Resumen de Dynamics of metallic glasses explored by mechanical relaxation
Chaoren Liu
Dynamic mechanical analysis (DMA) is often used to explore the relaxation dynamics of metallic glasses (MGs). Secondary relaxations in metallic glasses appear as an excess wing, as a shoulder of the primary relaxation peak or as a differentiated low temperature peak on the loss modulus E''(T). In order to differentiate this relaxation from a-relaxation, which is due to collective movement and directly related to the elastic (solid glass) to viscous (supercooled liquid) transition, it is termed as ß-relaxation. Its origin and main characteristics are still not clear and the understanding of the ß-process is still developing. Early results based on DMA suggested that it is the result of anelastic events, as the system is in a metastable state. It might be originated from diffusion processes, resembling Zener or Snoek relaxation in crystalline materials. In the energy landscape picture it is attributed to jumps between close energy minima separated by a low energy barrier. It is also treated as a process related to the activation of shear transformation zones (STZ) or flow units. In this thesis, the mechanical relaxation of MG is explored by quasi-static measurements like creep and stress relaxation, and by DMA. The current theoretical models and experimental data available in literature are revised and discussed. Following, three different MG systems, namely, Cu46Zr46Al8, Pd42.5Ni7.5Cu30P20 and Fe55Cr10Mo14C15B6 are analyzed and it is clarified that the different sub-Tg relaxations manifested on the DMA behavior have different origins. Finally, we discuss the implications of the relaxation dynamics characterization presented in this work on the mechanical properties of these materials. The understanding of the relaxation behavior and some related phenomena like physical aging, mechanical deformation and internal damping is seen as fundamental to improve our knowledge of MGs, this leading to new alloys with improved mechanical performance.
