Stress relaxation behavior of Fe–Co–Si–B–Nb metallic glassy alloys in their supercooled-liquid state

• Viscoelasticity of [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glass could be measured.
• We successfully measured the relaxation time and observed the viscoelastic behavior.
• The relaxation behavior of the glassy alloy was in good agreement with the KWW model.
• Our findings reveal the importance of a strict procedure for viscous-flow processing.

Iron-based metallic glasses have attracted much attention for use as microscale components in many devices because they provide unique processability with nanoscale precision. Fe-based metallic glasses, however, do not have enough resistance against crystallization, and therefore their incubation time for crystallization is too short. To prevent crystallization, the processing temperature cannot be set too high within the supercooled-liquid region, and high processing speeds are required. Taking this into consideration, the viscoelastic properties were observed during the viscous-flow microprocessing of a single metallic glassy particle of [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glass to identify its relaxation behavior of on the micrometer scale during a compressive test at 833 K. The relaxation behavior is found to be well described by the Kohlrausch–Williams–Watt model. The dependence of the ratio of the viscosity to the modulus of rigidity on the compressive speed during deformation was also investigated by constructing the master curve describing the behavior of the elasticity and viscosity fractions versus the apparent relaxation time. At a compressive strain rate of 4.5 × 10−2 s−1, the Fe-based metallic supercooled liquid behaved as a viscoelastic fluid, although at a compressive strain rate of 4.5 × 10−3 s−1, its behavior was almost viscous. From the master curve, it can be concluded that the effect of the compressive speed on the ratio of the viscosity to the modulus of rigidity can clearly be observed, and the viscous-flow behavior of a [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic supercooled liquid can be predicted at any processing speed.