Extrusion Doubles Strength of Heat-Resistant Aluminum-Cerium-Magnesium Alloy

A new generation of aluminum alloys that refuse to weaken in the heat is edging closer to the factory floor. Researchers led by Gaurav Singh and Catalin R. Picu of Rensselaer Polytechnic Institute, together with colleagues at Iowa State University, Ames National Laboratory and GE Aerospace Research, have shown that a simple, industrially standard extrusion process can transform a close-to-eutectic aluminum-cerium-magnesium alloy from a mediocre casting into a high-strength, damage-tolerant structural material. The alloy, designated Al-10Ce-4Mg, was squeezed through a steel die at 300 degrees Celsius, a deformation equivalent to nearly 200 percent plastic strain, and the result was dramatic: tensile strength roughly doubled to 390 megapascals at room temperature, and the strain at failure increased fourfold compared with the as-cast state. Just as importantly, the refined microstructure proved remarkably stubborn in the face of prolonged heat exposure, retaining most of its strength and nearly all of its fracture toughness after 100 hours at 300 degrees Celsius.

The motivation behind the work lies in one of the most stubborn limitations of conventional lightweight alloys. Aluminum alloys in the AA2XXX and AA7XXX families, the workhorses of the aerospace industry, rely on fine precipitates for their strength, but those precipitates dissolve or coarsen above roughly 200 degrees Celsius, causing properties to collapse. For applications such as supersonic aircraft, where skin temperatures can climb well beyond that threshold, designers have had no choice but to turn to heavier titanium or nickel-based materials, sacrificing fuel efficiency and increasing carbon dioxide emissions. Cerium-containing aluminum alloys offer a way out of this bind. When cerium is added to aluminum, it forms an Al11Ce3 intermetallic phase whose slow diffusion and low solubility in the aluminum matrix make it exceptionally resistant to coarsening, with stability demonstrated up to at least 350 degrees Celsius. Cerium is also a relatively abundant rare earth element, currently lacking high-volume applications, and is recovered as a byproduct of refining more valuable rare earths, which makes the alloy family economically attractive.

The alloy was prepared by melting high-purity aluminum with aluminum-cerium and aluminum-magnesium master alloys at 850 degrees Celsius and casting the melt into a graphite mold. Inductively coupled plasma analysis confirmed the composition at 10.096 percent cerium and 4.676 percent magnesium by weight, with only trace iron and silicon. The researchers deliberately chose the 4 percent magnesium content based on CALPHAD thermodynamic modeling, which showed that deforming the material at or just below the solvus of a τ-Al13CeMg6 intermetallic and above the metastable β-AlMg solvus would allow the high dislocation density generated during deformation to stimulate the nucleation of fine precipitates either during processing or upon subsequent cooling. The cast rods were then hot extruded at 300 degrees Celsius to a final diameter of 9.5 millimeters, an 86 percent reduction in cross-sectional area, using conditions deliberately kept within the envelope of conventional industrial practice.

Microstructural characterization revealed exactly why the extrusion was so effective. X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, differential scanning calorimetry and high-resolution transmission electron microscopy together painted a detailed picture of the transformation. In the as-cast condition, the alloy displayed the classic script-like eutectic structure of Al11Ce3 intermetallics distributed unevenly through a large-grained aluminum-magnesium matrix, with an average grain size of about 23.6 micrometers. After extrusion, the grains shrank to roughly 7.2 micrometers and the intermetallic particles were broken down and aligned into the extrusion direction, with the average dispersoid size falling from 3.57 to 1.91 micrometers. The dislocation density, calculated from kernel average misorientation measurements, roughly doubled, and a brass-type crystallographic texture emerged. Crucially, when the extruded material was annealed at 300 degrees Celsius for up to 100 hours, neither the grain size nor the dispersoid size changed to any statistically significant degree, confirming that the refined structure is thermally stable.

The mechanical property gains were striking across the board. Yield stress and tensile strength increased by factors of roughly 2.5, while elongation at failure jumped from 3.8 percent to 17.2 percent. Hardness rose from 98 HV in the cast state to 114 HV after extrusion. The simultaneous improvement in strength and ductility, a combination that is often difficult to achieve, was attributed to the uniform refinement of the intermetallic distribution, the increased dislocation density and the development of texture. Fractographic analysis told a consistent story: cast samples failed along cleavage facets in a brittle manner, while extruded samples fractured through dimpled, ductile surfaces, reflecting intense plastic deformation before failure. The extruded material also reached a Young’s modulus of about 75 gigapascals.

