Non Newtonian viscosity of bulk metallic glass forming liquids and the ordering and shear rate induced crystallization of undercooled Zr₄₁.₂Ti₁₃.₈Cu₁₂.₅Ni₁₀.₀Be₂₂.₅ metallic glass forming melt Public Deposited

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  • The influence of shear rate and temperature on the viscosity and fragility of the Zr₄₁.₂Ti₁₃.₈Cu₁₂.₅Ni₁₀.₀Be₂₂.₅ (Vit1) metallic glass forming liquid is measured in the liquid and undercooled liquid state between 907 and 1300 K. The results show a complex rheological behavior of Vit1. The viscosity of Vit1 is about three orders of magnitude higher than simple metallic liquids with strong shear thinning behavior upon first heating from the amorphous state above the liquidus temperature. The shear thinning behavior decreases with increasing temperature and this kinetically strong liquid transforms to a more fragile liquid with no shear rate dependence of viscosity. The Newtonian temperature (T[subscript Newtonian]) for Vit1 is measured as 1225 K. Upon cooling this liquid the strong liquid behavior is only re-established when the melt is deeply undercooled below the liquidus temperature. This gives rise to the "hysteresis effect" in the viscosity of Vit1 as a function of temperature. The difference in viscosity and the shear thinning behavior is also seen depending on the initial state of the material (amorphous or crystalline). The shear thinning and strong to fragile transition is attributed to the destruction of medium range order (MRO) in the liquid state whereas the reformation of order in the supercooled region is much more complex. Secondly, the effect of ordering and shear rate on the crystallization kinetics of undercooled Zr₄₁.₂Ti₁₃.₈Cu₁₂.₅Ni₁₀.₀Be₂₂.₅ metallic-glass-forming melts is investigated. The study quantitatively shows the shift of the TTT diagram of Vit1 to shorter crystallization times with increasing shear rates. The classical nucleation and growth theory is applied by incorporating the change in various factors such as viscosity, driving force for crystallization and entropy change as a function of shear rate. It is found that the change in crystallization kinetics can not be explained quantitatively by the classical nucleation and growth theory. The order present in the melt immensely influences the crystallization and the material with higher order crystallizes sooner than the material with smaller order. The ordered clusters are formed faster at higher shear rates and hence the material crystallizes sooner at higher shear rates making the TTT diagram shift to the left. Finally, the viscosities of two bulk metallic glass forming liquids Zr₅₇Cu₁₅.₄Ni₁₂.₆Al₁₀Nb₅ (Vit106) and Pd₄₃Ni₁₀Cu₂₇P₂₀ are measured and compared with the viscosity of Vit1. The main goal of this study was to investigate whether high viscosity and shear thinning behavior is just associated with Vit1 or the other BMG forming liquids also show such behavior. It is shown that all these glass formers show higher viscosities than monoatomic metallic liquids and surprisingly the non-Newtonian shear thinning behavior. Vit1 exhibits the highest viscosity with the largest drop in viscosity on shearing from 0.1 s⁻¹ to 250 s⁻¹ at the temperatures above T[subscript liq.] Vit106 shows lower viscosities than Vit1, however, a stronger non-Newtonian behavior on shearing from 0.1 s⁻¹ to 143 s⁻¹ at the temperatures between 1130 K and 1305 K is seen. Surprisingly, this non-Newtonian behavior disappears at higher shear rates where the viscosity stays constant. The melt viscosity of the Pd₄₃Ni₁₀Cu₂₇P₂₀ shows a lower viscosity and much smaller shear thinning behavior than the Zr-based alloys. The viscosity data of Vit106 and Pd₄₃Ni₁₀Cu₂₇P₂₀ indicate a possibility to reach a steady state viscosity if high enough shear rates are applied to the melt of Vit1 at the temperatures between 1075 K and 1225 K. In all the three alloys, this non-Newtonian behavior gets weaker with increasing temperature and the material starts to behave like a Newtonian liquid. This behavior is attributed to the presence of MRO and short range order (SRO) in the melt of these alloys. The MRO present in the melt can be reduced by shearing and more effectively destroyed to SRO by increasing the temperature which leads to a Newtonian liquid with smaller viscosity.
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