Nanocrystalline AZ91 alloy: Preparation, exceptional thermal stability and enhanced hardness

Nanocrystalline (NC) magnesium alloys can improve energy efficiency through enhanced mechanical properties as applying to traffic field. In this work, NC AZ91 alloys with high hardness were prepared by mechanical milling. The stability of this alloy was subsequently tested through isochronal annealing at 573 K–723 K for 1 h. The microstructural evolutions, including milled and annealed alloys, were characterized by SEM, XRD, and TEM. The activation energies at the low temperature region (573 K–673 K) and high temperature region (673 K–723 K) were 95–108 kJ/mol and 134–190 kJ/mol, respectively. After annealing at 723 K for 10 h, the average grain size increased from 91 nm to 256 nm. The results revealed that the thermal stability is superior to Mg–Al series and even Mg-RE series alloys, which could be attributed to the drag effect and thermodynamic stabilization of the grain boundaries. Nanoscale Mg17Al12 precipitates with particle sizes smaller than 20 nm were observed at annealing temperatures higher than 623 K. The main contributions to the strength of annealed AZ91 alloys could be attributed to the GBs strengthening and the precipitate strengthening. The corresponding contribution ratios were 54% and 45%, respectively.