C.J. Rossouw

Statistical ALCHEMI: Aeneral formulation and method with application to Ti[sbnd]Al ternary alloys

Abstract Recent advances in the theory of characteristic X-ray emission from crystals under dynamic electron diffraction conditions are used in developing a new fully quantitative statistical method for atom location by channelling-enhanced microanalysis (ALCHEMI). Limitations imposed by delocalization, disorder and occupancy of interstitial sites are clarified. A fully computerized method for automated data collection and analysis from incoherent channelling patterns formed by the variation in characteristic X-ray emission as a function of orientation is used for the fist time. The precision and accuracy of this method are investigated using both experimental and simulated data from the ordered γ phase in ternary Ti[sbnd]Al alloys. It is shown that 1 at. % Ga, Hf and Mo additions partition onto Al sites, Ti sites and both sites respectively, and contrast in X-ray incoherent channelling patterns conforms that these ternary atoms are not located on interstitial sites. The traditional ratio ALCHEMI techniqu...

Zone-axis convergent-beam electron diffraction and ALCHEMI analysis of Ti[sbnd]Al alloys with ternary additions

Abstract A new statistical method for atom location by channelling-enhanced micro-analysis is applied to γ [sbnd] α2 duplex Ti[sbnd]Al based alloys containing ternary element additions of V, Cr, Mn, Ga, Y, Zr, Nb, Mo, La, Hf, Ta and W at the 1 at.% level. Incoherent channelling patterns, formed by the variation in characteristic X-ray emission as a function or orientation, are collected from a number of different zone axes from both phases. Comparisons of experimental patterns with calculated maps for various crystallographic sites indicate that the ternary atoms are incorporated substitutionally in the atomic structure of both the γ and the α2-phases and do not occupy interstitial sites. The crystallographic site distribution of the ternary elements is determined through χ2-minimization techniques, together with quantitative error limits. The structure of the non-stoichiometric α2-phase is investigated by convergent-beam electron diffraction.

Generation and absorption of characteristic X-rays under dynamical electron diffraction conditions

Abstract The theory for formation of incoherent channelling patterns (ICPs), produced by variations in characteristic X-ray emission in response to a systematic scan in the direction of the incident electron beam for orientations near a zone axis, is extended to account for absorption of X-rays within the specimen. This is achieved by calculating the probability of X-ray generation as a function of depth within a crystal prior to the inclusion of an absorptive term which is dependent on the geometry of the X-ray detector relative to the crystal as well as the absorption characteristics of the specimen for the particular X-ray energy. ICPs from three major zones of the L1 0 phase in Ti Al and Ti Ga alloys are compared and effects of different absorption due to variation of goniometer tilt with respect to the X-ray detector are calculated. Although fast electron quantum states that are peaked on Ti atomic sites have similar eigenvalues (binding energies) in both alloys, it is shown that differences in complex eigenvalues (including absorption) occur between states that are localized on Al or Ga, and that this results in substantially different contrast between the two alloys in ICPs from Al and Ga characteristics X-rays.

Lattice Images of Solid Xenon Precipitates in Aluminum at Room Temperature

Small solid precipitates (bubbles) of xenon in an aluminum matrix have been formed by ion implantation. Lattice images of this room-temperature inert gas solid were obtained using high-resolution phase-contrast electron microscopy. Many bubbles showed a high degree of crystalline perfection, but regions of defective crystallinity were observed in several cases.

Use of MeMn(CO)5 in the low temperature MOCVD growth of Mn containing alloys

Abstract The new organometallic feedstock MeMn ( CO ) 5 has been used for the MOCVD growth of Cd 1− x Mn x Te (111B) epilayers on GaAs (100) substrates. The epitaxial relationship was established by HRTEM and X-ray diffraction (XRD). The quaternary alloy Hg 1−x−y Cd x Mn y Te was grown by in-situ annealing og HgTe overlayers on Cd 1−x Mn x Te epilayers. Adherent films of Hg 1−x Mn x Te on glass were grown by MOCVD. The electrical properties show semi-metal behaviour, and XRD and other properties indicate a zinc-blende structure.

