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 Ti _41.5Zr _2.5Hf ₅Cu _42.5Ni _7.5Si ₁ at% and Cu ₆₀Zr ₃₀Ti ₁₀ at% systems by copper mold casting. Nanocrystals were dispersed in the metallic glassy phase in both BMGs. The tensile strength (σ _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 50-100 μm. The specimens were tested at a stress ratio of 0.1 and a frequency of 5-8 Hz. The fatigue limit (σ _w) (half a total stress amplitude) and fatigue ratio (σ _w/σ _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, σ _w≈500 MPa).