Modification of Az91 Magnesium Alloy for High Temperature Application

Abstract

In recent years, the lightweight requirement has become crucial in aerospace and automobile application. Mg alloys provide an excellent alternative to conventional structural metals and alloys as it has the highest specific strength. Mg-alloy AZ91 (Mg-9Al-0.8Zn-0.2Mn) is the most favored alloy, being used in approximately 90% of all magnesium cast products. It has good castability, excellent room temperature (RT) mechanical properties and has low cost compared to other Mg-alloys. However, above 120°C its mechanical properties deteriorate because of the low softening temperature of same ß-Mg17Al12 which provides it better RT mechanical properties. The present study is an attempt to improve the creep strength of AZ91 through the addition of alloying elements which form thermally stable intermetallics and pins the ß-Mg17Al12. The effects of individual and combined additions of Sb and Sr on the microstructure and creep properties of squeeze-cast Mg-AZ91 alloy have been investigated. For comparison, the same has also been studied on the as-cast AZ91 alloy without any addition. The results indicate that individual and combined addition of Sr and Sb refine the grain size, ß-Mg17Al12 phase, and it affects the dynamic precipitation of ß-Mg17Al12 during creep. All the alloys have been creep tested at a temperature of 175°C and a stress level of 70 MPa. The lowest steady state creep rate is obtained in the AZ91 alloy pertaining combined addition of both Sb and Sr owing to the presence of relatively lower amount of ß-Mg17Al12 phase and high melting point intermetallics Mg3Sb2 and Al4Sr phase.