Size Effect On Deformation And Fracture Behavior Of Zr-and Ti-based Metallic Glasse

The arrangement of atoms in metallic glasses is disorder, resulting in their novelmechanical properties and exhibiting deformation and fracture behaviors different from thosein convenitional crystalline materials. Due to their high fracture strength and hardness,metallic glasses have been regarded as potential engineering materials since its birth in1960.Though their glass-forming ability (GFA) has been studied for a long time, and many alloysystems with good GFA have been developed, the mechanical properties and deformationmechanisms of metallic glasses were not well understood so far. In the present thesis, thedeformation and fracture behaviors of a series of metallic glasses and their composites wereinvestigated systematically under different loading modes.The effect of sample size on the shear deformation and compressive plasticity ofdifferent metallic glasses were investigated. The experimental results showed that thedeformation and fracture behaviors of samples prepared from chemically identical Zr-, Ti-,Fe-, or Mg-based metallic glass ingots were strongly dependent on the sample size andmachine stiffness, and a super-high compressive plasticity was achieved in the Zr-basedmetallic glasses with sample size of1.0mm in width. It is also found that the sample size cansignificantly influence the density of elastic energy dissipated in the shear band: with samplesize decreasing and machine stiffness increasing, the density of the elastic energy dissipated inthe shear band of metallic glasses is prominently decreased, thus the shear deformation turnsto be more stable, resulting in the improvement of plasticity in ductile metallic glasses and thetransition from fragmentation fracture to shear fracture in brittle metallic glasses. This findingis important for the potential applications of the present metallic glasses and for designingnew metallic glasses with better mechanical properties.The deformation and fracture behaviors of a Ti-based metallic glass under multiaxialstress state were investigated by using small punch test. It was found that controlling both theinitiation and propagation of a shear crack can significantly stabilize the plastic deformationof Ti-based metallic glass by forming multiple shear bands, and delaying the shear crack fromreaching the critical crack length. Radial, circumferential, and spiral shear band patterns andcorresponding fracture modes were observed. The relationship between the shear band patternand the stress state was established. This finding implies that metallic glass could be stabilizedand embody itself ductility in nature under suitable stress conditions. The size-dependentplastic deformation of a Zr-based metallic glass under biaxial loading was investigated byusing small punch (SP) test. Unlike under uniaxial tension or compression, under biaxialloading, both the critical shear offset and the density of shear bands decrease with thereduction of the sample size. However, it was found that the normalized critical shear offsetkeeps constant, which can well explain the worsened plastic deformation behaviors of the thin sample under biaxial loading. This finding indicates that metallic glass possesses goodductility in nature, yet it is influenced significantly by the loading mode and sample size.