Thermal endurance proved to be the alloy’s signature quality. After exposure to 300 degrees Celsius for 100 hours, the as-cast alloy retained 82 percent of its room-temperature strength, and the extruded version retained 88 percent. Fracture toughness, measured for the first time in any Al-Ce-Mg alloy using the single-edge bend method of ASTM E1820, reached 23 kilojoules per square meter in the extruded state, a value comparable to some of the highest figures reported for aluminum alloys, including aluminum-lithium systems and wire-arc additively manufactured Al-Si. After the same 100-hour anneal, 97.6 percent of that toughness was preserved. Compared with the commercial high-temperature alloy AA2618, which loses about 90 percent of its room-temperature strength at 300 degrees Celsius, the ternary alloy held on to roughly 40 percent of its strength, although it fell short of the binary Al-10Ce alloy, which retains about 70 percent.

The reason for that trade-off lies in the magnesium. Transmission electron microscopy revealed fine, nanoscale magnesium-rich precipitates both within the aluminum grains and along their boundaries, too small to be detected by X-ray diffraction or EBSD. Differential scanning calorimetry showed an endothermic dissolution event with an onset at about 215 degrees Celsius, and a weak exothermic peak on cooling at roughly 360 degrees Celsius, indicating that these precipitates dissolve as the temperature climbs and re-form when it falls. The researchers estimate that the magnesium-rich precipitates account for about 55 percent of the room-temperature flow stress, which explains why strength drops rapidly in the 150 to 250 degrees Celsius range and why the extra strength of the ternary alloy over the binary Al-10Ce is essentially erased above 250 degrees Celsius. The reversible nature of the dissolution and re-precipitation, however, means the material recovers its properties on cooling, ensuring excellent property retention through thermal cycling.

The study also documented an unusual deformation behavior: negative strain rate sensitivity at room temperature. Tensile tests across strain rates from 10^-5 to 10^-2 per second revealed serrated flow curves characteristic of the Portevin-LeChatelier effect, driven by dynamic strain aging as magnesium atoms diffuse toward and interact with moving dislocations. The strain rate sensitivity parameter reached about minus 0.03 at intermediate rates, a range similar to that observed in the commercial alloy AA5182, which contains 4.5 percent magnesium. The serrations vanished at both the fastest and slowest rates and disappeared entirely at 150 degrees Celsius, confirming the diffusion-controlled origin of the phenomenon and demonstrating that sufficient magnesium remains in solid solution even after processing.

By decomposing the strength contributions of different mechanisms, the team showed that the refinement from extrusion contributes about 106 megapascals to the yield stress, split roughly equally between the increased dislocation density and the finer grains and intermetallics, while magnesium in solid solution adds another 32 megapascals in the cast state. The most impressive single contribution, however, is strain hardening: the extruded ternary alloy hardens by about 190 megapascals between yielding and failure, nearly four times the strain hardening of the equivalent binary alloy, an effect the authors link directly to the nanoscale magnesium-rich precipitates. Taken together, the results establish extrusion-processed Al-10Ce-4Mg as a serious candidate for lightweight, temperature-resistant structural applications, from supersonic airframes to powertrain components, and demonstrate that the path from laboratory curiosity to industrial adoption may run through the most conventional of metalworking processes.

Subject of Research: Thermomechanical extrusion processing of a ternary Al-Ce-Mg alloy to enhance strength, toughness and thermal stability.

Article Title: Effect of thermomechanical processing on mechanical properties and the microstructure of ternary Al-Ce-Mg alloy

Article References: Effect of thermomechanical processing on mechanical properties and the microstructure of ternary Al-Ce-Mg alloy. (n.d.). https://doi.org/10.1007/s44492-026-00011-3

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00011-3

Keywords: Al-Ce-Mg alloy, extrusion, thermomechanical processing, Al11Ce3 intermetallic, fracture toughness, high-temperature strength, Mg-rich precipitates, microstructure, dynamic strain aging, thermal stability, lightweight alloys, tensile properties