Magnesium (Mg) alloys are attractive materials for the automotive industry due to their comparatively low density and high strength to weight ratio. However, the wider applications of Mg alloys, especially in wrought applications, are restricted by the combination of relatively poor corrosion resistance and limited formability.
In this work, low-level additions of Zn, Ca, Gd, Sr and Zr were studied as alloying additions to Mg, owing to their beneficial effects on improving the strength and formability. The alloys designed and presented herein are unique to date, owing to: (a) the total alloy loading being relatively low, resulting in the low density and low cost; (b) the microstructures being nominally homogeneous, essential for good formability; (c) the alloys are heat treatable and amenable to an increase in strength after wrought processing, for example a paint bake cycle (noting that the existing commercial wrought alloys such as the ZK and AZ alloys are not heat treatable).
Due the alloy compositions used in this work being new, the electrochemical properties of such alloys were not previously understood. Since the corrosion of Mg alloys depends on the alloying additions, the study herein aims to principally investigate the compositional and processing effects on the corrosion of Mg. To do this, significant aspets of alloy production and processing on the laboratory scale are required. Herein, additions of Zn, Ca, Zr, Gd and Sr to Mg were deliberately made in binary, ternary, quaternary and quinternary combinations to systematically explore the effects of altering bulk chemistry on the corrosion of Mg. Microhardness testing was employed to estimate the strength of Mg alloys. Results were correlated with microstructure analysis. This thesis contributes to an understanding of the corrosion of Mg alloys with relatively low alloy loadings, which is also lacking in the literature.
The results reveal that (i) hardness increased with alloy loading; whilst the corrosion rates did not show clear relationship with alloy loading; (ii) Mg alloys instead had an ultra-sensitivity to the type of alloying additions, and most elements promoted higher corrosion rates than pure Mg due to enhanced cathodic kinetics; (iii) alloys with multiple alloying additions (quaternary and quinternary) tolerated relatively higher alloy loading without much deteriorations on the corrosion resistance. An artificial neural network (ANN) model was used to manage, interpret and disseminate the datasets in a holistic framework
