AMORPHOUS alloys formed by rapid solidification of a metallic melt are of considerable technological interest as high-strength materials1–8. As they are not in thermodynamic equilibrium, these materials tend to crystallize on heating9,10. A high degree of crystallization leads to embrittlement, but if it can be arrested when the crystallites are of only nanometre dimensions, the resulting amorphous–nanocrystalline composite actually has greater strength than the original amorphous material11. There is consequently much interest in understanding the mechanisms of crystallization. Previous studies have suggested that mechanical deformation can induce crystallization12–16. Here we report the direct observation of crystallization within the shear bands of aluminium-based amorphous alloys induced by bending. The crystals are face-centred cubic aluminium, 7–10 nm in diameter, and seem to form as a consequence of local atomic rearrangements in regions of high plastic strain. We suggest that mechanical deformation might therefore be used to form high-strength amorphous–nanocrystalline composites.

Deformation-induced nanocrystal formation in shear bands of amorphous alloys

The effect of compression and tension on shear-band structure and nanocrystallization in amorphous Al90Fe5Gd5: a high-resolution transmission electron microscopy study

Mechanical properties of partially crystallized aluminum based metallic glasses

Various experimental techniques have been used to investigate the effect of mechanical milling on the structural stability of rapidly solidified aluminum-based metallic glasses. Using transmission electron microscopy (TEM) and X-ray diffraction methods, the formation of nanocrystalline Al particles in some ball-milled Al-rich metallic glasses (such as Al90Fe5Gd5 and Al90Fe5Ce5) is clearly observed. For other compositions with lower Al concentration such as Al85Ni5Y10, no such phase transformation can be detected by TEM or X-ray. However, differential scanning calorimetry (DSC) measurements show that the crystallization peaks of the ball-milled Al85Ni5Y10 metallic glass shifted to higher temperatures, while the crystallization enthalpy associated with the first exothermic peak decreased to a lower value, revealing that certain structural changes have taken place as a result of mechanical deformation. The compositional dependence of the structural stability of Al-based metallic glasses against mechanical deformation suggests that the nanocrystal formation induced by a deformation process is different from that caused by a thermal process. The large plastic strain induced atomic displacements and the enhancement of atomic mobility during the deformation process, are the possible mechanisms of mechanical deformation-induced crystallization. Our results demonstrate a new way of obtaining nanophase glassy composite alloy powders which are suitable for engineering applications upon further consolidation processing.

S.J. Poon

Papers

Formation and stability of aluminum-based metallic glasses in Al-Fe-Gd alloys

Mechanical properties of aluminum-rich AlFeGd metallic glass