| In this thesis, the rheology dynamics and flow mechanism of the supercooled bulk metallic glass forming liquids near glass transition temperature(Tg) have been investigated by thermomechanical analysis (TMA), differential scanning carlorimetry(DSC), scanning electron microscopy(SEM), material property testing machine, nanoindenter, micro hardness measurement and small-angle X-ray scattering(SAXS). The effect of metal element on the flow dynamics of supercooled liquid was also investigated.Creep behavior of Pd40Cu30Ni10P20,Cu46Zr42Al7Y5, Zr58.5Cu15.6Al10.3Ni12.8Nb2.8, Zr65Cu15Al10Ni10bulk metallic glasses (BMGs) were investigated by TMA in their respective supercooled liquid region. The fragility parameter m was calculated to be42(Pd40Cu30Ni10P20),33(Zr65Cu15Al10Ni10),24(Zr58.5Cu15.6Al10.3Ni12.8Nb2.8) and17(Cu46Zr42Al7Y5), from fragile to strong. It is found that the data scattering near Tg is correlated with the fragility of the supercooled liquids and reflects the characteristics of the relaxation dynamics of supercooled liquids. Viscosity measured at different heating rates show that the strong liquid will behave similar to a fragile liquid at higher heating rates.The structural relaxation time of Vit106a (Zr58.5Cu15.6Al1o.3Ni12.8Nb2.8) was investigated by differential scanning carlorimetry at different heating rates. The temperature dependent structural relaxation time can be fitted by Vogel-Fulcher-Tamman (VFT) equation. The fitting parameter D was calculated to be18.1reflecting its strong liquid behavior. The enthalpy relaxation time was measured by isothermal annealing to follow Arrhenius equation with an activation energy value of292KJ/mol. A crossover of the two relaxation times was detected near Tg. The correlation between the structural relaxation and enthalpy relaxation with a and β relaxation was recognized.Flow behavior of Vit106a (Zr58.5Cu15.6Al10.3Ni12.8Nb2.8) BMG was investigated by compression in the supercooled liquid region. With increasing strain rate, the rheolgoy behavior of the supercooled liquid can be divided into two regimes:homogeneous flow and inhomogeneous flow. In homogeneous flow regime, the flow shows two instinct transitions:the stress overshoot indicating the shear thinning where the index of strain rate sensivitvity ms gets smaller than1and the strain rate softening where ms gets smaller than0. In the inhomogeneous flow regime, the supercooled liquid fractured similar to its failure at glassy state. The enthalpy relaxation time reflects the annihilation rate of the excess volume and the structural relaxation time reflects the diffusion and percolation rate of the flow units. The shear thinning is attributed to the fact that the strain rate induced time scale (1/ε) is shorter than the enthalpy relaxation time which leading the accumulation of free volume (ms become less than1). The strain rate softening is attributed to the fact that the whole sample is correlated as a large flow unit, softening taking place macroscopically (ms become0and negative). The fracture was attributed to the fact that the strain rate induced time scale is shorter than the structural relaxation time.Samples of Vit106a after deformation at703K,(2×10-3s-1～5×10-2s-1) and of different deformation amout were tested by DSC. The enthalpy relaxation of samples indicated that with increasing strain rate, the relxation enthalpy of the supercooled liquid increased. The oscillation on the stress-strain curves indicated the continuous softening and hardening of the supercooled liquid. The sample deformed in Newtonian flow mode contained less enthalpy than the annealed sample. This proves the existence of the negative and positive free volume. The annihilation of free volume corresponding to the excess volume moving into negative free volume region, while the diffusion of the free volume corresponding to the excess free volume moving into positive free volume region. It was found that with the effect of flow stress, mechanical annealing will lead to the annihilation of the free volume similar to the case at isothermal annealing.The effect of metal element on the structure of the supercooled liquid was investigated by the compression test of several BMGs Vitl(Zr41.2Ti13.8Cu12.sNi10Be22.5, at%),Zr55Cu30Al10Ni5, Zr35Ti30Be27.5Cu7.5. As Vit106a and Zr55Cu30Al10Ni5show continuous oscillation during strain rate softening, while Vitl and Zr35Ti30Be27.5Cu7.5didn’t, further study on (Zr50Cu50)100-xAlx(x=3,5) indicates that this is attributed to Al elment. It is concluded that the addition of Al will induce a higher packing density of the atoms of the supercooled liquid. The dense packing will lead to a deeper minimum and a craggy surface on the surface of potential energy landscape. As strain rate increasing, stress oscillation will present as the model predicted. Without Al, the potential energy landscape is flat and the system can easily hop out of the minimun. In this case, oscillation will get weaker or even disappear. |
PDF Full Text Request