Thermal stability of nanostructured aluminum powder synthesized by high-energy milling

The thermal stability of nanostructured aluminum powder synthesized by high energy milling was studied through isothermal annealing at high temperatures for various times. Strain relaxation and grain growth of milled powders were studied at different milling times by differential scanning calorimetry (DSC). The results showed a high level of stored enthalpy due to milling procedure. After 25 h milling, powder particles reached a steady state with equiaxed morphology and 90 nm crystallite size in average. Isothermal grain growth kinetics of nanocrystalline Al powder was investigated using X-ray diffraction (XRD). A critical temperature (∼0.8 of melting point) was distinguished at which a considerable increase was observed in the grain size. At below this temperature, the mean grain size remains almost stable for long annealing times due to small amounts of interstitial and substitutional impurities. However, grain growth was pronounced significantly depending on settling time above it. Stability of powder hardness after annealing was evaluated by micro-indentation. The results revealed a down-shift of the hardness beyond the critical temperature.

► Thermal stability of nanostructured Al was investigated using DSC curves.
► Three kinds of peaks were determined: strain relaxation, grain growth and melting.
► A temperature (Tc) was defined at which grain size transformed to unstable status.
► Above Tc, hardness was dropped significantly with respect to Hall–Petch relation.