Age-hardening and age-softening in nanocrystalline Mg-Gd-Y-Zr alloy

Coarse-grained Mg-Gd-Y-Zr alloys are typical age-hardenable alloys. Here we report temperature-dependent age hardening and age-softening effects in nanocrystalline (NC) Mg-Gd-Y-Zr alloy. NC Mg-Gd-Y-Zr alloy can be strengthened as ageing at 175 degrees C via intergranular solute segregation hardening or softened as ageing at 200 and 225 degrees C due to depletion in intragranular solute atoms accompanied with formation and growth of intergranular precipitates. Microstructural characterizations reveal that solute segregation takes place at grain boundaries (GBs) of nanocrystalline alloy during ageing, stabilizing nanograins and strengthening the alloy. However, ageing at high temperature and/or over-time promotes fast formation of beta phases and depletion in intragranular solute atoms, weakening solid solution strengthening effect and leading to age-softening. Thus, age-hardening and age-softening in nanocrystalline alloy are ascribed to the competition between intergranular solute segregation hardening and weakened intragranular solid solution strengthening.