Magnesium Alloys: From Bombs to Cars and Biodegradable Implants

In 1908 German engineers created the “Electron” alloy, mixing 9 % aluminium with magnesium to tame the metal’s extreme reactivity. Pure magnesium burns fiercely at high temperatures and resists extinguishment, making it ideal for incendiary bombs. During World War I and the Spanish Civil War, these magnesium‑based weapons overwhelmed city fire departments, with the bombing of Gernika becoming a notorious example.

Material Science of Magnesium

Magnesium crystallizes in a hexagonal close‑packed (HCP) lattice, which offers few slip planes and therefore behaves brittly. Aluminium, by contrast, forms a face‑centered cubic (FCC) lattice that provides many slip systems and high ductility. When aluminium atoms substitute into the magnesium lattice, they generate tensile stress that hinders atomic movement, raising strength. Excess aluminium precipitates as hard intermetallic compounds at grain boundaries, further stiffening the alloy. Refining crystal grains through precise alloying also boosts strength, because smaller grains impede dislocation motion.

Engineering Challenges

When magnesium contacts a more noble metal in an electrolyte, galvanic corrosion initiates. Magnesium, the least noble material on the galvanic series, acts as the anode and corrodes rapidly, especially if the cathodic metal presents a larger surface area. Plasma Electrolytic Oxidation (PEO) combats this by submerging magnesium parts in a silicon‑based bath and applying high‑voltage arcs. The arcs drive dopants into the surface, converting it into a hard ceramic layer that resists electrolyte attack.

Modern Applications

Automotive engineers exploit magnesium’s low density to cut unsprung weight—wheels, suspension components, and motor housings. Reducing unsprung weight improves vehicle dynamics four times more effectively than trimming weight elsewhere. Corvette hybrid electric motors now house magnesium cases, saving mass while maintaining durability; a typical motor unit weighs 37 kg compared with a 46 kg hybrid package that includes the REZ battery. Across the board, magnesium alloy parts can be about 25 % lighter than comparable aluminium components.

In the biomedical field, the WE43 alloy—comprising magnesium, yttrium, neodymium, and zirconium—enables biodegradable implants. These screws dissolve as bone heals, eliminating the need for removal surgery. Designers avoid aluminium in medical alloys because of its neurotoxic reputation. WE43 received European approval in 2013 and FDA clearance in 2023, marking a milestone for resorbable implant technology.

Mechanisms in Detail

• Galvanic Corrosion: Dissimilar metals immersed in an electrolyte exchange electrons; the less noble metal (magnesium) loses atoms and corrodes, while the more noble metal remains protected. The corrosion rate accelerates when the cathode’s surface area exceeds that of the magnesium anode.
• Alloying Mechanism: Aluminium atoms embed in the magnesium lattice, creating internal tension. Upon cooling, surplus aluminium forms intermetallic compounds at grain boundaries, restricting slip and enhancing stiffness.
• Plasma Electrolytic Oxidation: High‑voltage electrical arcs strike the submerged magnesium, forcing silicon‑based dopants into the surface and forming a ceramic coating that shields the metal from corrosive environments.
• Unsprung Weight Dynamics: Heavier wheels increase rotational inertia, demanding more energy to spin and more force from the suspension to maintain road contact after bumps. Lightening these components yields disproportionate gains in acceleration, handling, and fuel efficiency.

“Removing unsprung weight is four times more effective than removing it anywhere else in the vehicle.”

“The further away two materials are from each other, the more important it is that they do not come in contact.”