Study On Morphology And Formation Mechnism Of Precipitates In Al-2Sc Alloy

The Morphology of precipitates in the as-cast Al-Sc alloy and annealed alloy were investigated by the optical microscope (OM), X-ray diffraction(XRD), scanning electron microscope (SEM), high-resolution transmission electron microscope(HRTEM) techniques. Combining with the solidification theory, the classical nucleation theory and the theory of crystal growth, the formation mechanism of precipitates with different morphology were discussed. The conclusions were obtained as follows:(1) The results of XRD analysis show that, Al and Al3Sc are in the as-cast Al-2Sc alloy. The lattice mismatch between Al and Al3Sc is small.(2) Through the OM analysis, all the primary particles exhibit faceted growth, forming cubic shaped. But the distribution of the primary pricipitates in each region at different cooling rate is different. Most of the particles are connected at corners to form clusters.(3) According to the theory of cycle chain, lattice theory and solid-liquid genetic phenomenon, stuying the formation process of primary phase in melt. First of all is to form primitive groups. The accumulation of the primitive groups makes the growth of primary particles. On the basis of interface dynamics, the{100} planes grow at the slowest rate resulting in the formation of cubic primary particles.(4) Dispersion of small precipitated phase particles have been observed in the matrix of the as-cast Al-2Sc alloy and annealed alloy. After homogenization, the distribution density of the dispersion precipitation is larger than the as-cast alloy.(5) The morphology of the Al3Sc primary particles and the Al3Sc precipitation particles are different, but they have the same lattice structure.