Thermal stability of nanocrystalline Mg-based alloys prepared via mechanical alloying

The thermal stability and the kinetics of grain growth of nanocrystalline Mg-6Al-1Zn and Mg-6Al-1Zn-1Si alloys prepared via mechanical alloying were investigated. It started with elemental powders, using a variety of analytical techniques including differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometry. The kinetics of grain growth in isothermal annealing was investigated. The XRD results show that, although the grain sizes of both material systems increase as the annealing temperature rises, the Si-containing system displays a relatively smaller grain size, i.e., 60 nm compared with 72 nm in Mg-6Al-1Zn system, after being exposed to 350 °C for 1 h. The second-phase intermetallic particle Mg2Si formed during the isothermal annealing of Mg-6Al-1Zn-1Si system could influence not only the activation energy but also the exponent of kinetic equation. Higher hardness values obtained in the Si-containing system would be due to the formation of Mg2Si intermetallic phase.