Making metallic glasses less brittle at room temperature could allow their use as strong, springy alloys for biomedical implants and structural materials. A team of researchers led by Zhaoping Lu from the University of Science and Technology in Beijing, China1, have developed a metallic glass composite with high ductility that can be transformed into an extremely durable material by pulling it apart under controlled tensile strain — a process known as work-hardening.

“Stress-induced transformations have been observed in metallic glasses under compression, but not under tensile deformation,” says Lu. “Work-hardening capabilities and high tensile ductility are critical for applications involving these alloys.”

Metallic glasses derive their incredible resilience from a non-periodic structural arrangement of metal atoms that does not contain the packing defects usually seen in metal crystals. However, because the atoms within metallic glasses are connected randomly, these materials normally fail catastrophically and snap apart under tension, limiting their engineering applications.

Lu and his colleagues set out to improve the ductile behavior of metallic glasses by developing a composite alloy of copper and zirconium (Cu–Zr) crystallites embedded homogenously in a non-crystalline, glassy matrix. After casting this material into millimeter-sized rods, the researchers conducted tensile tests by straining the samples lengthwise at controlled rates. Instead of fracturing catastrophically, the composite metallic glass deformed and stretched non-reversibly. This action was accompanied by a significant increase in the engineering stress — that is, the metallic glass became tougher.

Fig. 1: Scanning electron microscopy image of a copper–zirconium crystal (oval shape, center) embedded within a metallic glass that has been stretched into a much harder structural arrangement by tensile strain experiments.© 2010 Z. Lu

X-ray diffraction experiments revealed that the tensile experiments transformed the Cu–Zr crystals into a new structural arrangement known as a martensite phase, which prevents the metal atoms from moving easily. The team also observed that the initially spherical Cu–Zr crystals had been stretched into oval shapes by work-hardening, and that mechanical shearing was limited to the surrounding glassy matrix (Fig. 1).

“The concept applied in this work can be also utilized to promote ductility and toughness in other metallic glass systems,” says Lu. “This could be a route for developing metallic glasses with improved mechanical properties into practical engineering materials.”