The structure of the hardening eta'-phase precipitates in Al-Zn-Mg alloys has been investigated by using precession electron diffraction and X-ray synchrotron radiation diffraction. The latter was recorded as a three-dimensional, continuous intensity distribution from a single alloy grain, from which patterns of remarkable sharpness from the precipitate particles could be extracted. High resolution electron microscopy revealed extensive structural variations with prolific faults in the eta' precipitates. The eta'-structures are described in terms of two structure models based on two icosahedral elements that are inherent in the equilibrium eta-MgZn(2) structure. The role of these icosahedra in the transformations of the alloy system is discussed. (C) 2010 Elsevier B.V. All rights reserved.

The precipitation behavior, especially the early nucleation stages, of the industrial strip-cast Al3003 alloys was investigated by using transmission electron microscopy (TEM). An icosahedral quasicrystalline phase was found as secondary particles in these strip-cast alloys after heat treatment for a few seconds. Three different nucleation paths are proposed based on the TEM observations. They have the same origin, viz. (Mn, Fe)-containing Mackay icosahedra, and are governed by the composition of alloys, especially the Mn and Si content.

HgTe nanowires have been grown by molecular beam epitaxy (MBE). They are nucleated at Au particles on Si or GaAs substrates and subsequently self-organize and grow laterally on the surface into 20–50 nm wide, 0.5–1 μm long twisted, but single-crystal, wires. Further growth gives longer, wider, and straighter polycrystalline wires. When unimpeded by Au particles on the surface, the wires become straight and consist of segments of cubic 〈111〉 HgTe and hexagonal 〈001〉 Te parallel to the wire. Te nanowires and Au␣nanowires have also occasionally been formed. All attempts to grow CdHgTe on Si substrates with or without Au particles have resulted in polycrystalline layers. The phase diagrams and diffusion coefficients imply that CdHgTe or HgTe nanowires will not grow by the vapor–liquid–solid technique at the low MBE growth temperatures. SiO2 functions as a mask for selective growth of HgTe, but not for CdHgTe.

Age hardening in industrial 7xxx alloys at the temperature 100 degrees and 150 degrees C up to 144 hrs, after solid solution treatments at 450 degrees and 550 degrees C, has been followed by measurements of Vickers hardness, scanning and transmission electron microscopy. The influence of silicon on phase and kinetic of age hardening zones and precipitates has been studied. High iron and silicon content increase the number of primary particle in the alloy. Size distribution of eta'-precipitates has been determined.

HgTe nanowires nucleated by Au particles have been grown on Si and GaAs substrates by molecular beam epitaxy. The wires are polycrystalline. They evolve from crooked to straight during growth and have rounded to rectangular cross-sections. The widths are in the range 20 - 500 nm, with lengths up to 4 mu m. The height of the nanowires is typically less than the width. The nanowires have been characterized by scanning electron microscopy, x-ray photoelectron spectroscopy, transmission electron microscopy and atomic force microscopy. The effects of substrate material, substrate preparation and growth conditions have been investigated.

The Vincent-Midgley precession technique has been used to collect three-dimensional electron diffraction intensity data from a dispersion of coherent precipitates in a matrix. In order to suppress severe effects from multiple diffraction via matrix reflections, a fairly large precession (tilt) angle had to be used. This implied a high background from the surrounding matrix, and limited the number of reflections that could be measured from patterns on image plates. The heavily faulted hexagonal η′-precipitates (a=0.496nm, c=1.405nm) with thickness 3–5nm occur in four equivalent orientations relative to the aluminium matrix; with frequent overlap of reflections. A model of the average structure in the space group P63/mmc with assumed composition Mg2Zn5−xAl2+x, have been derived by Patterson analysis and intensity comparisons.

Resolution Microscopy and Atom Probe Analysis of Nano-Size Precipitates in Al-Zn-Mg Industrial Alloys. V. Hansen, A. Kverneland, R. Vincent, X.Z. Li, K. Stiller and J. Gjonnes, Faculty of Science and Technology, Stavanger University College, N-4068 Stavanger, Norway, H.H.Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL UK, Center for Materials Science, University of Oslo, N-0439 Oslo, Norway, Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588-0113, Department of Physics, Chalmers University of Technology, S-4196 Goteborg, Sweden. E-mail: vidar.hansen@tn.his.no

Age hardening of an industrial 7000 series alloy in the temperature range 70-150degreesC has been followed by mechanical testing, electrical conductivity measurement, differential scanning calorimetry and extensive electron microscopy (TEM). The property changes during aging are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable hardening precipitate eta' and the stable phase eta-MgZn2, as well as coarsening, changes of composition and internal order within zones and precipitates. Time-temperature ranges of the transformations during aging, and its dependence on quenching temperature, are estimated from TEM observations. The role of the GP(II) zones in the aging of alloys quenched from temperatures above 450degreesC is emphasized.

