Positive effect of natural pre-ageing on precipitation hardening in Al-0.44 at% Mg-0.38 at% Si alloy

Age hardening in a purely ternary Al-Mg-0.4-Si-0.4 (0.44 at% Mg, 0.38at%Si) alloy that is similar to AA6060 was investigated by hardness measurement, TEM and three-dimensional atom probe (3D-AP). In particular, the effect of natural pre-ageing before artificial ageing, which is known to have a positive effect in this alloy, was studied by comparing three different conditions: natural ageing only, artificial ageing for 1.5h at 180 degrees C only and combined natural pre-ageing and subsequent artificial ageing for 1.5h at 180 degrees C. Natural ageing influences the mechanical properties significantly. Naturally aged alloys exhibit a hardening response that is governed by the presence of small clusters. Subsequent artificial ageing of naturally aged specimens increases the value of peak hardness, which is attributed to the increase of the number density of needle-shaped precipitates as compared to the samples without natural ageing. It is assumed that besides these precipitates, the small Si clusters formed at room-temperature storage remain stable during artificial ageing. (C) 2008 Elsevier B.V. All rights reserved.