Al11Mn4 formation on Al8Mn5 during the solidification and heat treatment of AZ-series magnesium alloys

Abstract

The crystallography and kinetics of Al11Mn4 formation on Al8Mn5 have been studied in magnesium alloys AZ80 / AZ91 and AZ31. During solidification, Al11Mn4 formation was promoted by low cooling rates where triclinic Al11Mn4 nucleated on rhombohedral Al8Mn5 particles with one of multiple related orientation relationships (ORs) and their variants. Al11Mn4 grew as (010) plates that were commonly twinned and the interrelationships amongst Al11Mn4 twins, Al8Mn5 twins and Al8Mn5-Al11Mn4 ORs are discussed. During solid state heat treatment at 410 °C, Al8Mn5 particles transformed into Al11Mn4 by a core-shell reaction with cracking in the Al11Mn4 shell. The solid-state reaction kinetics were consistent with interface reaction controlled growth. The results show that heat treatment can be used to tailor the Al-Mn compound in contact with the matrix (Mg) phase which may enable some control of corrosion performance.