Abstract:
The influence of temperature history on clustering and precipitation is studied. Al-0.4 wt.% Mg-1.0 wt.% Si alloy with a very low level of impurities and Al-0.4 wt.% Mg-1.0 wt.% Si with 0.25 wt.% Fe and 0.08 wt.% Mn are the main alloys in order to understand the mechanism and kinetics of clustering and precipitation and the influence of intermetallics. Other alloys with additions of 0.01 wt.% Cr or 0.04 wt.% Cu are also used to elucidate the role of small additional elements on clustering and precipitation. The main findings of this work are: Two main processes occur during clustering at room temperature. During the first, small clusters form by diffusion of quenched-in vacancy and solute atoms. As the solute concentration in the matrix is reduced, vacancies are then trapped by the clusters. During the second process, coagulation of existing clusters occurs and big clusters form. Upon natural ageing, the number density of the precipitates formed during AA decreases while their size increases. The main reason causing a negative effect on subsequent artificial ageing is the formation of smaller clusters which lower the solute concentration in the matrix and capture vacancies. The presence of intermetallics stimulates recrystallisation and finer grains can be obtained after solutionising. However, the Fe-rich intermetallics do not completely dissolve during solutionising, and lower the Si concentration in the matrix, thus reducing the volume fraction of both clusters and precipitates. Cr addition has no direct influence on clustering and precipitation, but Cu hinders the first clustering process and therefore the negative strength response by natural ageing is reduced. Cu also influences the precipitation sequence because more kinds of precipitates are formed.