V. Hansen

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.

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.

Microstructure and catalytic activity towards the hydrogen evolution reaction of electrodeposited NiPx alloys

Abstract Electrodeposited NiP x alloys have been studied using electrochemical techniques, transmission electron microscopy and electron diffraction. The results show that crystals in the nanometer range are ‘floating’ in an amorphous phase. The crystal size is correlated with the catalytic activity of hydrogen evolution in an alkaline solution. It was found that the activity increased when the crystal size decreased. Electron diffraction patterns of the crystals showed that when few crystals were observed, they were found to be pure Ni. Ni 3 P was found only when considerable amount of crystals had been formed. With the formation of Ni 3 P the activity was reduced. It is proposed that the increased catalytic activity is caused by an increased amount of amorphous phase surrounding the Ni crystals. This phase is able to absorb large amounts of hydrogen, thereby changing the electronic structure of the electrode material.

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.

Growth of HgTe nanowires

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 μm. 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.

Metallurgical reactions in two industrially strip-cast aluminum-manganese alloys

Precipitation, phase transformation, subgrain growth, and recrystallization that occur during heat treatment of two strip-cast, cold-rolled, high manganese aluminum alloys have been studied mainly by transmission electron microscopy (TEM). The alloys differ in silicon content. The isothermal heat treatments have been performed in a salt bath at temperatures between 330 °C and 530 °C for times up to 1000 hours. Size distributions for each type of secondary particle have been determined. After short annealing times, small quasicrystals precipitated and subsequently transformed to α phase. The densities of these precipitates controlled dislocation movement and regulated subgrain sizes. Prolonged heating resulted in peritectoid reactions to Al6Mn or Al12Mn. Recrystallization, which is associated with the formation of Al12Mn, is advanced by increasing the silicon content; the nucleation and growth of Al12Mn occurs only at the expense of other phases that stabilize the subgrain network.