Theoretical considerations and experiments on microstructural stability regimes in monotectic alloys

Al–Bi and Al–In alloys of monotectic composition were directionally solidified with solidification velocities from 0.3 μm/s to 5 μm/s and temperature gradients from 2 K/mm to 6.5 K/mm. The monotectic reaction leads to three distinct microstructural morphologies: fibers, strings of pearls and irregular arrays of second phase droplets, depending on processing conditions. The transition from fibers to string of pearls seems to be a Nichols and Mullins instability. The formation of irregularly arranged drops is interpreted as an effect of repeated nucleation ahead of the solidification front.