Stereophotogrammetric investigation of overload and cyclic fatigue fracture surface morphologies in a Zr-Ti-Ni-Cu-Be bul
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lbert and V. Schroeder Department of Materials Science and Mineral Engineering, University of California, Berkeley, California 94720-1760
R. Pippan Austrian Academy of Sciences, Erich-Schmid-Institut fu¨r Materialwissenschaft, Leoben, Austria
R.O. Ritchie Department of Materials Science and Mineral Engineering, University of California, Berkeley, California 94720-1760 (Received 2 September 1999; accepted 28 January 2000)
Fracture surfaces of a recently developed Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 (at.%) bulk metallic glass were investigated using a three-dimensional surface reconstruction technique. Stereoscopic scanning electron microscopy of both fatigue and overload fracture surfaces permitted the creation of digital elevation models that were used to quantify important fracture surface features. Characterization of the surfaces revealed striations of nearly constant spacing on fatigue surfaces and a vein morphology characteristic in amorphous metals on the overload fracture surfaces. Additionally, at the onset of critical failure, crack-tip openings of ∼16 m were observed that were consistent with measured values of fracture toughness. Interestingly, at the onset of fracture, deformation was confined to one side of the fracture plane, possibly because of the asymmetric emission of shear bands from the crack tip, consistent with the highly inhomogeneous nature of deformation in this alloy.
I. INTRODUCTION
The recent discovery of a number of highly glassforming alloy groups (or bulk metallic glass) has rekindled interest in amorphous metals.1– 4 Low critical cooling rates of 60 MPa√m at a loading rate, K˙ , of ∼ 0.01 MPa√m/s, producing the overload frac-
ture surface on the right). Surface profiles [Fig. 5(d)] were extracted from the digital elevation model (Fig. 6), permitting the determination of the crack-tip opening displacement, ␦. Specifically, the profile on one specimen half was positioned relative to the profile on the other specimen half by three conjugate points on each side of the fracture surface [marked by arrows in Figs. 5(a) and 5(b)]. The vertical distance (or y-axis separation) between the points on either side of the onset of overload fracture [Fig. 5(d)] yielded the value of ␦ at this critical condition. The crack-tip opening displacement values, ␦, at fracture ranged from 15 to 19 m, with a mean value of 16.5 m. An estimate for the fracture toughness can be obtained by assuming the dominance of a Hutchinson–Rice– Rosengren singularity in the crack-tip region. Under small-scale yielding conditions the applied stress intensity, K, is related to ␦ by the following equation:24 ␦ = dn
K2 , E0
(2)
where E ∼ 90 GPa, 0 is the flow stress (∼2 GPa), and (for a given state of plane stress or plane strain) dn is a constant dependent on the work-hardening coefficient and yield strain. For elastic-perfectly plastic constitutive behavior with a yield strain corresponding to this mate-
FIG. 3. Scanning electron micrographs of matching fracture surfaces [S1 in (a) and S2 in (b)] developed during fati
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