Effect of thermal treatments on mechanical behaviour of thixoformed magnesium alloy

The effect of thermal treatments on the microstructure and subsequent mechanical behaviour of a thixoformed AZ91 magnesium alloy has been investigated. After thixoforming, the microstructure consists of globules of the primary Mg rich phase surrounded by an interglobular matrix originating from the solidification of the liquid and containing the intermetallic gamma phase. Thermal treatments lead to partial or almost complete dissolution of the intermetallic phase depending on the temperature of the treatment and to the development of grains with various sizes. At room temperature, the mechanical behaviour is strongly influenced by the temperature and duration of the thermal treatment since the elongation to failure is controlled by the fracture of the brittle intermetallic phase. At high temperature, this phase is no longer at the origin of fracture. Deformation results mainly from dislocation creep controlled by solute drag but a contribution of grain boundary sliding is likely to occur for the microstructures heat treated at the lower temperature, resulting in large elongations to failure. MST/5175.