Macro-and Micro-deformation Behavior Of Amorphous Alloys

Bulk metallic glasses (BMGs) have attracted a great attention on fundamental science and practical applications due to their promising physical, chemical and mechanical properties. However, the lack of any significant plastic deformation at room temperature of BMGs is one of the main obstacles to their application in practice. Thus, deeply understanding the macro- and micro- plastic deformation mechanisms of BMGs is always important to the improvement of the ductility of BMGs. In this thesis, by using micro- and nano- indentation methods and uniaxial compression test, the plastic deformation behavior of several BMG systems was investigated, focusing on the physical nature of the pressure sensitivity index and the strain hardening index of the BMGs. The morphology and evolution of the shear bands around the indentation obtained by the different indentation tests conditions were observed in the BMGs with the different pressure and the relationship between the pressure sensitivity index and the shear band patterns of the BMGs was set up. Moreover, the serrated flow feature at different loading conditions in several BMG systems was characterized by nanoindentation measurements, and the effect of structural relaxation on the plastic deformation behavior was investigated. Finally, the extrusion behavior of a Zr-based BMG was investigated. The primary conclusions are summarized as follows:1. The deformation of BMGs exhibits the pressure sensitivity which is reflected by the pressure sensitivity index. The pressure sensitivity index is strongly dependent on the structure of BMGs. With the increase of the degree of the structural relaxation, the pressure sensitivity index increased. Physically, the pressure sensitivity index reflects the atom friction.2. The morphology and evolution of the shear bands around the indentation correlate to the pressure sensitivity index: Under the Vickers and Rockwell indentation conditions, the include angle between two family shear bands decreases with the increase of the pressure sensitivity index, while, under the nano-indentation condition, the curvature of the load-depth curve changes with the change of the pressure sensitivity index. The pressure sensitivity index can be obtained through the measurement of the include angle between two family shear bands around the Vickers indentation.3. During nano-indentation test, some BMGs with the proper glass transition temperature exhibit the serrated flow feature. The appearance of the serrated flow phenomenon is not only dependent on the loading conditions, but also dependent on the structure of BMGs. With the increase of the structure relaxation degree, the critical loading rate for the appearance of the serrated flow decreases.4. Pile-up of materials was observed around the nano-indentation in La-, Zr- and Ni-based BMGs. The altitude of pile-up of materials related to the hardening index.5. The Nd- and Zr- based BMGs with the presence of the in-situ crystalline phases exhibit high strength and room-temperature ductility, compared to the monolithic BMGs. The feature of the deformation of the composites is strongly dependent on the shape and size of the crystalline phase in the BMGs.6. The extrusion deformation of a Zr-based BMG is strongly dependent on the temperature and the strain rate. At the temperature in the range of 355～415℃, the stress for the steady flow decreased from 1.5KN to 0.5KN with the temperature increasing. Under the temperature of 395℃and the strain rate of 0.05mm/s, the stress for the steady flow is up to 4KN.