By controlling the specimen aspect ratio and strain rate, compressive strains as high as 80% were obtained in an otherwise brittle metallic glass. Physical and mechanical properties were measured after deformation, and a systematic strain-induced softening was observed which contrasts sharply with the hardening typically observed in crystalline metals. If the deformed glass is treated as a composite of hard amorphous grains surrounded by soft shear-band boundaries, analogous to nanocrystalline materials that exhibit inverse Hall-Petch behavior, the correct functional form for the dependence of hardness on shear-band spacing is obtained. Deformation-induced softening leads naturally to shear localization and brittle fracture.

Softening caused by profuse shear banding in a bulk metallic glass.

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.

Ball milling-induced nanocrystal formation in aluminum-based metallic glasses

The atomic structure of liquid-quenched amorphous Al{sub 90}Fe{sub {ital x}}Ce{sub 10{minus}}{sub {ital x}} ({ital x}=5, 7) was studied by pulsed neutron and x-ray scattering. The atomic pair--density function determined by pulsed neutron diffraction indicates that a significant portion of Al--Fe distances are anomalously short, while some part of Al--Al distances are anomalously long. Both neutron and x-ray scattering showed the presence of a prepeak in the structure factor. These results suggest that a strong interaction between Al and Fe modifies the structure of this glass, leading to chemical and topological short-range ordering.

Atomic structure of amorphous Al90FexCe10−x

Hardness and shear band evolution in bulk metallic glasses after plastic deformation and annealing

The mechanical properties (i.e., tensile fracture strength and Young’s modulus) of eight different alloys of a new class of metallic glasses containing up to 90 at. % aluminum are reported along with crystallization temperatures of these alloys. The Al90Fe5Ce5 material has a tensile fracture strength of 940 MPa (1 MPa=145 psi), while two others (Al87Fe8.7Gd4.3 and Al87Ni8.7Y4.3) exceed 800 MPa. Young’s modulus measurements for three of these exceed 60 GPa with a high value of 66 GPa for the Al90Fe5Ce5 glass. These unusually high strengths of the aluminum glasses can be of significant importance in obtaining high‐strength, low‐density materials.

S. J. Poon

Papers

Mechanical properties of a new class of metallic glasses based on aluminum