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

The texture and anisotropy of magnesium–zinc–rare earth alloy sheets

Effect of Zn additions on the age-hardening of Mg-2.0Gd-1.2Y-0.2Zr alloys

Effects of rare-earth elements Gd and Y on the solid solution strengthening of Mg alloys

Enhanced age hardening response and creep resistance of Mg-Gd alloys containing Zn

Strengthening effect of Zn in heat resistant Mg-Y-Zn solid solution alloys

Compressive creep behavior and deformation substructures of binary Mg–Y alloys containing 0.2–2.4 mol% Y were investigated at 550–650 K under 4–200 MPa. The addition of yttrium improves creep strength of magnesium more efficiently than aluminum and manganese. This efficiency decreases with increasing temperature. The apparent activation energy for creep is substantially greater than that for self-diffusion in magnesium. Transmission electron microscopy revealed that the non-basal slip systems are activated at 650 K although the basal slip systems mainly operate at 550 K. Both the high activation energy and the cross-slip of dislocations from the basal to the non-basal planes can be explained consistently by the cross-slip mechanism.

Creep deformation behavior and dislocation substructures of Mg–Y binary alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys.

Mayumi Suzuki

Papers

Strengthening effect of Zn in heat resistant Mg-Y-Zn solid solution alloys

Creep deformation behavior and dislocation substructures of Mg–Y binary alloys

Creep strength of magnesium-based alloys

Creep Behavior and Deformation Substructure of Mg-Y Alloys Containing Dilute Content of Zinc

Effects of Zinc on Creep Behavior and Deformation Substructures of Mg-Y Alloy