Real-time synchrotron X-ray radiography of nucleation, growth and phase selection of FE-rich intermetallic compounds in Al alloy solidification

In-situ synchrotron X-ray radiography has been used to study the dynamics of nucleation, growth and other temperature related phe- nomena of Fe-rich and analogous faceted IMCs. Measurements from radiographs have been combined with post-solidification microscopy. A method was developed to estimate the temperature at any time and any position in the radiography field of view and applied to obtain tip temperatures of 203 secondary Fe-rich IMCs growing in an Al- Cu-Fe alloy. The cooling rate and grain refiner additions affected the IMC tip temperature, phase selection and the final population, primarily by affecting the inter-dendritic liquid channel tortuosity. Adding grain refiners and using a higher cooling rate delayed the growth of secondary IMCs to lower temperatures. The formation dynamics of Fe-rich IMCs were investigated at different cooling rates, thermal gradients and inoculation conditions (no inoc- ulation, TiB2 and TiC) in a hypereutectic Al-Fe alloy that formed Al13Fe4 as a primary phase. Without inoculation, IMCs formed on the oxide of sample surface; with inoculants, IMC formation in the melt was increased. When a thermal gradient was imposed/increased, the IMC number density and average formation rate decreased, and it was inferred that their formation temperature also decreased. A conceptual model for IMC nucleation was proposed. Last, faceted growth of primary and secondary IMCs was studied in three model alloys: Al-Cu-Fe, Al-Fe and Al-Pt-Er that formed primary Pt-rich IMCs related to the Fe-rich IMCs while providing stronger X-ray absorption imaging contrast. Time-resolved radiogra- phy measurements suggested that growth of both secondary IMCs in constrained liquid and primary IMCs in non-constrained liquid, were governed by solute diffusion effects in the liquid. Repeated forma- tion of twin plane re-entrant (TPRE) corners facilitated preferential anisotropic growth and resulted in IMC platelets with a high aspect ratio of up to 10. In contrast, the formation of layered twins perpen- dicular to the preferential growth direction, promoted crystals with a much lower aspect ratio. The findings suggest that if "modifying" ele- ments could be added to poison the TPRE mechanism of IMC growth, layered twins might be promoted to obtain IMCs with a lower, more uniform aspect ratio. In this scenario, the usual embrittling effects of high-aspect ratio, platelet IMCs may be reduced.