Preliminary Study On The Mechanical Behavior And Mechanism Of Metallic Glasses

Metallic glass is also often referred to as amorphous alloy or non-crystalline alloy,it is a kind of emerging member of metal materials family.With extraordinary mechanical and physical properties,metallic glass is very promising as the structure and functional materials.Moreover,it simultaneously obtains characters of glass,metal,solid and liquid.This unique structure allows metallic glass to become an ideal model for exploring some important issues in the condensed matter physics and material science field.As emerged for more than eighty years,metallic glass has overcome some challenges such as the limited critical size,and gradually got rid of bad reputation of "stupid alloy" at very beginning.Currently,it has been a research focus in material science.From the view of fundamental science and industrial application,there are still some problems to be solved:for the mechanical branches,the researchers need to explore the deformation mechanism of metallic glass at different states;And the researchers need to deeply understand the relationship between external performance and mechanical behavior with micro-mechanism and internal structure.In this thesis,we carried out a primary research to try to answer these significant questions.We focus on the size effect of deformation mode by the deformation behavior in metallic glass films.We adopt appropriate method to study the evolution of mechanical properties at wide temperature range.On the other side,we measure the volume and activation energy of "shear transformation zone" in metallic glasses at different state by experimental methods.Thus we could analyze the deformation behavior at the point of the basic rheology unit.The main results are summarized as follows.(1)The size effect on the bending deformation behaviour of magnetron-sputtered La57Al25Co18,CusoZrso and Fe70Y8B22 glassy films was investigated.The transition of the deformation mode from highly localized to non-localized occurs as the film thickness reduces below the critical value,which does not exhibit a distinct dependence on Poisson's ratio.By combining the already-reported critical size for deformation mode transition in various metallic glasses,it is found that the critical size is primarily proportional to the homologous temperature.(2)Stress effect on shear transformation zone(STZ)in Ni60Nb40 thin film metallic glass was investigated.Film/substrate co-bending was exerted to produce a symmetrical tension/compression stress distribution.Through nanoindentation tests on the bent specimen,it was found that compressive stress decreased the STZ volume and activation energy in comparison with tensile stress and hence easy activation of STZs lowered the tendency of shear localization,which resulted in increased number and reduced length of serration.Therefore,easier STZ activation conduced to the larger critical size for deformation mode transition from highly-localized to non-localized.(3)The holding time effect on the mechanical property,creep behavior and shear transformation zone was systematically investigated by performing a series of nanoindentation creep measurements in magnetron-sputtered La-Co-Al,Zr-Cu-Ni-Al,Ni-Nb and W-Ru-B thin film metallic glasses(TFMGs).The hardness,Young's modulus and strain rate were found to decrease with increasing holding time,after considering the variation of contact stiffness with different indentation depth.Meanwhile,strain rate sensitivity,m,and the volume and activation energy of shear transformation zone(STZ)were analyzed,according to the cooperative shear model,at different stages of creep to explain the time-dependent mechanical behavior and properties.The higher strain rate leads few potential STZs with larger STZ volume and activation energy to be activated and lower plastic stability of deformation,manifested by smaller m,at the primary creep stage,as compared with those at apparent steady-state creep.(4)Indentation size effect of magnetron-sputtered thin film metallic glasses(TFMGs)was investigated with a Berkovich indenter.Unlike the crystalline metals and bulk-sized MG,the hardness in TFMGs is smaller in the lower indentation depth,which accords with the lower creep resistance and activation energy of shear transformation zone(STZ).The intrinsic profuse fertile sites in TFMGs limit the strengthening effect from scarcity of STZ nucleation sites and highlight the softening effect from geometrically necessary STZs.(5)While metallic glasses(MGs)exhibit extraordinary strengths,their ductility--an essential property for widespread utilization of them as structural materials--has been enigmatic in terms of understanding.Here we report that it is not only sensitive to the test temperature(T),but also exhibits a minimum,wherein the MG is essentially brittle,at intermediate temperatures of about 0.6 Tg(Tg = glass transition temperature).This behavior is exactly similar to the 'ductility minimum'observed in a wide variety of polycrystalline metals that is attributed to dislocation dynamics.In MGs,which do not contain dislocations in them,the intermediate temperature ductility minimum(ITDM)is a consequence of the decreased shear band nucleation rate with increased T,within the T regime over which shear band mediated heterogeneous plastic flow dominates.The observation of brittleness at intermediate temperatures highlights the importance of the homologous temperature(T/Tg)to any MG's ductile versus brittle behaviour.