Effect Of Heterogeneous Microstructure On Room Temperature Plasticity And Deformation Behavior In Zr-based Bulk Metallic Glasses

In this dissertation, the effect of heterogeneous microstructure on room temperature plasticity and deformation behavior in Zr-based bulk metallic glasses have been inverstigated by the means of X-ray diffraction (XRD), optical microscopy (OM), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), mechanical testing system, nanoindenter, thermomechanical analysis (TMA) and mcrohardness measurement, and then the deformation mechanism of metallic glasses is also discussed.Zr-based bulk metallic glassees (BMGs) were synthesized by addition an amount of Ta and Fe into ZrCuAl alloys by copper mould casting. It is found that the plasticity can be enhanced by the addition of positive elements, from 0.59% to 2.62% and 4.81% for Cu46Zr47Al7, Zr45Cu46Al7Ta2 and Zr47Cu44Al7Fe2 alloys, respectively. The effect of Fe content on the deformation behavior had been further studied. The plasticity increased with Fe content at frist, and reached the maximum of 7.47% for Zr47Cu45Al7Fe1, and the decreased until to the appearance of crystalline with the Fe content of 2.5%. TEM showed the occurrence of phase separation with Fe-riched and Cu riched phases in ZrCuAlFe metallic glasses is the reason of enhancement of plasticity. The size increased with the Fe content, and the plasticity is best with the Fe content of 1 at%.The mechanical properties of Zr65Cu17.5Ni10Al7.5 and Zr69.5Cui2Ni11Al7.5BMGs have been comparatively studied. The crystallization process of the two BMGs during continuous heating shows quite difference:Zr65Cu17.5Ni10Al7.5BMG undergoes one-step crystallization with the formation of intermetallic compounds of Zr2Cu and small amount Zr2Ni, while Zr69.5Cu12NinAl7.5 BMG follows two-step crystallization with a preferential formation of icosahedral phase, indicating that Zr6.5Cu17.5Ni10Al7.5 BMG may involve a strong icosahedral midium-range ordering structure. It was also found crystallization activation energy and nucleation activation energy were nuch lower in Zr69.5Cu12NinAl7.5 BMG, which further proved the existence of strong clusters in the alloy, which is also confirmed by SAXS results. Comparing with the former BMG,Zr69.5Cu12Ni11Al7.5 BMG exhibited large plastic strain and extremely good toughness. The compressive strain is more than 25%, while the bending degree can reach as large as 120 degree without failure. The noth toughness is as high as 86MPam1/2, while the Zr65Cu17.5Ni10Al7.5 BMG is only 60MPam1/2. It is suggested that the existence of icosahedral midium range ordering clusters could cause a heterogeneous distribution of free volume, which contributed the enhancement of plasticity and toughness of Zr69.5Cu12Ni11Al7.5 BMG.The deformation behavior of Zr69.5Cu12Ni11Al7.5 BMG was investigated at different strain rate, from 10-4 to 1s-1. It is found that there is critical strain rate form cold shear to hot shear. When the strain rate is higher than Is-1 for the 2 mm samples, the alloys fractured with litter plastictity due to the temperature rise above glass transition temperature, which can be well explained by temperature rise model. The serrated flow is also dependent on the strain rate, which decreased with the increasing of strain rate, and disappeared at 10-1s-1, which is consistent with the Stick-Slip model.Through the above studied, we have conducted a controlled experiment to form a single shear band in the specimen which enabled us to probe shear induced dilatation and softening directly on the shear band itself. Extreme dilatation and free volume increase as high as 1.14% and 1.40% respectively, have been observed resulted from a drastic structure change due to severe plastic flow in the band. The nano-indentation on the individual shear band revealed significant softening of 36% and unexpected wide width up to 160μm, three magnitudes higher than what has been reported. The minimum hardness was independent of plastic strain, which indicated that the free volume reached saturated with the plasti strain of only 2%. These prove beyond doubt the dilatation as the mechanism for softening rather than temperature rise in this study. The correlation between the free volume content and softening is discussed, and can be expressed as H=0.56+0.04/ζwith the activation volume of 1.61×10-28 m3.