X-ray diffraction with hard X-rays (E = 70 keV) was used to investigate the grain nucleation and grain growth during solidification of a grain refined Al-0.3Ti-0.02B (wt.%) alloy. The investigations showed for the first time the nucleation profile during solidification and how nucleation rate increases with cooling rate. The results indicate that the nucleation process is complete for solid fraction below 30 %, irrespective of the cooling rate. This is explained in terms of the release of latent heat during solidification. The growth of individual aluminium grains during solidification is experimentally observed and compared to model predictions for the diffusion limited grain growth. The experimental results are only in agreement with the theory in the first stage of the transformation. The difference between the experiment and the theory is discussed qualitatively.