The research team, led by Rakesh K.R. of the National Institute of Technology Karnataka together with Pratyush Mohanty, Srikanth Bontha, Ramesh M.R., and Vamsi Krishna Balla of CSIR-Central Glass and Ceramic Research Institute, focused on two alloy compositions: Mg-1Zn-2Gd-0.4Zr and Mg-1Zn-2Dy-0.4Zr, all concentrations given in weight percent. Gadolinium and dysprosium are rare earth elements prized for their high solubility in magnesium at eutectic temperature, roughly 23.49 and 25.8 weight percent respectively, which makes them ideal for precipitation hardening. Zirconium was included in both alloys as a grain refiner, a role it performs more effectively than any other known refining method in zinc-containing magnesium melts. The alloys were prepared by conventional casting under a protective argon atmosphere containing two percent sulfur hexafluoride, with the melt held and stirred at 750 degrees Celsius before being poured into a cast iron mold.

The centerpiece of the study was the classic T6 heat treatment schedule: solution treatment at 500 degrees Celsius for twelve hours, water quenching, and then artificial aging at 225 degrees Celsius for periods of twelve, eighteen, or twenty-four hours, designated T6-12, T6-18, and T6-24. Hardness measurements using a Brinell tester with a ten millimeter steel ball indenter under a 250 kilogram load revealed that aging for twelve hours produced the most significant gains. The Mg-Zn-Gd alloy climbed from 37 BHN in the as-cast state to 47 BHN after T6-12, while the Mg-Zn-Dy alloy rose from 39 BHN to 46 BHN. Extending the aging time to eighteen or twenty-four hours offered no meaningful further improvement, so the researchers adopted T6-12 as the optimum condition for all subsequent wear and corrosion comparisons.

The microscopic origin of these gains lies in a dramatic restructuring of the secondary phases. In the as-cast condition, both alloys displayed dendritic grains with thick eutectic phases segregated along grain boundaries in a highly non-uniform fashion. After solution treatment, these coarse eutectic networks largely dissolved into the alpha-magnesium matrix, and fine lamellar precipitates of the long period stacking ordered type, known as LPSO phases, nucleated within the grains. Transmission electron microscopy confirmed the lamellar LPSO morphology in both alloys, with energy dispersive spectroscopy showing the phases were strongly enriched in gadolinium at 78.39 weight percent in the Mg-Zn-Gd alloy and dysprosium at 82.99 weight percent in the Mg-Zn-Dy alloy. X-ray diffraction added corroborating evidence: new MgGd3 peaks appeared in the heat-treated Mg-Zn-Gd alloy at approximately 33 and 57 degrees two-theta, while in the heat-treated Mg-Zn-Dy alloy the Mg24Dy5 peaks vanished entirely, indicating dissolution of the eutectic phases.

These microstructural changes translated directly into superior tribological performance. Dry sliding wear tests were conducted on a pin-on-disc apparatus against an EN-24 steel counterface at applied loads of 10 and 20 newtons, temperatures ranging from 200 to 400 degrees Celsius, a sliding velocity of 1.25 meters per second, and a total sliding distance of 1500 meters, following the ASTM G-99 standard. Across every condition, the heat-treated alloys exhibited lower wear rates than their as-cast counterparts, consistent with Archard’s law, which states that wear rate is inversely proportional to hardness. The Mg-Zn-Gd alloy consistently outperformed the Mg-Zn-Dy alloy, and for the gadolinium-bearing composition the wear rate actually fell from 1.6 times ten to the minus three cubic millimeters per millimeter to 1.2 times ten to the minus three as the test temperature rose from 200 to 400 degrees Celsius under a 10 newton load.

