The effect of submicron SiC particles on the thermal stability of nanocrystalline AZ91 alloy

Nanocrystalline (NC) magnesium alloys and composites present potential for improving energy efficiency in transportation applications due to outstanding strength. Unfortunately, softening owing to grain growth at elevated temperature would induce fatal invalidity. The NC SiCp/AZ91 composite was isothermally annealed. A study on the thermal stability, including phase transformation, grain growth, softening, and strengthening mechanisms was carried out. After annealing treatment at 673 K for 240 min, the average grain size of AZ91-15vol.%SiCp composite increased from 29.0 nm to 53.1 nm. Coherent interface between nano-scale Mg17Al12 precipitates and Mg matrix was certified by HRTEM. Pinning effect due to the dispersing submicron SiC particles with size of 164 nm and nano-scale Mg17Al12 precipitates and dragging effect due to dissolving Al element were the reasons for excellent thermal stability. After annealing treatment, the hardness values of AZ91-xvol.%SiCp (x = 5, 10 and 15) composites were 1.61 GPa, 1.48 GPa and 1.78 GPa. Taking annealed AZ91-15vol.%SiCp composite as an example, there was no evident softening phenomenon and the decreasing ratio was ∼2% in comparison with milled composite with hardness of 1.82 GPa. Non-thermal sensitive SiC particles and high thermal stability Mg matrix played a key role in non-softening of composites.