinic unit cell of a?=?10.32?Å, b?=?8.1?Å, c?=?4.05?Å, γ?=?101°.     

High-resolution observation of basal-plane C-core edge dislocations in 4H–SiC crystal by transmission electron microscopy

High-resolution transmission electron microscopy was used to analyze basal-plane dislocations, which display very characteristic contrasts in grazing incidence monochromatic X-ray topographic images, on the Si-face of 4H–SiC. Grazing incidence monochromatic synchrotron X-ray topography, which is a lattice defect observational technique, has been used in power devices made from 4H–SiC. This technique is useful in analyzing lattice defects near the surface but without the contrast of high-density lattice defects inside the wafer. Basal-plane dislocations exhibit several distinct types of contrast: dark, bright, asymmetric dark/bright and intermediate contrast. Dark and bright contrast areas have been reported to be the edge dislocation regions of basal-plane dislocations. Nevertheless, it remains unclear whether the dark contrast regions are edge dislocations with extra half-planes on the surface side, i.e. Si-core edge dislocations, or those with extra half planes on the deeper crystal side, namely C-core edge dislocations on the Si-face. In this paper, basal-plane dislocations with dark contrast edge dislocations in grazing incidence X-ray topographic images around the (0001) surface were observed via high-resolution transmission electron microscopy, and it was determined that the extra half planes are located on the deeper side against the Si-face. This indicates that the dark contrast edge dislocations are those with a C-core structure on the Si-face. This conclusion is important in establishing the analytical procedure for dislocation contrast in grazing incidence monochromatic X-ray topography on Si face images.     

Experimental examination of strain field within GP zone in an Al–Zn–Mg–Cu alloy

The strain field of GP zone plays a very important role in strengthening of the precipitation-hardened aluminum alloys by prohibiting movement of dislocations; however, quantitative analysis about the strain field of the GP zone in the aluminum alloys has been seldom reported elsewhere. In this paper, the microstructure of GP zone in an Al–Zn–Mg–Cu alloy was explored by using high-resolution transmission electron microscopy (HRTEM), and the displacement field of lattice planes within the GP zone was experimentally measured by geometric phase analysis (GPA) technique; then, the quantitative results about strains of the distorted lattice planes within the GP zone were also obtained. It is found that the GP zone core is convergence region of the strains, and the maximum value of the compressive strains within the GP zone is about 7.6%.     

A transmission electron microscopy study of the deformation behavior underneath nanoindents in nanoscale Al–TiN multilayered composites

Nanoscale multilayered Al–TiN composites were deposited using the dc magnetron sputtering technique in two different layer thickness ratios, Al : TiN = 1 : 1 and Al : TiN = 9 : 1. The Al layer thickness varied from 2 nm to 450 nm. The hardness of the samples was tested by nanoindentation using a Berkovich tip. Cross-sectional transmission electron microscopy (TEM) was carried out on samples extracted with focused ion beam from below the nanoindents. The results of the hardness tests on the Al–TiN multilayers with two different thickness ratios are presented, together with observations from the cross-sectional TEM studies of the regions underneath the indents. These studies revealed remarkable strength in the multilayers, as well as some very interesting deformation behavior in the TiN layers at extremely small length scales, where the hard TiN layers undergo co-deformation with the Al layers.     

Effects of minor additions of Mg and Ag on precipitation phenomena in Al–4 mass% Cu

Calorimetric measurements and transmission electron microscopy have been used to study the effects of minor additions of Mg and Ag on mechanism and kinetics of precipitation in Al–4 mass% Cu. Isothermal studies at 30°C show that the rate of Guinier–Preston (Cu) zone formation is progressively reduced by the presence of Mg, or Mg?+?Ag, which is attributed to preferential trapping of vacancies so that their ability to assist diffusion of Cu atoms is reduced. At elevated temperatures, Mg enhances precipitation of θ′, whereas Mg?+?Ag stimulates formation of the Ω phase. Precipitation of S′ follows θ′ if Mg or Mg?+?Ag are present and pre-ageing increases the quantity of θ′ precipitated at the expense of S′. It has been confirmed that secondary precipitation occurs at low temperatures if alloys are first aged at elevated temperatures.     