The seemingly paradoxical improvement of wear resistance at higher temperatures is explained by the behavior of oxide debris. Scanning electron microscopy of the worn surfaces revealed parallel ridges and grooves characteristic of abrasive wear, along with sheet-like delaminated particles and craters at the heavier 20 newton load, signatures of delamination wear in which subsurface cracks propagate parallel to the surface before shearing off thin wear sheets. At elevated temperatures, however, frictional heating oxidized both the sliding surfaces and the wear debris. X-ray diffraction of the worn pins detected zinc oxide on both alloys and magnesium oxide on the dysprosium alloy, confirming that a compact oxide glaze had formed. This oxidized debris fills the valleys of the worn surface, prevents direct metal-to-metal contact with the steel disc, and acts as a solid lubricant, simultaneously reducing the coefficient of friction and the wear rate.

Corrosion testing delivered perhaps the most striking results of the study. The team immersed polished specimens in 3.5 weight percent sodium chloride solution for 72 hours at 30 degrees Celsius, collecting the evolved hydrogen and measuring weight loss to compute corrosion rates. The as-cast Mg-Zn-Gd alloy corroded at 3.86 millimeters per year, but after T6-12 treatment the rate dropped to 1.48 millimeters per year, a reduction of roughly sixty-one percent. The Mg-Zn-Dy alloy improved from 3.92 to 2.62 millimeters per year, a thirty percent reduction. Electron microscopy of the corroded surfaces showed severe galvanic attack and deep pitting penetrating to the subsurface in the as-cast samples, whereas the heat-treated Mg-Zn-Gd surface displayed only mild filiform corrosion with large areas left entirely untouched.

The mechanism behind the corrosion improvement is a textbook illustration of how microstructure governs electrochemistry. In the as-cast alloys, the large, sparsely distributed eutectic phases act as cathodes adjacent to the anodic alpha-magnesium matrix, driving aggressive galvanic corrosion. After solution treatment, these cathodic eutectic phases dissolve, and the resulting fine, homogeneously distributed precipitates including the LPSO phases raise the anode-to-cathode area ratio and act as barriers that impede corrosion propagation. Gadolinium-bearing secondary phases in the treated alloy form a more continuous network along grain boundaries that effectively blocks corrosive attack, while the dysprosium alloy retains some clustered intermetallic precipitates that continue to behave as localized galvanic cathodes, explaining why Mg-Zn-Gd outperformed Mg-Zn-Dy in both as-cast and heat-treated conditions.

The implications reach well beyond the laboratory. Magnesium alloys containing rare earth elements are already irreplaceable in aerospace and defense applications despite their cost, and the automotive industry continues to push magnesium components as a route to lighter, more fuel-efficient vehicles. By demonstrating that a simple, industrially routine T6 treatment, twelve hours of solutionizing at 500 degrees Celsius followed by twelve hours of aging at 225 degrees Celsius, can simultaneously harden these alloys, reduce their wear rates across a broad temperature window, and cut their corrosion rates by up to sixty percent, the researchers have provided a low-cost, scalable route to magnesium alloys that are far more durable in service. The study also underscores the special value of LPSO phases, whose high hardness, thermal stability, and coherent interfaces with the magnesium matrix make them ideal strengthening agents for both tribological and corrosion performance, pointing the way toward the next generation of lightweight magnesium engineering alloys.

Subject of Research: Heat treatment effects on the microstructure, wear, and corrosion behavior of Mg-Zn-Gd and Mg-Zn-Dy magnesium alloys

Article Title: Heat treatment of Mg-Zn-Gd and Mg-Zn-Dy alloys for enhanced wear and corrosion properties

Article References: K.R, R., Mohanty, P., Bontha, S., M.R, R., & Balla, V. K. (2026). Heat treatment of Mg-Zn-Gd and Mg-Zn-Dy alloys for enhanced wear and corrosion properties. Journal of Materials Science: Metallurgy, 1(1), Article 7. https://doi.org/10.1007/s44492-026-00008-y

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00008-y

Keywords: magnesium alloys, Mg-Zn-Gd, Mg-Zn-Dy, heat treatment, T6 temper, LPSO phase, precipitation hardening, wear resistance, corrosion resistance, rare earth elements, microstructure, galvanic corrosion