Adding a new flexibility to metallic glasses

Mechanical behavior

March 7, 2008

Strips of the new BMG composites >3 mm thick can be bent to a 90° angle or more. Standard BMGs can normally only be bent with a thickness <1 mm. (Courtesy of Douglas C. Hofmann.)

California Institute of Technology researchers have made designer bulk metallic glass (BMG) materials with massively increased tensile ductilities [Hofmann et al., Nature (2008) 451, 1085].

“BMGs are known for their high strengths but their use in practical engineering applications has been limited, owing to brittle failure during unconfined loading,” explains Douglas C. Hofmann.

The researchers overcome the brittle failure of BMG materials in tension by adding crystalline inclusions into the metallic glass. This more ductile phase reinforces the BMG in the same way as plastics that are toughened by adding soft rubber particles.

“Tensile ductility is increased typically to >10%, and fracture toughness is increased to an extent where the new BMG matrix composites are comparable to common engineering Ti alloys and steels,” says Hofmann.

By understanding the mechanisms that lead to ductility in BMGs, the researchers were able to calculate the size of inclusions needed to give the best fracture toughness.

“The fracture toughness of a BMG is related to the size of the plastic zone in front of a crack tip, which is in turn related to the critical size of a shear band before it develops into a crack,” explains Hofmann. If the inclusions, which grow as dendrites in the BMG alloys, match this critical size they can suppress shear band opening and avoid crack development.

The Caltech group created three Zr-Ti-Nb-Cu-Be alloys using semi-solid processing. The alloys were heated between the solidus and liquidus temperatures to allow the dendritic phase to coarsen, before quenching to vitrify the glass matrix. This provided the necessary microstructure for a tough BMG composite material.

“Working closely with our collaborators in industry, we are currently developing the technology to commercially manufacture these alloys,” Hofmann adds.

Jonathan Wood