Hardening and thermal stability of nanocrystalline AlMg4.8 powder

Abstract

Ball milled nanocrystalline AlMg4.8 powder was investigated in terms of hardening and thermal stability. The validity of the Hall–Petch relation was confirmed down to the minimum grain size of ∼44 nm. Prolonged milling in the range of the minimum grain size still increased the hardness. This development is discussed in terms of contamination effects and the influence of full and partial dislocations. Concerning thermal stability, recovery processes occur in the range of 100–230°C, whereas substantial grain growth starts at a temperature of ∼250°C. The enthalpy release for recovery was detected to be ∼39 J mol⁻¹ and ∼208 J mol⁻¹ for grain growth. Dynamic strain ageing was indicated by an activation energy for recovery of Q ∼ 120 kJ mol⁻¹. The activation energy of grain growth was calculated by means of the Kissinger theory (Q = 200–210 kJ mol⁻¹) and using the results of static grain growth (Q = 204 kJ mol⁻¹).

Keywords:

• aluminium alloys
• ball milling
• DSC
• nanostructured materials
• Hall-Petch