Crystal structure of nano-scale precipitates in age-hardening aluminum alloys is a challenge to crystallography. The utility of selected area electron diffraction intensities from embedded precipitates is limited by double scattering via matrix reflections. This effect can be signally reduced by the precession technique, which we have used to collect extensive intensity data from the semicoherent, metastable eta(')-precipitate in the Al-Zn-Mg alloy system. A structure model in the space group P-62c is proposed from high-resolution microscopy and electron diffraction intensities. The advantages of using the precession technique for quantitative electron diffraction is discussed.

A series of structure analyses during 19942001 by electron crystallographic techniques applied to phases in aluminum alloys are reviewed. Methods for structure solution employ electron diffraction intensity data collected by the precession technique, by selected area micro-diffraction and by the convergent-beam technique. High-resolution electron microscope images (HRTEM) are treated by a different kind of processing, including exit wave reconstruction. Crystallographic calculations are performed either by direct method or Patterson and Fourier procedures, assuming kinematical scattering, or by refinement from models derived from HRTEM images. Dynamical scattering calculations can be introduced in the refinement stage or as a correction procedure applied to part of the intensity data. The phases studied include primary Al-Fe-(Si) particles, Al-Mn-Si dispersoids, Al-Co quasicrystals and two types of precipitate phases in age-hardening Al-Mg-Si and Al-Zn-Mg alloys.

Transmission electron microscopy and atom probe field ion microscopy have been used to study the effect of heat treatment at 150 °C and material composition on precipitation in Al–Zn–Mg alloys. Two Cu-free materials with different Zn:Mg ratios (1.7 and 3.2) and one material with low Zn:Mg ratio but with small additions of Cu have been investigated. Selected area electron diffraction patterns of Cu-free materials revealed the presence of GP II zones for all heat treatments used. The number of η particles increased for longer ageing times, but η′ is the dominant precipitate phase up to 6 h at 150 °C. In the material containing Cu, occurrence of GP I zones was also observed. One dimensional atom probe analyses showed that Zn:Mg ratio in the precipitates decreased with heat treatment time increase and that particle number density got lower. Investigation also revealed that Cu is incorporated into the precipitates. An attempt to estimate the contribution of Al excess into η′ due to ion trajectory aberration has been made. The influence of Cu and excess Zn on the precipitation process is discussed.

Journal Article The collection of electron diffraction intensity data and their use in structure determination Get access J Gjonnes, J Gjonnes Center for Materials Science, University of Oslo, Gaustadalleen 21 N-0349 Oslo, Norway Search for other works by this author on: Oxford Academic Google Scholar V Hansen, V Hansen Stavanger University College, Departement of Technology and Natural Sciences, P.O. Box 2557 Ullandhaug N-4091 Stavanger, Norway Search for other works by this author on: Oxford Academic Google Scholar XZ Li XZ Li Walter Scott Engineering Center, University of Nebraska, Lincoln, NE 68588-0656. USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 8, Issue S02, 1 August 2002, Pages 100–101, https://doi.org/10.1017/S1431927602102017 Published: 01 August 2002

A TEM and atom-probe (APFIM) survey of GP-zones and precipitate phases that occur in a 7108 Al-Zn-Mg-Zr age hardening alloy is presented, encompassing structure, composition and range of existence along the transformation paths during aging. Aging treatment included single-step aging at temperatures between RT and 150 degreesC, and double aging at 100degrees/150degreesC. Quenching temperatures were varied between 450 and 550 C. A structure model for the transformation: solid solution (alpha)--> GP(II) --> eta' --> eta-MgZn2 is shown. The connection between high solution temperature, GP(II)-zones and strength is discussed.

The effects of zirconium and copper on the early stages of the precipitation processes in an Al-5.5 wt pct Zn-1.2 wt pct Mg alloy have been studied by differential scanning calorimetry (DSC) thermal analysis. Electron diffraction has been used as a complementary technique to aid in the interpretation of the thermal effects observed in the DSC thermograms. The results show that the initial stages of Guinier-Preston zone I (GP(I)) formation at room temperature are not affected by the presence of zirconium, but the rate of Guinier-Preston zone II (GP(II)) precipitation is slowed down significantly. For aging at 100 °C, the stability of GP zones is reduced by the addition of zirconium, and this leads to a reduction in the amount of η ′ produced during aging. The addition of copper to an Al-5.4 wt pct Zn-1.2 wt pct Mg-0.2 wt pct Zr alloy intensifies the electron diffraction spots from GP(I), suggesting that the strong electron-scatterer copper may be incorporated into GP zones. The rate of growth of GP(I) at room temperature is unaffected by the presence of copper, but the rate of formation of GP(II) at room temperature is retarded. For artificial aging at 100 °C, the development of GP(I) and GP(II) is not affected significantly by the presence of copper, but the formation of η ′ is stimulated, producing a high number density of very fine η ′ precipitates. Preaging at room temperature results in accelerated η ′ formation during subsequent aging at 100 °C in the zirconium-containing alloy. However, this acceleration of η ′ formation is absent when copper is present in the alloy.

