Periodic structural fluctuations during the solidification of aluminum alloys studied by neutron diffraction

Time-resolved neutron diffraction measurements have been carried out to study the crystallization dynamics during the liquid-solid phase transformation in an Al-0.3Ti-0.02B (wt.%) alloy. Starting from the liquid phase, the temperature was reduced to below the thermodynamic transition temperature of T₀=933 K by continuous cooling at rates of 0.6 and 0.06 K/min and by step-wise cooling in temperature steps of 1 K. After nucleation of the solid phase on the micron-size TiB₂ particles in the liquid alloy, pronounced fluctuations in the Bragg-peak intensity of the growing crystallites are observed during solidification. These fluctuations have been analyzed by a time- correlation function to extract the frequency and correlation time of the fluctuations. The deduced correlation time and oscillation frequency strongly depend on the cooling rate. The correlation time increases and the oscillation frequency decreases for decreasing cooling rate. For step-wise cooling a monotonic increase in correlation time is observed for decreasing temperatures demonstrating a slowdown of the dynamics during the liquid to solid phase transformation.