Thermal Stability of Nanocrystalline AZ31/TiB₂ Magnesium Matrix Composites Prepared via Mechanical Milling

In this work, the thermal stability of nanocrystalline (NC) AZ31/TiB₂ magnesium matrix composites was investigated, while the microstructure evolution and mechanical properties were analyzed. The results indicated the AZ31/TiB₂ still maintained NC structure after annealing at 350 °C for 180 min. Even at the high annealing temperature of 400 °C and 450 °C for 180 min, their average grain size just reached about 124 nm and 155 nm, indicating excellent thermal stability. The TiB₂ particles with sub-micron size uniformly distributed in Mg matrix had no change in size and no reaction with matrix during the annealing treatment. Due to the strong Zener pinning effect of TiB₂ particles, the grain growth of Mg matrix at high temperature was effectively inhibited. Meanwhile, the solution and precipitation behavior of Al atoms were completed in a short time, due to the existence of many grain boundaries and structural defects. By calculation, the grain growth kinetics was described by the kinetics equation \(D^{8} - D_{0}^{8} = kt\) and the activation energy E_g for grain growth was 131.6 kJ/mol, which was much higher than that of pure Mg (92 kJ/mol). Due to their excellent thermal stability, the decrease in both compressive yield strength and ultimate compressive strength was no more than 12.2% after annealing treatment. Even annealing at 450 °C for 180 min, the CYS and UCS of the samples were still above 283 MPa and 295 MPa, respectively.