The structure of an Al–Rh–Cu decagonal quasicrystal studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy

The structure of an Al–Rh–Cu decagonal quasicrystal formed with two quasiperiodic planes along the periodic axis in an Al₆₃Rh_18.5Cu_18.5 alloy has been studied by spherical aberration (Cs)-corrected high-angle annular detector dark-field (HAADF)- and annular bright-field (ABF)-scanning transmission electron microscopy (STEM). Heavy atoms of Rh and mixed sites (MSs) of Al and Cu atoms projected along the periodic axis can be clearly represented as separate bright dots in observed HAADF-STEM images, and consequently arrangements of Rh atoms and MSs on the two quasiperiodic planes can be directly determined from those of bright dots in the observed HAADF-STEM image. The Rh atoms are arranged in pentagonal tiling formed with pentagonal and star-shaped pentagonal tiles with an edge-length of 0.76 nm, and also MSs with a pentagonal arrangement are located in the pentagonal tiles with definite orientations. The star-shaped pentagonal tiles in the pentagonal tiling are arranged in τ²(τ: golden ratio)-inflated pentagonal tiling with a bond-length of 2 nm. From arrangements of Rh atoms placed in pentagonal tilings with a bond-length of 2 nm, which are generated by the projection of a five-dimensional hyper-cubic lattice, occupation domains in the perpendicular space are derived. Al atoms as well as Rh atoms and MSs are represented as dark dots in an observed ABF-STEM image, and arrangements of Al atoms in well-symmetric regions are discussed.     

Percolation and the electron–electron interaction in an array of antidots

A square lattice of microcontacts with a period of 1 μm in a dense low-mobility two-dimensional electron gas is studied experimentally and numerically. At the variation of the gate voltage V _g , the conductivity of the array varies by five orders of magnitude in the temperature range T from 1.4 to 77 K in good agreement with the formula σ(V _g ) = (V _g ?V _g ^* (T))^β with β = 4. The saturation of σ(T) at low temperatures is absent because of the electron–electron interaction. A random-lattice model with a phenomenological potential in microcontacts reproduces the dependence σ(T, V _g ) and makes it possible to determine the fraction of microcontacts x(V _g , T) with conductances higher than σ. It is found that the dependence x(V _g ) is nonlinear and the critical exponent in the formula σ ∝ ? (x - 1/2) ^t in the range 1.3 < t(T, V _g ) < β.     

Observations of the stress developed in Si inclusions following plastic flow in the matrix of an Al–Si–Mg alloy

Abstract

Measurements are made of the stress developed in near-spherical elastic inclusions in an elastic plastic matrix under both tension and compression loading. Two experimental conditions are reported. The first case is where no thermal mismatch exists between the inclusions and the matrix, so that the stress in the inclusion is purely a result of the misfit in the elastic moduli and of the distortion of the plastic slip-line field around the inclusion. The observations are believed to be the first such and are in qualitative agreement with finite-element modelling for idealised inclusion distributions. The second case is the more usual one where a thermal misfit stress exists and observations are reported of the stress relief effects caused by the interaction of the plasticity-induced stress with the thermal and elastic misfit stresses.     

HRTEM and HAADF-STEM tomography investigation of the heterogeneously formed S (Al2CuMg) precipitates in Al–Cu–Mg alloy

The distribution of variants and three-dimensional (3D) configurations of the heterogeneously formed S (Al₂CuMg) precipitates at dislocations, grain boundaries and the Al₂₀Cu₂Mn₃ dispersoid/Al interfaces were studied in this research. By means of high resolution transmission electron microscopy, we systematically investigated the orientation relationships (ORs) between these heterogeneously formed S precipitates and the Al matrix, and further unraveled that the preferred orientation of S variants at grain boundaries and at dispersoid/Al interfaces are respectively associated with the OR between the precipitate habit plane and the grain boundary plane, and the OR between the precipitate habit plane and the interface plane. The inherent characteristic of the crystal structure of the S phase, i.e. the symmetry of the pentagonal subunit, was considered to be the fundamental factor determining the preference of the variant pair. By using high angle annular dark field scanning transmission electron microscopy tomography, we successively obtained the 3D reconstruction of the S precipitates at these defects. Both the morphology of an individual S precipitate and the overall configuration of the S precipitates nucleated at these defects can be clearly observed without misunderstandings induced by the overlap and projection effects of the conventional two-dimensional methods.