Convergent-beam Diffraction Technique Research Articles

Novel hafnium containing steels for power generation

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and microcracks. The formation of M23C6 carbides as grain boundary precipitates can also lead to grain boundary chromium depleted zones which are susceptible to corrosive attack. Such precipitates are the causing loss of creep life in the later stages of creep because of their very high coarsening rate. Through Monte Carlo based grain boundary precipitation kinetics models, combined with continuum creep damage modelling it is predicted that improvements in creep behaviour of power plant steels can be achieved by increasing the proportion of MX type particles. Studies of a Hf containing steel have produced improvements in both creep and corrosion properties of 9%Cr steels. Hf has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study its effect on precipitation. Two new types of precipitates are formed, Hf carbide, (an MX type precipitate) and a Cr–V rich nitride, with the formula M2N. The Hf carbide particles were identified using convergent beam diffraction techniques, and micro-analysis. The nanosized particles are present in much higher volume fractions when compared to VN volume fractions in conventional power plant ferritic steels. Furthermore it is confirmed that the Hf causes the removal of M23C6 grain boundary precipitates. This has led to an increased concentration of Cr within the matrix, reduced chromium depleted zones at grain boundaries, and increased resistance to intergranular corrosion cracking.

Characterisation of defects and piezoelectric fields in InGaN/GaN layers by transmission electron microscopy

Transmission electron microscopy is used to examine the microstructure of high quality hexagonal InGaN/GaN layers grown by MOCVD. It is shown that a combination of imaging and convergent beam diffraction techniques gives information on the types of threading dislocation present, and on the structure of nanopipes, inversion domains and inversion domain boundaries. Recent results which throw new light on the nanopipe formation mechanism are also reported. We also present preliminary work in which electron holography has been used to investigate piezoelectric fields generated across strained InGaN quantum wells.

TEM studies of ETA carbide precipitate particles in a DS cast Ni 3Al base superalloy

Relatively large needle-shaped precipitate particles with a rectangular cross section have been observed in a DS cast Ni 3Al base superalloy aged at 1100 °C for 200 h. Thin film transmission electron microscopy studies, using energy dispersive spectroscopy, selected area diffraction, and convergent beam diffraction techniques were successful in identifying these particles as eta carbides. They have a definite orientation relationship with the γ′ matrix; this has been unambiguously determined to be: (11̄1̄) η − c//(1̄11) γ′;[211] η − c//[211] γ′

III-V ternary semiconductor heterostructures: The choice of an appropriate compositional analysis technique

The composition of ternary III-V semiconductor heterostructures can now be measured in a number of ways. Conventional spectroscopic probe techniques are inappropriate for fine (nanometer scale) heterostructures: Here, those transmission electron microscopy techniques which are appropriate, are compared emphasizing that different alloy systems often require different approaches. The three representative methods discussed in detail are kinematic convergent beam diffraction, 200 dark field intensity measurement, and [001] bright field thickness fringe matching. Dark field intensity measurement is found to be the most sensitive analytical method for many alloys, but the accuracy that may be achieved by thickness fringe matching is generally little worse, and sometimes better. Those convergent beam diffraction techniques which rely on lattice parameter measurement would not appear to be particularly useful except for the purposes of distinguishing nonunique interpretations of other measurements. It is also noted that no technique currently available is capable of measuring the composition of AlxGa1−xSb alloys to an accuracy of better than ±0.05 in x, so that further technique development will be required if heterostructures in this alloy system are to be studied seriously.

Microanalysis of inclusion/matrix interfaces in weld metals

AbstractThe effects of non‐metallic inclusions on the microstructure of as‐deposited C–Mn weld metals have been investigated in a series of welds produced using different electrode coatings to obtain inclusions of different chemical compositions. The inclusions were analysed on carbon extraction replicas or in thin foils in a 120 kV (scanning) transmission electron microscope (TEM (STEM)) equipped with energy dispersive x‐ray microanalysis facilities. Both x‐ray microanalysis and convergent beam diffraction techniques, with probe sizes down to 10 nm, were used to characterize the inclusions and the inclusion/matrix interfaces.It was found that the ‘cores’ of the inclusions were mainly oxides, such as MnSiO3 or MnAl2O4. The surfaces of the inclusions were either coated with thin layers of copper sulphide, or mixtures of copper sulphide and titanium carbonitride when both copper and titanium were present as impurities in the welds. Effects of non‐metallic inclusions on weld metal microstructure are discussed with respect to the above‐mentioned coatings and the heterogeneous nucleation of the fine acicular ferrite microconstituent.

The effect of temperature on the chemistry and morphology of the interphase in an SCS6/Ti–6Al–4V metal matrix composite

The changes in the chemistry and morphology within the interphase region of an SCS6/Ti–6A1–4V metal matrix composite upon exposing the samples to varying heat treatments have been studied using Auger electron spectroscopy, TEM and convergent beam diffraction techniques. These changes, such as the formation of small TiC particles at one interface and the narrowing of a protective pyrocarbon layer at another, induce fracture to occur at different places within the interphase upon heating. The reasons for this are explained. Evidence for a change in phase of a TixSiy(C) layer to the more thermodynamically stable Ti5Si3 is also given.

Analytical electron microscopy of materials

Analytical electron microscopy enables combined crystallographic and chemical information with a high spatial resolution to be gained from microregions of electron-transparent specimens. This is reached by the combined application of imaging, diffraction and spectroscopic methods, using either a dedicated scanning transmission electron microscope or a conventional high-resolution electron microscope (having a strong objective lens) equipped with suitable X-ray or electron spectrometers. Of the diffraction methods especially the technique of convergent beam diffraction is used, yielding valuable information on crystal structures, lattice parameter changes, symmetry variations and crystal perfection, respectively. For chemical analysis, either energy-dispersive X-ray spectroscopy (EDX) is used or electron energy loss spectroscopy (EELS). Finally, high-resolution electron microscopy in the lateral resolution range of some 0.1 nm allows the reliable geometrical inspection of extreme microregions.

Struktur- und Absorptionspotentiale von KCl und NaCl aus Beugungsaufnahmen im konvergenten Elektronenbündel

By improvement of an earlier published preparation method for producing thin single crystals soluble in water, it is possible to determine structure and absorption potentials of some reflections of KCl and NaCl crystals by a convergent beam diffraction technique.