Microsegregation of Si, Cu, Mn, P, and Sn in Graphitic Cast Irons

Microsegregation in cast materials is important to their solidification, solid-state transformation, microstructure and material properties. This work studies quantitatively the microsegregation of Si, Mn, Cu, Sn, and P in graphitic cast irons using an electron microprobe with wavelength dispersive spectrometry. The alloys contain [mass%] C: 3.86, Si: 2.59, Mn: 0.64, P: 0.03, S: 0.01, Sn: 0.098, Cu: 0.84, Mg: 0.065, include graphite morphologies ranging from ductile iron to compacted graphite iron and solidified with a solidification time of 10 min. Concentration maps are presented, showing that microsegregation patterns provide detailed information about the solidification chronology of the metal matrix. Sequencing the measurements into concentration profiles showed that, despite large differences in microstructure and cooling curve characteristics, the severity of microsegregation was similar in the studied materials. Scheil simulation of concentration profiles provided decent prediction of concentration profiles, given appropriate thermodynamic data. Numerical simulation of isothermal diffusion suggested that, for about 10 min of solidification time, diffusion in austenite mainly affected the last 10% of the matrix to freeze. Effective partition coefficients extracted from the concentration profiles varied slightly through solidification. The estimated mean effective partition coefficients for the first 90% of the alloy to freeze are ${\overline{k}}_{Si}^{\gamma /L}=1.124\pm 0.006$, ${\overline{k}}_{Mn}^{\gamma /L}=0.696\pm 0.008$, ${\overline{k}}_P^{\gamma /L}=0.15\pm 0.03$, ${\overline{k}}_{Sn}^{\gamma /L}=0.50\pm 0.02$, ${\overline{k}}_{Cu}^{\gamma /L}=1.35\pm 0.01$, where $\pm$ indicates standard deviation.