Studying the nucleation behavior of immiscible aluminum alloys with X-ray tomography

Kurzfassung

There is a class of aluminum alloys, including Al-Pb, Al-Bi and Al-In, which exhibit a miscibility gap in the liquid state such that during cooling of a melt through this gap droplets of one phase are nucleated, which grow within milliseconds to micrometer size. Due to many experimental difficulties, the mechanism of nucleation in such alloys is unknown. Therefore we study the nucleation and growth of droplets in Al-Pb alloys using a combination of 3D-X-ray microtomography (XCT) and 2D Scanning Electron Microscopy (SEM). The nucleation is artificially induced by inoculants like TiB2, TiC or ZrO2 added to the melt before casting and cooling. The microstructure of the as-cast state is then analyzed. The figure below shows an example of a tomography without and with a nucleation promoter. The image shows large lead droplets in the case without ZrO2 particles and a much refined structure if small amounts are added. Comparing 3D XCT of alloys with and without inoculants and with different contents clearly reveals for the first time, how effective TiB2, TiC or ZrO2 act in nucleating Pb droplets from an Al-melt. This was essentially unknown and only guessed before 3D XCT could be made. SEM images made in parallel on all alloys and samples show in addition small crystallites, only slightly heavier than aluminum. They could mostly be identified as the inoculants or Al3Ti grown on them and then acting as a promoter of nucleation. Even though their size is larger than the resolution limit of our tomographic facility, the absorption contrast to the surrounding is not enough for finding these particles in the 3D image. To even resolve these, further developments in XCT technology are necessary.