Growth Behavior,Mechanical Property And Low-temperature Resistivity Of MgZnCa Metallic Glass Thin Films

Metallic glass(MG),completely lacking long-range periodic order,is often referred to as amorphous alloy or non-crystalline alloy.Metallic glass exhibits attractive properties compared to their crystalline counterparts,such as high strength,large elastic strain limit,superior corrosion resistance.Nevertheless,bulk MGs still suffer from macroscopic brittleness and poor workability at room temperature.Thus,thin film metallic glasses(TFMGs)have been investigated in recent years as a possible route for making use of unique MG properties while minimizing issues related to extreme brittleness.In this work,kinetic surface roughening of Mg-Zn-Ca metallic glass thin films was investigated to quantitatively describe the surface evolution.Then the size effect of deformation mode transition in Mg-Zn-Ca metallic glass thin films was researched.Finally,the low-temperature resistivity of Mg-Zn-Ca metallic glass thin films was also studied.The main results are summarized as follows.(1)The growth behavior of Mg-Zn-Ca metallic glass thin films was investigated by scanning electron microscope and atomic force microscope.All thin films with the thicknesses ranging from 49 nm to 1786 nm have a columnar structure with size increasing with the thickness,and the local roughness at typical length scales of below 100 nm rises with thickness,indicating an anomalous growth mode.And the enhanced structural heterogeneities in thinner film manifested by more interfacial fraction between adjacent columns serve as fertile sites for shear transformations and facilitate distributed plastic flow,resulting in the occurrence of deformation mode transition.(2)The as-deposited Mg-Zn-Ca metallic glass thin films with Ti substrate were co-bent using a mandrel with a radius of 40 mm,which enables the positions for SEM observation to experience about 2%tensile strain.By reducing the thickness of thin films,a transition in deformation mode from highly localized to non-localized deformation was observed in Mg-Zn-Ca metallic glass thin films.It is revealed that increasing temperature and decreasing strain rate can promote the deformation mode transition from highly localized to non-localized.The lower critical thickness of Mg-Zn-Ca metallic glass thin films observed experimentally cannot be explained from the perspective of the elastic energy,therefore,the extreme brittleness rather than higher homologous temperature T/Tg may be responsible for the lower critical thickness,which can facilitate shear banding turn into open cracking.(3)The low-temperature electrical resistivity measurements of Mg-Zn-Ca metallic glass thin films were carried out in the temperature range 4-300 K using the four-probe direct current technique.For the thin film with the thicknesses ranging from 94 nm to 1786 nm,Ziman-Faber diffraction mode can explain their low-temperature properties.For the thin film with the thickness of 49 nm,both Ziman-Faber diffraction mode and two-level systems(TLS)in disordered structures can be responsible for its low-temperature properties.More interface fraction between the columnar structures with enhanced heterogeneity could be the possible origin of TLS scattering in thinnest film,instead of denser packing.