Effects of cyclic extrusion and compression on the microstructure and mechanical properties of AZ91D magnesium composites reinforced by SiC nanoparticles

Cyclic extrusion and compression (CEC) was implemented on the SiC nanoparticles reinforced AZ91D magnesium composites at 300 °C, 350 °C and 400 °C. Up to 8 passes were performed. Effects of CEC parameters on the microstructure and mechanical properties of the n-SiCp/AZ91D composites were investigated. Results show that fine grains appear immediately after 1 pass of CEC and invade continuously with the progress of CEC from grain boundaries and twin bands to the center of the grain. After 8 passes of CEC at 300 °C, the matrix grains are effectively refined down to ~ 130 nm, with Mg17Al12 uniformly distributed along grain boundaries. Increasing in the CEC temperature leads to an obvious growth of the matrix grain and coarsening of the Mg17Al12. Clusters of SiC nanoparticles are remarkably dispersed after 8 passes of CEC. Besides the normal extrusion component, multi-pass of CEC induces other complex texture components, which is attributed to the activation of various twinning, especially 101−2 twinning, 101−1 twinning and 101−1−101−2 double twinning. The grain refinement mechanism is verified as twinning-assisted rotational DRX accompanied by continuous DRX at relatively low temperature (< 350 °C). The dynamically precipitated Mg17Al12 and the incorporated SiC nanoparticles act as obstacles for the growth of the fine grains. Eventually, an increase of ~ 43.57% is achieved in the hardness of the n-SiCp/AZ91D composites after 8 passes of CEC at 300 °C.