Influence of thermal treatments and plastic deformation on the atomic mobility in Zr_50.7Cu₂₈Ni₉Al_12.3 bulk metallic glass

The atomic mobility in Zr_50.7Cu₂₈Ni₉Al_12.3 bulk metallic glass has been evaluated as a function of temperature and the influence of different treatments (thermal annealing, plastic deformation) has been investigated using mechanical spectroscopy and nanoindentation technique. In particular the loss factor has been measured. This parameter is connected to the energy loss during the application of a periodic stress and therefore is sensitive to atomic movements. Master curves can be obtained, confirming the validity of the time-temperature superposition principle. The atomic mobility is reduced during physical aging (also called structural relaxation) but increased after a plastic deformation (a rejuvenation of the material is then induced). In the framework of the nanoindentation tests and mechanical spectroscopy, the concentration of "defects" in metallic glasses increases by deformation (i.e. cold-rolling) while decreases after structural relaxation and crystallization. These results are discussed using the concept of quasi-point defects, which assist the atomic movements.