Nanocrystallized Al₉₂Sm₈ Amorphous Alloy Investigated by High‐Resolution Microscopy and 3D Atom‐Probe Analysis

Abstract

The discovery of Al-based metallic glasses has generated fundamental and practical interest since these amorphous alloys crystallize to a nanostructured composite containing face-centred-cubic (fcc) nanometre-sized aluminium crystallites (diameter < 20-30 nm) embedded in an amorphous matrix. [1,2] They show remarkable mechanical properties, potentially useful for engineering applications. [3][4][5][6] For example, the nanocomposite aluminium alloys produced by rapid solidification and subsequent annealing of Al-TM-RE alloys (TM: transition metal; RE: rare earth) show ultra-high tensile fracture strength: for the Al 88 Ni 9 Fe 1 Ce 2 nanophase composite, the strength at room temperature is as high as 1.56 GPa far above the highest strength of conventional wrought aluminium alloys. [5] The properties are strongly related to the volume fraction of the nanocrystallized fcc-Al particles in the amorphous matrix. We have previously presented our investigations [7][8][9] concerning the glass-forming ability, nanocrystallization and the mechanical properties (hardness and wear resistance) of alloys prepared within the Al-Ni-Sm system. Because of the excellent mechanical properties, enhanced by a nanocrystallite dispersion, one strategy has been to control the primary crystallization (devitrification) by annealing the fully amorphous as-quenched state. [10] The nanocomposites obtained by devitrification of amorphous Al 88 Ni 4 Sm 8 were found to provide very stable materials with rather good ductility in the temperature range corresponding to the primary crystallization. In this alloy, a high density of nanocrystallites with a dendritic-like shape of about 20-30 nm was observed uniformly distributed in the remaining amorphous matrix. [11] A very similar observation on the Al 92 Sm 8 alloy showed the density of nanoparticles crystallized under thermal treatment to be ∼ 10 21 m -3 . [12] Several proposals involving solute effects, phase separation or quenched-in nuclei and heterogeneous nucleation have been advanced to account for the high nanocrystal density. [13][14][15][16] The strengthening in such materials has been explained in terms of the defect-free nanoscale Al particles (which would have higher flow stress than the amorphous phase) homogeneously dispersed in the amorphous matrix acting as effective barriers to the shear banding of the matrix. [17] However, it was also suggested that the hardening effect could be attributed to solute enrichment of the remaining glassy matrix and not exclusively due to the presence of the nanoparticles themselves. [6,18] Unfortunately, the rather good room-temperature bending ductility of the fully amorphous as-quenched state is progressively reduced by annealing into the nanocrystalline state. Plasticity is very important for many shaping and forming operations and for avoiding catastrophic failure in load-bearing applications. Although these nanostructured alloys possess exceptionally high strength, a compromise must be found in the development of such materials for engineering ap...