PhD ceremony Mr. O. Kuzmin: Intrinsic and extrinsic size effects in metallic glasses

PhD ceremony: Mr. O. Kuzmin

Dissertation: Intrinsic and extrinsic size effects in metallic glasses

Promotor(s): prof. J.T.M. de Hosson

Faculty: Mathematics and Natural Sciences

A metallic glass represents an exceptional class of materials which possesses rather unique properties for metals. Size effects in metallic glasses can be potentially divided into intrinsic or extrinsic. The deformation is defect-nucleation-controlled in larger pillars but becomes propagation-controlled in smaller pillars. Pillars with a diameter smaller than a certain diameter show a homogeneous flow behavior without shear banding during compression. The initial structure and chemical composition of MGs also affect the deformation behavior at the nanoscale. The Al86Ni9Y5 taper-free metallic glass showed the highest transition threshold for brittle behavior, i.e. above pillar diameter of 300 nm. This composition shows plastic deformation becoming even more apparent through softening and toughening effects.

Extrinsic size effects as opposite to intrinsic are caused by dimensional constraints due to preparation methods or influence of preparation technique. These extrinsic constraints arise due to geometry of the specimens, aspect ratio or are influenced by the FIB interface layer. The pillar aspect ratio has a large influence on deformation behavior representing a clear extrinsic size effect.

Tensile experiments were performed in order to separate the difference between extrinsic and intrinsic size effects. The transition threshold from inhomogeneous flow which is characterized by shear banding events is shifting compare to compression experiments towards a lower diameter of the specimens. It indicates the separation of volumetric extrinsic size effect which was concluded by compression experiments and intrinsic size effects on MG composition which is related to collective behavior of STZs.