Effect Of Ambient Environment On The Fracture And Fatigue Properties Of Zr Based Bulk Metallic Glasses

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 ([Delta]K[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[square root]m. However, crack branching was discovered to have occurred during each K[subscript C] measurement, making the accuracy of the measurements difficult to ascertain.