Microsegregation in Al–4.5Cu wt.% alloy: experimental investigation and numerical modeling

Microsegregation in Al–4.5 wt.%Cu alloy was investigated experimentally using directional solidification and electron probe microanalysis (EPMA). The degrees of dendrite tip undercooling were measured for growth rates from 0.0038 to 0.2 mm s−1 with a temperature gradient of 50°C cm−1 at the liquid–solid interfaces. A modified Scheil model incorporating back diffusion, undercooling and dendrite arm coarsening was used to calculate the degrees of microsegregation. The partition coefficients obtained from the thermodynamic models of the solid and liquid phases and the measured cooling curves were used as the inputs to carry out the microsegregation calculation. While the calculated results using the Scheil model deviate significantly from the experimental data, those from the modified Scheil model are much better. Out of the three geometrical models, i.e. plate, sphere and cylinder, to approximate the shapes of the dendrites, the sphere is the best. However, the calculated results using the spherical model is near accord with the data for small fractions of solids and those using the cylinder is better at large fractions of solids. Two different thermodynamic descriptions of the Al–Cu system were used to demonstrate the importance of reliable phase diagram data in studying the degree of microsegregation.