Glass Forming Ability, Thermal Stability And Mechanical Properties Of Zr-Cu-based Bulk Metallic Glasses

The glass forming ability (GFA), thermal stability, microstructure and mdchanical properties of Zr-Cu-based bulk metallic glasses (BMGs) were studied in this work. The details are as follow:(1) The GFA of binary alloys was investigated based on our previous work. A metastable phase was found in the rapidly quenched Cu-Hf rod and its effect on GFA was studied. A previously defined parameter 1/? was employed to evaluate the compositional range of good glass former in Zr-Cu-Al ternary alloy system. As a result, we selected alloys within the compositional range of ZrCu-Zr2Cu-ZrCuAl accordingly for further investigations. The metstable phases existed in the rapidly quenched samples of these alloys were observed and then the GFA of these alloys was predicted by the revised parametersγ*. The prediction was proved by the experiments and thus we have established a valid system for the prediction of GFA in ternary alloys.(2) The glass transition and crystallization behaviors as well as their kinetics of the Zr_49.7Cu_50.3 and Zr₅₀Cu₄₂Al₈ BMGs were studied. It was found the addition of Al not only change the crystallization behaviors but also improve the thermal stability of the BMGs; The continous heating transition (CHT) curves of Zr_49.7Cu_50.3 and Zr₅₀Cu₄₂Al₈ BMGs were constructed from the Vogel -Fulcher-Tammann (VFT) fittings of the crystallization temerature of the BMGs. The curves are very similar to the TTT curves of the BMGs obtained from the isothermal annealing curves. The method provides a simple and useful way to evaluate the long-term stability of the BMGs; Diferences on the DSC curves between Zr₅₀Cu₄₂Al₈ as-spun ribbons and as-cast rods were observed. It was found that the microstructure of the ribbons changed from fully amorphous to a complex structure with nano-scaled clusters embedded on the glassy matrix, accompanied with the DSC traces of the ribbons becoming similar to the those of the rods. Hence, we concluded that the differences on glass transition, crystallization behaviors and thermal stability are mainly contributed by the structural diference between glasses.(3) Room-temperature mechanical properties of Zr₅₀Cu₅₀-xAlx(x=2, 4, 6, 8, 10, 12) were tested and a nearly linear relationship between the Tg/m and E was found. The Zr₅₀Cu₄₂Al₈ BMG with best room temperature plastility amongst these BMGs was employed for the investigation of the mechanism of high room temperature plasticity of the BMGs. A lot of nano-scaled twins were found in the post-deformation sample of the Zr₅₀Cu₄₂Al₈ BMG and were supposed to be closely related to the work-hardening behavior on the true stress-strain curve of the glassy rod. By careful examination, we found the hierarchical structure from surface to central region of the as-cast rod and phase separation in the central region. These inhomogeneous structures are supposed to be induced by the continuously decreased cooling rate from surface to center of the rod. Nano-indentation tests have revealed the heterogeneity in modulus of the region from surface to central region of the rod. It is suggested that the inhomogeneous deformation induece by structural hierarchy / phase separation as well as the heterogeneity in modulus in different area of the rod could be the main reason for the high room temerature plasticity of the rod.