The penetration of liquid metals into the grain boundaries of polycrystalline solid metals may in some cases lead to the formation of micrometre-thick liquid phase layers and thereby to severe degradation of the mechanical properties of the solid metal. The underlying mechanisms of this grain boundary penetration and wetting process are still poorly understood and depend on the specific metal couple under investigation.

Synchrotron radiation X-ray microradiography and microtomography have been used on ID19 to investigate the penetration of liquid gallium into the grain boundaries of aluminium - a system known for very high penetration rates. Due to the large difference in absorption, sub-micrometre thick Ga layers in the bulk of millimetre-sized Al samples can be detected. Figure 107 shows a reconstructed cut through an Al sample which has been exposed to liquid Ga. A comparison of the tomographic reconstruction before and after the penetration revealed that the individual grains have moved with respect to each other by several micrometres - in other words, the liquid phase has replaced the grain boundaries and separates the grains [1].

Comparison of the tomographic data with electron backscattering diffraction mappings recorded on the surface of the same sample indicates that the wetting and the thickness of the liquid phase layer depend on the relative grain misorientation. A more detailed analysis is foreseen, taking advantage of the complete 3-D grain orientation information provided by the tracking technique recently developed at ID11.

In situ radiographic observation turned out to be a promising tool for the characterisation of the kinetics of the propagating liquid metal films. The optimised setup at ID19 takes advantage of the 4 channel parallel read-out capability of the FRELON camera and allows the recording of sequences of up to 400 high resolution images (spatial resolution 1 µm) with frame-rates of up to 15 images per second. Measurements on well-defined grain boundaries (bicrystals) should allow the investigation of the influence of various parameters like temperature and liquid metal composition.

In addition the microtomographic observation of Ga decorated grain boundaries might be used as a unique technique for three-dimensional visualisation and analysis of polycrystalline aluminium samples: shape and size of the individual grains are readily obtained from the segmented volume data. Figure 108 shows a 3-D rendition of a small part of the same polycrystalline sample presented in Figure 107. An application of this technique is shown in Figure 109: the propagation of cracks with respect to grain boundaries can be studied within the bulk of the sample [2].

Principal Publications and Authors
[1] W. Ludwig (a), D. Bellet (b), J. Mater. Sci. Eng., in press.
[2] W. Ludwig (a), J-Y. Buffière (c), S. Savelli (c), S. Bouchet (a,d), D. Bellet (b), to be published.

(a) ESRF
(b) Laboratoire GPM2, ENSPG, INPG, Grenoble (France)
(c) Laboratoire GEMPPM, INSA, Lyon (France)
(d) CMM, Ecole Nationale Supérieure des Mines de Paris (France)