Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted efforts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identified and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys via precipitation hardening.

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Precipitation and Hardening in Magnesium Alloys

Magnesium casting technology for structural applications

Towards magnesium alloys for high-volume automotive applications

Recent research and developments on wrought magnesium alloys

The structure of long period stacking/order Mg–Zn–RE phases with extended non-stoichiometry ranges

The use of magnesium alloys in structural applications has great potential for the lightweighting of transportation vehicles. Research within the CAST Cooperative Research Centre has tackled some of the important issues related to the use of magnesium in structural applications. To this end, a new alloy with extrudability and properties similar to 6000 series aluminum alloys has been developed. Furthermore, a method of laser heating magnesium alloys before self-piercing riveting has enabled high-integrity joining between magnesium components or between magnesium and dissimilar metals. In this paper, new technologies and improved understanding of the deformation behavior of wrought magnesium alloys are discussed in light of key metallurgical features such as alloy composition, grain size, and work hardening rate.

Magnesium alloy applications in automotive structures

The age-hardening response of two cast Mg-Zn-RE alloys has been investigated and compared with that of a binary Mg-Zn alloy. The microstructures of the aged specimens were examined by analytical electron microscopy. Formation of a fine dispersion of rodlikeβ1′ precipitates is the main cause for age hardening, while extensive precipitation of disc-shapedβ2′ coincides with the onset of overaging. Rare earth additions retard the formation ofβ2′ precipitates and thus postpone overaging. Four different orientation relationships betweenβ2′ precipitates and matrix were found and explained in terms of the near-CSL model.

Precipitation Hardening of Mg-Zn and Mg-Zn-RE alloys

The solidification path and microstructure of cast Mg-9Zn and Mg-8Zn-1.5MM (misch metal) alloys have been investigated by a combination of thermal analysis and analytical electron microscopy. The addition of 1.5 wt pct MM had a strong influence on the as-cast microstructure with the introduction of new ternary interdendritic phases and structural modification of known binary phases. The temperature ranges for formation of these phases from the melt were identified, their crystal structures determined, and their compositions analyzed. Products from eutectoidal decomposition of the interdendritic phase in the binary Mg-9Zn alloy were also identified.

The intergranular microstructure of cast Mg-Zn and Mg-Zn-rare earth alloys

Microstructural analysis of the improved creep resistance of a die-cast magnesium–aluminium–rare earth alloy by strontium additions

The effect of increasing levels of silicon on the microstructure and creep properties of high-pressure die-cast Mg-Al-Si (AS) alloys has been investigated. The morphology of the Mg2Si phase in die-cast AS alloys was found to be a function of the silicon content. The Mg2Si particles in castings with up to 1.14 wt pct Si have a Chinese script morphology. For AS21 alloys with silicon contents greater than 1.4 wt pet Si (greater than the alpha-Mg2Si binary eutectic point), some Mg2Si particles have a coarse blocky shape. Increasing the silicon content above the eutectic level results in an increase in the number of coarse faceted Mg2Si particles in the microstructure. Creep rates at 100 hours were found to decrease with increasing silicon content in AS-type alloys. The decrease in creep rate was most dramatic for silicon contents up to 1.1 wt pct. Further additions of silicon of up to 2.64 wt pct also resulted in significant decreases in creep rate.

G. L. Dunlop

Papers

Magnesium alloy applications in automotive structures

Precipitation Hardening of Mg-Zn and Mg-Zn-RE alloys

The intergranular microstructure of cast Mg-Zn and Mg-Zn-rare earth alloys

Microstructural analysis of the improved creep resistance of a die-cast magnesium–aluminium–rare earth alloy by strontium additions

The effect of silicon content on the microstructure and creep behavior in die-cast magnesium AS alloys