Unique energy-storage behavior related to structural heterogeneity in high-entropy metallic glass

Structural heterogeneity dominates energy-storage behavior of metallic glasses. Ti20Zr20Hf20Be20Cu7.5Ni12.5 high-entropy bulk metallic glass (HE-BMG) shows extraordinary energy-storage behavior under cryothermal cycling: the relaxation enthalpy monotonically increased with cryothermal cycling cycles, even after 240 cycles of treatment. By comparison, the relaxation enthalpy of Ti41Zr25Be22Ni12 BMGs reached a maximum only at 30 cycles and then dramatically decreased as prototypical BMGs reported. The as-cast HE-BMG demonstrates an extremely low fraction of loose-packing regions (~7.8%, fractions for Ti- and Zr-based BMGs are above 15.6%), indicating degraded structural heterogeneity that would induce smaller local internal stress during cryothermal cycling and eventually lead to the sluggish energy-storage behavior. In addition, the increase in the fraction of loose-packing regions accompanied by the energy storage also lead to significant improvement in ductility of the HE-BMG, the plastic strain increased from 0.6% (the as-cast sample) to 5.2% (the sample treated 240 cycles).