HRTEM study of the {252}γ austenite–martensite interface in an Fe—8Cr–1C alloy

Abstract

High-resolution transmission electron microscopy has been used to image the atomic structure of the (2S2)_γ austenite-martensite interface. By imaging along [101]_γ ∥[111]_α, the interface was viewed edge-on and seen to consist of facets on the close-packed (111)_γ, planes. From the correspondence of atoms in the close-packed planes across the austenite-martensite interface, the magnitude of the shear can be analysed as (a/24)<112> on every close-packed plane in the plane of projection. Comparison with theory indicates that this is an (a/12)<112> Burgers vector out of the plane of projection. Hence, each atomic facet can be viewed as a structural ledge containing an (a/12)<112> transformation dislocation.     

{111} Glide in Ni3Al at room temperature. In situ observations under weak-beam conditions

Abstract

In situexperiments are performed in Ni₃AL at room temperature. We observe reversible changes of antiphase boundary (APB) planes between {111} and {100}, and movements in {111} planes controlled by the crossing of sessile positions with the APB plane in {111}.     

HREM study of the chemical nature of twin planes in CuAlO2

Abstract

High-resolution electron microscopy has been used to study the atomic structure of twins in the binary oxide CuAlO₂. This compound is an occasional product of the bonding between pre-oxidized copper and alumina; its peculiarity is to exhibit a twinned microstructure, owing to the occurrence of two enantiomorphic right- and left-handed configurations. The microstructural examination at high resolution reveals that twin planes are located on copper basal planes rather than aluminium planes; this result is discussed on the basis of an analysis of the various possible stacking sequences which can reasonably be considered to occur at the twin interfaces.     

Deformation mechanism of long-period TiAl 2

The microstructure of long-period TiAl 2 deformed at room temperature has been studied by means of transmission electron microscopy. Dislocations, stacking faults and twins were found to contribute to the deformation. A screw superdislocation with Burgers vector d 110] dissociates into two ½ d 110] super-partial dislocations associated with an antiphase boundary. The ½; d 110] super-partial dislocation further dissociates into two Shockley partial dislocations associated with a portion of complex intrinsic stacking fault on the closest-packed {111} plane. The propagation of stacking faults on successive {111} planes yields an order twin.     

Determination of atomic structure of phason planes in the ξ′-Al–Pd–Mn phase

The atomic structures of specific types of linear defects (phason lines) and planar defects (phason planes) in the complex metallic alloy phase ξ′-Al–Pd–Mn have been determined by high-resolution electron microscopy (HREM) and theoretical HREM simulation. The results show that a representational atomic structural model for phason planes can be constructed by introducing a shift between two parts of the perfect crystalline structure using a translation vector of r ?=?(1/2) a ?+?(1/2τ) c . This typical phason plane is normally parallel to the (001) plane of the ξ′-Al–Pd–Mn phase and consists of phason lines, which are arranged side-by-side with their linear direction parallel to the [010] axis. HREM simulations, based on the structural model for both edge-on and inclined types of phason lines, agree well with the experimental results. Taking into account the fact that the structural difference between various curved phason planes arises from the variation in the arrangement of individual phason lines, the atomic structures of the edge-on and inclined phason lines can be used to explain the various curved phason planes frequently observed in the ξ′-Al–Pd–Mn phase.     

Observation of planar stacking faults in a Ti-rich two-phase Ti-Al alloy after deformation at elevated temperatures

It is a common observation that in two-phase Ti-Al-based binary alloys, deformation of the gamma phase occurs by 1/2<110]-type ordinary dis-location activity and twinning associated with 1/6<112] type partials. In the present study the microstructure of a new Ti-Al-based alloy (Ti-47at.% Al-2at.%Mn-2at.%Nb+ 0.8 vol.% TiB₂) with a duplex microstructure consisting of primary equiaxed gamma grains and lamellar alpha₂+ gamma colonies was studied by transmission electron microscopy (TEM) after deformation at elevated temperatures. Planar stacking faults were found in the gamma laths. A detailed contrast analysis by TEM shows that these planar stacking faults lying on {111} planes are bound by all the fcc variants of the Shockley partial dislocations of type 1/6<121>, in contrast with the observations in stoichiometric binary TiAl alloys, where only 1/6<112]-type Shockley partials are found to be associated with the true twins. It is proposed that the addition of ternary and quaternary elements such as Mn and Nb promotes the other variants of the fcc-like dissociations (not common in L1₀structure) in the present alloy.     

