Investigation Of Flow And Relaxation Behavior In Metallic Glasses

Bulk metallic glasses(BMGs)are useful in a wide range of potential applications due to their unique properties,including high strength,large elastic strain limit,high hardness,and good soft magnetic properties,which have attracted more and more extensive attentions.However,because of the lack of microstructure information,the origin of flow behavior,glass forming ability,glass transition is still a mystery.We try to explain the flow behavior and mechanical relaxation behavior of MGs from the flow units point of view,and find new MGs system with excellent glass formation ability and low glass transition temperature.The microscopically localized flow under a constant applied strain in apparent elastic region or before yielding in MGs is hard to detect.We present a stress relaxation approach to reveal the evolution of the localized flow with loading time or loading strain in MGs.The unstable nano-scale liquid-like regions acting as flow units in glasses can be activated by loading time and loading strain,and their evolution with loading time or loading strain shows a crossover from localized activation to cascade as the proportion of the flow units reaches a critical percolation value.The evolution of flow units with loading time or loading strain finally lead to a mechanical elastic-to-plastic transition or macroscopic plastic flow.The fractions of flow units or free volume in MGs can be controled by cooling rates.The cooling rate has the dramatic effect on the ?-relaxation behaviors and the mechanical properties of the MGs.The results indicate that the ?-relaxation has correlation with the inhomogeneous microstructure and/or the density of the potential flow units in MGs.We find a series of Fe-based MGs with unusual distinct ?-relaxation peaks in their relaxation spectra.The investigation on the ?-relaxation behavior of Fe-based MGs suggests that the magnetic element Fe plays a critical role in the pronounced ?-relaxation behaviors in the Fe-based MGs.There is an unusual fast dynamic mode in a series of rare earth based MGs,manifested as a distinct fast ?'-relaxation peak in addition to slow ?-relaxation and ?-relaxation peaks.The activation energy of the fast ?'-relaxation is about 12 RTg,which is almost equivalent to the activation energy of localized flow event,and the observed fast ?'-mode is attributed to the most mobile atoms of Co/Ni or RE-Co/Ni pairs in the flow units in the MGs with microstructural heterogeneous.We report the formation of LaGa-based bulk MGs with low glass transition temperature and good glass forming ability.With minor addition of extra elements such as Co,Ni,Fe,Nb,Y,and Zr,the critical diameter of the fully glassy rods of the La-GaCu matrix can be enhanced to at least 5 mm in diameter.The La-Ga-Cu MGs show the abnormal crystallization induced by volume expansion behavior,and the abnormal volume expansion behavior is attributed to the liquid metal element of Ga.