Abstract:

A typical thermal scan by differential scanning calorimetry (DSC) for glass-forming materials consists of an endothermal plateau arising from the glass transition, followed by one or more sharp exothermal peaks due to crystallization.  An anomaly in DSC has been reported in a number of metallic glass materials in which a broad exothermal peak was observed between the glass and crystallization temperatures.  The mystery surrounding this DSC anomaly is epitomized by nearly half a century long studies of Pd-Ni-P metallic glasses, arguably the best glass-forming alloys.  We have used a suite of in-situ experimental techniques to determine the structural origin of this anomalous exothermal peak, including high-energy synchrotron X-ray diffraction, small angle neutron scattering coupled with simultaneous DSC, and high-resolution transmission electron microscopy.  Here we show [1] that Pd-Ni-P alloys have a hidden amorphous phase in the supercooled liquid region.  The anomalous exothermal peak is the consequence of a polyamorphous phase transition between two supercooled liquid phases, involving a change in the packing of atomic clusters over medium-range length scales as large as 18 Å.  With a further increase in temperature, the alloy reenters the supercooled liquid phase which forms the room-temperature glass phase upon quenching.  ([1] S. Lan, Y. Ren, X. Y. Wei, B. Wang, E. P. Gilbert, T. Shibayama, S. Watanabe, M. Ohnuma, and X.-L. Wang, Nature Communications 8, 14679 (2017).)