Bulk metallic glasses (BMG) can exhibit favorable properties relative to traditional crystalline metals such as near theoretical strength coupled with metal like fracture toughness values. However, BMGs also exhibit unique characteristics such as 1) fatigue endurance strengths that can range widely for different compositions with similar tensile strength and 2) ductility and strain softening/hardening behavior that is dependent on both sample size and loading configuration. In order to expand the application of BMGs into more structural and mechanical components designers need to ensure the components will not fail unexpectedly. To achieve this a detailed understanding of the unique fracture and fatigue characteristics of BMGs is needed. The BMG with composition Zr52.5Cu17.9Ni14.6Al10Ti5 exhibits excellent fatigue endurance strength in four point bending with a ratio of 107 cycle endurance strength to ultimate tensile strength of ~0.25, although small amounts of casting porosity can reduce the endurance limit by ~60%. This behavior was attributed to a relatively high fatigue threshold (ΔKTH ≈ 2 MPa√m) and an insensitivity to degrading environmental effects in ambient air compared to other Zr-based BMGs that exhibit very poor fatigue endurance strengths. Also, other factors found to influence the fatigue response include structural relaxation and residual stresses. Next, the issue of size dependent fracture behavior was also examined for the Zr52.5Cu17.9Ni14.6Al10Ti5 BMG and it was found that when sample size requirements of ASTM standard E399 were met the fracture toughness values are sample size and geometry independent. However, if ASTM standard E1820 is applied to allow smaller sample dimensions the results become sample size dependent in a manner that is distinct from crystalline metals.

 

Research has been funded in part by the Alexander von Humboldt Foundation Friedrich Wilhelm Bessel Research Award and the Arthur E. Hitsman Faculty Scholarship