MEASUREMENTS OF ULTRARAPID SOLIDIFICATION RATES IN GREATLY UNDERCOOLED BULK MELTS WITH A HIGH-SPEED PHOTOSENSING DEVICE

Solidification rates of pure nickel and Cu70Ni30 bulk melts have been investigated as a function of undercooling ΔT. Large undercoolings up to ΔT = 323 K have been achieved by the use of a containerless electromanetic levitation technique. For a controlled variation in ΔT, nucleation of the suspended droplet was stimulated by touching the surface with a needle. The dependence of the recalescence time τ down to 25 μs om ΔT was monitored using a silicon photodiode and a high speed data acquisition system with a total response time of 1 μs. From the measured recalescence times, growth velocities up to 70 m s-1 were reduced. The growth velocity was found to rise with increasing ΔT, whereby two different regimes can be distinguished. The results are discussed within theoretical predictions for dendritic growth in highly undercooled melts.