STABILITY OF MICROSTRUCTURES IN DIRECTIONALLY SOLIDIFIED MONOTECTIC ALLOYS

摘要

Al-Bi of exact monotectic composition was directionally solidified with solidification velocities from 0.3 mu m/sec to 5 mu m/sec and temperature gradients from 2 K/mm to 6.5 K/mm. Three distinct microstuctures could be observed: fibers, strings of pearls and irregular arrays of second phase droplets. The transitions from fibers to strings of pearls or irregular arranged drops are discussed theoretically and compared with the experimental results. One transition seems to be a Nichols and Mullins instability of the fibers, the other an effect of periodic nucleation ahead of the solidification front.