Microstructure evolution and solidification behaviour of ZrB₂-SiC composite ceramics fabricated by laser surface zone-melting

Microstructure evolution and solidification behaviour of ZrB₂-SiC composite ceramics fabricated by laser surface zone-melting were investigated. Microstructure coarsening at high scanning speed and microstructure refining after turning off the laser was observed due to the changes in the solidification rate. The solidification behaviour from bottom to top of the molten pool was studied, where there are some coarsen eutectic bands caused by the secondary heating of the melting pool on the solidified eutectic zone in the molten pool. The deviation of melt composition from the eutectic ratio due to the volatilization of SiC can form a coarse primary ZrB₂ phase among fine eutectic structure (single-phase instability), and the constitutional supercooling due to the accumulation of impurity elements can form coarse eutectic dendrites among fine eutectic structure (two-phase instability). Both single-phase instability and two-phase instability are adverse to the mechanical properties, which should be prevented by adjusting the composition of raw materials and the solidification process.