Combined growth of α-Al and Bi in a Al-Bi-Cu monotectic alloy analyzed by in situ X-ray radiography

Al-based monotectic alloys show an interesting combination of wear resistance and mechanical strength. While self-lubricating bismuth (Bi) guarantees an adequate wear resistance, the modification with copper (Cu) can increase the ability to support load. Tailoring the micromorphology of the α-Al phase as well as the distribution of the inclusions of Bi embedded in the α-Al matrix is of prime importance in order to improve the properties. In this paper, various solidification experiments are performed with the Al-3.2wt.%Bi-3wt.%Cu monotectic alloy. Real-time radiographs could be acquired during solidification by using the laboratory device SFINX (Solidification Furnace with IN situ X-radiography). The processing conditions could be chosen in such a way that both cellular and dendritic morphologies of the α-Al phase could be clearly seen in a single test. Micromorphological transitions from dendrites to cells are recognized for samples solidified from different cooling rates, i.e. 0.025 K/s and 0.01 K/s. Measurements of the α-Al dendrite velocity and growth parameters of the Bi droplets during solidification have also been extensively performed. A comprehensive model for the cellular-to-dendritic (CDT) transition of Al-based monotectic alloys is outlined. The model considers the interaction between α-Al and Bi droplets during solidification and its impact on the microstructure.