Transition Of Deformation Modes In Amorphous Alloy Film And Its Intrinsic Mechanism

Amorphous alloy also often referred to as metallic glass, is a relative new member of amorphous solid materials family. As a new-structure metal, amorphous alloy integrates characters of glass, metal, solid and liquid. Amorhpous alloy exhibits very unique properities, meanwhile this new material could be an important part of model system in the material science and condensed matter physics reaserches. As emerged more than half a century, it has been at the cutting edge of material researches though it was treated as a kind of "stupid alloy" at the very beginning. From the sight of industrial application, amorphous alloy researchers are faced with two basic scienctific issues:what is the mechanism of deformation; what is the relation between inernal structure and external performance such as mechanical property and deformation behavior. In this thesis, the main research contents are carried out around these two key questions. We focus on the size effect of deformation mode by the defomation behavior in amorphous alloy film. On the other side, we measure the size of "shear transformation zone" in amorphous alloy by experimental methods. Thus we could analysis the deformation behavior at the point of the basic rheology unit. The main results are summarized as follows.(1) By depositing on the polished Ti substrate, Ni-Nb amorphous allopy thin film could be deformed with the bending of substrate. We study the deformation behavior of5%tensile area of thin film by the SEM. As the thickness decreases, defomation mode could be changed from localized shearing to non-localized deformation. In addition, there exists a critical transition zone for the two deformation modes, i.e within a thickness range, the deformation mode is neither complete shearing or non-localized deformation. By annealing of the Ni-Nb thin film, we find the structure relaxation could reduce the critical thickness for non-localized deformation, the "thickness window" is also shorten. Based on the Griffith crack theoy, we theoretically calculate the critical thickness for non-localized deformation at the angle of driving energy for shear banding. Considering the substrate constraint effect,we introduce a paremeter to describe the transfer ratio of thin film elastic energy. Annealing would enlarge the Young’s modulus and weaken the adhension between film and substrate,thus results in the reduction of critical thickness for non-localized deformation. (2) The transition in deformation mode from highly localized to non-localized deformation was investigated in Ni6oNb4o glassy film bymonitoring the reduction in thickness during film/substrate co-bending. It is revealed that in addition to the film thickness, the mode of plastic deformation depends on the stress state. With the reduction in thickness of thin film, tensile stress can efficiently suppress the change in deformation mode from highly localized to non-localized deformation in comparison with compressive stress. A mechanism for the stress-state-dependent deformation mode change in glassy alloys is discussed on the basis of the pressure/stress effect of plastic deformation and Griffith’s crack-propagation criterion. This study provides distinct evidence of the deformation mode change in metallic glassy film via the variation in stress state, and also sheds light on the deformation mechanism of glassy alloys.(3) Both the effects of temperature and strain rate on the deformation behavior of metallic glass thin films (Ni60Nb40, Gu44Zr44Al12Zr55Cu15Ni13Al17, Pd79P21, and Pd41Ni39P20) were systematically investigated. The evolution of surface morphology of magnetron sputtered thin films co-bent with the Ti substrate was monitored. A transition from highly localized deformation to non-localized deformation was observed at various temperatures and strain rates by reducing the film thickness to a critical thickness, which can be understood on the basis of Young’s modulus, Poisson’s ratio, Griffith’s crack-propagation criterion and elastic energy transferring efficiency of the material. Temperature and strain rate dependence of the critical thickness for transition are discussed to shed light on the deformation mechanism of glassy thin films.(4) By nanoindentation technique, we study the creep behaviors of eight kinds of amorphous alloy films. A phenomenon was observed that the higher Tg or Young’s modulu an amorphous alloy film has, the stronger resistance to creep could be detected, and also a lower strain rate sensitivity. The size of "shear transformation zone" could be calculated from the steady-state creep. We found that the material has better fluidity during the non-localized deformation (creep), its STZ size is smaller. This result indicates the STZ size not only influences the plasticity of amorphous alloy but also plays an important part on the transformation of deformation modes. Weather in our work or other groups’researches on the mechanical properties of amorphous alloy nanopillars, there exists a tendency that if an amorphous alloy owns higher Tg, its critical size for non-localized deformation would be larger. That is to say, smaller STZ size could promote the non-localized deformation.(5) Using nanoindentation technique, we study the STZ siz of Ni-Nb amorphous alloy film in different structure state and experiment methods. The results show that structure relaxation could remarkably enlarge the STZ size in all the tesing. Meanwhile larger STZ size would be detected if the strain rate during the deformation of thin film is faster. This could explain the reason why structure relaxation and strain rate would affect the critical size for non-localized deformation in amorphous alloy in theaspect of internal rheology unit.