Grain-boundary precipitation in an Al4.0Cu0.5Mg0.5Ag alloy

Transmission electron microscopy and electron diffraction were used to study grain-boundary precipitation in an Al-4.0Cu-0.5Mg-0.5Ag (wt%) alloy. Low-angle grain-boundaries were found to nucleate Ω precipitates on the {111}_α planes even when the {100}_α habit planes of the competinng θ′ metastable phase were closer to the grain-boundary plane. High-angle grain-boundaries, which were random in nature and had relatively large energy, nucleated Ω precipitates predominantly. A few S precipitates and a θ precipitate (G IV/V orientation) were found to co-exist with Ω in these boundaries. The proximity of the grain-boundary plane to the {111}_α plane on which grain-boundary Ω nucleated was found to be particularly important in both low and high-angle grain-boundaries, similar to results for the θ′ phase in AlCu alloys.     

Precipitate stability in AlCuMgAg alloys aged at high temperatures

A study has been made of the thermal stability of the Ω phase in AlCuMgAg alloys aged at high temperatures (200 to 350°C). This phase, which precipitates as thin plates on the {111}_α planes, has been shown to be replaced by the equilibrium precipitate θ (Al₂Cu) after prolonged ageing (e.g. 2400 h at 250°C). Measurements have been made of the thickening behaviour of the Ω plates and the various orientations and morphologies of the θ phase have been characterised. Whilst there is some evidence for the direct allotropic transformation of Ω to θ, it is concluded that a gradual dissolution/re-precipitation mechanism dominates the changes to microstructure at these high temperatures. Although magnesium and silver are known to segregate to the Ω phase, they were not detected in association with θ. Rather they were found to partition to sites of the S phase (Al₂CuMg) which forms as a minor precipitate under these ageing conditions.     

Study of the new Frank-Kasper phases in AlLiCuMg alloys

A new group of Frank-Kasper phases has been observed in AlLiCuMg alloys using high resolution electron microscopy (HREM). The Y phase, which has a face-centre cubic structure with a = 2.0 nm, occurs in grain boundaries during precipitation heat treatment of supersaturated solid solution. Considerable amount of intrinsic faults, extrinsic faults and microtwins were observed in the Y phase. A new domain denominated as D intergrowth with Y phase along {111} planes has been found. By combination of Y phase and D domain, new domains can be constructed.     

Anomalous X-ray scattering study of early-stage precipitation in AlZnAg

Initial stages of phase separation in an Al14%Zn4%Ag alloy was studied by anomalous small-angle X-ray scattering. Although the three partial structure functions could not be determined individually, it was observed that composition tie-lines rotated in the course of aging time. The rotation was interpreted in terms of a change of the Ag/Zn concentration ratio within the evolving precipitates. An early stage stability analysis was also performed based on an approximate sub-regular solution model for the coherent AlZnAg ternary miscibility gap.     

Numerical modelling of dendritic solidification in aluminium-rich AlCuMg alloys

By modification of a numerical iterative computer simulation worked out for binary alloys, the solidification behaviour of ternary alloys can be quantitatively predicted. The program finds the cooling curves, the amount of binary and ternary eutectic, the concentration distribution of the solute elements in the dendrite arms and the spacing of the dendrite arms during and after complete solidification. Comparison of the numerical predictions with experimental results obtained for five alloys from the aluminium rich corner of the AlCuMg system at four cooling rates shows very good agreement.     

Coherent precipitation effect on thermo-power of AlCu alloys

In situ thermoelectric measurements of AlCu alloys are described for several types of heat treatments. The thermoelectric power shows deviations from the solid solution matrix during the sequence of precipitations occurring in these alloys (i.e. GP zones, θ′',θ′ and θ). The sign and the amplitude of these deviations can be modeled on the basis of the HEW theory for the resistivity maximum. Extension to the diffusion thermopower displays a size and morphology (isotropic, platelike) effect in the electron Bragg scattering by the coherent precipitates, corresponding rather well with the experimental data.     

Discontinuous precipitation in CuCo alloys

This contribution deals with the cooperative discontinuous growth of coherent rod-like Co particles from supersaturated copper-rich CuCo solid solutions, and the subsequent spheroidization of these rods into columnar arrays. Electron microscopy has been used to determine the precipitate morphology, size, and distribution. A model is presented for the discontinuous reaction in which a local equilibrium is assumed between the product rods and a uniformly depleted matrix. It is concluded that two major kinds of forces act on the grain boundary during rod growth : A chemical force, predominant at lower temperatures, and a precipitate traction, thought to be available at higher temperatures, where the chemical force is dissipated by volume diffusion. The apparent mobility of the grain boundary, as inferred from this work is low; this is attributed to the presence of a general coherent precipitate, which forms ahead of the reaction front. The kinetics of the rod spheroidization process are also considered. and shown to be consistent with the present observations.     

Direct observation of coarsening in AlCu alloys

Observations have been made on coarsening of θ-CuAl₂ precipitates at the surface of AlCu alloys. One specific area was examined over extended periods so that dissolution and growth of individual precipitates could be followed. It was found that nearly all precipitates shrank on ageing, even those larger than the average particle size. Only the largest 8% increased in dimension. A simple theory was used which gave good agreement with the observed dissolution kinetics of all precipitates except the very largest. This theory assumes a solute flux from the precipitate to a larger precipitate located a significant distance away. The theory gave a mean value for interfacial energy of 0.81 J/m². Particles were observed to spheroidise during ageing, and by the time their dimension had decreased 50%, they were completely spherical. Particle coalescence took place in a few cases but was generally of minor importance in the overall coarsening process.     

Structure-property relationships in AlLiCuMgAgZr alloy X2095

Although aluminum-lithium alloys showed initial promise for aerospace applications, implementation has not proceeded swiftly. In this study, efforts were made to design and develop microstructures with good fracture and fatigue crack propagation resistance to achieve a better balance of mechanical properties in the high strength alloy X2095. Lower aging temperatures were employed, resulting in precipitation of shearable δ' (Al₃Li) particles and reduced subgrain boundary T₁ precipitation. Although fracture toughness was not significantly altered in the 1.6 Li variant, improvements approaching 50% were achieved in the 1.3 Li alloy. Intrinsic fatigue crack propagation resistance was also slightly improved due to reduced environmental interactions. These improvements were made without altering the 660 MPa yield strength.     

Microstructural aspects of strain localization in AlMg alloys

The literature contains a number of continuum plasticity models describing the onset of strain localization but little definitive work which describes the microstructural transitions which accompany localization. In the present study a range of metallographic methods have been used in order to observe the progression of localization from events within single grains to the spatial organization of these events across the entire sample. The deformation mode used was cold rolling and observations were made using a variety of orientations relative to the rolling direction. It is concluded that in the AlMg system, localization begins by a structural instability in the accumulated dislocation substructure and later becomes organized into macroscopic bands due to local stress concentrations. The structure of the macroscopic bands is complex. They contain some high angle boundaries suggesting that they form due to the rapid and cooperative action of a number of slip systems over distances of the order of 0.2 μm. The bands cross grain boundaries and are organized in a cooperative sense because they represent local softening events. Thus, the shear bands in AlMg appear to form without texture softening or the need for the precursor of a lamellar structure. They involve a dramatic local change in the process of dislocation accumulation which is essentially a form a local dynamic recovery. The events become spatially organized to form macroscopic bands inclined at approx. 35° to the rolling plane as required by continuum plasticity.