In situ DSC investigation into the kinetics and microstructure of dispersoid formation in Al-Mn-Fe-Si(-Mg) alloys

Abstract

Dispersoid strengthening in Al-Mn-Fe-Si(-Mg) aluminum wrought alloys can play an important role in applications at elevated temperatures. Fine and evenly distributed dispersoids are needed to increase alloy strength, which can be achieved by conducting an adapted homogenization heat treatment. In this work for the first time, the precipitation behavior of dispersoids in direct chill casted Al-Mn-Fe-Si(-Mg) alloys was systematically investigated in situ during heating by means of differential scanning calorimetry. In situ analysis of the phase transformation kinetics during heating and homogenization was complemented by ex situ testing of the electrical conductivity and Vickers hardness, as well as micro- and nano-structural analyses by optical light and transmission electron microscopy. The influence of the heating rate on precipitation behavior was determined by calorimetric in situ experiments, covering a wide heating rate, ranging from 0.003 to 2 K/s. The influence of 400 and 550 °C homogenization temperatures and soaking durations on hardness and electrical conductivity was also investigated. The highest dispersoid strengthening was obtained after heat treating the as-cast Mg-containing alloy at 400 °C for 3 to 10 h. Furthermore, the Mg content heavily influenced the precipitation behavior by forming β-Mg2Si precursor phases, which can act as nucleation sites for dispersoid precipitation.