Study Of The Mechanical Behavior And Its Mechanism Of Bulk Amorphous Alloys

In the past decades, the discovery of novel multi-component alloys with extremely good glass forming ability (GFA) enables the synthesis of bulk amorphous alloys at very low cooling rates. Compared to conventional amorphous alloys, the bulk ones usually exhibit a distinct glass transition process, large supercooled liquid region and high thermal stability with respect to crystallization. This offers us a perfect opportunity to investigate in much more detail the glass transition, the inherent nature of supercooled metallic liquid and the correlation between mechanical behaviors and microstructure of amorphous alloys over a sufficiently wide time–temperature region.In this dissertation, Zr based bulk amorphous alloy forming systems, such as Zr₅₅Cu₂₅Ni₅Al₁₀Nb₁₀, Zr_41.2Ti_13.8Cu_12.5Ni_10-xBe_22.5Fe_x (x=0,2), which exhibit good GFA and high thermal stability against crystallization at glass transition as well as in supercooled liquid region, have been chosen to be the model materials for us to study various mechanical behaviors of amorphous alloys. In order to well understand and therefore improve the GFA, thermal stability and mechanical properties of these alloys, the present research activities are mainly focused on the dynamic mechanical behaviors during glass transition, crystallization or viscous flow and their dependence on micro-alloying, the correlation between the high temperature homogeneous plastic flow behavior and the microstructural evolution in supercooled liquid region and the effect of microstructural evolution on the inhomogeneous plastic flow at room temperature by the measurements of mechanical spectroscopy, uniaxial compression tests and nanoindentation tests combined with differential scanning calorimetry (DSC), X-ray diffraction(XRD) and high resolution transmission electronic microscopy (HRTEM). Some of main experimental results and conclusions are listed as follows:1) Like in other conventional glass forming systems, a distinct main (α) relaxation with typical kinetic features of non-Debye relaxation is experimentally detected for the Zr₅₅Cu₂₅Ni₅Al₁₀Nb₁₀ bulk amorphous alloy. During the main mechanical relaxation, the variation of dynamical mechanical properties of the Zr base bulk amorphous alloy is found to follow the biparabolic equation derived from the quasi-point defect (QPD) physical model, which can characterize the atomic mobility and mechanical response of disordered condensed materials. Moreover, the temperature dependence of the characteristic relaxation time,Ï„relax(T) is theoretically confirmed for the alloy. It is found that there is a good agreement between the well-known fragility parameter, m , and the parameter a of the physical model, when characterizing the degree of deviation from the Arrhenius law for theÏ„relax(T).2) The internal friction behavior of Zr_41.2Ti_13.8Cu_12.5Ni_10-xBe_22.5Fe_x(x=0,2) bulk amorphous alloys has been analyzed in terms of QPD model. It is noted that the activation energy Uβ, which is the height of the energy barrier for the elementary atomic movement in the Fe-containing alloy (1.20eV) becomes larger than that in the Fe-free alloy (0.68eV), while the correlation parameterχis slightly decreased from 0.41 to 0.38. As a consequence, the presence of a larger glass transition and supercooled liquid region for the Zr–Ti–Cu–Ni–Be bulk amorphous alloy containing 2 at.%Fe element can be attributed to the micro-alloying, which leads to the enhancement of activation energy and degree of short range order and therefore the lowering of atomic mobility and increase of structural relaxation time. 3) The isothermal dynamic mechanical experiments on the Zr-Ti-Cu-Ni-Be bulk amorphous alloy containing 2%at Fe lead to the determination of metastable equilibrium internal friction Q e?1, as function of temperature, which can be well described by the Maxwell model with the viscosity,η(T), following a Vogel-Fulcher-Tammann(VFT) relation. It is worth noting that while the VFT temperature of Zr-Ti-Cu-Ni-Be BMG is lowered by 80.9 to 331.6K, the value of strength parameter, D *increases from18.5 to 22.3 due to the addition of 2at%Fe. This means that the Zr based alloy becomes stronger as a result of Fe addition, which correlates with its better glass forming ability and higher thermal stability.4) From the Arrehnius plot of isothermal internal friction data, the activation energy, cE , for primary crystallization of the Zr-Ti-Cu-Ni-Be-Fe supercooled metallic liquid is derived to be 3.8eV, which is comparable to that reported on atomic mass transport in the Zr based bulk amorphous alloy, e.g. Q_Ti =4.09(±0.46) eV for diffusion of Ti element in the alloy. Taking into account previous experimental data on the crystallization, it is thus proposed that the isothermal primary crystallization of supercooleld liquid of the studied Zr based alloy is an atom diffusion-controlled process, in which Ti element undoubtedly plays a key role。5) High temperature homogeneous plastic flow of a Zr-Ti-Cu-Ni-Be bulk amorphous alloy has been investigated in the supercooled liquid region. At low strain rates or high temperature, the studied alloy exhibits a Newtonian behavior whereas at high strain rates and low temperature, a non-Newtonian behavior is observed. It is shown that the experimental results can be successfully described not only by the empirical model frequently used for amorphous alloys (stretched exponential function) but also by various physical models, such as the transition state theory based on a free volume concept. From the experimental data, the activation volume V act and activation energy for the homogeneous plastic flow of the supercooled metallic liquid are determined to be 160Aand 4.6eV, respectively. Furthermore, it is found the value of activation energy is comparable to that for the atomic diffusion in the alloy, indicating that the homogeneous plastic flow in the supercooled metallic liquid might be mainly controlled by the atom diffusion, which likely follows the large-scale atomic cooperative movement mechanism. More importantly, it is verified that the QPD model initially developed for glassy polymers is also applicable to describe the homogeneous plastic flow of the Zr based bulk amorphous alloy in supercooled liquid region. The relavant physical parameters of atomic mobility are deduced from systematically analyzing the dynamic mechanical property data of the alloy in terms of the same physical model.6) The microstructural and thermal analysis of the samples before and after deformation clearly indicate that the strain hardening subsequent to the steady state Newtonian flow is closely associated with the phase separation of initial homogeneous supercooled liquid into two different liquid phases, which causes the significant increase of viscosity and thus the dramatic slowdown of kinetics of the alloy. On the other hand, it is found that during Newtonian viscous flow, the presence of applied stress may have limited influence on the kinetics of the structural evolution related to the phase separation and subsequent nanocrystallization.7) The influence of microstructure on the inhomogeneous plastic deformation of Zr_41.2Ti_13.8Cu₁₂₅Ni_10.0Be_22.5 bulk amorphous alloy at ambient temperature has been investigated by nanoindentation tests. It is found that the dependence of hardness on indent depth of the phase separated Zr based alloy is different from that of the as-received or relaxed one, which can be well described by a developed clustered-model. Consequently, this indicates that the phase separation remarkably modifies the mechanism of the inhomogeneous plastic deformation in the alloy, which may be attributed to the decomposition of the initial homogeneous glassy phase into two amorphous phases differing in chemical composition and stability with respect to crystallization.