Fatigue strength in nanocrystalline Ti- and Cu-based bulk metallic glasses

Fatigue tests were carried out on nanocrystalline Ti- and Cu-based bulk metallic glasses (BMGs) for which there are still no reports on fatigue strength. Test alloy rods with a diameter of 2 mm were prepared in nanocrystalline Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 at% and Cu60Zr30Ti10 at% systems by copper mold casting. Nanocrystals were dispersed in the metallic glassy phase in both BMGs. The tensile strength (sigma(B)) in the nanocrystalline Ti- and Cu-based BMGs were 2.04 GPa and 2.00 GPa, and Young's modulus were 95 GPa and 114 GPa, respectively. The test specimens were machined to hourglass shape type (the radius in axial direction; 4.45 mm, the minimum diameter; 0.9 mm), and after machining the specimen surfaces were electropolished by 50 similar to 100 mu m. The specimens were tested at a stress ratio of 0.1 and a frequency of 5-8 Hz. The fatigue limit (sigma(w)) (half a total stress amplitude) and fatigue ratio (sigma(w)/sigma(B)) in the nanocrystalline Ti- and Cu-based BMGs showed very large values (Ti-based alloy; 806 MPa and 0.40, Cu-based alloy; 490 MPa and 0.25). This experimental result indicated a possibility that the nanocrystal dispersed BMGs have higher fatigue strength than the high strength crystalline alloys with high fatigue strength (e.g. Cr-Mo steel, sigma(w) approximate to 500 MPa).