DUCTILITY ENHANCEMENT AND STRUCTURAL ORIGIN OF DUCTILE-BRITTLE TRANSITION IN BULK METALLIC GLASSES

Brittleness largely restricts promising applications of the metallic glasses as a new engineering material. Understanding fundamental amorphous structure, deformation mechanisms and search for ways to enhance its ductility is imperative. Among these, establishing a valid structure-property relationship is particularly important. Following these thoughts, a series of research works are conducted. Both the finite element simulation and ins-situ transmission electron microscopy were conducted to investigate the size effect in amorphous ZrCu nanopillars. Studies demonstrate that the deformation is localized near the top of the metallic glass pillars, which looks absent from outside, but form inside. By assigning the free volume constitutive relation to the metallic glass, the radial shear bands were observed when indenting directly a bulk metallic glass, while extra semi-circular shear bands were found when a bonded-interface is introduced as in experiments. Ductility enhancement mechanisms in the titanium thin film coated bulk metallic glasses were investigated with both the Rudnicki-Rice instability theory and free volume model. Reflection of the shear band at the film/substrate interface and shear band branching were observed. On top of that, the effect of adhesion between the film and substrate and the film thickness were also investigated. Shear bands in the BMG composites are found initiate from the second phase/matrix at an angle or ~45 o, forming a blocking mechanism to the shear bands propagation, contributing to ductility improvement. Finally, statistical nanoindentation experiments were employed to study the structure-mechanical property relationship of the metallic glass. The statistical nanoindentation technique finds that the pop-in load and the corresponding maximum shear stress increases gradually with increasing degree of structural relaxation, accompanied with a decrease in the statistical variation. A quantitative model incorporating both thermally-activated and defect-assisted processes is developed to understand the pop-in statistics, in which the pre-existing defects, or soft zones, are distributed randomly in the hard amorphous matrix. Before performing nanoindentation tests, for reliability of the results, the spherical indenter tip radii were calibrated by taking the machine stiffness into the classic Hertzian solution rather than assuming a constant machine stiffness. By this method, the machine stiffness of the nanoindentation system was also explicitly evaluated