Partitioning of titanium during solidification of aluminium alloys

Abstract

The purpose of the present work is to compare the segregation behaviour of Ti solute in low solute alloys, such as nominally pure Al, and high solute alloys typical of Al–Si casting alloys. Microprobe measurements of Ti segregation within grains show that, under the solidification conditions employed, the measured partition coefficient of Ti in pure Al is 6.7 compared with the phase diagram prediction of ～7.5. Similar measurements in an Al–7Si–0.3Mg alloy resulted in a partition coefficient of 3.2. Based on the measured partition coefficients, this translates into a reduction in the growth restriction factor (a measure of the segregating power of solute elements) from 8.75 K in pure Al to 3.36 K in Al–7Si–0.3Mg with an addition of 0.05 wt-%Ti. This may explain why Ti in solution is a less effective grain refiner in casting alloys than in low solute content wrought alloys. In addition, the measured Ti segregation profiles were compared with predicted profiles based on the Scheil or similar solidification relationships. It was found that the predictions of the Scheil equation produced a good fit with the measured Ti segregation profiles once it was adapted for the geometrical nature of dendritic solidification.