The effect on yield stress of predeformation and natural aging prior to a two-stage artificial hardening treatment has been investigated for the 7xxx series alloys AA7108 and AA7030. It was found that 10% predeformation in tension reduced the yield stress measured in the T6-state by 7–10%. About half of this reduction could be regained when a preaging period was inserted between the deformation and the artificial aging treatment. The natural aging response was followed also by measurement of electrical conductivity and hardness. Precipitate phases and GP (Guinier–Preston)-zones occurring during the aging treatments were investigated by transmission electron microscopy. The lower yield stress in predeformed samples is explained by early nucleation of the equilibrium phase η on the dislocation network, leaving less solute for formation of the main hardening phase η′. The reduction in yield stress was partly offset by the preaging, due to recovery processes in the dislocation network.

The structure of GP-zones in an industrial, 7xxx-series Al–Zn–Mg alloy has been investigated by transmission electron microscopy methods: selected area diffraction, conventional and high-resolution imaging. Two types of GP-zones, GP(I) and (II) are characterized by their electron diffraction patterns. GP(I)-zones are formed over a wide temperature range, from room temperature to 140–150°C, independently of quenching temperature. The GP(I)-zones are coherent with the aluminum matrix, with internal ordering of Zn and Al/Mg on the matrix lattice, suggested to be based on AuCu(I)-type sub-unit, and anti-phase boundaries. GP(II) are formed after quenching from temperatures above 450°C, by aging at temperatures above 70°C. The GP(II)-zones are described as zinc-rich layers on {111}-planes, with internal order in the form of elongated <110> domains. The structural relation to the η′-precipitate is discussed.

Two spinel dunite xenoliths (Fo(89.8-91.2) in olivine) from La Palma contain minor amounts (<1%) of a pale-blue sodalite-group mineral with hauyne/lazurite chemistry. Selected-area electron diffraction (SAED) patterns of this phase indicate a cubic unit cell with dimensions 9.12 +/- 0.02 Angstrom, and space group P (4) over bar 3n. Superstructure spots along three <110> directions are common, implying commensurate or incommensurate modulations along <110> directions. Raman spectra show peaks typical of both lazurite and hauyne. It is concluded that the mineral has a structure intermediate between those of pure lazurite and pure hauyne, and it is here referred to as hauyne(ss). The hauyne(ss) occurs together with strongly nepheline-normative glass in thin veinlets (<0.1 mm), in interstitial glass pockets, and as inclusions in olivine porphyroclasts. To our knowledge lazurite or hauyne has not previously been described in mantle rocks. The hauyne(ss) is strongly depleted in REE and most other highly lithophile elements relative to the coexisting glass, whereas D-mineral/glass for Sr is approximate to 1.0, and D-Eu higher than the other REE. The hauyne(ss) crystallized from a melt now present as phonolitic glass, probably in response to rapidly decreasing pressure during transport of the xenoliths to the surface. The coexistence of hauyne(ss) and FeS-rich sulfide globules in some samples suggests slightly more oxidizing conditions than for samples in which the glass contains sulfide globules alone.

Transmission electron microscopy of minerals at high resolution documents mostly retrograde transformations in rocks.When metamorphic and igneous rocks cool down, ongoing mineral reactions are incipient and fine-grained products are located within, or close to, prograde phase boundaries.Combined high resolution transmission electron microscopy imaging (HRTEM), selected area electron diffraction (SAED), and analytical electron microscopy (AEM) provide invaluable informations about the reaction mechanisms at the quasi atomic scale.This works when the mother phase(s) and the daughter phase(s) coexist, i.e. for reactions arrested before completion at some closing temperature.At low solid/fluid ratios, the dissolution / cristallization mechanisms predominate whereas high solid/fluid ratios favour solid-state transformation mechanisms.In the latter case, coexisting species display tight crystallographic relationships (topotaxy) which largely illustrate the principle of saving the nature and/or orientation of structural modules of silicates as far as possible.Metastable structural states often come in between mineral reactants and products.The oxygen framework is virtually unaffected during most reactions.Such structural inheritance is more frequent than maintenance of local chemistry.This indicates that .microstructuresmight be more efficient than chemical composition in tracing back the history of a rock.However several reaction mechanisms coexist frequently side-by-side for one and the same mineral system.It is not clear in many cases whether they were simultaneously used by Nature or were operating successively during the cooling of the host rock.The above conclusions will be illustrated mainly from hydrothermally altered and/or hyrothermally grown silicates: pyroxenes, amphiboles, biopyriboles, planar and rolled layer silicates, and their association.s5.m1.