Magnesium is the lightest structural metal available commercially making it an attractive alternative to aluminium, titanium or steel in weight critical applications. In more recent years, light-weight materials have become desirable for use in aerospace and automotive applications to increase fuel efficiency. The magnesium-rare earth alloy family is of interest due to the age-hardening properties, producing high strength at high temperatures combined with good corrosion resistance.

With the addition of certain transition metals the hardness, proof stress, elongation, and tensile stress can be further increased. This is due to the formation of the long period stacking ordered (LPSO) phases. The present study controls and characterises the LPSO_14H phase in a Gd containing Zr refined LPSO alloy system. These “type two” LPSO forming alloys allow for significant control of the phase since the LPSO phase can be precipitated in a controlled way through heat treatment.

The microstructures and morphologies of the LPSO phase can be highly complex, which will influence the ability of LPSO to provide strengthening. There remains limited understanding of the optimum distribution of LPSO to control complex properties such as fracture toughness. By changing alloy chemistry and heat treatment, different LPSO microstructures have been obtained. These have been characterized with high resolution scanning and transmission electron microscopy to understand the relationships between composition, heat treatment and the morphology, fraction, and distribution of LPSO. The relationship between the microstructure and complex properties such as fracture toughness has been investigated and used to propose strategies for optimising the LPSO microstructure to give a desired property balance.