Quasi-point Defect Research Articles

Investigation on the creep mechanism of PA6 films based on quasi point defect theory

Polyamide 6 (PA6) films with significant α relaxation process was selected as the model system. The creep behavior and rheological mechanism during deformation in the amorphous regions of semi-crystalline polymers are systematically investigated by carrying out creep experiments. Based on the quasi point defect (QPD) theory, the complete physical process of PA6 film creep behavior from elasticity to viscoelasticity and viscoplasticity was analyzed and modeled from the perspective of structural heterogeneity. The results demonstrate that the creep deformation of PA6 film is a typical thermo-mechanical coupling and nonlinear mechanics process, and potential creep mechanisms corresponds to stress-induced local shear deformation enhancement and thermal activation-induced particle flow diffusion. The elastic-plastic transition involved in the creep deformation process of semi-crystalline polymer originates from the activation of quasi-point defective sites in the amorphous region, the expansion of sheared micro-domains and irreversible fusion. The generalized fractional Kelvin (GFK) model is proposed, and the physical meaning of parameters is explained by combining the quasi point defect theory and creep delay spectrum(L(τ)). Finally, the effectiveness of the GFK model and the QPD theory in studying the deformation behavior of PA6 films was validated by comparing experimental data with theoretical results, which theoretically reveals the structural evolution of PA6 film during creep process.

Probing microstructural heterogeneity of La-based amorphous alloy under versatile mechanical stimuli

The intrinsic structural heterogeneity of amorphous alloy is closely related to the thermodynamics and dynamical behavior, such as relaxation/crystallization, glass transition and plastic deformation. However, the structural information is submerged into the meta-stable disordered long-range structure, which makes it very difficult to explore the structural heterogeneity of amorphous alloy. A mechanical excitation factor is insufficient to effectively describe the heterogeneity of the microstructure in amorphous alloy, particularly the correlation between structure and dynamics. To explore the essence of the structure in amorphous alloy, it is necessary to consider the different mechanical stimuli. La<sub>62</sub>Cu<sub>12</sub>Ni<sub>12</sub>Al<sub>14</sub> amorphous alloy is selected as the model system, dynamic mechanical process is probed by dynamic mechanical analyzer (DMA). The contributions of <i>α</i> relaxation process and <i>β</i> relaxation process are described in the framework of the quasi-point defect theory. Based on the quasi-point defect theory, the <i>α</i>-relaxation and <i>β</i>-relaxation in the La-based amorphous alloy are separated. Tensile strain rate jump measurements are conducted to study the high temperature rheological behavior of amorphous alloy. The contributions of elasticity, anelasticity, and plastic deformation during the homogeneous flow of amorphous alloy are determined within the framework of quasi-point defect theory. The present work aims to reveal the structural heterogeneities of amorphous alloys under the action of dynamics on various temporal scales. The physical background of the activation, propagation and coalescence of defects in amorphous alloy under different mechanical stimuli are reviewed.

Physics-motivated fractional viscoelasticity model for dynamic relaxation in amorphous solids

Dynamic mechanical relaxation is an important metric to understand the mechanical/physical properties of amorphous solids which are of viscoelastic nature. Due to the heterogenous microstructure, the relaxation behavior of amorphous solids usually shows strong deviation from the Debye relaxation. The distribution of relaxation time derived from either the stretched exponential function (KWW function) or the power law form is probably the most adopted paradigm to describe the non-Debye relaxation. They are essentially the continuous spectrums given in analytical forms. However, whether a real amorphous material conforms to such distribution law remains to be discussed. Here we test the assumption in typical metallic glasses (MGs) as representatives of the general amorphous solids. The mechanical spectrum of a Cu46Zr47Al7 MG in wide frequency domain is probed by the dynamic mechanical analysis technique. It is found that both the KWW function and the modified fractional (MF) model based on power law can well describe the experimental data. As a step forward, we combine the quasi-point defect theory with the MF model to theoretically reveal the feature of temperature-dependent structural evolution in MG. Finally, the distribution of relaxation time corresponding to the experimental data is discretized to argue the theoretically predicted microstructural heterogeneity in the MGs.

Influence of Microalloying Process on Dynamic Mechanical Relaxation of ZrCo-Based Amorphous Alloy

In this study, the dynamic mechanical spectroscopy was used to characterize the effects of minor Ni addition on the relaxation behavior of ZrCoAl bulk metallic glass (BMG). For this purpose, the Kohlrausch–Williams–Watts (KWW) function and quasi-point defect (QPD) model were used for evaluation of relaxation under the different aging temperatures. The results indicated that the Ni addition shifted the relaxation process to the higher temperatures. Moreover, the estimations showed that the activation energy of relaxation was 5.951 eV and 6.205 eV for ZrCoAl and ZrCoAlNi, respectively. It was also revealed that the microalloying process enhanced the structural defects in the system and led to the improvement of dynamic heterogeneity in the BMG. Comparing the physical models, it is suggested that a small change in the structural defects intensifies the dynamic heterogeneity in the material.