HREM examination of [101] screw dislocations in Ni3Al

Abstract

The dissociation of [101] screw dislocations in Ni³Al has been examined using high-resolution electron microscopy. [101] superdislocations are found to be dissociated into (a/2)[101] superpartial dislocations on the (010) cube cross-slip plane. These superpartials in turn dissociate into complex stacking faults on the (111) or (111) which are bounded by Shockley partials in agreement with theoretical predictions. The degree of antiphase boundary spreading on (010) was found to increase with deformation temperature while the superpartial core dissociations remain unchanged.     

Hydrogen-assisted damage in austenite/martensite dual-phase steel

For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1?1?0}_M localized slip and {1?1?2}_M twin and (2) cracking of martensite–martensite grain interfaces. The former resulted in nanovoids along the {1?1?2}_M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking.     

Dissociation of dislocations in MgAl2O4 spinel deformed at low temperatures

Abstract

When spinel is deformed in compression at 400°C along 〈110〉, the primary slip plane is found to be {111} with cross-slip occurring on a {001} plane. A comparison of weak-beam images of dislocations from both systems indicates that all dislocations which belong to the primary slip plane are dissociated out of the {111} plane independent of the character of the dislocation. It is proposed that deformation occurs by motion of dislocations in their dissociated state and that the partial dislocations actually glide on parallel glide planes. Movement of these dissociated dislocations is then accompanied by a concurrent migration of the stacking fault which takes place by a local shuffling of the cations. A stacking fault energy for conservative dissociation at 400°C on {001} of 530±90mJ m^?2 has been determined from weak-beam images of screw dislocations.     

Stacking faults in pseudomorphic ZnSe-GaAs and latticematched ZnSe-In0.04 Ga0.96 As layers

ABSTRACT We report transmission electron microscopy studies of native extended defects in pseudomorphic ZnSe/GaAs (001) and lattice-matched ZnSe-In0.04 Ga0.96 As (001) heterostructures. The dominant defects present in the layers were identified as Shockley stacking fault pairs lying on (111) and (111) fault planes and single Frank stacking faults lying on (111) or (111) fault planes by comparing experimental images with the predictions obtained with the g b = 0 rule as well as with simulated images.     

Atomic structure and solute segregation of a Σ = 3, [110]/{111} grain boundary in an yttria-stabilized cubic zirconia bicrystal

The atomic structure and chemical composition of a =3, [110]/{111} symmetric tilt grain boundary in an yttria-stabilized cubic zirconia bicrystal has been investigated by high-resolution transmission electron microscopy (HRTEM) and nanoprobe energy-dispersive X-ray spectroscopy. The experimental HRTEM images are compared with simulated images for a model obtained by lattice statics calculations. It is found that the grain boundary has two individual mirror symmetrical planes in cation and anion sublattices. In this case, the cations are forced to form sevenfold coordination in the vicinity of the boundary owing to the restraint of the boundary structure, while the cations in the cubic fluorite structure have eightfold coordination. The segregation of yttrium ions was experimentally detected at the = 3 boundary, a finding that is considered to be closely related to the change in the local coordination at the boundary.     

High-angle annular dark field scanning transmission electron microscopy of the antiphase boundary in a rapidly solidified B2 type TiPd compound

High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with Z-contrast is applied to characterize the antiphase boundary (APB) of the B2 structure in a rapidly solidified TiPd melt-spun compound. The atomic shift associated with the R ?=?(1/2) a ₀ ?111? type displacement vector is directly observed at the boundary. A microstructure modification of the melt-spun compound with the cooling rate during solidification is also described.     

Grain-boundary sliding and accommodation mechanism during creep of yttria-partially-stabilized zirconia

The role of grain-boundary sliding in deformation of ceramic materials is analysed from an atomic force microscopy investigation of the change in surface topography during high-temperature deformation of yttria-partially-stabilized zirconia polycrystals. From transmission electron microscopy observations of strained specimens, we attempt to model the accommodation mechanism of grain-boundary sliding.     

The order–dsorder transition at twin boundaries in Cu3Au as studied by TEM

Abstract

The order-disorder phase transition at ∑ = 3{111}- and {211}-type twin boundaries has been studied in the L1²-ordered alloy Cu³Au employing in situ heating in a transmission electron microscope. Evidence is presented for an order-disorder phase transition occurring in these boundaries prior to the bulk transition. The temperature difference ΔT between the transition temperature of both boundary types and the bulk is estimated as 0.5K <ΔT<2K. No difference in T _c for the twin boundaries can be established as yet. The nature of the order-disorder transition in both twin boundaries is presumably a second-order phase transition.     

