Room Indentation Behavior Of Bulk Amorphous Alloy

When the melted alloy undergoes the rapid cooling, since there is nosufficient time for ordered arrangement of atoms, the irregular structure in liquidstate is frozen in solid state, the metallic glass comes into our view. The plasticdeformation of metallic glass is highly localized into shear bands, for whosestructure is in long-range disordered and short-range ordered, and has nodislocations and grain boundaries. To understand the micro-mechanism plasticdeformation of metallic glass does great scientific significance.The paper mainly researches the Nanoindention behavior ofZr52.5Cu17.9Ni14.6Al10Ti5metallic glass in room temperature, and tries to give areasonable physical explanation on typical serrations phenomenon. The strainrates were settled as0.002s-1,0.02s-1,0.2s-1, and it is found that serrated flowbehaviors increasingly become obviously with increasing strain rate, and showsrate sensitivity. Serrations of metallic glass are resulted from the shear bandingprocess, and the concentration of the free volume regions are preferential sitesfor shear band origin, as the stress applying, the free volume increases, the shearband zone can be extended, proliferated. Two kind of serrations are observed inthe experiment, one is flat serration, mainly attributed to single shear band, and the other is upswept serration, may ascribed to multiple shear bands mutualinteractions. Under high strain rate, due to the propagation of a good deal ofshear bands, there still exist upswept serrations, but they are not enough to beobserved.Three cylindrical Vit105specimens with different diameters, such as,2mm,3mm,6mm, were tested from the center to the border in cross section usingNanoindentation experiments, respectively. Results show that cylindrical samplewith larger dimension possesses a larger elastic modulus and higher hardnessaccordingly, and so is the center region compared to boundary in the crosssection of a cylindrical specimen. To a certain extent, cylindrical metallic glassis radial gradient structure. The serrations in the position which has a smallelastic modulus and hardness are obvious, implying that the position has a highcontent of free volume induced by high cooling rate. The temperature rise,calculated as0.06K via low strain rate, is thought to be dispensable. It isconsidered that localized shear deformation mechanism is dominated by a freevolume increase model.The finite element analysis software, ABAQUS, was used to investigate theyield criteria for amorphous alloy. The non-associated flow rule inMohr-Coulomb and Drucker-Prager criteria can reasonably describe themechanical behavior of amorphous alloy, while the associated flow rule willmake the material excessive dilatation. It will make a big error to define aconstitutive model of metallic glass with Mises criterion.