| Several dendritic monocrystals of Al-3.75%Cu-1.5%Mn and Al-1.0%Cu-1.5%Mn were directionally solidified at various growth rates under a constant thermal gradient. Solidification was interrupted at a given time by quenching the remaining liquid. For isothermal coarsening kinetic studies dendritic monocrystal pulling was arrested for various lengths of time prior to quenching. Transverse sections were used for evaluating coarsening kinetics, primary dendrite arm spacing and solute distribution within the dendrites and the surrounding liquid, and longitudinal sections were used for measuring dendrite tip radii and secondary dendrite arm spacings, as a function of temperature, holding time, or time during solidification.;Combination of isothermal coarsening Models III and IV which are valid for the earlier and later stages of coarsening, respectively, yielded the dependence of S;A modified Kirkwood model ("Regular Model") was introduced to predict the secondary dendrite arm spacing during isothermal coarsening, as well as during solidification.;The dependence of dendrite tip radius on growth rate at constant temperature gradient was modelled in Al-3.75%Cu-1.5%Mn alloy. Dendrite tip radius decreased with increasing growth rate and copper concentration.;For a given alloy composition isothermal coarsening rate, expressed as the time variation of the normalized specific dendrite surface area, S;Analytically derived copper distribution profiles across transverse dendritic sections at various temperatures in the semi-solid region show an increase in copper concentration with increasing fraction solid and decreasing temperature. Below the ternary eutectic temperature they show a decrease in copper concentration at a given fraction solid with increasing growth rate. Measured solute concentrations agree reasonably well with analytical predictions, especially if the model assumes the operation of coarsening with a linear increase of copper concentration in the liquid with time. |
