Energy Storage Behaviors In Metallic Glasses During High Intensity Elastic Treatments

Metallic glasses?MGs?are characterized by yield stresses and elastic strain limits far exceeding those of conventional polycrystalline alloys,yet they show strain softening and consequent localized flow in shear bands in compression?poor plasticity?and catastrophic failure in tension?zero ductility?.For bulk MGs,strain softening is the principal hindrance to their wider application as structural materials.To improve the mechanical properties,one of the main approaches has been to attain higher-energy states of the glass by rejuvenation through thermomechanical processing?TMP?.TMP of MGs can,remarkably,induce significant changes in structure and properties,even when the imposed macroscopic strains are well within the elastic regime.The changes can take the glasses to higher-energy‘rejuvenated'or lower-energy‘aged'states,with rejuvenation being of particular interest as it improves their mechanical properties.It has generally been assumed that the induced property changes would evolve monotonically with the extent of processing.In this dissertation,we treat various MGs with ultrasonic hammering process and show that with sufficiently intense ultrasonic elastic processing,the intrinsic structural competition between?i?damage,and?ii?repair facilitated by increased atomic mobility,can lead to oscillatory energy storage.This mimics that seen in viscous flow of glass-forming liquids,but processing of the glasses in the elastic regime gives much higher amplitude of oscillation.Interestingly,we observe energy oscillation through cryogenic thermal cycling?CTC?of a La-based MG,which suggests that the energy oscillations might be a generic feature of MGs during intense elastic treatments.The uncovering of stored energy oscillations forces reconsideration of how to optimize thermomechanical processing of metallic glasses,and may provide new opportunities.We also study the role of chemical heterogeneity on rejuvenation behavior during CTC.Structural heterogeneity of metallic glass is critical in cryogenic rejuvenation but chemical heterogeneity is seldom explored.Here we show that cryogenic rejuvenation of a nominal La₆₀Al₂₅Ni₁₅?at.%?metallic glass can be enabled with little impurities of Ce and Fe inside the material.Besides,these impurities can significantly enlarge the plastic strain conveyed by the material in bending.In addition,the heterogeneous structure is observed in the sample containing these chemical heterogeneities.Although the glass forming ability of the metallic glass is worsened,our study highlights the impact of chemical heterogeneity on the cryogenic rejuvenation of metallic glasses.