High-resolution characterization of the precipitation behavior of an Al–Zn–Mg–Cu alloy

The metastable particles in an Al–Zn–Mg–Cu alloy have been examined at atomic-resolution using high-angle annular dark field (HAADF) imaging. In under-aged conditions, thin η′ plates were formed with a thickness of seven atomic planes parallel to the {111}_Al planes. The five inner planes of the η′ phase appear to be alternatively enriched in Mg and Zn, with two outer planes forming distinct Zn-rich interfacial planes. Similar Zn-rich interfacial enrichment has also been identified for the η phase, which is a minimum 11-plane thick structure. In rare instances, particles less than seven planes were found indicating a very early preference for seven-layer particle formation. Throughout the aging, the plate thickness appears constant, while the plate radius increases and no particles between 7 and 11 planes were observed. Based on the HAADF contrast, our observations do not support the η′ models previously set forth by other authors. Clear structural similarities between η′ and η were observed, suggesting that drawing distinctions between η′ and η phases may not be necessary or useful.     

Anisotropic BaTiO3 thin films grown on MgO-buffered R-plane sapphire

Epitaxial BaTiO₃ thin films grown on MgO-buffered R-plane-cut Al₂O₃ substrates show a non-uniform distribution of planar defects. Transmission electron microscopy allows the determination of the distributions of planar defects with respect to the Al₂O₃ substrate. The BaTiO₃ film, the MgO buffer and the Al₂O₃ substrate have an orientation relationship of [100]_BaTiO3//[100]_MgO//[1120]_Al2O3, (010)_BaTiO3//(010)_MgO//(1104)_Al2O3 and (001)_BaTiO3//(001)_MgO// ≈ (1102)_Al2O3 (about 5°; deviation). Under the above relationship, most of the {111} stacking faults occur on (111) and (111) planes, that is two of four sets of {111}_BaTiO3, whereas cracks in the BaTiO₃ film (and MgO buffer) are situated on the (100) planes. The anisotropic distribution of planar defects is regarded as a consequence of an anisotropic stress in the films generated during their fabrication.     

The atomic structure of the dislocation cores in a small-angle grain boundary in BaTiO3 thin films

The atomic structure of the cores of the dislocations forming a [110] tilt boundary of 10^o misorientation in a BaTiO₃ film is characterized by means of focal-series reconstruction. Along the small-angle grain boundary, facets with different inclinations are accompanied by different types of dislocation. The dislocation with a Burgers vector of a [001] appears as a perfect edge type. Those with a Burgers vector of a [111] are dissociated into three partials with a Burgers vector of (a/3)[111], leading to two segments of stacking faults. Structural models of these dislocation cores are suggested on the basis of the phase of the reconstructed wavefunction and tested by simulation of the wavefunction.     

Transformation of Shockley into Frank stacking faults in a ZnS 0.04 Se 0.96 /GaAs (001) heterostructure

We report the transformation of Shockley partial dislocations (PDs) into Frank PDs in lattice-matched ZnS 0.04 Se 0.96 /GaAs(001) as investigated by transmission electron microscopy. The ZnS 0.04 Se 0.96 layers, with a nominal thickness of 70 nm, were grown on GaAs(001) by metal-organic chemical vapour deposition at 350°C. We mainly find stacking-fault pairs on the (111) and planes that are bound by Shockley PDs with Burgers vector . Different reactions are observed between PDs taking place in situ in the electron microscope, leading to the transformation of Shockley PDs into Frank PDs with and stacking faults on the or planes.     

On the atomic structure of an asymmetrical near Σ = 27 grain boundary in copper

The atomic structure of an asymmetrical near Σ = 27 {525} tilt grain boundary (GB) in copper is determined by coupling high-resolution transmission electron microscopy and molecular dynamics simulation. The average GB plane is parallel to {414} in crystal (1) and {343} in crystal (2). The detailed GB structure shows that it is composed of facets always parallel to {101} and {111} in crystals (1) and (2), respectively. The atomic structure of one facet is described using the structural units model. Each facet is displaced with respect to its neighbours by a pure step, giving rise to the asymmetry of the GB plane orientation. The energy of this asymmetrical GB is significantly lower than that of both the {525} symmetrical and the {11,1,11}/{111} asymmetrical Σ = 27 GBs. One GB region displays another atomic structure with a dislocation that accounts for the misfit between interatomic distances in the {414} and {343} GB planes.