Viscosity and fragility of the supercooled liquids and melts from the Fe-Co-B-Si-Nb and Fe-Mo-P-C-B-Si glass-forming alloy systems

The dynamic viscosity of four Fe-based bulk metallic glass-forming alloys, [(Fe<inf>0.5</inf>Co<inf>0.5</inf>)<inf>0.75</inf>B<inf>0.2</inf>Si<inf>0.05</inf>]<inf>96</inf>Nb<inf>4</inf> (alloy A), {[(Fe<inf>0.5</inf>Co<inf>0.5</inf>)<inf>0.75</inf>B<inf>0.2</inf>Si<inf>0.05</inf>]<inf>0.96</inf>Nb<inf>0.04</inf>}<inf>99.5</inf>Cu<inf>0.5</inf> (alloy B), Fe<inf>74</inf>Mo<inf>4</inf>P<inf>10</inf>C<inf>7.5</inf>B<inf>2.5</inf>Si<inf>2</inf> (alloy C) and (Fe<inf>0.9</inf>Ni<inf>0.1</inf>)<inf>77</inf>Mo<inf>5</inf>P<inf>9</inf>C<inf>7.5</inf>B<inf>1.5</inf> (alloy D), was investigated as a function of temperature in the supercooled liquid region, as well as above the melting point. The alloy B is Cu-added alloy A, while the alloy D was obtained upon fine-tuning the alloy C composition. All these alloys may form bulk metallic glasses upon copper mold casting. The viscosities in the supercooled liquid region were calculated using the data obtained upon parallel plate rheometry measurements, as well as upon differential scanning calorimetry (DSC). The values of the supercooled fragility parameter m, 61, 66, 52 and 60 for the alloys A, B, C and D respectively, indicate that these alloys are intermediate glass formers. The behavior of the same alloys, in the molten state, was studied using a high temperature torsional oscillation cup viscometer. The values of the corresponding fragility parameter M was calculated as 5.03, 5.91, 4.25, 4.93 for the alloys A, B, C and D, respectively. They confirm the supercooled liquid behavior and predict that the alloys A and C may form glasses easier than the fine-tuned compositions B and D. Angell plot is constructed for the entire range of viscosities and the values from both regions, i.e. above melting point and supercooled liquid region, fit well with the model.