Newly Discovered Ultrahigh Strength Solid Solution (Al)—Intermetallic (Al_xGe_y) Metastable Nano-Eutectics

Metastability-aided designing, which is either circumstantially induced during synthesis or purposefully engineered, has significant impacts in invigorating the performance of structural materials - mainly metals and alloys. Laser rapid solidification of cast Aluminum (Al) – Germanium (Ge) eutectic alloy is shown to produce ultrafine lamellar eutectics with interlamellar spacing refined up to ~60 nm and composed of FCC Al-rich solid solution and unusual Al_xGe_y intermetallic-rich phases that do not form during near-equilibrium solidification. The crystallographic attributes of the intermetallics are characterized in detail using a combination of selected area electron diffraction (SAED), high-resolution scanning transmission electron microscopy (HR-STEM), energy dispersive X-ray spectroscopy (EDX) to obtain high-resolution elemental maps, and atomistic modeling using density functional theory followed by atomic-scale image simulation. Depending on the undercooling and cooling rate imparted during laser processing, the crystal structure of the Al_xGe_y intermetallic phases was either monoclinic (C 2/c) or monoclinic (P 2₁), with high densities of rapid quenching instigated defects, such as dislocations and planar faults. The phase evolution after laser processing is in sharp contrast to the as-cast alloys that exhibited nominally pure Al and Ge phases with prominent solute partitioning and FCC and diamond cubic crystal structures of those phases, respectively. The mechanism for the formation of these metastable eutectics under rapid solidification have been revealed as greater solidification velocity than atomic diffusion velocity in a complete solute trapping condition that results in significant entrapment of Al atoms in Ge and the corresponding kinetic phase diagrams are proposed to interpret the metastable phase equilibria, to understand the evolution of nano-lamellar eutectic morphologies with equilibrium Al and metastable Al_xGe_y phases, and to explain compositional metastability in the Al phases manifested by precipitation of ultrafine clusters of Ge. These metastable nano-eutectics when subjected to micromechanical testing exhibit extraordinary compressive strength of up to 1.2 GPa with a stable plastic flow up to 14% plastic strain. Deformed microstructures have been thoroughly investigated to establish the deformation mechanisms in these microstructures. Primary strengthening comes from (i) stacking faults that nucleate in the Al-rich phases in presence of coherent Al(Ge) precipitates and from (ii) arrays of single dislocations that form locally to give rise to confined Al-layer slip owing to the interfaces across soft Al and extremely hard intermetallic phases. The monoclinic intermetallics are hard to deform plastically because of inadequacy of active slip systems and cracked easily beyond a certain strain. However, significant detwinning of the process-induced twins in one of the intermetallic phases contribute to prolonged softening during deformation, unlike the other intermetallic that did not produce solidification instigated twins. The findings of this work help understand how structural and compositional metastability in eutectics with ultrafine length-scale can result in extraordinary mechanical properties with unusual deformation mechanisms.