Detector Dark-field Scanning Transmission Electron Research Articles

Effect of Different Scale Precipitates on Corrosion Behavior of Mg–10Gd–3Y–0.4Zr Alloy

A large amount of directional and willow-like β′ phase was precipitated in Mg–10Gd–3Y–0.4Zr (GW103K) alloy after solution treatment and subsequently aged treatment (T6). In order to explore the effect of the precipitates on the corrosion behavior of the GW103K alloy, the alloy was subjected to solution treatment (T4) at 773 K for 4 h at first, subsequently aged at 498 K for 193 h (T6). The microstructure evolution of the GW103K alloy after this treatment was investigated by scanning electron microscopy and transmission electron microscopy. The high-angle annular detector dark-field scanning transmission electron microscopy was used to observe the typical corrosion morphologies of the nanoscale precipitation phases (β′) in the T6-treated alloy. The corrosion rate was measured by potentiodynamic polarization test. Combining with the potential measurement results by scanning Kelvin probe force microscopy, the effects of the skeleton-like Mg24(Gd, Y)5 and β′ precipitates on the corrosion behavior of GW103K alloy were explored. The results showed that the corrosion rate of the GW103K alloy in different conditions was ranked as: as-cast alloy> T4-treated alloy> T6-treated alloy, attributing to the fact that the relative potential differences of skeleton-like Mg24(Gd, Y)5 were lower than those of the matrix, therefore Mg24(Gd, Y)5 phase formed micro-galvanic coupling with the matrix and corrosion dissolution occurred. The nanoscale β′ precipitates in T6-treated alloy can retard the cathodic process.

Effect of microstructure on exfoliation corrosion resistance in an Al-Zn-Mg alloy

In this work, the effect of microstructure on exfoliation corrosion (EXCO) resistance in an Al-Zn-Mg alloy were investigated using standard EXCO test, together with detailed microstructural examinations mainly by scanning electron microscopy (SEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). An EXCO product model was built based on morphologies of EXCO product on longitudinal section and cross section for further measurement and analysis. A quantitative characterization of EXCO resistance by measuring average thickness of EXCO product was conducted, showing average results of 6.94μm, 10.21μm and 18.14μm, which are corresponding to EXCO ratings of P, EA and EB under ASTM G34 standard, respectively. It was concluded that recrystallization, high fraction of high-angle grain boundaries (HAGBs) and high coverage ratio of η phases on HAGBs reduced EXCO resistance.

Intergrowth structure of α-phase in β-type TmAlB4 compound studied by high-angle annular detector dark-field scanning transmission electron microscopy

Nanostructure of a ThMoB4-type (β-type) TmAlB4 compound, in which YCrB4-type (α-type) domains are locally intergrown, is studied by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Z-contrast images by HAADF-STEM directly represent the arrangements of Tm atoms located at centers of heptagonal atomic columns of B atoms as bright dots, and give us detailed information of intergrowth of type domains in the matrix of the β-type phase, which coherently occurs. Structural and bonding analyses for β-TmAlB4 point out the closeness in atomic interactions and energy of the α- and β-type structures which support the easy formation of such nanostructure intergrowths. From combination between HAADF-STEM and electronic structure calculation, a detailed local crystal structure with intrinsic building defects is effectively revealed.

Characterisation of precipitates in a Mg–7Gd–5Y–1Nd–0.5Zr alloy aged to peak-ageing plateau

The precipitates in a Mg–7Gd–5Y–1Nd–0.5Zr alloy aged to peak-ageing plateau (aged at 210°C from 18h to 180h) were studied using transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Precipitation at the peak-ageing plateau involves the formation of the β′ phase and a new type of plate-shaped phase precipitated along the (0001)α habit plane. The ellipsoid morphology of β′ was illustrated according to [0001]α and [101¯0]α zone axes observations. The orientation relationship between the β′ precipitates and matrix satisfies [001]β′//[0001]α and (100)β′//(112¯0)α, respectively. The TEM and HRTEM results indicate that the β′ precipitates have a base-centred orthorhombic structure with lattice parameters a=0.64nm, b=2.22nm and c=0.52nm. Further, atomic-scaled HAADF-STEM observations suggest a Mg7RE-type and not a Mg15RE-type structure for the β′ phase. The positive thermal stability of the alloy at 210°C ageing is attributed to the coherent relationship with the matrix and the formation of an interlaced network of the β′ precipitates. Aside from the β′ phase, a large amount of precipitate particles were observed to form on the (0001)α habit plane in the peak-aged samples. The new type of precipitates have a plate shape, of approximately 5nm wide along the [101¯0]α direction of the matrix and approximately 5nm long along the [2¯110]α direction. A completely coherent relationship with the matrix can also be observed.

