On the destabilisation of fibrous growth in monotectic alloys

Al–Bi alloys of exact 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. At large gradients and low solidification velocities the monotectic reaction leads to Bismuth fibres in an Aluminium matrix. The Bi fibres are replaced by a string of pearls of Bi droplets at larger velocities or smaller gradients. The transition between both structures is discussed theoretically and compared with the experimental results.