Glassy metals are tougher than steel?

Researchers in the US have developed a class of materials that have the potential to be tougher than the best titanium or steel alloys, ranking them among the toughest known

Bulk metallic glasses (BMGs) have shown promise in engineering for the past 15 years but have been let down by poor ductility, making them brittle and prone to fracture. But Douglas Hofmann, William Johnson and colleagues from the California Institute of Technology in Pasadena have found a way of modifying the structure of BMGs so that their ductility increases significantly.

BMGs are like metals in that they contain metallic bonds and conduct, yet their atoms are disordered like glass. A disordered structure cannot contain defects, which means that BMGs should withstand high loads before fracturing. This is as least true when they are bent or compressed, but when they are pulled their poor ductility fails them — parts of the materials begin to slide past each other in “shear bands”, which grow quickly into cracks.

One way of improving ductility and avoiding cracks, which has been adopted by the California team, is to introduce elements in the hot, initial BMG alloy that will nucleate dispersed crystals or “dendrites” when finally cooled. These dendrites act as a barrier to growing shear bands, preventing them from every developing into cracks. Instead, pulling the composite BMG will simply produce more, small shear bands, allowing the material to stretch and withstand a greater load.