Effect of melt convection on primary dendrite arm spacing in directionally solidified Pb-26%Bi hypo-peritectic alloys

Primary dendrite arm spacing (PDAS) of α phase in directionally solidified Pb-26%Bi (mass fraction) hypo-peritectic alloys was measured by considering the effect of melt convection in cylindrical samples with different diameters. The experimental results show the measured PDAS increases with increasing diameter of the sample. At the growth velocity of 5 μm/s, its value changes from 161.5 μm for the sample with 1.8 mm in diameter to 240.4 μm for the sample with 7 mm in diameter. The strong melt convection in large diameter samples causes a high bulk alloy composition and a high concentration gradient in peritectic β phase, resulting in a larger PDAS. Simultaneously, the high concentration gradient could effectively promote the peritectic transformation, enhancing the dissolution of the thin a dendrite.