Microstructural and compositional transients during accelerated directional solidification of Al-4.5 wt pct Cu

The transient behavior of mushy-zone velocities, primary dendrite arm spacings, and microsegregation effects have been investigated for an Al-4.5 wt pet Cu alloy by instantaneous velocity changes in a standard Bridgman furnace. After suddenly imposed velocity changes, the mushy-zone velocities, dendrite arm spacings, and compositions exponentially adjust to new steady-state values. Good agreement was found between the transient mushy-zone positions and velocities and predictions from the theoretical model of Saitou and Hirata. The primary dendrite arm spacings appear to adjust to changed velocity conditions about as rapidly as the mushy-zone velocity adjusts. Steady-state arm spacings agree very well with corresponding steady-state data from the literature. However, the observed composition profiles in the dendrite core and the interdendritic liquid appear to adjust more slowly than the corresponding adjustment of the mushy-zone velocity and arm spacings. Our observation of the sluggish response of the compositional profiles is consistent with an estimated Lewis number (Le = k/c(p)rho D) of 9.4 x 10(3) for the aluminum-copper system. The diffusivity of heat, thus, greatly exceeds the diffusivity of solute in this system. These results indicate that testing for the steady state during directional solidification experiments by looking for constant primary dendrite arm spacings can lead to errors, since the microsegregation profiles adjust more slowly than the spacings. It is suggested that constancy of composition also be tested for critical experiments investigating steady-state microsegregation effects.