  Many glass-formers exhibit phase transitions between two distinct liquid
states. For some metallic glass-formers, the liquid-liquid transition is
experimentally found in the supercooled liquid at intermediate temperature
between the melting point and the glass transition temperature Tg. We report
here on a liquid-liquid transition in an ultra-viscous metallic glass-former,
accessed during long-time annealing. This study is conducted on the
Au49Cu26.9Si16.3Ag5.5Pd2.3 composition with a liquid-liquid transition
temperature slightly lower than Tg. The consequence is that the
high-temperature kinetically fragile liquid freezes into the glass during
conventional processing and the underlying liquid-liquid transition is thus
accessed by the system during annealing below Tg. Upon reheating, the reverse
transformation is observed by calorimetry. This conclusion is supported by a
broad collection of complementary laboratory and synchrotron-based techniques,
such as differential- and fast- scanning calorimetry, and x-ray photon
correlation spectroscopy. Our findings support the big-picture proposed by
Angell that liquids with different fragility occupy different flanks of an
underlying order-disorder transition. Furthermore, our multiscale analysis
reveals the existence of multiple decays of the enthalpy recovery, which is
reflected in the observed microscopic ordering and aging mechanism of the glass
through distinct stationary regimes interconnected by abrupt dynamical aging
regimes.