Dynamical electron diffraction analysis of lattice parameters, Debye—Waller factors and order in Ti-Al and Ti-Ga alloys

Abstract It is shown that higher order Laue zone (HOLZ) measurements under strong zone-axis dynamical diffraction conditions can be used to measure crystal lattice parameters with relatively high precision (∼1 in 10 3 ). This method is illustrated for γ -phase Ti-Al and Ti-Ga alloys, and results for Ti-Al are compared with those obtained from X-ray measurements. The relative positions of quantum state excitation rings in the HOLZ beams are used to demonstrate that both TiAl and TiGa are fully ordered. In addition, a novel way is described for the determination of Debye—Waller factors from intensities in the different quantum state rings. This has the particular advantage of enabling independent assessment of Debye—Waller factors for the various atomic species.

Polarity determination of epitaxial structures of CdTe on GaAs by channeling techniques

Abstract RBS spectra analysis near the (111) planar channeling direction is used for polarity determination of MOCVD-grown epitaxial layers of (111) and (100) CdTe on sapphire and GaAs substrates. Extensive multiple twinning limits the application of RBS channeling analysis for polarity determination. Comparison is made with polarity determination by X-ray diffraction and two electron diffraction techniques.

Location of Zn within the Mg12(LaxCe1−x) lattice by X-ray incoherent channelling patterns

Systematic electron diffraction studies on intermetallic precipitates formed within a lightweight Mg-RE-Zn alloy (RE = La or Ce) identify these to be of structural type Mn(12)Th (space group I4/mmm). Analytical electron microscopy yields an overall composition of Mg(12)(La(x)Ce(1-x)) with x ~ 0.43, with 1 at.% Zn incorporated within the lattice. Variations in characteristic X-ray emission rates, as an electron beam is rocked near zone-axis orientations, are used to form two-dimensional channelling patterns, termed X-ray incoherent channelling patterns. This channelling contrast enables a specific sublattice site that is occupied by Zn to be unambiguously identified within the Mg(12)RE lattice. The particular sublattice site is denoted by the Wyckoff letter f, and is one of the three different Mg sublattice sites f, i and j. Of these three sites, the Wigner-Seitz cell that is centred on the f sublattice site has the largest Mg-RE interatomic distance, and therefore the f site is expected to be favoured for accommodating the substitution of a larger Zn atom.

Convergent beam electron diffraction zeroth order Laue zone and higher order Laue zone analysis of order in TiAl alloys

Abstract Order in γ-phase TiAl alloys is investigated by convergent beam diffraction of 200 keV electrons. The separation of different branches of the dispersion surface, as observed in quantum state rings in beams from higher order Laue zones, is shown to provide a direct measure of order. Theory and experiment are shown to be in good agreement.

Atomic site occupancy in D022 and L12 titanium trialuminides

Abstract X-ray incoherent channelling patterns (ICPs) are generated by variations in characteristic X-ray emission rates induced by a systematic scan of incident fast electron beam orientation near low index zone axes. ICP contrast is used to derive relative sublattice site occupancies of alloyed atoms in Al3Ti and related intermetallics. Depending on composition and alloying atoms, these have crystal structures of type D022 or L12. When V is added to form the D022 intermetallic A13(V0.6Ti0.4), it is shown that Ti and V share the same sublattice site. The L12 structure is stabilized by additions of Cr (which induces the transformation from the D022 structure and stabilizes the L12 phase). Elements X = Hf, Zr and W were added at the 0.5at.% level to form separate quaternary alloys of overall composition A162Cr10.5Ti27X0.5 with the L12 crystal structure. The sublattice site partitioning of these minority atoms is investigated by the formation of X-ray ICPs and subsequent statistical analysis of these patterns, related to atom location by channelling enhanced microanalysis (or ALCHEMI). It is shown that Hf and Zr are accommodated exclusively on Ti sublattice sites whereas W favours the A1 sublattice site with about 10% of the alloyed W atoms accommodated on Ti sublattice sites. Cr is shown to partition exclusively onto the A1 sublattice site. Contrast from a ⟨110⟩ zone confirms that these minority atoms do not occupy the interstitial site at the body centre position of the L12 unit cell. Convergent beam electron diffraction data from the zero-order Laue zone and higher order Laue zone are shown and discussed for the ⟨110⟩ projection of the L12 structure.