Confining cracks in metallic glass

Lightness, strength, and moldability are among the most desired material properties for aircraft, sporting equipment, and many structural applications. Those sometimes opposing properties converge in bulk metallic glasses—supercooled amorphous metal alloys that can be cast into complex shapes and are resilient under large elastic strains. However, their toughness is suspect: Under repeated stress, BMGs fatigue and develop fatal cracks much more quickly than crystalline metal alloys do. To control crack propagation, Caltech’s William Johnson, Lawrence Berkeley National Laboratory’s Robert Ritchie, and their collaborators focused on controlling the microstructure of a particularly tough BMG composite made of zirconium, titanium, and other metals. Its fingerlike crystalline dendrites (67% by volume) are surrounded by an amorphous matrix, as seen in this optical micrograph. By heating the precursor alloys between their melting points then rapidly quenching the solution, the researchers were able to control dendrite size and the spacing between the glassy and crystalline phases. The width of the glassy region between the much-tougher dendrite fingers was tailored to be short enough to serve as a “microstructural arrest barrier” for just-formed cracks. Compared with existing dendrite-containing BMGs, the new material holds up under three times more stress cycles and is comparable in toughness to high-strength steel or aluminum. (M. E. Launey et al., Proc. Natl. Acad. Sci. USA 106 , 4986, 2009. http://dx.doi.org/10.1073/pnas.0900740106 )