Study On Mechanical Behavior And Dynamic Of Metallic Glass Near Its Surface

Metallic glass(MG)is a metastable metallic material and an excellent candidate structural material for fabricating micro/nano-devices due to its unique structural features and outstanding mechanical,physical,chemical properties and excellent thermoplasticity in the supercooled liquid region.However,size effect emerges with decreasing MG size and the mechanical behavior of MGs is very different from that of bulk samples at micro-and nano-scale.Specifically,with the increase of the specific surface area in small-sized samples,surface effect overwhelms bulk effect and becomes the dominant factor to determine the mechanical properties of MGs.Therefore,elucidating the nature of the mechanical behavior of MGs near its surface region not only expands our understanding of MG at fundamental research level,but also advances the current technology of nano-manufacturing based on crystalline.In this thesis,we firstly studied in details the nanoscale plasticity in MGs near its surface region via dynamic nanoindentation.Our experiments clearly show that nano-scale shear-banding in different MGs undergoes a transition from the “distributed”(at and near the surface)to “localized”(interior)mode when the resultant plastic flow extends over a critical length scale.In the “distributed” regime,multiple shear bands with a constant but small shear offset are activated while in the localized regime one or a few dominant shear bands with large and varying shear offsets prevail.Through the extensive experimental and theoretical efforts,we,for the first time,unveil an intrinsic interplay between elasticity and fragility that governs the nanoscale plasticity crossover in MGs.In the second part we focused on the surface dynamic features of MGs by employing atomic force microscopy technique and provide the first report of how plasticity initiates and proceeds on metallic glass surface.For bulk samples,plasticity in MGs is associated with the increase in the proportion of soft regions and the proliferation of these regions leads to structural rejuvenation,whereas at the surface of MGs,a four-step evolution process was revealed,which starts from a stochastic process featured with local structural rearrangements,towards a deterministic process of structural relaxation(densification & hardening)till reaching an apparent dynamic equilibrium,and finally to a deterministic process of structural rejuvenation(dilation & softening).This surface plasticity resembles the general behavior of soft glassy materials and demonstrates that the surface dynamics of MG is faster than the interior.Finally,we studied the effect of the fast surface dynamics on the friction behavior of MGs.We find that the mobile surface layer acts as a lubricating layer in friction experiments,which leads to a significant reduction of frictional coefficient by almost a factor of 2.This obvious reduction arises from the homogenous plastic flow of the mobile surface layer,which facilitates self-lubrication on MG surface.When the plastic flow extends over a critical length scale,localized shear bands prevail and result in stick-slip instabilities in friction data.Through the extensive experimental efforts,we finally unveil an interplay between the critical length scale and the homologous temperature T/Tg,in which Tg is glass transition temperature.Our work not only sheds qualitative light on the deformation behavior of small-sized MGs and nanomechanical behavior of metallic glass near its surface region but also provides experimental basis for perfecting and developing deformation mechanism of MGs.Meanwhile,our experimental results also suggest that nano-sized MG can be further explored for important applications,such as self-healing and self-lubrication.