Gordon Dunlop

The precipitation sequence in Al–Mg–Si alloys

Fine-scale precipitation that occurs during age hardening of Al alloy 6061 has been studied using differential scanning calorimetry (DSC), atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). It was found that the precipitation sequence is: independent clusters of Mg and Si atoms {yields} co-clusters that contain Mg and Si atoms {yields} small precipitates of unknown structure {yields} {beta}{double_prime} needle-shaped precipitates {yields} B{prime} lath-shaped precipitates and {beta}{prime} rod-shaped precipitates. A new structure is proposed for the {beta}{double_prime} precipitate. It was found that the Mg:Si ratio in the intermediate precipitates and co-clusters was close to 1:1.

The interfacial morphology of strained epitaxial InxGa1-xAs/GaAs

The microstructure of strained layers of InxGa1−xAs/GaAs grown by molecular‐beam epitaxy has been investigated by transmission electron microscopy. lt was found that the formation of irregular interfacial morphologies of the InxGa1−xAs layers was due to a transition in growth mode from two‐dimensional (layer‐by‐layer growth) to three‐dimensional nucleation via island formation. It was also found that the occurrence of irregular growth surfaces of epitaxial layers was dependent upon inhomogeneous lattice strains induced by the formation of islands. A possible role of lattice strain for the formation of irregular growth surfaces was also discussed.

APFIM investigation of fine-scale precipitation in aluminium alloy 6061

The results of an APFIM investigation of precipitation during ageing of aluminium alloy 6061 at 70°C and 175°C are presented. The first stage of precipitation was found to involve the separate clustering of Si atoms and Mg atoms. This was closely followed by movement of Mg atoms to the Si clusters. The Mg:Si ratio in these small precipitates increased slowly during ageing at 70°C and more rapidly during ageing at 175°C, with most precipitates attaining a Mg:Si ratio of close to one at both temperatures. A needle-shaped precipitate formed during ageing for 24 h at 175°C was also found to have a Mg:Si ratio of close to one.

Interfacial Microstructures in InxGa1-xAs/GaAs Strained Layer Structures

The interfacial microstructures of lattice strained In x Ga l-x As/GaAs multiple layer structures, that were grown by molecular beam epitaxy (MBE) on GaAs (100) substrates, have been investigated and characterised by transmission electron microscopy (TEM). A g 3ii weak beam imaging technique has been used to study structural imperfections at the heterointerfaces. The morphology of rough heterointerfaces, which resulted from the growth of the In x Ga l-x As layers (strained layer) either in a two dimensional (2D) or in a three dimensional (3D) growth mode via island formation, was imaged using this technique. A transition from 2D to 3D growth was found to occur at a certain critical layer thickness which decreased with increasing indium fraction. In thicker layers, dislocation complexes, which may have been caused bythe formation of islands, were also observed. These complexes were primarily composed ofstacking faults bounded by partial dislocations.

Interfacial Microstructure of InxGa1-xAs/GaAs Strained Layers

The topography and defect structure of interfaces in InGaAs/GaAs strained-layer structures grown by molecular beam epitaxy were investigated by transmission electron microscopy. A large range of indium fractions, x, was investigated: 0.16 ≥ x ≥ 1.00, and three different critical layer thicknesses, corresponding to the formation of three different types of defects, were observed with increasing thicknesses of the strained layers. These defects were: rough interfacial topographies resulting from an onset of 3-dimensional growth for the InGaAs layers; misfit dislocations of the 60° mixed type; and dislocation complexes consisting of planar defects on {111} planes. Compared with results obtained from photoluminescence measurements it was found that both types of defects involving dislocations resulted in severe degradation of the optical properties of the strained-layer structures.

The AMC-CAST alliance for advanced magnesium research and development

Australian Magnesium Corporation (AMC) and the Cooperative Research Centre for Cast Metals Manufacturing (CAST) have strengthened their relationship by creating an alliance to further R & D on the processing and applications of magnesium alloys. This research is necessary in order to address a number of issues that are critical for continued growth of the market for magnesium alloys in the automotive industry. The paper describes recent progress that has been made in R & D on such important issues as: continuous direct chill casting; cover gas protection solidification; mechanical properties, high temperature alloy development and design. Many aspects of this research program are being conducted in close collaboration with manufacturers and end-users of magnesium alloys.