New family of aluminum-cerium alloys shows significantly improved high-temperature performance, economic benefits
The high strength-to-weight ratio, corrosion resistance, and high thermal conductivity of aluminum alloys have made them important for the automotive and aerospace industries—and even more so now due to the promise of improved performance and fuel economy from lightweighting. However, while aluminum alloys offer outstanding castability, excellent mechanical properties, and low cost, they have lacked high-temperature mechanical performance.
Now, a multi-laboratory research team led by Oak Ridge National Laboratory has described a new family of economically competitive aluminum alloys containing 6-16 wt. % cerium which exhibits significantly improved high-temperature mechanical properties, in addition to improved castability and thermal stability when compared to existing aluminum alloys.
These alloys show room temperature ultimate tensile strength of 400 MPa and yield strength of 320 MPa, with 80% mechanical property retention at 240°C. The microstructure remains stable to above 500 ˚C—corresponding to a homologous temperature (T/TMelt) greater than 0.84, which rivals the stability observed in heat-tolerant materials such as superalloys. The properties may enable applications without the need for heat treatment. A paper on the work is published in the RSC journal Materials Horizons.
Castable engineering alloys are typically strengthened through precipitation of intermetallic phases from alloying elements dissolved during the casting process or driven into solution by heat-treating. These strengthening precipitates improve alloy performance by increasing stiffness and strength, while lowering thermal expansion; however they reside in kinetically frozen high energy architectures along chemical potential gradients that lead to instabilities at elevated temperatures.
The high mobility of traditional alloying elements leads to coarsening through processes such as Ostwald ripening; thus, prolonged exposure to high temperatures leads to dramatic changes in the microstructure and a corresponding degradation of mechanical properties. The loss of mechanical performance bounds the maximum operating temperature near the alloy aging temperature during the final step of heat treatment (155-190 °C for most Al alloys). This limitation becomes particularly significant for internal combustion engines, which benefit from light-weight materials compatible with higher temperatures for both the engine and nearby components.—Sims et al.
Most research so far on high-temperature aluminum alloys has focused on systems such as Al-Sc, Al-Zr, and Al-V which form stable L12 precipitates. However, the lattice coherence breaks down above about 300 °C (for Al-Sc6), resulting in the loss of high-temperature performance.
Aluminum-cerium alloys, however, remain thermodynamically stable, regardless of the mode of preparation. The ORNL-led study focused on alloys formed by casting rather than alternative processing methods due to the application versatility arising from their ability to adopt a greater range and complexity of structures.
The team characterized and correlated the composition, microstructure and mechanical behavior of cast Al-Ce-based alloys. They found that the strong reaction affinity between the lanthanide (the fifteen metallic chemical elements with atomic numbers 57 through 71; cerium is the second element of the lanthanide series), aluminum, and alloying elements provide large chemical driving forces that activate potent strengthening mechanisms which remain thermodynamically stable at elevated temperatures.
The Al-Ce base alloy forms architectures that are inherently stable at elevated temperatures and under load. We found that adding minute amounts of other elements, such as 0.4 percent weight magnesium, achieved significant increases in alloy strength.—ORNL’s Orlando Rios, whose team also experimented with the alloy’s composition
In addition to the highly desirable attributes of the Al-Ce alloys—high ductility, robust room-temperature mechanical properties, exceptional high-temperature mechanical property retention, high tolerance to casting defects, and excellent castability across a broad range of compositions—there is an economic benefit as well. Cerium metal is highly available and low cost.
Given the high availability and low cost of cerium metal, these alloys are economically viable for large volume industries such as the transportation sector, where their properties make them ideally suited for vehicle lightweighting. Elimination or reduction of heat-treatment amplifies the economic and environmental benefits of lightweighting in the transportation sectors.
Adoption of these alloys by industry will not only impact current technologies, but will provide the basis with which to develop the next generation of high temperature aluminum alloys. Finally, by creating demand for Ce, which is overproduced, the economics of rare-earth mining improve. In a typical deposit, one-third to one-half of the rare-earth content by weight is cerium, so converting a by-product into a co-product will help stabilize global production and encourage diversification of the rare-earth supply chain.—Sims et al.
The research was conducted through the Critical Materials Institute, which has been developing high-value applications for cerium to boost the economics of mining rare-earth materials.
Zachary C. Sims, Orlando R. Rios, David Weiss, Patrice E. A. Turchi, Aurelien Perron, Jonathan R. I. Lee, Tian T. Li, Joshua A. Hammons, Michael Bagge-Hansen, Trevor M. Willey, Ke An, Yan Chen, Alex H. King and Scott K. McCall (2017) “High performance aluminum–cerium alloys for high-temperature applications” Mater. Horiz. 4, 1070-1078 doi: 10.1039/C7MH00391A