Shear band nucleation and indentation size effect in metallic glass

PolyU Library Call No.: [THS] LG51 .H577P ME 2015 Wangxv, 141 pages :color illustrationsSince the Bronze Age, the search for strong-and-ductile metals has been one of the central themes in all kinds of tool-making activities. After centuries of dedicated efforts, plasticity in crystalline metals has now been well understood; however, it still remains unclear in many fundamental aspects for amorphous metals, i.e. metallic-glasses. Despite the absence of microstructural features, metallic glasses (MGs) could display size-dependent hardness at the submicron scale. While most early studies attributed this size effect to Weibull statistics, here I proposed a shear-band nucleation controlled mechanism giving rise to a deterministic indentation size effect in MGs. In line with this mechanism, an explicit relation is derived linking the size dependency of hardness to a critical shear-band nucleation length in MGs. Through a series of carefully designed spherical indentation tests, this mechanism is experimentally justified, from which we are able to extract the shear-band nucleation lengths for a Zr-based MG at different indentation strain rates. On the basis of the combined theoretical/experimental efforts, our current work provides quantitative insights into the shear-band nucleation mechanism in MGs. Then spherical indentation was also conducted on the bent MG samples to study the effect of residual stress on the shear band nucleation. The Young's modulus was measured via the Joslin-Oliver based Berkovich nanoindentation approach. It is shown that the elastic modulus keeps at a constant while its hardness drops more significantly in the compression region than that in the tension region. The trend of indentation size effect for varies residual stress was found. It is obvious that the shear band nucleation in the Zr-based MG influenced by the presence of compression or tension residual stress, which can be rationalized by the different shear softening rate in MGs. Spherical indentation was further carried out on the ribbon and annealed MG samples to explore the thermal history effect on shear band nucleation. The hardness increases with annealing time and the ribbon one lower than the other three, demonstrating that denser structure has a higher hardness. Meanwhile, with the increasing annealing time, the Young's modulus increases. We can found that thermal history effect on the nucleation length which is not caused by the significantly change the softening rate, but the variation of Young's modulus. At last, various MGs with nine different spherical indenters were studies by spherical nanoindentation. The size effect is same as previous findings for large indenters while the reverse size effect was found for smaller ones. And the changing point is different for different MGs.Also the shear band nucleation length is correlated with the fragility and Poisson's ratio. The MGs with a higher fragility and Poisson's ratio represent a higher possibility for a BMG to have better plasticity and to form shear band.Department of Mechanical EngineeringPh.D., Department of Mechanical Engineering, The Hong Kong Polytechnic University, 2015Doctorat