The role of solute titanium and TiB₂ particles in the liquid - Solid phase transformation of aluminum alloys

The nucleation and growth kinetics of α-Al grains in the systems Al-0.1Ti and Al-0.15TiB 2 (wt.%) have been studied by time-resolved neutron diffraction measurements during the liquid-solid phase transformation under continuous cooling. The evolution of the static structure factor S(Q) was monitored for different cooling rates. The evolution of the solid fraction f_S for both samples during the transformation was determined from the normalized variation of the height of first peak in the liquid structure factor. The transformation kinetics was analyzed in terms of the Johnson-Mehl-Avrami model, and compared for both samples. The evolution of Bragg peaks emerging after the nucleation of the solid phase was monitored. The results reveal that the TiB₂ particles in pure aluminum are not effective nucleation sites for α-Al grains during solidification. However, the presence of solute titanium in the Al-0.1Ti alloy changes the growth rate of crystallization during solidification. In the early stages of the phase transformation in the Al-0.1Ti alloy, pronounced oscillations in the Bragg peaks intensity are observed. These observations are discussed in the light of the present grain refinement theories.