J.J. Blandin

X-ray micro-tomography an attractive characterisation technique in materials science

X-ray tomography is a non-destructive technique which provides 3D information of materials. It is consequently very attractive in Materials Science since the relation between macroscopic properties and the micro-structure of a material is very frequently required. The aim of this paper is to present selected results obtained in various investigations of metallic materials such as superplastic deformation, materials in the semi-solid state and metallic foams. Depending on the studied features, several tomography analysis modes were used: conventional absorption mode, phase contrast and holotomography, a new technique, which provides the 3D distribution of the electron density in the bulk of the material. Furthermore micro-tomography enables one to perform in situ experiments either by using a mechanical test machine or a furnace.

Grain refinement in AZ91 magnesium alloy during thermomechanical processing

Abstract Microstructural changes during high-temperature extrusion and torsion of an AZ91 alloy (Mg–9Al–1Zn, wt.%) were investigated. In the experimental domain studied, dynamic recrystallisation (DRX) occurs and the effect of temperature and strain rate on the resulting recrystallised grain size was investigated. Complete recrystallisation in torsion is associated with the development of a stress plateau after softening from the peak stress, which is systematically observed in the first steps of straining. The resulting grain size can be related to the value of the peak stress. It appears that the precipitation of the Mg 17 Al 12 phase does not affect significantly the torsion behaviour of the alloy in the experimental domain investigated here. This study supports the idea that very fine-scale microstructures (i.e. with a mean grain size smaller than 5 μm) can be easily produced by DRX during high-temperature extrusion of the AZ91 alloy.

Effect of alloying elements on the ignition resistance of magnesium alloys

Abstract The ignition resistance of pure Mg, AZ91 and WE43 alloys is investigated. For pure Mg, ignition occurs in the solid state whereas it requires the presence of liquid in the AZ91 alloy. The WE43 alloy is ignition-proof. The ignition resistance is attributed to the role of yttrium in the oxidation process.

Effect of the nature of grain boundary regions on cavitation of a superplastically deformed aluminium alloy

Abstract Superplastic deformation of aluminium alloys induces cavity formation throughout the material, so that superplastic forming usually requires to be carried out under superimposed gas pressure to minimize strain-induced damage. This paper deals with the beneficial effects of heat treatment at high temperature for several hours before deformation on cavitation behaviour of a superplastically deformed 7475 alloy. Transmission electron microscopy observations show that several microstructural transformations are induced by superplastic deformation and affected by the heat treatment. At first, the generation of dispersoid free zones at the periophery of the grains is observed, the composition of which depends on the prior history of the specimen. Secondly, the formation of long thin fibres extending in the cavities in the as received specimens, these fibres being no longer present in the heat-treated conditions. A TEM characterization of the fibres is presented and a mechanism of their formation is discussed. Such a reduction of the cavitation level for a given strain is interesting in view of superplastic forming of aluminium alloys under atmospheric pressure.

Variations in microstructure and texture during high temperature deformation of Al–Mg alloy

Abstract The effect of strain rate on the superplastic deformation of a fine-grained aluminium–magnesium alloy has been investigated. Microstructural transformations were quantified by crystallographic texture, the study of the size of the grains, their morphology and their spatial distribution. At low strain rate a progressive reduction of the initial texture is observed, resulting from the predominance of grain boundary sliding. But even for large strains the texture randomisation is not complete, which confirms the role played by dislocation creep in superplasticity of the alloy. At high strain rate textural components of plasticity develop and the grains become elongated along the tensile direction. Nevertheless, transmission electron microscopy observations and a calculation of the expected variation with strain of the shape of the grains, suggest that continuous recrystallisation is likely to take place during deformation.

Effects of zirconia additions on the superplasticity of aluminazirconia composites

Abstract The superplastic properties in compression of alumina zirconia composites (0–20 vol.% of zirconia) were investigated at constant strain rate, in air and at temperatures ranging from 1300 to 1400°C. The as-sintered materials have identical grain sizes and significant amounts of residual porosity. In the strain range where no appreciable densification occurs, the influence of zirconia additions was determined. The compressive stresses of the zirconia-containing materials (composites) are higher than in the case of alumina for identical conditions of deformation. This difference is interpreted in terms of intergranular segregation of Zr 4+ ions in the composites. For the composites, the compressive stresses are roughly independent of the zirconia content. This result may be attributed to comparable sliding capacities of alumina/zirconia interfaces and alumina grain boundaries.

Towards stiffness prediction of cellular structures made by electron beam melting (EBM)

Abstract Additive manufacturing is a novel way of processing metallic cellular structures from a powder bed. However, differences in geometry have been observed between the CAD and the produced structures. Struts geometry has been analysed using X-ray microtomography. From the 3D images, a criterion of ‘mechanically efficient volume’ is defined for stiffness prediction. The variation of this criterion with process parameters, strut size and orientation has been studied. The effective stiffness of struts is computed by finite element analysis on the images obtained by X-ray tomography. Comparison between the predicted stiffness and the effective one tends to show that the efficient volume ratio leads to a slight underestimation of the stiffness. Finally, the effective stiffness is used at the scale of a unit cell. This can help define the build orientation and loading direction that lead to the highest stiffness.

Effect of thermomechanical processing on the microstructure and mechanical properties of AlMg (5083)/SiCp and AlMg (5083)/Al2O3p composites. Part 1: Dynamic recrystallization of the composites

Abstract Three types of composites, reinforced by as-received SiC, artificially oxidized SiC or Al 2 O 3 particles, were fabricated by a compocasting method and extruded at 480 °C with an extrusion ratio of 16:1. Extrusion of these composites causes large scale dynamic recrystallization resulting in a fine matrix microstructure. The extrusion increases not only the ultimate tensile strength, but also the elongation to fracture of the composites. In the first part of our series of three articles, the microstructure of as-cast composites, extruded composites and extruded monolithic 5083 aluminum alloy is studied. The effects of reinforcing particles and other parameters on the dynamic recrystallization behaviour of the composites are discussed.

Correlation between ignition and oxidation behaviours of AZ91 magnesium alloy

Abstract The ignition resistance of AZ91 alloy is investigated during continuous heating experiments and when held at constant temperature in the semisolid range. It is found that during continuous heating, ignition takes place close to the liquidus temperature when a significant amount of liquid is present. Similarly, ignition occurs during an isothermal hold in the semisolid region, but after an incubation time that depends on the temperature. Resistance to burning is closely related to high temperature oxidation, which is particularly complex in the case of this alloy. The non-protective nature of the oxides together with the liquid oxidation and the possible role of Mg vapour seem to be the reasons for the AZ91 ignition.

Interactions between high-temperature deformation and crystallization in zirconium-based bulk metallic glasses

High-temperature deformation of a ZrTiCuNiBe bulk metallic glass (BMG) is investigated by compression tests in the supercooled liquid region. When the temperature is decreased or strain rate increased, the amorphous alloy exhibits the usual Newtonian/non-Newtonian transition behaviour. Using specific heat treatments, partially crystallized alloys are produced, the associated microstructures characterized and the volume fractions of the crystal measured. The interaction between high-temperature deformation and crystallization is investigated by appropriate mechanical testing. According to these measurements, partial crystallization is responsible for a significant increase in flow stress and the promotion of non-Newtonian behaviour. Deformation does not significantly change the volume fraction, composition or size of the crystal. The flow-stress increase with crystallization is analyzed under different hypotheses. We conclude that the flow-stress increase cannot be interpreted through a compositional change in the residual amorphous matrix, either by reinforcement due to hard crystallites or by connections between crystals. It appears that the effect is due to the nanometric size of the crystals alone.