Viscous flow and crystallization of bulk metallic glass-forming liquids.

摘要

An experimental setup was designed and implemented to measure the flow behavior of liquids in the viscosity range from 10−1 Pa s to 104 Pa s. The viscosity of the Zr_41.2 Ti_13.8Cu_10.0Ni_12.5Be_22.5 (V1) bulk metallic glass forming alloy was measured over a temperature range from 927 K to 1173 K. At the liquidus temperature, the viscosity is 2.3 Pa s, which is about three orders of magnitude larger than the viscosity of a pure metallic liquid. The free volume theory as formulated by Cohen and Grest describes the temperature dependence of the viscosity of V1 over 14 orders in magnitude.; The high viscosity of V1 above the liquidus temperature stabilizes the liquid against convective flow due to temperature gradients and allows for diffusion experiments in the equilibrium liquid. The temperature dependence of the diffusivity of large atoms like Al or Au scales with the viscosity. The time scales obtained from the viscosity measurements suggest that above the calorimetric glass transition region the diffusion of small and medium sized atoms is governed by thermally activated jumps.; Liquid V1 could be successfully supercooled inside high purity graphite crucibles without changing the stability of the supercooled liquid with respect to crystallization compared to levitated samples. The sluggish kinetics that are reflected in the high viscosity in the supercooled liquid state contribute significantly to the good glass forming ability of the alloy. The critical cooling rate is about 1 K/s.; The onset of crystallization under isothermal conditions as well as upon heating from the amorphous state was studied in detail. The critical heating rate to bypass crystallization was measured to be 200 K/s and the difference between the critical cooling and critical heating rate can be qualitatively understood in the framework of nucleation and growth.; However, the observed deviations from classical steady state nucleation behavior indicate a more complex crystallization mechanism. Rheological and crystallization studies at constant shear rate suggest that changes in the morphology of the supercooled liquid of V1 occur as a precursor of crystallization.