J. Gjønnes

GP-zones in Al–Zn–Mg alloys and their role in artificial aging

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 domains. The structural relation to the η′-precipitate is discussed.

THE DEFECT STRUCTUFE OF SrTi1−xFexO3−y (x = 0–0.8) INVESTIGATED BY ELECTRICAL CONDUCTIVITY MEASUREMENTS AND ELECTRON ENERGY LOSS SPECTROSCOPY (EELS)

Abstract The electrical properties and defect structure of selected compositions in the SrTi1−xFexO3−y system (x = 0–0.8) have been studied using van der Pauw 4-point conductivity measurements and electron energy loss spectroscopy (EELS). Using X-ray powder diffraction and selected area electron diffraction (SAD), the basic crystal structure was determined to be cubic perovskite for all the investigated compositions. A superstructure cell 2·2·1 times the ordinary perovskite cell was found in materials with x = 0.6 and x = 0.8. The conductivity was measured on sintered tablets as a function of the partial pressure of oxygen (pO2 = 10−25 to 1 atm) at 600–1100°C. The materials investigated are predominantly p-type electronic conductors at high, n-type conductors at low, and ionic conductors at intermediate oxygen partial pressures. All conductivity contributions increase with increasing iron content. This can be attributed to the acceptor role of the iron, decreased band gap and decreased activation energy for oxygen vacancy migration. The EELS spectra show a shoulder below the oxygen K-edge, increasing in magnitude with increasing iron content and oxygen partial pressure. This shoulder is assigned to empty electron energy states at some of the oxygen atoms, indicating that electron holes are associated with lattice oxygen in the structure. Spectra from the Fe L-edge showed small changes, suggesting that there are few or no empty states at the iron atoms.

Investigation of precipitation in an Al–Zn–Mg alloy after two-step ageing treatment at 100° and 150°C

Abstract Fine-scale precipitation of the metastable Zn- and Mg-rich η′ phase and its precursors is essential for the mechanical properties of Al–Zn–Mg alloys. However, at present neither the precipitation sequence nor the structure and composition of the intermediate precipitate phases are completely clear. This paper deals with an investigation of precipitation in an industrial Al–Zn–Mg alloy at various stages of a conventional two-step ageing treatment at 100° and 150°C. Studies were performed using both transmission electron microscopy and atom-probe field ion microscopy. Transmission electron microscopy (TEM) analysis revealed two parallel precipitation paths; one involving formation and dissolution of the ordered GP (I) zones, the other involving formation of clusters (type II), having a different atomic arrangement compared to the Al-matrix, which transform to the η′ phase. Atom-probe study of the material after short time ageing at 100°C did not show any observable distinction between GP (I) and type II precipitates. In the peak-aged material the best classification of precipitates was obtained using their morphology (the cigar-like and the plate-like) because there was significant overlap in the range of total solute contents of each type of precipitate. Generally the Zn:Mg ratio in all observed types of precipitates was close to 1:1 and the total solute atom content increased with ageing time. Distribution of alloying elements in the precipitates and in the surrounding matrix is discussed.

HREM study and structure modeling of the η′ phase, the hardening precipitates in commercial Al–Zn–Mg alloys

Abstract The structure of the η ′ phase, one of the most important age-hardening precipitates in commercial Al–Zn–Mg alloys, has been studied at the atomic level by means of high-resolution electron microscopy (HREM). A structural model of the η ′ phase has been constructed on the basis of the structural characteristics in the observed images and the structure of the η -MgZn 2 phase. Image simulation of this model shows a good agreement between calculated and experimental images. Comparison of this model with the early existing model on the basis of the X-ray diffraction is also given.

Effect of predeformation and preaging at room temperature in Al–Zn–Mg–(Cu,Zr) alloys

Abstract 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.

Measurement of three-dimensional intensity data in electron diffraction by the precession technique

Abstract The precession technique, recently devised by Vincent and Midgley at the University of Bristol, has been used to acquire three-dimensional intensity data of sufficient quality for structure determination of the intermetallic phase Al m Fe. Dynamical interactions are effectively reduced using the precession method, and the intensity data can be treated within a kinematical approximation. Diffraction patterns from a total of eight projections were digitised by a CCD camera coupled to an image analysis system, and by combining equivalent reflections from different projections, the intensity data was merged into a three-dimensional data set by a least-squares procedure based on experimental intensity ratios between reflections common to each projection. A three-dimensional set of intensity data is needed for calculations of Patterson maps, and eventually Fourier maps, as a guide for a structure model. The quality of the quantitative data and the factors affecting the methods and calculations are discussed. The preliminary conclusion is that the intensities are expected to be useful for calculating Patterson and Fourier maps, but less so for least-squares refinement.

Crystal structure of β-Al4.5FeSi

By a combination of X-ray and electron diffraction, the average structure of the intermetallic phase β-Al 4.5 FeSi has been determined. The crystals grown from the melt were generally of poor quality; the monoclinic space group A2/a was deduced from electron diffraction patterns obtained from small domains. X-ray diffraction data measured at T= 298 K with Mo Kα radiation (λ=0.71069 A) out to sin θ/λ=0.8 A -1 resulted in 1739 observed reflections, of which 980 were unique. Of these, 244 weak reflections affected by disorder were removed. Cell dimensions are a=6.161 (3), b=6.175 (3), c=20.813 (6) A, β=90.42 (3) o

β-Al4.5FeSi: A Combined Synchrotron Powder Diffraction, Electron Diffraction, High-Resolution Electron Microscopy and Single-Crystal X-ray Diffraction Study of a Faulted Structure

A previous single-crystal X-ray and electron diffraction structure study [Romming et al. (1994). Acta Cryst. B50, 307–312] of the heavily faulted alloy phase β-Al4.5FeSi has been extended by synchrotron powder data and further electron microscopy and diffraction observations. Reflections that were omitted in the single-crystal work could be included in the powder refinement, which resulted in some adjustment of cell parameters and atom coordinates. The double c axis reported by some authors is explained by periodic faults in the structure, which is described in terms of a tetragonal sub-unit. Apparent discrepancies between refinement from single-crystal and powder data are discussed briefly.

The Precession Technique in Electron Diffraction and Its Application to Structure Determination of Nano-Size Precipitates in Alloys

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.

Structure refinement of Al3Zr using single-crystal X-ray diffraction, powder neutron diffraction and CBED

The structure of the intermetallic compound A13Zr has been studied at 293 K by single-crystal X-ray diffraction (Mo Ka radiation, ,~ -- 0.71069 A), powder neutron diffraction {A[Ge(711)] = 1.0867 A} and convergent-beam electron diffraction (CBED) (200 keV, A = 0.0251/k). The structure of AI3Zr comprises four close-packed metal sub-lattices and has the tetragonal space group 14/mmm with a = 3.9993 (5), c = 17.283 (2) A, V= 276.43 (6) ,~3, Z= 4, Dx=4.136gcm-3, /z=45.11cm-l. The new z coordinates of four A1 and four Zr atoms on the e position [ZA~(e)and Zzr(e)] were determined by singlecrystal X-ray diffraction: Zn~(e)=0.37498(5) and