Diffusion measurements of Si in liquid Al-Cu-Si alloy using X-ray radiography and shear cell techniques

• Impurity diffusion coefficient of Si in liquid Al alloys at 973 K was measured. • Measurement method is combined with X-ray radiography and shear cell technique. • Suppressing natural convection by adding Cu to the Al-Si system. • Shear convection can be corrected using the XRR and ICP results of Cu. • Validity of adding Cu is confirmed by negligible thermodynamic coupling. Impurity diffusion coefficients of Si in liquid Al alloys are essential for understanding solidification but are difficult to measure due to convection and the ineffectiveness of X-ray radiography (XRR) for elements with similar atomic numbers. To address this problem, this study aims to propose a methodology for determining diffusion coefficients by using a tracer to quantify and correct for additional mass transport. Diffusion experiments were conducted on Al-Cu-Si alloys at 973 K, combining XRR and the shear cell techniques with stable density layering by adding Cu to the Al-Si system. Using Cu as a tracer allowed for the quantitative evaluation of shear convection via in-situ XRR. Based on the impurity diffusion coefficients of Cu obtained from XRR results, it was confirmed that the diffusion experiments were conducted under conditions where natural convection was effectively suppressed during the entire diffusion time. Furthermore, the additional mass transport induced by shear convection was quantitatively corrected by isolating the initial mass transport from cell insertion and the final mass transport from cell separation. The initial mass transport was derived from the intercept of in-situ XRR data. The final mass transport was determined by the difference between the final XRR and inductively coupled plasma optical emission spectroscopy (ICP-OES) results for Cu. The obtained impurity diffusion coefficient of Si was determined to be (7.81 ± 0.90) × 10 –9 m 2 s -1 at 973 K. Additionally, the validity of adding Cu was confirmed by thermodynamic calculations, which showed that the thermodynamic coupling between Cu and Si fluxes in the ternary system is negligible.