In situ investigation of the crystallization kinetics and the mechanism of grain refinement in aluminum alloys

The crystallization kinetics during the liquid to solid phase transformation of pure aluminium and various Al-Ti-B alloys is investigated using differential thermal analysis (DTA) and three-dimensional X-ray diffraction (3D XRD). A reduced undercooling required to activate nucleation of aluminum grains is observed, when both solute titanium and TiB₂ particles are present in the liquid. The cooling rate dependence of the onset temperature T_o, the crystallization peak temperature T_P, and the latent heat ΔH are evaluated and compared for all samples. The DTA curves for slow cooling of an Al-0.3Ti-0.02B (wt.%) alloy illustrate the formation of an aluminide phase (TiAl₃) upon solidification. A comparison of the DTA curves during slow cooling of the hypoperitectic Al-0.1Ti-0.1TiB₂ (wt.%) and the hyperperitectic Al-0.3Ti-0.02B (wt.%) alloys seem to exhibit a kinetic similarity at the onset of the solidification. In situ 3D XRD measurements clearly exhibit the formation of a metastable TiAl₃ phase prior to solidification of both alloys. This explains the mechanism of grain refinement in the presence of solute titanium and TiB₂ particles in the grain refined aluminum alloys. The influence of titanium diffusion, latent heat, and cooling rate on the growth behaviour of individual aluminium grains during the phase transformation is further quantified.