  We report results of statistical and dynamic analysis of the serrated
stress-time curves obtained from compressive constant strain-rate tests on two
metallic glass samples with different ductility levels in an effort to extract
hidden information in the seemingly irregular serrations. Two distinct types of
dynamics are detected in these two alloy samples. The stress-strain curve
corresponding to the less ductile $Zr_{65}Cu_{15}Ni_{10}Al_{10}$ alloy is shown
to exhibit finite correlation dimension and a positive Lyapunov exponent,
suggesting that the underlying dynamics is chaotic. In contrast, for the more
ductile $Cu_{47.5}Zr_{47.5}Al_{5}$ alloy, the distributions of stress drop
magnitudes and their time durations obey a power law scaling reminiscent of a
self-organized critical state. The exponents also satisfy the scaling relation
compatible with self-organized criticality. Possible physical mechanisms
contributing to the two distinct dynamic regimes are discussed by drawing on
the analogy with the serrated yielding of crystalline samples. The analysis,
together with some physical reasoning, suggests that plasticity in the less
ductile sample can be attributed to stick-slip of single shear band, while that
of the more ductile sample could be attributed to the simultaneous nucleation
of large number of shear bands and their mutual interactions.
