Research on grain stability under room temperature of nanocrystalline Al-Zn-Mg-Cu alloy cryomilled powders

Nanocrystalline Al-10. 0Zn-3. 0Mg-1. 8Cu (wt. %) alloy powders were fabricated by mechanically milling at cryogenic temperature (cryomilling) in liquid nitrogen. By XRD, TEM and DSC, the grain stability under room temperature of the cryomilled alloy powders was studied. The results show that the milled powders,with the average grain size of 34 nm, was composed of high angle grains with the size of submicron and the subgrains with the size of more than 10 nm rather than wholly composed of the uniform nanocrystalline high angle grains. After a long time (two years) of placement at room temperature, the subgrains disappeared and the high angle grains were kept. That the subgrain is not stable and the high angle grain is comparatively stable is due to the combined actions of the mircostrain of the powders introduced during cryomilling and the Zener pinning force of the nano-scale particles on grain boundary.