Transition from diffusive to ballistic capture related to hydrogen incorporation in amorphous silicon

Three intersection mechanisms with the gliding planes (111)_TB, (001)_TB and (115)_TB respectively have been observed by high-resolution electron microscopy in the type-I twin intersection of gamma-TiAl. It was found that the intersection mechanism that occurred was related to the thickness of the incident twin. The accommodation mechanism on the (111)_TB atomic plane is preferred when the incident twin becomes very thin. The dislocation dissociations of the (111)_TB plane accommodation are the most energetically unfavourable of the dissociations of the three intersection mechanisms; however, the resultant dislocations on the (111)_TB planes are the easiest to propagate away from the intersection area. Accordingly, (111)_TB atomic plane accommodation is considered to be the only mechanism allowing shear transmission under the small local stress of the pile-up of the incident twinning partials.     

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.     

Stress-corrosion cracking in YBa2Cu3O7−x ceramic superconductor

Abstract

Thin-foil transmission electron microscopy specimens of orthorhombic YBa²Cu³O^7?x subjected to residual elastic strains have shown slow crack growth in an atmosphere with low moisture content. Microcracks, initiated by amorphization at the strained edges, propagate alternately along (110) twin boundaries and (001) stacking faults, where strain energy is accumulated locally. The formation of carbon-rich amorphous layers along the strained lattice planes, followed by residual stress-assisted crack propagation, resembles stress-corrosion cracking in this ceramic superconductor.     

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.     

{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}.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.     

Evidence of carbon ordering and morphology change in a cubic carbide phase

Abstract

The crystallographic and microstructural changes of the ordered cubic carbide (K) phase on ageing have been studied in rapidly solidified Fe-30.9 wt% Mn-8.9 wt% AI-2.8 wt%C ((Fe_0.65, Mn_0.35)_0.83 Al_0.17–12 at.%C) alloy. In the as-rapidly solidified condition, an austenitic phase coexists with the carbide phase which is composed of antiphase domains in the interior of the solidification cell. The antiphase domain boundaries were aligned nearly parallel to the {100} planes. By visually comparing the intensity ratio (I₁₁₀/I₁₀₀) of the selected-area diffraction pattern in the transmission electron microscope and by quantitatively comparing the same X-ray diffraction intensity ratio, we have deduced that the K phase tends to consolidate into the L'1₂ structure on ageing at 450°C but into the L1₂ structure on ageing at 700°C.     

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.     

Twin formation and structural modulations in orthorhombic and tetragonal YBa2(Cu1-xCox)3O7-δ

Abstract

The microstructure of YBa₂(Cu_1?xCo_x)₃O_7?δ, prepared by the standard ceramic method, shows lamellar twin structures with decreasing spacings between twin walls with increasing Co content for x?0·02, developing into {110}-type cross-hatched ‘tweed’ modulation for x?0·02. Several wall junctions are found for x=0·02. The structural phase transition between macroscopically orthorhombic and tetragonal material occurs at x≈0·025; structural modulations (λ≈20Å) persist in the samples with high Co content (x>0·25). The modulations lead to a considerable broadening of the X-ray lines affected by orthorhombic splitting, and show maximum amplitude at the critical composition x≈0·025.     

The crystallographic structure of λ transition-alumina compared with the periodic antiphase boundary structure of a Ga2O3-rich Mg gallate

One of the high-temperature metastable phases found in the MgGa₂O₄–Ga₂O₃ system is the ε_Mg non-stoichiometric Mg gallate. It has a one-dimensional periodic antiphase boundary (PAPB) structure based on the spinel structure. The APBs are parallel to the {3?1?0} planes and the APB vectors are of the 1/4?1 1 0? type when referred to the spinel structure. This generally leads to the existence of 12 monoclinic twin variants in the studied samples. By comparing the X-ray or electronic diffraction patterns of both phases, it will be shown that the recently discovered?λ?transition-alumina has the same type of structure.     

