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

Microsegregation in Al-4.5 wt.%Cu alloy was investigated experimentally usi ng 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 degreesC cm(-1) at t he liquid-solid interfaces. A modified Scheil model incorporating back diff usion, undercooling and dendrite arm coarsening was used to calculate the d egrees of microsegregation. The partition coefficients obtained From the th ermodynamic models of the solid and liquid phases and the measured cooling curves were used as the inputs to carry out the microsegregation calculatio n. While the calculated results using the Scheil model deviate significantl y from the experimental data. those from the modified Scheil model are much better. Out of the three geometrical models, i.e. plate, sphere and cylind er, to approximate the shapes of the dendrites, the sphere is the best. How ever, the calculated results using the spherical model is near accord with the data for small fractions of solids and those using the cylinder is bett er at large fractions of solids. Two different thermodynamic descriptions o f the Al-Cu system were used to demonstrate the importance of reliable phas e diagram data in studying the degree of microsegregation. (C) 2001 Elsevie r Science B.V. All rights reserved.