Remelting and solidification of a 6082 Al alloy containing submicron yttria particles: 4D experimental study by in situ X-ray microtomography
Graphical abstract
Introduction
Aluminium- or magnesium-based Metal Matrix Nano-Composites (MMNCs) are believed to be innovative and promising materials to find applications in many domains such as transportation where lightweighting and pollution reduction are important issues. In this context, the challenge taken up by the European project ExoMet is to design and study these new materials in terms of microstructural features and mechanical properties as well as prototyping end-user applications [1]. The recent possibilities of using novel reinforcers, such as ceramic particles of very small size (< 1 μm), allow enhancing the mechanical properties of the material without inducing detrimental effects on the ductility by contrast with standard Metal Matrix Composites containing larger particles. This was made possible by the development of new processing techniques that prevent the natural agglomeration of nanosized particles in the melt. They usually involve the application of external fields (mechanical stirring, magnetic field or ultrasonic melt treatment) aiming to homogeneously disperse the particles into the matrix [2], [3]. Extensive theoretical [4], computational [5] as well as experimental [6], [7] research has been carried out to investigate the relationship between microstructural features and mechanical properties of MMNCs containing particles of different types (Al2O3, TiB2, Y2O3, AlN etc.…) and sizes (30 nm to 1 μm). However, very few in situ studies have been performed to understand the behaviour of the particles inside molten matrices as well as during solidification and casting [8]. The recyclability of MMNCs is also rarely addressed despite the fact that material recycling represents an important societal challenge. The modelling and design of nanocomposites are very complex and experimental data on the direct observation of solidification mechanisms is extremely valuable. There is a wide range of parameters governing the interactions between particles and liquid–solid interface. The velocity and shape of the solid front, the morphology of the particles, their density as well as the melt viscosity and the potential chemical reactions have to be considered [9], [10]. A general model is therefore difficult to derive and can be achieved only through restrictive assumptions. For example, some theoretical studies considered the interaction particle-planar front [11] whereas others focused on the growth dynamics of dendrites into a field of particles [12], [13]. Moreover very few observations have been performed in conventional metallic alloys and most of in situ studies were performed using transparent organic materials [14]. Nearly all of them involve 2D systems where the Hele-Shaw confinement effect can potentially impact the quality and relevance of the results. X-ray microtomography is an efficient non-destructive technique allowing direct 3D observation inside the matter. Indeed, with sufficient resolution and adequate absorption contrast, the presence and precise 3D location of particles can be recorded as a function of time [15]. X-ray in situ microtomography has been widely used now for solidification studies in aluminium alloys [16], [17], [18] and some concerning solidification of aluminium composites [19], [20]. However very few studies concern in situ solidification of nano-composites. In this paper, we present an in situ tomography experiment performed during melting and solidification of an MMNC. The material investigated is a commercial 6082 Al alloy1 containing Y2O3 particles. The behaviour of the Y2O3 particles is characterized during melting, in the fully molten state and in interaction with the solid–liquid interface upon solidification.
Section snippets
Materials and method
The material was provided by BCAST institute from Brunel University. The incorporation of 1 wt.% (0.54 vol.%) Y2O3 particles (~ 500 nm) in the molten Al-6082 matrix was performed under mechanical stirring (at about 350 rpm). Ultrasonic melt treatment was applied for 5 min at a frequency of 17.5 kHz. The detailed procedure can be found in [3]. The samples were machined from the as-received ingots (12 × 12 mm2 in width and 100 mm in length) into cylindrical-shaped specimens with two different section sizes.
Results and discussion
Fig. 1a (top) is an extracted tomography slice of a scan recorded at room temperature (RT) showing the ‘as-received’ state. As regions locally rich in absorbent elements are displayed in white, the porosity appears in black, the primary phase of the matrix containing in majority Al atoms in grey and the interdendritic region where most of the heavy elements are segregated in white. At this stage it is difficult to unambiguously determine the location of the high-absorbent Y2O3 particles.
Conclusion
The remelting and solidification behaviour of a 6082 + 1 wt.%Y2O3 composite was investigated by in situ synchrotron microtomography. The main results of this study are:
- 1.
The Y2O3 particles can be revealed by remelting the sample and dissolving the eutectic phase. The majority was located at the grain boundaries suggesting that most of them were pushed by the solidification front during casting. Few yttria particles were nevertheless found in the matrix, i.e. engulfed in the primary solid phase.
- 2.
Above
Acknowledgements
The authors wish to acknowledge financial support from the ExoMet Project, which is co-funded by the European Commission in the 7th Framework Programme (contract FP7-NMP3-LA-2012-280421), by the European Space Agency and by the individual partner organizations.
They also wish to acknowledge the ESRF-MA1876 long term project for providing strong support as well as efficient and dedicated tools to perform such in situ experiment.
Wim Sillekens (European Space Agency) is also gratefully acknowledged
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