Viscosity in metallic supercooled liquids obtained by fast scanning calorimetry and its application to thermoplastic forming

Viscosity is a crucial dynamic property for alloy liquids. For alloy liquids with glass-forming ability, their viscosity undergoes a change of more than ten orders of magnitude upon cooling, and eventually, a transition from liquid to glassy state occurs when viscosity reaches 10¹² Pa s. Nevertheless, the viscosity measurement in the supercooled liquid region (SLR) has been an experimental challenge. Here, in this work, by combining conventional and fast scanning calorimeters, the accurate viscosity of supercooled liquids for Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 and Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ metallic glasses (MGs) was measured over a wide viscosity range of 10⁶–10¹² Pa s. Moreover, the viscosity range of 10⁶–10⁸ Pa s is generally selected for thermoplastic forming (TPF), which takes advantage of the dramatic softening that MGs exhibit after being heated into the SLR. We further gave the TPF process window covering the range of processing temperature and time by utilizing the obtained viscosity data and time-temperature-transformation curves. The optimal time window avoiding property degradation was also determined, based on that there was no marked effect on the mechanical properties of MGs when crystallinity was below ∼5%. Our findings not only provide the accurate viscosity data in the SLR, but also determine the optimal process parameters for the TPF process.