Abstract:
The effect of ambient environment on the fatigue behavior of two Zr-Ti-Cu-Ni-Be bulk metallic glasses (BMGs) is examined in the present study. The first metallic glass, Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅, was tested to establish the presence of an environmental effect in the fatigue growth curve measured in ambient air. Fatigue growth curves produced from Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅ in ambient environment were shown to exhibit a plateau where the fatigue growth rate was independent of stress intensity range near 10⁻⁹ m/cycle, which are typically characteristic of aggressive environment, affecting crack growth. Experiments performed in inert atmosphere (dry N₂) demonstrated a higher threshold stress intensity (ΔK[subscript TH]), near-threshold crack growth rates decreased significantly, and no stress intensity independent plateau was measured. It was therefore confirmed that the fatigue behavior of Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅ is significantly affected by aggressive ambient air environment. Both oxidation and hydrogen embrittlement processes are discussed as possible causes for the observed environmental effect. An initial study of the fatigue properties of the second metallic glass, Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈, was performed. A fatigue growth curve was produced in ambient atmosphere and a plateau in the fatigue growth curve near 2.5 x 10⁻⁸ m/cycle was measured. In the plateau produced by Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈ the growth rate was independent of stress intensity, like in the fatigue curve measured from Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅ under similar conditions. It is speculated that Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈ is susceptible to an environmental species more diffusive in Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈ than that found to affect Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅, because the characteristic plateau occurs at a higher growth rate. The plane strain fracture toughness (K[subscript C]) was also measured for Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈ and found to equal ~26 MPa√m. However, crack branching was discovered to have occurred during each K[subscript C] measurement, making the accuracy of the measurements difficult to ascertain.
