Correlation Between Relaxation And Mechanical Behavior In Metallic Glasses Through Molecular Dynamics

Dynamic relaxation is an inherent and universal feature of amorphous materials.As the material with unique microstructure and mechanical and functional properties,metallic glasses(MGs)offer a simple and effective system for studying relaxation and related issues in glass science.The mechanical properties of MGs are the most concerned properties in amorphous materials.At the same time,it has long been recognized that the relaxation of MGs is essentially related to their disordered structural characteristics and mechanical behaviors.Therefore,understanding the internal relationship between the mechanical behavior and dynamic relaxation mechanism of MG is of great significance for its application and understanding the nature of amorphous materials,which is also one of the current important scientific issues in this field.However,due to the long-range disorder of the intrinsic structure of MG and its complex dynamic relaxation behavior,it is difficult to capture the structural characteristics of MG from both experiments and computer simulations.Therefore,it is hard to understand the structural origin of the correlation between dynamic relaxation and mechanical behavior from microscopic perspective.Meanwhile,due to the limitation of technical level,it is currently difficult to carry out the long-time scale computer simulations,which sets another shackles for the study of the‘mechanical behavior-structure-relaxation'correlation in MG.In this paper,the mechanical behaviors of multiple metallic glass systems under different mechanical loading conditions are studied by systematic and long-time molecular dynamics simulation.Based on the quantitative analysis of the structural characteristics of its micro dynamics and combined with the relevant characteristics of dynamic relaxation,the‘mechanical behavior-structure-relaxation'correlation in MG was established.The main works of this paper are summarized as follows:The potential energy changes of two typical metallic glass systems,Cu₆₅Zr₃₅ and Ni₈₀P₂₀,under periodic shear vibration were systematically studied.It is found that for an appropriately small deformation,the MG might lower its potential energy at a rate very much faster than iso-thermal annealing,which is called accelerated ageing.On the other hand,large deformation could render the MG with higher energy,which is called rejuvenation.It is also found that both of these two phenomena have obvious temperature effect,and there is a characteristic temperature(0.6T_g),at which the accelerated ageing effect is the most significant.In the meantime,it is proved that vibration induced accelerated ageing is correlated strongly with the strain fields of the string-like motions of?relaxation,whereas the rejuvenation is associated with the activation of?relaxation exhibited by the formation of nano-scale shear bands.Finally,the internal relationship between dynamic relaxation,accelerated ageing and rejuvenation was revealed.The mechanical behavior and microstructure of Al₉₀Sm₁₀ MG during tensile deformation were studied systematically.It is demonstrated that the transition from stress-overshoot to nonovershoot-yielding(SNT)is caused by the mobility decoupling of solute and solvent atoms.In particular,the SNT and the?₂ relaxation follow the same time-temperature relation and microscopic origin,which reveals the close link between the yielding behaviors and the newly identified?₂ relaxation in MG.In order to get rid of the shackles of classical molecular dynamics,the activation process of atoms involved in STZ(STZ_atoms)was studied in laboratory time scale by well-tempered metadynamic molecular dynamics.It is found that in Al₉₀Sm₁₀MG,after the STZ_atoms are activated,they will form a string-like motion and drive the surrounding atoms to move together,and finally form a complete long string.This process has the same characteristic time as the?relaxation.However,in the Y₆₅Cu₃₅ MG,due to the limitation of its frozen substrate,the atoms involved in STZ have the trend of forming string-like motion,but such string-like motion is difficult to really form.Therefore,it shows that Al₉₀Sm₁₀MG has more obvious?relaxation and is easy to deform than Y₆₅Cu₃₅ MG.It reveals the internal correlation among STZ,?relaxation and string-like motion.The results of this study show that it is feasible and effective to establish the‘mechanical behavior-structure-relaxation'correlation by quantitatively analyzing the structural origin of mechanical behavior and relaxation in glass system,which provides new challenges and opportunities for further understanding of mechanical behaviors and glass relaxations and has positive significance for promoting the development of glass materials.