Abstract
An anomaly in differential scanning calorimetry 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 calorimetric anomaly is epitomized by four decades long studies of Pd-Ni-P metallic glasses, arguably the best glass-forming alloys. Here we show, using a suite of in situ experimental techniques, 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 liquids, involving a change in the packing of atomic clusters over medium-range length scales as large as 18 Å. With further temperature increase, the alloy reenters the supercooled liquid phase, which forms the room-temperature glass phase on quenching. The outcome of this study raises a possibility to manipulate the structure and hence the stability of metallic glasses through heat treatment.
Highlights
An anomaly in differential scanning calorimetry has been reported in a number of metallic glass materials in which a broad exothermal peak was observed between the glass and crystallization temperatures
Pd-Ni-P is an excellent glass-forming system, for which centimetre-sized metallic glasses can be made at a cooling rate as slow as 0.17 K s À 1
In 1976, Chen[4] observed a broad exothermic peak, which is far below the crystallization temperatures in the differential scanning calorimetry (DSC) curves of (Pd0.5Ni0.5)[100] À xPx (x 1⁄4 17B19) amorphous alloys. He proposed that this anomalous exothermal peak, hereafter abbreviated as AEP, corresponds to crystallization catalysed by amorphous phase separation[4]
Summary
An anomaly in differential scanning calorimetry 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 anomalous exothermal peak is the consequence of a polyamorphous phase transition between two supercooled liquids, involving a change in the packing of atomic clusters over medium-range length scales as large as 18 Å. In 1976, Chen[4] observed a broad exothermic peak, which is far below the crystallization temperatures in the differential scanning calorimetry (DSC) curves of (Pd0.5Ni0.5)[100] À xPx (x 1⁄4 17B19) amorphous alloys. Subsequent study using atom probe tomography[6] and element-specific transmission electron microscopy (TEM)[7] failed to provide evidence of compositional or microstructure modulations before the onset of crystallization Together, these findings appear to rule out the possibility of nanoscale phase separation. Our studies of Pd-Ni-P alloys show that at the transition temperature, the underlying atomic structure undergoes significant changes over the medium-range length scale, while the short-range order is little changed
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