The anelastic deformation behavior of Pd_(20)Pt_(20)Cu_(20)Ni_(20)P_(20) high-entropy metallic glass was probed by monitoring the stress relaxation and recovery processes. The stress relaxation under consecutive strai...The anelastic deformation behavior of Pd_(20)Pt_(20)Cu_(20)Ni_(20)P_(20) high-entropy metallic glass was probed by monitoring the stress relaxation and recovery processes. The stress relaxation under consecutive strain steps can be described by the Kohlrausch-Williams-Watts(KWW) function. In addition, considering a hierarchy of relaxation processes related to the structural heterogeneity, a constitutive model is proposed in order to describe the whole process of stress relaxation and determine the contribution of different time scales. Moreover, a crossover from stochastic activation to percolation of flow defects with the ultimate strain can be observed during stress relaxation process. The anelastic recovery process after a strain step is studied as a function of the initial strain level and characterized by means of a direct spectrum analysis. The peaks in the recovery time-spectra revealed the evolution of flow defects in Pd_(20)Pt_(20)Cu_(20)Ni_(20)P_(20) high-entropy metallic glass. The understanding of the atomic free-volume zones effect and the anelastic deformation provides important insight into how atomic structural features affect the deformation behavior of high-entropy metallic glasses, and may provide a new avenue into the improvement of their mechanical properties.展开更多
Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25 - 500). It is observed that all the positron parameters are strongly dependent on N when N≤2...Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25 - 500). It is observed that all the positron parameters are strongly dependent on N when N≤250, and then tend to constant levels when N≥250. The variations of the longest-lived component (I3,τ3) show that the size of free- volume holes has a minimum at N - 100 and the hole concentration decreases with N. These trends may be due to the variations in crystallinity, the chain ends, entanglements and the Van der Waals interaction between segments. The variations of the second component (I2,τ2) with N reflect the changes of defect properties in crystalline regions of PEG.展开更多
Metallic glasses(MGs)exhibit exceptional mechanical properties,but their application is often limited by brittleness.At elevated temperatures near the glass transition(T_(g)),they undergo homogeneous viscoplastic defo...Metallic glasses(MGs)exhibit exceptional mechanical properties,but their application is often limited by brittleness.At elevated temperatures near the glass transition(T_(g)),they undergo homogeneous viscoplastic deformation,a regime commonly described using free volume(FV)theory.Despite its prevalence,the quantitative accuracy and applicability of FV models,particularly for transient behaviors,remain under investigation.This study examines the homogeneous rheology of a LaCeYNiAl high-entropy MG(HEMG)between 475 K and 490 K,and critically assesses the relevance of two prominent FV model formulations.Experimental characterization includes dynamic mechanical analysis and uniaxial tensile tests across various strain rates.The tensile data are subsequently analyzed using two elasto-viscoplastic constitutive frameworks incorporating distinct FV evolution kinetics:Spaepen's original formulation(model 1),and the bimolecular annihilation kinetics proposed by Van den Beukel/Sietsma(model 2).Our analysis reveals that model 1,when applied to steady-state flow,yields physically inconsistent negative parameters,calling its validity for homogeneous deformation into question.Model 2 demonstrates better qualitative agreement with the experimental stress–strain curves but still fails to accurately reproduce the stress overshoot features.Moreover,fitting model 2 requires unphysically low Young's modulus values and produces unusual negative apparent activation energies for key kinetic parameters,suggesting limitations in the model structure(e.g.,neglecting explicit viscoelasticity)or possibly unique behavior in HEMGs.These findings highlight significant shortcomings of standard FV models in quantitatively capturing the homogeneous deformation of this HEMG,particularly its transient characteristics,and underscore the need for more refined constitutive descriptions.展开更多
基金supported by the NSFC (Grant No. 51971178)the Natural Science Foundation of Shaanxi Province (Grant No. 2021JC-12)+3 种基金financial support from MICINN (grant FIS2017–82625-P)Generalitat de Catalunya (Grant 2017SGR0042)sponsored by Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (No. CX202031)China Scholarship Council (CSC) under Grant 202006290092。
文摘The anelastic deformation behavior of Pd_(20)Pt_(20)Cu_(20)Ni_(20)P_(20) high-entropy metallic glass was probed by monitoring the stress relaxation and recovery processes. The stress relaxation under consecutive strain steps can be described by the Kohlrausch-Williams-Watts(KWW) function. In addition, considering a hierarchy of relaxation processes related to the structural heterogeneity, a constitutive model is proposed in order to describe the whole process of stress relaxation and determine the contribution of different time scales. Moreover, a crossover from stochastic activation to percolation of flow defects with the ultimate strain can be observed during stress relaxation process. The anelastic recovery process after a strain step is studied as a function of the initial strain level and characterized by means of a direct spectrum analysis. The peaks in the recovery time-spectra revealed the evolution of flow defects in Pd_(20)Pt_(20)Cu_(20)Ni_(20)P_(20) high-entropy metallic glass. The understanding of the atomic free-volume zones effect and the anelastic deformation provides important insight into how atomic structural features affect the deformation behavior of high-entropy metallic glasses, and may provide a new avenue into the improvement of their mechanical properties.
基金The Project Supported by National Natural Science Foundation of China
文摘Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25 - 500). It is observed that all the positron parameters are strongly dependent on N when N≤250, and then tend to constant levels when N≥250. The variations of the longest-lived component (I3,τ3) show that the size of free- volume holes has a minimum at N - 100 and the hole concentration decreases with N. These trends may be due to the variations in crystallinity, the chain ends, entanglements and the Van der Waals interaction between segments. The variations of the second component (I2,τ2) with N reflect the changes of defect properties in crystalline regions of PEG.
基金supported by the National Natural Science Foundation of China(Grant Nos.52271153 and 12472069)the Natural Science Basic Research Plan for Distinguished Young Scholars in Shaanxi Province(Grant No.2021JC-12)。
文摘Metallic glasses(MGs)exhibit exceptional mechanical properties,but their application is often limited by brittleness.At elevated temperatures near the glass transition(T_(g)),they undergo homogeneous viscoplastic deformation,a regime commonly described using free volume(FV)theory.Despite its prevalence,the quantitative accuracy and applicability of FV models,particularly for transient behaviors,remain under investigation.This study examines the homogeneous rheology of a LaCeYNiAl high-entropy MG(HEMG)between 475 K and 490 K,and critically assesses the relevance of two prominent FV model formulations.Experimental characterization includes dynamic mechanical analysis and uniaxial tensile tests across various strain rates.The tensile data are subsequently analyzed using two elasto-viscoplastic constitutive frameworks incorporating distinct FV evolution kinetics:Spaepen's original formulation(model 1),and the bimolecular annihilation kinetics proposed by Van den Beukel/Sietsma(model 2).Our analysis reveals that model 1,when applied to steady-state flow,yields physically inconsistent negative parameters,calling its validity for homogeneous deformation into question.Model 2 demonstrates better qualitative agreement with the experimental stress–strain curves but still fails to accurately reproduce the stress overshoot features.Moreover,fitting model 2 requires unphysically low Young's modulus values and produces unusual negative apparent activation energies for key kinetic parameters,suggesting limitations in the model structure(e.g.,neglecting explicit viscoelasticity)or possibly unique behavior in HEMGs.These findings highlight significant shortcomings of standard FV models in quantitatively capturing the homogeneous deformation of this HEMG,particularly its transient characteristics,and underscore the need for more refined constitutive descriptions.
