Modification of AZ91 Mg Alloys for High Temperature Applications

Abstract

The good specific strength and specific modulus of magnesium alloys had drawn the attention of the automotive manufacturers for use in fuel efficient vehicles. Among the cast magnesium alloys, AZ91 (Mg–9Al–1Zn) is the most sought alloy because of its good casting properties. However, this alloy loses its strength and creep resistance properties above 120 °C due to softening of the β phase (Mg17Al12). Hence, this alloy cannot be used for making heavier engine components (power train), which require the thermal stability up to about 250 °C. The paper discusses the approach of modifying the AZ91 alloy by minor alloying additions to improve the high temperature withstanding capability without significantly affecting its casting properties. Additions of Ca to AZ91 alloy to the levels of about 0.4 wt% increased the ambient and high temperature strength of the base alloy. Additions of other minor alloying elements such as Sb, Pb, rare earths etc. can also increase the high temperature capability of the AZ91 by further modifying the β phase structure. The paper overviews the work carried out by the authors on the role of different alloying additions on the microstructure and mechanical properties of AZ91 magnesium alloys.