Thermal analysis studies of the precipitation and dissolution processes of second phases in the Al-Si and Al-Si-Mg systems

Abstract

In the alloy systems Al-Si and Al-Si-Mg, where Si and Mg typically are in the range 0.5-1.5 w%, a number of second phase particles have been identified, with different composition and crystal structure. To control the conditions under which these particles precipitate and dissolve is very important for controlling the properties of the alloy, like fracture toughness and hardness. In this paper, particularly the influence of the cooling rate in the DSC instrument, with which alloy samples are cooled from a homogeneous phase at high temperature to subambient temperature, has been investigated. Also the total number of heating-cooling cycles applied to a sample has been used as a parameter to explain the different reactions as identified by DSC. The results show a close correlation between the cooling rate and the precipitation/dissolution processes. It was found that the characteristics after a certain cooling rate were different depending on how many cycles that material previously had been through. This can have some implication on estimating the properties of materials in applications were heat and load vary over time.