Partitioning of titanium during solidification of aluminium alloys

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 AI-Si casting alloys. Microprobe measurements of Ti segre gation within grains show that, under the solidification conditions employe d, the measured partition coefficient of Ti in pure Al is 6.7 compared with the phase diagram prediction of similar to 7.5. Similar measurements in an Al - 7Si - 0.3Mg alloy resulted in a partition coefficient of 3.2. Based o n 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 add ition of 0.05 wt-%Ti. This may explain why Ti in solution is a less effecti ve grain refiner in casting alloys than in low solute content wrought alloy s. In addition, the measured Ti segregation profiles were compared with pre dicted 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 g eometrical nature of dendritic solidification. MST/4517.