Effect of cooling conditions during casting on fraction of β-Mg17Al12 in Mg–9Al–1Zn cast alloy

The major second phase (β-Mg12Al17) that is present in the most commonly used cast magnesium alloy, AZ91, has a strong effect on various properties of the alloy. Thus how casting conditions affect the amount of this phase formed needs to be studied. In the present study, casting of AZ91 was conducted allowing various cooling conditions by employing a chill. Total fraction of β-phase (fβ), the fraction of the eutectic β phase (fβ-eut) formed during eutectic solidification, and the fraction of discontinuous β precipitates (fβ-d) formed during subsequent solid state cooling were quantitatively measured, respectively. It was found that fβ was not significantly affected by the variation of cooling history in different locations of the casting. However, fβ-eut and fβ-d are strongly location-dependent, decreasing and increasing, respectively, towards the chill end of the casting. Explanation on this is given taking into accounts the effects of casting/thermal conditions on solidification and subsequent discontinuous precipitation.