Elevating the fracture toughness of Cu49Hf42Al9 bulk metallic glass: Effects of cooling rate and frozen-in excess volume

A major challenge for the structural applications of bulk metallic glasses (BMGs) is to improve their fracture toughness. Here we demonstrate that by increasing the cooling rate during the casting of liquid Cu49Hf42Al9 into BMG, using a mixed argon and helium atmosphere, the notch toughness of the resultant BMG can be tripled relative to that obtained at slower cooling rates. The much elevated toughness is attributed to a ten-fold increase in the size of the plastic zone at crack tip, due to the proliferation of shear banding facilitated by enhanced propensity for shear transformations. The latter propensity is explained by the reduced shear modulus and microhardness, as well as increased enthalpy recovery, all of which are rooted in structural disorder as reflected by the lowered density and increased frozen-in excess volume. Such a structure-property correlation is systematically demonstrated by monitoring all these properties over a range of diameters of the as-cast BMG rods that correspond to cooling rate levels from 40 K/s to 103 K/s.