Papers by Author: Jean Jacques Blandin

Abstract: Superplasticity of metallic alloys has been widely investigated for many years but despite numerous studies, some important questions regarding superplastic behavior of metals can be considered as still open. Among others, one can mention the role of grain boundary sliding as contributing mechanism of deformation, the microstructural requirements for getting superplasticity in metallic alloys or the link between mechanisms of deformation and mechanisms of damage. Interesting information were recently obtained by the use of new experimental techniques. One can mention for instance the development of fine grids allowing more detailed studies of the respective contributions of intra-and inter-granular strains in superplastic alloys or fast X-ray micro tomography allowing in situ 3D damage studies. The aim of this paper is to give information about current findings related to superplastic deformation of metallic alloys and to suggest some perspectives for future.

Abstract: Due to limited deformability at room temperature, high temperature forming of magnesium alloys appears as an interesting alternative. Superplastic properties can be obtained in the case of fine grained magnesium alloys and in this regime, due to significant damage sensitivity, fracture strain is mainly controlled by nucleation, growth and coalescence of cavities. Magnesium alloys with large grained alloys can also exhibit interesting deformabilities at high temperature since dislocation movements can be controlled by a solute drag effect promoting plastic stability. Examples of such situations are presented in the case of wrought magnesium alloys, the associated damage mechanisms being investigated thanks to 3D X-ray micro tomography performed in continuous mode, namely directly during high temperature deformation tests.

Abstract: As for aluminium alloys, magnesium alloys are generally sensitive to strain induced cavitation when they are deformed in superplastic conditions. It has been widely shown that X-ray micro tomography is a particularly efficient tool for studying in 3D damage mechanisms during superplastic deformation. However, such characterisations are generally performed in post mortem conditions, namely on samples first deformed up to given strains and then characterised. In the present investigation, thanks to particularly short acquisition times offered by ESRF, damage induced by superplastic deformation of a magnesium alloy is studied thanks to tomography analyses performed in 4D conditions, namely directly during high temperature deformation tests. Such conditions provide unique opportunities for investigating nucleation, growth and coalescence of cavities since it is thus possible to follow each cavity up to the fracture process.

Abstract: A way to overcome the low deformability of magnesium alloys at room temperature is toincrease the temperature of forming operations. The stress exponent n, which is known to be a keyparameter in the control of plastic stability, generally decreases when temperature increases.Nevertheless, low n-values are not enough to ensure large capacity of deformation since fracturecan also result from strain induced cavitation. In the present investigation, both the mechanisms ofhigh temperature deformation and damage were studied in selected Mg alloys. Since damage datacan also give information on the deformation mechanisms, the strain induce cavitation behaviourwas mainly studied thanks to X-ray micro tomography which provides 3D information like thecavity shapes or the variation with strain of the number of cavities. Moreover, additionally toconventional post mortem analyses, it was attempted to perform the 3D damage characterisation inin situ conditions, namely directly during high temperature deformation tests.

Abstract: Due to their brittleness, bulk metallic glasses (BMG) are generally difficult to form at room temperature. Casting of BMG is one way to get components but an alternative route is to use the capacity to reach particularly large strains when the glasses are deformed in their supercooled liquid region (SLR). The experimental window (temperature, time) in which high temperature forming can be carried out is directly related to the glass resistance to crystallization. Such forming windows have been identified for various bulk metallic glasses (mainly zirconium and magnesium based BMG) thanks to compression tests in the supercooled liquid region. The effects of partial crystallization on the high temperature rheologies are also discussed. Finally, forming experiments were carried out in the selected windows.

Abstract: The deformability of wrought magnesium alloys at room temperature is limited and a way to overcome this limit is to carry out forming operations in warm or hot conditions. In the case of fine grained alloys, superplastic properties can be generally achieved but in this regime, the Mg alloys are sensitive to strain induced cavitation. However, large grained alloys can also exhibit quite large deformabilities when they are deformed at high temperature. This can be due to the fact that on one hand, the Mg alloys may quite easily dynamically recrystallize and on the other hand, that dislocation movements may be controlled by a solute drag effect leading to significant strain rate sensitivity parameters. These various mechanisms of deformation will depend on the composition, the mean grain size and the conditions of deformation (i.e. temperature and strain rate). In this work, the high temperature deformation mechanisms as well as the associated damage mechanisms of two wrought magnesium alloys are discussed.

Abstract: Superplastic forming (SPF) of magnesium alloys has received increasing attention in the recent past. The aim of this presentation is to review recent works dealing with SPF of Mg alloys with a three-fold objective: i. How to produce fine or ultra fine grained (UFG) microstructures ii. Are there specifities in superplastic deformation mechanisms iii. How SPF Mg alloys resist to cavitation Deformation mechanisms as well as damage variations in the superplastic regime will be preferentially discussed in relation with grain size, content in intermetallic particles and diffusion kinetics. For the sake of illustration, some results concerning the superplastic behaviour of UFG magnesium alloys produced by severe plastic deformation will be presented since such microstructures exhibit particularly attractive superplastic properties at quite low temperatures.

Abstract: The effect of partial crystallization on the mechanical properties of a Zr based bulk metallic glass (Vitreloy 1) is investigated. Viscoelastic properties are studied by mechanical spectroscopy in large frequency and temperature ranges, both below or above the glass transition temperature (Tg), whereas viscoplastic properties are investigated by compression tests . To study the interaction between crystallization and mechanical properties at high temperature, nanocomposites are produced thanks to appropriate heat treatments. Formation of nanocrystalline particles induces an increase of the storage elastic modulus, especially in the glass transition range, where this modulus is very low in the amorphous material. It also results in a decrease of the loss elastic modulus, corresponding to a decrease of the atomic mobility. Finally, partial crystallization induces very large hardening revealed by the compression tests but the hardening extent depends strongly on the applied strain rate.

Abstract: The microstructural evolution and mechanical response in compression in the semi-solid state of previously extruded AZ91 alloys containing two levels of Ca additions (1mass%Ca (AZC911) and 2mass%Ca (AZC912)) have been studied. Ca additions have a significant effect on microstructural evolution and compression behavior of the AZ91 alloy. At 515°C, the liquid fraction in AZC911 is larger than that in AZC912, so that the compression stress of the AZC912 alloy was found to be much larger than that of the AZC911 alloy. This behavior is explained through DSC analysis which suggests that some solid Al2Ca phase remains in AZC912 alloy at this temperature. Increasing the remelting temperature for this alloy leads to more liquid and coarsening of the solid particles occurs with increasing holding time.

Abstract: Microstructure and mechanical properties of AM50+xTi (x=0,0.01,0.1wt%) magnesium alloys extruded from as-cast and solution treated conditions have been studied. Results show that Ti element obviously refines the microstructure of AM50 magnesium alloy and Mg17Al12 phase. Only 0.01 wt% Ti addition can make the Mg17Al12 phase turn into particles and small rod-like shape. Ti addition improves tensile strength at room temperature, and obviously improves elongation at elevated temperatures up to 200°C. The AM50+xTi alloys extruded from as-cast have better tensile strength at room temperature and better elongation at 100°C, 150°C and 200°C than that of AM50+xTi alloys extruded from solution treatment; The plasticity of AM50 magnesium alloys increases with Ti content increasing and temperature increasing for the tensile fractograph.