The effect of cooling rates on the apparent fragility of Zr-based bulk metallic glasses

We investigate the behavior of the kinetic fragility parameter D* as different cooling rates are applied to samples of the Zr57Cu15.4Ni12.6Al10Nb5 and Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 bulk metallic glass formers. The glassy samples are heated into the supercooled liquid region using differential scanning calorimetry (DSC) and cooled back into the glassy state with sets of different cooling rates, qC. The shifts in the glass transition are measured by determining the onset glass transition temperature, Tgonset, as well as the limiting fictive temperature, Tf', upon reheating with a set of heating rates, qH. We then model the data by assuming a Vogel-Fulcher-Tammann-type behavior in the structural relaxation time, τ, and observe an apparent increase in the kinetic fragility parameter for slower cooling rates. These results show that fragilities calculated from DSC scans, where qH=qC are in good agreement with those from equilibrium viscosity data recently obtained by three-point beam-bending.