Structural Changes of Precipitates by Aging of an Mg-4 at%Dy Solid Solution Studied by Atomic-Scaled Transmission Electron Microscopy

Phase transformation of solid solution decomposition occurring in a 96 at%Mg-4 at%Dy alloy, which was solution-treated at 540°C and subsequently aged at 250°C for various lengths of time, has been investigated by conventional transmission electron microscopy (TEM) in combination with high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). The atomic-scaled observations based on both techniques provide the evidence that the first appreciable change in microstructure caused by aging is the occurrence of a short-range ordered state in Dy-segregated regions and that the short-range ordered state allows full of the nuclei of β′ phase associated with an Mg7Dy-type structure to occur in the domains, just as in cases of Mg-Gd and Mg-Y systems. With an increase of age-hardening effect, the β′ precipitates become larger and increasingly anisotropic in morphology, accompanying three orientation variants in coherent with the Mg-matrix. When reaching at the stage of hardness maximum (as-aged at 250°C for 100 h), the β′ precipitates, which have an orthorhombic structure with lattice parameters of a=0.659 nm, b=2.231 nm, c=0.523 nm, take the form of a thin disk-shape with a thickness of 20∼100 nm and a diameter of 200∼400 nm. With an advance of over-aging effect, the β′ precipitates are gradually reduced in volumes and replaced by β precipitates of cubic structure.

Microstructural changes of Guinier–Preston zones in an Mg–1.5 at% Gd–1 at% Zn alloy studied by HAADF-STEM technique

The microstructures of Guinier–Preston zones (GP-zones) precipitated in Mg 97.5Gd 1.5Zn 1 alloys annealed at 200 °C for various lengths of time have been thoroughly investigated by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) in combination with energy dispersive X-ray spectroscopy (EDS). The HAADF-STEM technique, which is capable of providing an atomic-scale Z-contrast image associated with heavier constituent elements in the alloys, has successfully allowed us to determine the atomic arrangement of Gd/Zn in the GP-zones. It is further evidenced that the GP-zones in the alloy have fairly variable microstructures with an advance of aging in the following way: wavy GP-zones → planar GP-zones → band-shaped (multi-layer) GP-zones (→ stacking faults).

Characterization of Precipitates in Mg-Sm Alloy Aged at 200°C, Studied by High-Resolution Transmission Electron Microscopy and High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy

Precipitates in an Mg-0.99 at%Sm (Mg 99.01 Sm 0.99 ) alloy aged at 200°C were studied by the combination of high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Fine precipitates of a meta-stable phase, which is called γ here, in the alloy aged at 200°C for 4 h have a thin lens-shape with a thickness of 2-5 nm and a diameter of 20-60 nm. The γ precipitate has an incommensurate structure with an orthorhombic unit cell of α = 2a 0 = 0.64 nm, b˙=. 6a 0 3 = 3.334 nm and c = c 0 = 0.52 nm, where a 0 and c 0 are lattice constants of a hexagonal unit of the Mg-matrix. In the early stage of aging at 200°C for 0.5 h, isolated structure units forming the γ structure are dispersed in an Mg hexagonal lattice. By annealing at 200°C for 100h, coarse precipitates of a stable Mg 3 Sm phase are formed along grain boundaries and inside grains of the Mg-matrix, and wide γ precipitate-free zones appear around them.

Stabilization of Stacking Faults and a Long Period Stacking Phase Dispersed in α-Mg Crystalline Grains of Mg-0.7 at%Zn-1.4 at%Y Alloy

Mg-0.7 at%Zn-1.4 at%Y alloys annealed at low temperatures after quenching in water from 520°C were studied by high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Stacking faults, thin bands of a 14H-type long period stacking (LPS) phase and relatively thick bands of LPS were precipitated in α-Mg crystalline grains by annealing at 300°C, 400°C and 500°C, respectively. The precipitation of stacking faults, LPS phase and a supersaturated solid solution without any precipitates were reversibly transformed by annealing at low temperatures. It can be concluded that the stacking faults and LPS phase are stabilized by the segregation of Zn and Y from a supersaturated solid solution.

HAADF–STEM study of B–Mg–Ru approximant crystals

Two-dimensional structures of decagonal approximant crystals have been studied in a B–Mg–Ru system where decagonal quasicrystals have been predicted. High-angle annular detector dark-field scanning transmission electron microscopy (HAADF–STEM) imaging confirms that four approximant phases, which form at high temperatures, can be described as tessellations of hexagon and boat subunits with the same atomic decorations as B11Mg5Ru13. The images also support the existence of bow-tie flip structures in the all four approximant crystals. Long periodicities in one direction up to 8.5 nm are interpreted to be caused by modulation of rows of boat tiles.

