Fatigue properties in high strength bulk metallic glasses

Fatigue tests were carried out on ultra-high strength Co- and Fe-based bulk metallic glasses (BMGs), and the fatigue results were investigated together with the ones which had already been obtained from the fatigue tests on the super-high strength Ni-based BMG and the high strength Ti- and Cu-based BMGs. The test alloy rods with a diameter of 2 mm were prepared in [(Co 0.6 Fe 0.4 ) 0.75 B 0.2 Si 0.05 ] 96 Nb 4 , (Fe 0.5 Co 0.5 ) 72 B 20 Si 4 Nb 4 , Ti 41.5 Zr 2.5 Hf 5 Cu 42.5 Ni 7.5 Si 1 , Ni 60 Zr 20 Nb 15 Al 5 and Cu 60 Zr 30 Ti 10 at% systems by the copper mold casting method. The test specimens were machined to hourglass shape type with a minimum diameter of 0.9 mm. The specimens were tested at a stress ratio of 0.1 and a frequency of 10 Hz. The fatigue limits in the Co- and Fe-based BMGs exceeded 2 GPa, and those in the Ni- and Ti-based BMG surpassed about 1.5 GPa. Fatigue ratios in all BMGs were found to be about 0.5. Exceptionally, only the Ti-based BMG showed the high ratio, which may indicate the possibility that the BMG does not cause the fatigue phenomenon. In front of the fatigue crack initiation sites, fracture surfaces showed a radial ridges and valleys morphology as well as a striation like pattern. Unstable fracture surfaces consisted of facets with vein patterns. As the facet and vein pattern sizes increase, the strength and fatigue limit decreased, whereas the fracture toughness increased. Fracture surfaces in front of the fatigue cracks in strong BMGs showed the clear elastic wave traces.