Effects Of Stress And Temperature On The Creep Behavior Of Metallic Glasses Measured By Nanoindentation

Metallic glasses（MGs）exhibit extremely high strength,hardness（H）,and excellent corrosion resistance.They are attractive as novel structural materials used in the field of three-dimensional（3D）printing,molding,embossing,imprinting,etc.It has been demonstrated that bulk MGs will be sharply softened when they are heated to a temperature close to the supercooled liquid region or treated by ultrasonic vibration at ambient temperature,which can enhance the thermoplastic formability of MGs by reducing resistant stress around their glass transition temperature（T_g）or the stress-induced deformation.The plastic flow of MGs is proposed to be closely related to structural rearrangement at the atomic scale during deformation.Despite these facts,the underlying mechanism for the thermoplastic deformation of MGs at the temperature around T_g is not clear.And the structural evolution of MGs under ultrasonic vibration as well as the underlying physical origin for ultrasonic-assisted plastic flow still remain unknown.It is proposed to be associated with the generation of free volume and/or the activation of flow defects during cyclic loading with high frequency.Whereas,it is still unclear how are those defects distributed in the glassy matrix and their influences on the deformation mechanism,especially in various MG systems.Nanoindentation has been proven to be an effective approach to determine the mechanical properties of structural materials at the microscopic scale,e.g.,H,elastic modulus（E）,as well as time-dependent mechanical responses such as indentation creep,and so on.In this work,the plastic flow of MGs at different temperatures,or treated by ultrasonic vibration was studied by nanoindentation,the Maxwell-Kelvin model and the relaxation-time spectra were introduced to detect the activation of defects and the structural evolution during creep deformation.Combined with thermodynamic parameters,the relationship between creep behaviors and the structural heterogeneity as well as the defect-activation mechanism in MGs was investigated.The softening mechanism of MGs under cyclic loading and at different temperatures was discussed.The mechanical behavior of the Pd₄₀Cu₃₀P₂₀Ni₁₀,Zr₃₅Ti₃₀Cu_8.25Be_26.75,and La₅₅Al₂₅Ni₅Cu₁₀Co₅MGs after ultrasonic vibration with the applied energy of 60 J,140 J,and 30 J were systematically investigated by nanoindentation,respectively.（1）After ultrasonic vibration,a notable mechanical softening is observed in both Pd-and La-based MGs,however,this phenomenon has not been found in the Zr-based MG.The plastic flow behaviors of these three MG systems are quite different.A notable softening after 60 J ultrasonic vibration treatment is demonstrated in Pd-based MG by the reduction of hardness and elastic modulus about 25%and 40%,respectively.The creep displacement,pile-up and maximum penetration depth only change slightly with the loading rate,exhibiting a steady flow behavior in the Pd-based MG.After ultrasonic-vibration treatment,the La-based metallic glass sample converts into a high-energy softening state along with a pronounced creep deformation as well as the reduced H and E.While the pile-up and maximum penetration depth exhibit a high loading rate sensitivity,displaying a quite unstable plastic flow behavior.Whereas,the H and E of Zr-based MG almost remain unchanged,and the creep displacement also shows a slight change with a low loading rate sensitivity,indicating that the plastic flow almost has not been affected by cyclic loading in Zr-based MG.（2）The activation processes of flow defects in Pd-,La-and Zr-based MGs after treated by ultrasonic vibration are quite different as well.In the Pd-based MG,the first peak of relaxation-time spectra becomes more intense after ultrasonic vibration,indicating that the activated defects with short characteristic relaxation time play a dominant role in cutting down the resistance to shear,which eventually leads to the evident mechanical softening during ultrasonic-assistant plastic deformation of the Pd-based MG.In the La-based MG,the intensity of both the first and second peaks of relaxation-time spectra alters remarkably as a function of loading rates.It demonstrates that both defects with the long and short characteristic relaxation time are activated in the La-based MG under ultrasonic-frequency cyclic loading.In the Zr-based MG,the intensity of both the first and second peaks of relaxation-time spectra increase obviously at the loading rate higher than 50 m N/s,suggesting that flow defects are intended to be activated at a high loading rate.It is also found in DSC traces that the change of excess free volume in Pd-,and Zr-based MGs after ultrasonic vibration is not significant,however,the underlying reasons are different.In the Pd-based MG,the free volume generated during vibration treatment almost annihilates at the same time.The activation of small defects is accompanied with consumption of the existent large inherent defects,which leads to a more homogenous glassy structure along with a stronger liquid behavior in the Pd-based MG.The stress-induced defect redistribution,rather than structure rejuvenation,leads to the stable plastic flow of the Pd-based MG.Compared with the Pd-based MG,the Zr-based MG exhibits a more homogenous glassy structure,which is barely affected by the external environment such as cyclic loading condition.Only a few defects are activated under ultrasonic-frequency cyclic loading.It is consistent with the high T_g and wide supercooled liquid region of the Zr-based MG.Unlike Pd-and Zr-based MGs,large amounts of defects with longer characteristic relaxation time are activated in the La-based MG.The excess free volume generated under ultrasonic-frequency cyclic loading improves the degree of structural disorders in the amorphous matrix,which leads to the pronounced mechanical softening accompanied by structural rejuvenation in the La-based MG.Besides the cyclic-loading effect induced by the ultrasonic-vibration treatment,the creep behavior of the Fe₆₆Tb₅B₂₃Nb₆ MG at different temperatures and its underlying deformation mechanism were investigated by using nanoindentation.It is found that both E and H decrease dramatically with increasing temperatures,especially at the temperature around 723 K,along with a notable mechanical softening.Anelastic deformation plays an important role in the plastic flow process of the Fe-based MG,where the number of activated defects depends on the temperature:the defects in the strongly-bonded hard region related to shorter characteristic relaxation time more prefer to be activated at the temperature close to T_g,e.g.,723 K,while the defects in the weakly-bonded soft region related to the longer characteristic relaxation time intend to be activated at room temperature.This is the underlying reason for the mechanical softening of the Fe-based MG at the temperature approaching the supercooled liquid region.With increasing loading rates,the viscoplastic flow gradually increases and internal friction due to the anelastic deformation decreases obviously,which leads to the enhanced thermoplastic deformation.In this work,we find that the Pd-based MG exhibits a more homogeneous and stable plastic flow behavior after treated by ultrasonic vibration.Besides,its T_g is close to the ambient temperature.It can be used as structural materials in the fields of molding,embossing,imprinting,etc.The Fe-based MG exhibits an excellent thermoplastic formability at the temperature approaching T_g,which is more suitable as a mold material for hot embossing.Our work provides a clear picture of the ultrasonic-assistant structural rearrangement in non-equilibrium-state MGs based on their anelastic creep response,which may enlighten the physical origin of the superplastic flow in the glassy system during stress-assisted molding and cold joining process.This work may help us reveal the relaxation mechanism of MGs as well as other disordered materials during the ultrasonic-assistant plastic flow process at ambient temperature,which can further promote industrial applications of glassy materials based on the cold processing with low in production cost.