Understanding Deformation Mechanism Of Bulk Metallic Glasses By Vickers Indentation Measurements

The obscure nature of glass physics has led to develop various correlations between different parameters and properties of metallic glasses.Despite these correlations the clear picture of plastic deformation is still lacking.Detailed topological analysis of Vickers indentation on different bulk metallic glasses(BMGs)has been performed by means of atomic force microscopy to understand the deformation mechanism under the sharp indenter.Elastic recovery,pile-up and indentation volume were found to correlate with different parameters and properties of bulk metallic glasses.These correlations help to understand the prevailing mechanism in that process.The microstructure of metallic glasses is homogenous at meso-scale,local heterogeneities are,however,present at micro-scale.These local heterogeneities play a vital role in different phenomena is metallic glasses such as deformation and relaxation.The extent of elastic recovery and plastic deformation in terms of pile-up and indentation volume,under the Vickers indenter,was found to depend on these local heterogeneities or more precisely termed as flow units.Residual stress was found to inversely correlate with Tg whereas plastic zone scaled with v,beneath the indenter.BMGs with high v showed less elastic recovery as compared to BMGs with low v.MGs having high v possess more flow units can exhibit large amount of plastic deformation under the application of load and show less elastic recovery.Initially sharp and followed by less pronounced decline in elastic recovery was observed with increasing the loading time.This result indicates that activation of these flow units is a time dependent process,which reaches to saturation at longer loading times.In quasi-static cyclic loading,elastic recovery was found to reduce suggesting Vickers indentation produce low strain rate,which provides enough time to activate more flow units to further plastically deform the metallic glass.BMGs with low v,showing more strain softening behavior developed larger pileup as compared to BMGs with high v.This trend suggests that deformation behavior changes from cutting wedge mechanism to expanding cavity model with increasing v.Indentation volume dependence on shear modulus(G)suggests G is the most important elastic constant for describing the response to deformation in MGs.A hint of relation between plastic deformation and β-relaxation was found.The BMG compositions with pronounced β-relaxation produced considerably larger more amount of indentation volume as compared to MGs without β-relaxation.Load as well as load time has more influence on pile-up volume than indentation volume.Therefore,it can be inferred that Vp is the controlling parameter.Annealing and high pressure has similar effects on bulk MGs.All these observations are well accounted for flow units model which could provide insight into the plastic deformation beneath the sharp indenter and nature of the metallic glasses.