Microstructural development in Al-Ni alloys directionally solidified under unsteady-state conditions

Three Al-Ni hypoeutectic alloys were directionally solidified under upward unsteady-state heat-flow conditions. Primary (lambda (1)) and secondary (lambda (2)) dendrite arm spacings were measured along the castings for all alloys and correlated with transient solidification thermal variables. A combined theoretical and experimental approach was used to quantitatively determine such thermal variables, i.e., transient metal/mold heat-transfer coefficients, tip growth rates, thermal gradients, tip cooling rates, and local solidification time. The article also focuses on the dependence of dendrite arm spacings on the alloy solute content. Furthermore, the experimental data concerning the solidification of Al-1.0, 2.5, and 4.7 wt pct Ni alloys are compared with the main predictive dendritic models from the literature.