PRECIPITATION HARDENING OF AN AL-4.2WT-PERCENT-MG-0.6WT-PERCENT-CU ALLOY

The precipitation hardening of an experimental Al-4.2 wt% Mg-0.6 wt% C u alloy has been studied. After a first initial jump, the yield streng th increases almost linearly with the logarithm of the ageing time and a peak of hardness is reached after 11 days at 180 degrees C. Special attention is given to the precipitation hardening during the early st age of ageing. it has been shown that S'' phase can be formed heteroge neously on dislocation loops and helices and a new mechanism of precip itation hardening due to this S'' phase precipitation is proposed. The precipitation of S'' on dislocations is the predominant cause of stre ngthening during the initial stage of precipitation hardening (up to 3 0 min at 180 degrees C). Guinier-Preston-Bagaryatsky (GPB) zones (or b etter, the recently introduced ''Cu/Mg clusters'') also appear in the initial stage, but their contribution to the hardness. which up to now was considered to be predominant, is shown to be smaller then the one of the S'' precipitates. Since the density of the S'' nucleation site s is related to the amount of dislocations, this mechanism is importan t in the case of a bake hardening treatment when ageing is preceded by cold deformation. Uniform S'' precipitation has also been found at th e later ageing stage, which suggests that the contribution of S'' to t he precipitation hardening at that stage is not less important. (C) 19 98 Acta Metallurgica Inc.