The role of composition control on the structure property relationships in Mg-Nd-Zn alloys

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Mg alloys are ideal for weight critical applications, where the reduction in weight correlates with a reduction in energy consumption. However, the property profiles of commercially viable magnesium alloys are not sufficient of many of the applications envisaged. The design of Mg alloys through control of both the alloy composition and distribution of secondary phases is important in realizing the full potential of these alloys. It has been reported that the addition of Zn to Mg-RE alloys (RE=rare earth) results in modification of the microstructures and the resulting mechanical properties at relatively low RE content. However, to present only RE additions with high solubility in Mg, such as Gd and Y, has been investigated systematically to understand the role of RE:Zn ratio on the microstructure and resultant mechanical properties in a range of processing conditions. Nd is a lower cost and low solubility RE addition that has shown to enhance the strength of Mg alloys through formation of fine scale particles that hinder plastic deformation effectively in Mg-Nd and Mg-Nd-RE alloys. It is expected that the presence of Zn will enhance the mechanical properties through the modification of microstructure at a relatively low Nd content.The ultimate goal of this investigation is to develop a lower cost Mg alloy system for structural applications with tunable mechanical properties through modification of the alloy composition and controlled heat treatments. The ability to design the microstructure and resultant mechanical properties through formation of solid-state precipitation along with intermetallic particles at the grain boundaries and would allow the development of alloys with a synergistic combination of enhanced strength and ductility.A range of alloys with varying Nd: Zn ratios will be investigated to understand the microstructural evolution during solidification using in situ X-ray diffraction and subsequent ex situ electron microscopy. The intermetallic phases, their formation temperature and their volume fraction of these phases will be used to improve the Mg rich side of the Mg-Nd-Zn phase diagram that is currently available. The role of heat treatments on modifying the intermetallic phases formed during solidification and formation of new solid-state precipitates during such heat treatments is to be investigated with the aid of electron microscopy. This would provide for a further step during which the microstructure and the resultant mechanical properties could be modified. The mechanical properties of the cast and the heat treated alloys will be evaluated both at room and at elevated temperature to understand the role of alloy composition, intermetallic phases and the thermal history on such properties.