Transmission Electron Microscopy for Precipitate Phases in Rapidly Solidified Mg-2 at%Ce-1 at%Zn and Mg-2 at%Ce Alloys

Precipitate phases in Mg-2 at%Ce-1 at%Zn (Mg 97 Ce 2 Zn 1 ) and Mg-2 at%Ce (Mg 98 Ce 2 ) alloys, prepared by a melt-quenching method, have been examined by high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). A precipitate phase formed at grain boundaries has an ordered structure with a hexagonal unit cell of √2a 0 and c 0 , in which a 0 and c 0 are lattice parameters of an h.c.p. structure of the Mg-matrix. On the other hand, a precipitate phase embedded in Mg-matrix grains has a long-period modulated structure formed by the appearance of two-dimensional anti-phase boundaries with a hexagonal symmetry in the fundamental hexagonal structure with lattice constants of √3a 0 and c 0 .

Characterization of β′ Precipitate Phase in Mg-2 at%Y Alloy Aged to Peak Hardness Condition by High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM)

The β′ phase precipitated in an Mg-2 at%Y (Mg98Y2) alloy aged to a peak hardness condition at 200°C for 336 h has been studied by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). The β′ precipitates have an about 20 nm size and definite facets parallel to the (010) plane, and Y-enriched double atom planes parallel to the (010) plane grow long and sometimes individually along the [100] and [001] directions. This morphology of the β′ precipitates in the Mg98Y2 alloy is remarkably different from that in an Mg95Gd5 alloy.

Structural Changes of Precipitates in an Mg-5 at%Gd Alloy Studied by Transmission Electron Microscopy

Structural changes of precipitates in an Mg-5 at%Gd (Mg 95 Gd 5 ) alloy by aging at 200°C and 250°C have been studied by ordinary high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). In the early stage of precipitation by aging at 200°C up to 5 h, a short-range ordered structure exists in Gd-enriched regions with an about 2 nm size, and nuclei of an Mg 7 Gd structure (β' phase) occur in the short-range ordered structure by aging at 200°C for 5 h. The β' phase with the Mg 7 Gd structure is formed as definite precipitates by aging at 200°C for 10 h. The β' precipitates are joined along the b-axis and form a plate-shape with a thickness of about 20 nm along the a-axis, and lengths of about 200 nm along the b-axis and 200 nm along the c-axis, and the connection of planar precipitates along the three directions, which are parallel to the [210] m ,-typed directions of the Mg-matrix, forms a two-dimensional cell structure, by top-aging at 200°C for 100h.

Molecular beam epitaxial growth of GaN thin film on Si substrate with InN as interlayer

Low-temperature interlayer InN was used to fabricate GaN thin film on Si (1 1 1) substrate by molecular beam epitaxy method. Surface scanning electron microscopy image shows a uniform polycrystalline structure. Transmission electron microscopy and high-angle annular detector dark-field scanning transmission electron microscopy images indicate column GaN crystal microstructure with the diameter between 100 and 150 nm. Air-gap structure is formed between the high-temperature GaN film and the underlying buffer layer. Photoluminescence measurement shows a small peak of 357 nm at room temperature and an increase of peak intensity about 40 times at temperature 8 K. Room temperature Raman spectra of the GaN column crystals shows narrow full-width at half-maximum of 2 cm −1 and E 2 mode peak line of 565.9 cm −1. A shift of 1.1 cm −1 from the intrinsic position may suggest relaxed residual strain of the GaN column crystals. The liquid In metal derived from the decomposed InN interlayer diffuses into the GaN buffer layer and high-temperature GaN thin film, forming InGaN layer. The application of InN as interlayer offers another effective way for the heteroepitaxial growth of GaN thin film on Si substrate.

Evidence of phase separation correlated with nanocrystallization in Al85Ni5Y6Fe2Co2 metallic glass

After carefully ruling out the known sample preparation artifacts, high-angle annular detector dark-field scanning transmission electron microscopy, nanobeam composition analysis and energy-filtered imaging suggest that phase separation occurs upon heating in the Al85Ni5Y6Fe2Co2 amorphous ribbon prior to the onset of primary crystallization, but not in its as-prepared state. The length scale of the phase separation appears to correlate with the dimensions of the nanocrystals formed upon crystallization. (c) 2006 Acta Materialia, Inc. Published by Elsevier Ltd. All rights reserved.

Structure of type I and type II modifications of Al–Co–Ni decagonal quasicrystals studied by HAADF-STEM technique

The structure of type I and type II modifications of Al–Co–Ni decagonal quasicrystals was studied by using the high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) technique. It was found that the quasi-unit-cell should be selected as columnar decagon of 3.2 nm in diameter and 0.82 nm in height, and its center consists of a bright circle with 0.76 nm of the outer diameter, indicating that atomic distribution in the quasi-unit-cell is disordered. Lattices obtained by connecting the centers of the atom clusters in HAADF images reveal that the lattice of the type I modification is rhombic Penrose tiling and the lattice of the type II is a mixture of the rhombic and pentagonal tilings. Namely, the lattice of the type II modification is a mixture of the lattices of the type I and S1 modifications.

