A recent article in Communications Materials proposed a novel approach to enhance the elastic modulus of a cast aluminum alloy by incorporating isostructural Laves phases to form a multi-component, high-symmetry, isotropic phase. The structure and mechanical properties of this rhombicuboctahedron (RCO) phase were analyzed using flux-grown single crystals.
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Background
Aluminum-rare earth element (REE) alloys are known for their high yield strength, tensile strength, and strength retention at room temperature and elevated temperatures. For instance, Al-Ce alloys leverage the high reactivity of Ce to form strengthening intermetallics. However, despite REE additions, these alloys typically show no increase in elastic modulus.
Al-Ce intermetallics usually have anisotropic crystal structures and tend to precipitate in high aspect ratio morphologies. This complicates traditional Al alloying due to the presence of stable ternary and quaternary Al-Ce-X phases. Low-symmetry intermetallic morphologies, such as needle-like and plate-like structures, often result in weak interfaces, low dislocation densities, and limited slip systems, which hinder performance.
In contrast, high-symmetry, isotropic cubic phases with strong mechanical properties can strengthen Al alloys without secondary processing. This study aimed to design an isotropic intermetallic phase that can be easily incorporated into traditional casting techniques.
Methods
The alloy design method combined the components of isostructural Fd-3m (cubic Laves) compounds in CeNi, CuMg, and AlMnCe alloys to create an isotropic Al-REE-based high symmetry phase.
The cast alloy samples were characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Single crystals of the cubic Al-Ce-Ni-Cu-Mn phase were grown through gradually cooling high-temperature aluminum melt.
The master alloy had a composition of Al53.7 %-Ce18.14 %-Cu19.08 %-Ni7.44 %-Mn2.86 %, while the single crystals were grown using additional aluminum metal (Al72.38 %-Ce10.74 %-Cu10.75 %-Ni4.43 %-Mn1.70 %). Additionally, crystals of CeAlSi were grown from an aluminum-rich melt with a composition of Al86.99 %-Ce5.01 %-Si8.00 %.
X-ray diffraction (XRD) patterns were used to determine the crystal structures, refined with FullProf software. Neutron powder diffraction was conducted using a nanoscale-ordered materials diffractometer (NOMAD), followed by co-refinement of the XRD and neutron patterns with FullProf software.
Further characterization of the cast specimens was performed using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Nanomechanical properties were assessed through the continuous stiffness measurement (CSM) nanoindentation method.
Thermal properties were analyzed using differential scanning calorimetry (DSC), and mechanical properties (elastic modulus) were determined through compression and tensile tests. In-situ high-energy synchrotron X-ray diffraction experiments were carried out during tensile loading.
Results and Discussion
XRD studies confirmed the RCO phase structure (Pm-3m cubic space group) of the fabricated alloy, which was analogous to an AlCuCeMn phase. Additionally, there are three main phases, including Al-FCC at 74(2)wt.%, Al8Cu4Ce at 6(1)wt.%, and RCO at 21(1)wt.%, were revealed in the cast alloy. Notably, the co-refinement showed the composition of the RCO phase was estimated to be Al25.3Ce3Cu3.6Ni3.1Mn.
STEM and EDS measurements verified that the refined structure of the single crystal matched the RCO phase in the alloy. The measured lattice mismatch between the (400)RCO and (200)Al planes was only 2 %. The cast aluminum alloy containing the RCO phase exhibited an average modulus of 91.5 ± 7.4 GPa in tensile tests, which is the highest value reported for a cast aluminum alloy with the RCO phase without using beryllium or other processing methods.
All specimens displayed predominantly brittle failure with limited ductility. While the aluminum matrix exhibited ductile dimpling, indicating plastic deformation, the RCO grains showed brittle fracture with radial marks, indicating effective stress transfer and crack propagation through the RCO crystals. This behavior demonstrated a robust interface between the phases.
The alloy retained 94 % of its compressive strength at 100 °C and 60 % at 200 °C. The reduced strength at 200 °C was attributed to Cu precipitate coarsening, which allowed the Al-FCC matrix to flow more freely around the RCO phase.
Nanomechanical mapping of SEM images and EDS maps showed that the modulus of the RCO phase exceeded that of the Al matrix and other Al-Ce-X intermetallics. The average modulus from nanomechanical mapping was approximately 89 GPa, which is only 5 % different from the tensile testing measurements of the alloy.
Conclusion
The study successfully demonstrated that incorporating the RCO phase enhances the elastic modulus of Al alloys. This improvement is due to the presence of spherical, isotropic, high-modulus particles with a strong, strain-sharing interface with the matrix. A multi-component, isostructural alloy design method was used to cast a seven-element alloy, creating an Al-rich cubic phase with RCO crystals during primary solidification.
Modulus enhancement was achieved by exploiting load sharing during elastic deformation through a robust strain-sharing interface between the RCO phase and the Al matrix. This method provides control over phase size, morphology, and volume fraction using casting and solidification techniques.
Journal Reference
Neveau, M. L. et al. (2024). Secondary phase increases the elastic modulus of a cast aluminum-cerium alloy. Communications Materials, 5(1). DOI: 10.1038/s43246-024-00611-3, https://www.nature.com/articles/s43246-024-00611-3
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