Identification of planar defects in D019 phases using high-resolution transmission electron microscopy

An analysis based on crystal symmetry and high-resolution transmission electron microscopy (HRTEM) is presented as a general methodology to identify planar defects on the basal planes of D0₁₉ compounds. As a starting point, the possible (close-packing preserving) planar defects are classified in accordance with their visibility, and the magnitude of the displacement vector on prismatic projections. Analysis of experimental HRTEM images, obtained under two different viewing conditions, followed by matching with simulated images, enables unambiguous identification to be made. The methodology is applied to planar defects observed in D0₁₉ Co₃     

On the asymmetrical dissociation of Lomer dislocations

Abstract

Using anisotropic elasticity theory, Korner, Schmid and Prinz have shown that Lomer-Cottrell dislocations in the f.c.c. structure have asymmetrical equilibrium positions. This is also true for the isotropic theory, but in addition it can also be demonstrated in a very simple way that the ratio of the dissociation widths on the two {111} planes is material independent. This result seems to be general even for the anisotropic case. However, careful observations of Lomer dislocations in copper by weak-beam transmission electron microscopy indicate clearly that they are not dissociated.     

Formation and structure of misfit dislocations at the La2Zr2O7-Y2O3 -stabilized ZrO2 (001) reaction front during vapour-solid reaction

La₂ Zr₂ O₇ (LZO)-based pyrochlore islands were grown on a Y₂ O₃ -stabilized ZrO₂ (YSZ) (001) single-crystal surface by the reaction between La₂O₃ vapour and the crystal. A network of interfacial edge dislocations with line directions [100] and [010] and Burgers vectors ( a_s/2)[101] and ( a_s/2)[011] respectively ( a_s being the lattice parameter of the YSZ) was observed at the moving LZO-YSZ reaction front. The interface-parallel component of the Burgers vectors accommodates the LZO-YSZ lattice mismatch of +5.0%, while the perpendicular component causes a slight tilt of the LZO lattice with respect to the YSZ lattice. The dislocation half-loops nucleate and glide on inclined {101} planes at the edges of the four corners of the growing islands.     

Atomic configurations of twin boundaries and twinning dislocation in superconductor Y0.6Na0.4Ba2Cu2.7Zn0.3O7− δ

Two types of twin boundaries in superconductor Y_0.6Na_0.4Ba₂Cu_2.7Zn_0.3O_{7? δ} , the cation-centered and oxygen-centered types, and the associated twinning dislocation have been studied by high-resolution electron microscopy. The structure map projected in the [001] direction was obtained from a single image by means of the image deconvolution technique. In this map, all columns of metallic atoms appear as individual black dots, and hence the two types of twin boundaries are distinguished from each other at atomic level. It is seen that the twinning dislocation occurs when the two types of twin boundaries meet each other. The structure model of the twinning dislocation together with the two types of twin boundaries has been derived straightforwardly based on the positions of black dots seen in the deconvoluted image.     

On the dislocation mechanism of amorphization of Si by indentation

The formation of an amorphous phase underneath a Vickers indentation produced on a Si(001) surface at room temperature has been observed by cross-sectional transmission electron microscopy. Two types of location are observed for the amorphous phase. One is formed just underneath the image of the indentation and the other is parallel to the slip planes of Si. It is concluded that the latter type, at least, is formed as a result of activation of dislocations which is induced by an external shear stress combined with a hydrostatic pressure.     

Cross-sectional transmission electron microscopy of electrodeposited Ni-P alloys

Abstract

Ultramicrotoming has been employed to prepare thin cross-sections of Ni-P alloys electrodeposited on to a copper substrate. The sections, as thin as 10nm or even less, can be prepared readily with negligible damage to the alloys. Subsequent examination of the ultramicrotomed sections in an analytical transmission electron microscope has revealed that Ni-P alloys of low P content consist of fine layers of different composition and crystallinity. High-resolution transmission electron microscopy of ultramicrotomed sections has also revealed the presence of ordered regions, a few nanometres in size, in the Ni₇₇P₂₃ alloy which has been thought previously to be amorphous.     

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.     

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.     

Structural characterization of double stacking faults induced by cantilever bending in nitrogen-doped 4H-SiC

Defects in highly nitrogen-doped 4H-SiC deformed by cantilever bending at 550°C have been identified by weak-beam and high-resolution transmission electron microscopy techniques. The induced-defects consist of double stacking faults (DSFs) whose expansion produces a local 4H?→?3C phase transformation. Each DSF is bound by two identical 30° Si(g) partial dislocations which glide on two adjacent basal planes. The DSFs belong to three different populations which differ by their extension as a function of the applied-stress and the 30° Si(g) characteristics (line direction L , Burgers vector b , glide planes and glide direction). The external mechanical stresses are the main driving forces involved in the DSF expansion. However, extra driving forces such as thermodynamic or electronic forces are also likely to be involved.