  We report a wavelike fracture pattern in a Zr-based bulk metallic glass that
has been deformed under quasi-static uniaxial tensions at temperatures between
room temperature (300 K) and liquid nitrogen temperature (77 K). We attribute
this wavelike pattern to a Kelvin-Helmholtz flow instability that occurred at
certain interfaces between local cracking/softening regions. The instability
criterion for the pattern formation is achieved via a hydrodynamic perturbation
analysis, and furthermore an instability map is built which demonstrates that
the shear velocity difference on both sides of the interface is the main
destabilizing factor. Finally, the characteristic instability time (the inverse
of the instability growth rate) is explored by seeking the dispersion relation
in the dominant (fastest) instability mode. The results increase the
understanding of the flow and fracture of metallic glasses as well as the
nature of their liquid structures.
