The solidification process of alloys involves the continuous transition from the liquid to the solid state and the final properties of the solidified alloys strongly depend on the resulting microstructure. This microstructure is usually characterised on 2D sections by optical or scanning electron microscopy. Solidification models, developed on the basis of the physical mechanisms that are possibly occurring when solid and liquid are coexisting, are validated by these post-solidification observations. Direct characterisation of the dynamical phenomena occurring during solidification would greatly improve these models and our knowledge of the solidification process. This requires in situ 3D observations which are now possible by fast X-ray tomography experiments carried out at high resolution on both the ID15 and ID19 beamlines. Limitations, however, exist in terms of size of the specimens and in the kinetics of the evolution of the microstructure. Another limitation is that the liquid and the solid phases must exhibit sufficiently different absorption coefficients to be clearly distinguished when they are coexisting. Al-Cu based alloys are therefore good candidates for these experiments.

The solidification experiment was carried out with an Al-7%Si-10%Cu alloy using a cylindrical specimen of 1.5 mm in diameter and 3 mm in height which was glued with zirconia paste on the top of an alumina rod placed on the rotating stage. It was heated until it became completely liquid and then slowly solidified while being supported by its own oxide skin. The solidification experiment was carried out at the ID19 beamline and a gas blower was used to heat up and cool down the specimen at a controlled cooling rate. The scan time to take 400 projections over a 180° rotation of the specimen was 20.7 s and the cooling rate was 3°C per min. Radiographs were recorded using the FReLoN 2k14 CCD-camera developed at the ESRF.

This experiment allows the in situ observation of the continuous growth of the dendrites as shown in Figure 127. Several mechanisms occur concurrently, i.e. dissolution of small secondary arms, filling of the gap between two arms. These mechanisms have been proposed in the literature [1] but this is the first time that they have been observed directly during solidification. These morphological changes can be quantified in terms of variation of the solid-liquid interface area or local curvatures of the solid phase and compared with predictions of the models.

Fig. 127: Evolution of an individual dendrite in an Al-Si-Cu alloy with increasing solid fraction.

 

In situ tomography can also be carried out while maintaining an alloy isothermally in the semi-solid region to study the evolution of the microstructure when the solid fraction remains constant. Such experiments were carried out on the ID15 beamline for an Al-15.8%Cu alloy processed to produce globular solid particles rather than dendritic structures. For this experiment, a similar type of specimen was used with the same holding procedure but in this case a resistance furnace developed at ESRF was used. The maintain temperature was 555°C which leads to a solid fraction of 0.68 after a period of 1500 s required to reach thermodynamic equilibrium. In a similar way as for solidification, 400 projections were taken over a 180° rotation of the specimen using a DALSA CCD-camera. The scan time for this total rotation is smaller than 15 s. Again during these experiments local changes at the scale of the individual solid particles were observed in situ. Figure 128 shows these changes where coalescence of particles of similar size (1 and 2) occurs initially followed by dissolution of the smaller particle (1) for the benefit of the bigger ones (2 and 3).

Fig. 128: Morphological changes occurring while maintaining a constant temperature for a group of solid grains in an Al-Cu alloy: (a) 1560 s; (b) 3360 s; (c) 4260 s; and (d) 4560 s.

 

These experiments clearly demonstrate that fast X-ray tomography is a very powerful technique to characterise in situ and in real-time the microstructural evolution of a solid-liquid mixture during solidification and partial remelting. The basic mechanisms that are occurring at the scale of the solid grains or dendrites can be observed and the morphological changes can be quantified. There are still some limitations in the observations but it is anticipated that these limitations will be overcome in the next few years thanks to the continuous improvement in the available equipments and techniques.

 

References

[1] M. Chen and T.Z. Kattamis, Mater. Sci. Eng. A247, 239 (1998).

Principal publication and authors

N. Limodin (a), E. Boller (b), L. Salvo (a), M. Suéry (a), M. DiMichiel (b), Proc. 5th Decennial Inter. Conf. on Solidification Processing, Sheffield, 23-25 June 2007, Ed. Howard Jones, The University of Sheffield, 316-320; N. Limodin (a), L. Salvo (a), M. Suéry (a), M. DiMichiel (b), Acta Materialia 55, 3177-3191 (2007).
(a) SIMaP, INP Grenoble, Saint-Martin d’Hères (France)
(b) ESRF