One-Dimensional Anti-Phase Structures Based on an Mg<SUB>12</SUB>Ce (Mn<SUB>12</SUB>Th-Type) Structure in an Mg<SUB>91</SUB>Ce<SUB>6</SUB>Zn<SUB>3</SUB> Alloy, Studied by High-Resolution Transmission Electron Microscopy and High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy

The crystal structure of a new phase with an approximate composition of Mg 91 Ce 7 Zn 2 , which is formed as a main phase in an Mg 91 Ce 6 Zn 3 alloy, has been determined by atomic-scale observations of high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). The structure of this phase can be described as a one-dimensional incommensurate structure with an orthorhombic unit cell of a = 1.03 nm, b = 1.03 nm and c ≒ 3.7 nm, formed by insertion of anti-phase boundaries in the fundamental Mg 12 Ce (Mn 12 Th-type) structure with a tetragonal unit cell of a 0 = 1.03 nm and c 0 = 0.60 nm. The anti-phase boundaries are parallel to the (001) plane of the fundamental tetragonal structure, and an average interval of the boundaries along the [001 ] direction is about 3.1c 0 . Also, a commensurate structure with an interval of 5.5c 0 is observed as a coexisting phase with the incommensurate structure.

Characterization of β′ Phase Precipitates in an Mg-5 at%Gd Alloy Aged in a Peak Hardness Condition, Studied by High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy

The β' phase precipitated in a 95 at%Mg-5 at%Gd (Mg 95 Gd 5 ) alloy aged in a peak hardness condition (at 200°C for 100 hrs) is studied by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Atomic-scaled HAADF-STEM observations of the β' phase can propose a new structure model with an orthorhombic unit cell of a = 0.64 nm, b = 2.28 nm and c = 0.52 nm and a composition of Mg 7 Gd. The β' precipitates have a plate-shape with an about 40 nm width along the [001] m of the Mg-matrix and an about 100nm length along [210] m -typed directions, and joining of the plate-shaped precipitates establishes a two-dimensional cell structure parallel to the (001) m plane of the Mg-matrix.

Precipitates with Peculiar Morphology Consisting of a Disk-Shaped Amorphous Core Sandwiched between 14H-Typed Long Period Stacking Order Crystals in a Melt-Quenched Mg<SUB>98</SUB>Cu<SUB>1</SUB>Y<SUB>1</SUB> Alloy

The microstructure of a melt-quenched Mg98Cu1Y1 alloy has been studied by high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). We have found Cu- and Y-rich precipitates, which are uniformly dispersed in Mg-matrix grains. The precipitates are aligned parallel to the c-plane of the Mg-matrix crystal, and have the peculiar morphology consisting of a disk-shaped amorphous core sandwiched between 14H-typed long period stacking order (LPSO) crystals. A relatively stable supper-cooled liquid phase in an Mg-Cu-Y alloy system, and the formation and growth of the LPSO crystals from the amorphous phase are responsible for the peculiar morphology of the precipitates.

HAADF-STEM study on the early stage of precipitation in aged Al-Ag alloys

We report on the structure of Ag precipitates in aged Al-Ag alloys using transmission electron microscopy and high-angle annular detector darkfield scanning transmission electron microscopy (HAADF-STEM). Irregularly shaped small Ag particles of 1-2 nm dominate the alloy annealed at 140 degrees C for 10 h. These particles are present also within large precipitates (10-50 nm), which are often characterized by their {100} and {110} facets. In addition, atomic-resolution HAADF-STEM images revealed that Ag atoms tend to form {111} planar clusters, which criss-cross a colony of the irregularly shaped small Ag precipitates.

Reversible phase changes of Ni-rich Al?Co?Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

In the stability region of the Ni-rich Al–Co–Ni decagonal quasicrystals, phase relationships among three different modulations, i.e. the so-called Ni-rich basic, S1-state and Type-I phases, and their phase changes have been thoroughly investigated by various transmission electron diffraction and imaging techniques in combination with thermal analysis. Electron diffraction experiments have clearly indicated that the S1-state phase formed in the Al 70 Co 12 Ni 18 alloy, which is stable at 900 °C, transforms into the Type-I phase by annealing at 650 °C and that the Ni-rich basic phase formed in the Al 71.5 Co 9 Ni 19.5 alloy, which is stable at 900 °C, transforms into the S1-state phase by annealing at 650 °C. In addition, differential scanning calorimetry measurements have evidenced for the first time that both phase changes occur reversibly at certain temperatures ranged between 700 and 800 °C. The structural features of the modulations mentioned above have been distinguished and uniquely characterized by making the best use of high-angle annular detector dark-field scanning transmission electron microscopy as well as high-resolution transmission electron microscopy.