Abstract: Laser processing of Al-Ge eutectic alloys is used to produce two different metastable intermetallic-matrix nano-scale lamellar composite microstructures: (i) β₁ (monoclinic) Al₆Ge₅ (60 vol%)-α (FCC) Al (40 vol%), and (ii) β₂ (monoclinic) AlGe (67 vol%)-α (FCC) Al (33 vol%). Nanoindentation and micropillar compression tests were performed to characterize mechanical behavior and compare with the equilibrium structure of as-cast diamond cubic Ge (42 vol%)-FCC Al (58 vol%) micrometer-scale eutectic composite. The as-processed and deformed microstructures were characterized by scanning/transmission electron microscopy. Both β₁-α and β₂-α eutectics exhibit high compressive flow strengths of ≈ 1 GPa and 1.2 GPa respectively, whereas the maximum compressive flow strength of Al-Ge eutectic is about 450 MPa. In spite of complex monoclinic structures and higher volume fraction of intermetallic phase as compared to metallic, both β₁-α and β₂-α composite microstructures have higher level of plastic deformability than as-cast Al-Ge. The microstructure with the highest strength, β₂-α, also exhibits the highest plastic strain to failure attributed to a gradual strain softening behavior due to interaction of micro shear cracks with nano-twins in the β₂ (monoclinic) AlGe phase, whereas the β₁ (monoclinic) Al₆Ge₅ phase without the nano-twins exhibits sudden fracture by a sharp crack. In the Al-rich α phases in both β₁-α and β₂-α microstructures, nanoscale Ge-rich clusters were observed that led to profuse stacking faults post deformation. Density functional theory calculations suggest that Ge solutes can lower stacking fault energy of FCC Al, thereby promoting partial dislocation glide in Al. This study highlights unusual mechanisms that impart plastic deformability at ultra-high yield strengths in intermetallic-metal composites with a low-symmetry intermetallic matrix phase.

Key words: Eutectic nano-composites, Metastable intermetallic phases, Nano-twins, Dislocations, Scanning transmission electron microscopy, Density functional theory