Locating bulk metallic glasses with high fracture toughness: Chemical effects and composition optimization

We have developed new bulk metallic glasses (BMGs) with optimized glass-forming ability (GFA), in ternary Zr-Cu-Al and quaternary Zr-Ti-Cu-Al systems. The relatively large dimensions of these BMGs permitted fracture toughness tests, leading to the discovery of Zr₆₁Ti ₂Cu₂₅Al₁₂ (ZT1) that has a toughness among the highest for monolithic BMGs (fatigue pre-cracked fracture toughness, K _max, in excess of ∼100 MPa√m, with a nonlinear-elastic fracture behavior as seen in the resistance curve). We have also measured the toughness for Zr-TM-Al (TM = Co, Ni) BMGs, which have previously been optimized for GFA. Comparing the BMGs studied, in conjunction with literature data on Zr-based BMGs, we have identified trends in chemistry effects on BMG fracture toughness, in particular the role of Al as well as the effects of Cu when substituting Ni (or Co). These trends are explained in terms of not only the internal atomic packing structure, but more importantly the electronic structure and nature of bonding in light of the first-principles calculations. A strategy is then outlined to locate BMG compositions with high toughness. The correlations of the BMG toughness with the shear banding behavior, the Poisson's ratio (ν), the product of shear modulus and molar volume (μV _m), and the glass transition temperature (T_g), are also discussed.