Towards magnesium alloys for high-volume automotive applications

Advanced Mg alloys require a stable microstructure that maintains good mechanical properties when exposed to high temperatures. On the basis of this demand, the search for new Mg alloys has stimulated the imagination of material scientists. This review's discussion ranges from modifications of commercial Mg-Al- and Mg-Zn-based alloys by Ca and rare earth (RE) elements to the investigations of new families of Mg alloys based on Mg-Sn and Mg-Gd. It reports the fundamental findings of these efforts, including microstructure, mechanical properties, and strengthening mechanisms.

Trends in the Development of New Mg Alloys

The effect of 0.5, 1.2, and 2.0 wt.% Ca additions on the microstructure, thermal stability, and mechanical properties of the cast Mg–6Al–0.3Mn alloy (AM60) was investigated. Addition of 0.5 wt.% Ca did not form any new phase but suppressed the discontinuous precipitation of the β-Mg 17 Al 12 phase by being dissolved in both the second phase and magnesium matrix. In the materials containing higher amounts of Ca, however, the Al 2 Ca phase with a lamellar morphology appeared in the microstructure and the volume fraction of β-Mg 17 Al 12 diminished. It was found that Ca generally refines the microstructure and enhances the thermal stability of the alloy. This was documented by the retention of the initial fine dendritic microstructure, hardness, and ultimate shear strength (USS) of the Ca-containing materials after long-term annealing at 673 K. The observed improvements are believed to be caused by the solid solution strengthening of Ca in the Mg matrix, and the particle strengthening effects of the thermally stable Al 2 Ca phase which are formed as an interconnected network. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength and hardness after the same thermal treatment.

Effect of Ca additions on the microstructure, thermal stability and mechanical properties of a cast AM60 magnesium alloy

Metals are the backbone of manufacturing owing to their strength and formability. Compared to polymers they have high mass density. There is, however, one exception: magnesium. It has a density of only 1.7 g/cm3, making it the lightest structural material, 4.5 times lighter than steels, 1.7 times lighter than aluminum, and even slightly lighter than carbon fibers. Yet, the widespread use of magnesium is hampered by its intrinsic brittleness. While other metallic alloys have multiple dislocation slip systems, enabling their well-known ductility, the hexagonal lattice of magnesium offers insufficient modes of deformation, rendering it intrinsically brittle. We have developed a quantum-mechanically derived treasure map which screens solid solution combinations with electronic bonding, structure and volume descriptors for similarity to the ductile magnesium-rare earth alloys. Using this insight we synthesized a surprisingly simple, compositionally lean, low-cost and industry-compatible new alloy which is over 4 times more ductile and 40% stronger than pure magnesium. The alloy contains 1 wt.% aluminum and 0.1 wt.% calcium, two inexpensive elements which are compatible with downstream recycling constraints.

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A rare-earth free magnesium alloy with improved intrinsic ductility

A creep resistant Mg alloy ACM720 was subjected to laser surface treatment using Nd:YAG laser equipped with a fiber optics beam delivery system in argon atmosphere. This treatment was found to be beneficial for the corrosion and wear resistance of the alloy. Long-term linear polarization resistance and Electrochemical Impedance Spectroscopy measurements confirmed that the polarization resistance values of laser surface treated alloy were twice as high as that for the untreated alloy. The improved corrosion resistance was attributed to the absence of the second phase $Al_2Ca$ at the grain boundary, microstructural refinement and extended solid solubility, particularly of Al, in \alpha-Mg matrix owing to rapid solidification. The laser treatment also increased surface hardness two times and reduced the wear rate considerably due to grain refinement and solid solution strengthening.

Effect of laser surface treatment on corrosion and wear resistance of ACM720 Mg alloy

Effects of calcium and rare earth additions to alloy AZ91 on the microstructure and mechanical properties were investigated. The results indicated that small amounts of calcium addition to AZ91 did not cause the formation of any new phases in the microstructure, but refined the as-cast microstructure and increased the thermal stability of the β phase so that the yield strength and creep resistance of the alloy were significantly improved. Additions of lanthanum-rich misch metal (MM) resulted in the formation of needle-shaped particles, which showed very high thermal stability and did not dissolve into the matrix after the solution treatment at 420 °C for 20 h. The strength as well as creep resistance of the alloy at elevated temperatures was remarkably increased when MM was added combined with calcium. The highest creep resistance was obtained from the alloy with 3% of MM and 0.3% of calcium addition and its steady state creep rate reached as low as 2.69×10 −8 s −1 , one order of magnitude lower than that of alloy AZ91 without MM and calcium additions.

Wu Dengyun

Papers

Microstructure and mechanical properties of Mg–Al based alloy with calcium and rare earth additions