Based on internal friction theory: Investigating of Fe-based bulk amorphous alloys on mechanical properties with different Si content

In the current work, internal friction and mechanical properties of a Fe41Co7Cr15Mo14-xC15B6Y2Six (x = 0.5, 1, 1.5 and 2 at.%) bulk metallic glasses (BMGs) was investigated. Investigations demonstrated that the Si content could play an important effect on the mechanical properties of Fe-based bulk metallic glasses. The influence of Si on mechanical properties may be related to the structural heterogeneity in the metallic glasses. The quasi-point defects theory was used to describe the structural heterogeneity. Quasi-point defect concentration represents the degree of structural heterogeneity of amorphous alloys. The addition of Si reduces the quasi-point defect concentration in the amorphous samples, and the low quasi-point defect concentration increases the modulus, hardness and fracture strength of the samples. The quasi-point defect concentration of the sample with 1 at.% Si content is the lowest. Results shows that the mechanical properties of amorphous alloy can be improved by adding appropriate an amount of Si.

Abnormal change in dynamic mechanical behavior of metallic glass by laser shock peening

Laser shock peening (LSP) has been shown to be an effective technique to improve the plasticity of metallic glasses (MGs) by inducing residual stress and structural rejuvenation. However, the microstructural evolution and viscoelastic properties of MGs after LSP remain largely unknown. In this paper, the dynamic mechanical properties of Zr-based MG before and after LSP treatment were investigated. An abnormal increase of storage modulus near the glass transition temperature (Tg) was observed after LSP. An increased deviation between the dynamic mechanical result and the quasi-point defect (QPD) theory prediction was also observed after the LSP treatment. A competing mechanism between the liquid-like region change and the pre-existing fringe-like precipitations is proposed to explain the unusual dynamic behaviors of MGs after LSP treatment. The results provide insightful information about the microstructural changes and viscoelastic properties of the MGs after LSP.

Effect of minor addition on dynamic mechanical relaxation in ZrCu-based metallic glasses

The dynamic mechanical behavior of Zr50-xCu34Ag8Al8Pdx (x = 0 and 2) bulk metallic glasses was probed by mechanical spectroscopy. The results suggest that microalloying retards the relaxation dynamics. The physical aging was analyzed considering a model based on the variation of the concentration of flow units and the Kohlrausch-Williams-Watts (KWW) equation. In the current study, the characteristic parameters of the two models, βc and βaging, reflect the faster annihilation of flow units at higher temperature and the increase of dynamic heterogeneity with temperature, respectively. The mechanical relaxation spectrum was also analyzed. Based on the KWW equation and the quasi-point defect model, replacing Zr with a small amount of Pd can slightly increase the dynamic inhomogeneity and reduce the defect concentration. The analysis according to the HN function suggests that the addition of Pd increases the relaxation time. However, it does not significantly alter the shape of the relaxation time distribution.

Dynamic mechanical relaxation in Zr65Ni7Cu18Al10 metallic glass

Dynamic modulus and internal friction of Zr65Ni7Cu18Al10 metallic glass was probed by dynamic mechanical analysis (DMA) technique. The apparent activation energy of main α relaxation is 4.18 eV. Experimental results can be analyzed in the framework of the quasi-point defect (QPD) theory. Current investigations demonstrated that annealing below the glass transition temperature Tg leads to a decrease in loss factor tanδ of Zr65Ni7Cu18Al10 metallic glass. The evolution of loss factor tanδ with annealing time can be well described by the stretched Kohlrausch-Williams-Watts (KWW) exponential equation. The Kohlrausch exponent βaging is 0.43 of Zr65Ni7Cu18Al10 metallic glass annealed at 573 K.

Mechanical relaxation behavior of Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass

The dynamic mechanical relaxation behavior of Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass was investigated by mechanical spectroscopy. The activation energy of the main α relaxation was determined to be 4.24 eV. In order to well understand the α relaxation process, several physical and phenomenological models such as Debye model, Kohlrausch-Williams-Watt (KWW) model and quasi-point defect (QPD) model were employed to analyze the dynamic mechanical performance. It was found that Debye model was not so persuasive due to the asymmetry of the main relaxation of Zr64.13Cu15.75Ni10.12Al10bulk metallic glass. By contrast, KWW model agreed well with the experimental data if taking Kohlrausch parameter βkww as 0.571. The QPD model, which had a definite physical meaning, could not only match the experimental data perfectly but also correlated the relaxation behavior with the microstructure evolution versus temperature in this bulk metallic glass.

Effects of iron addition on the dynamic mechanical relaxation of Zr55Cu30Ni5Al10 bulk metallic glasses

Dynamic mechanical relaxation behavior of (Zr55Cu30Ni5Al10)100-xFex (x = 0, 1 and 2) bulk metallic glasses was investigated by mechanical spectroscopy. It is found that the main α relaxation of the Zr-based metallic glasses can be well described by the Kohlrausch-Williams-Watts (KWW) function. With the increasing of micro-alloying element content, i.e. iron, the value of Kohlrausch exponent βKWW diminishes for the (Zr55Cu30Ni5Al10)100-xFex (x = 0, 1 and 2) metallic glass, whereas the characteristic relaxation time increases. In the framework of the quasi-point defects theory, the physical correlation factor χ increases with higher iron content below the glass transition temperature Tg. In addition, the parameter a of the quasi-point defect theory can describe the degree of deviations from the Arrhenius law. Based on the theoretical and experimental analysis, it is implied that structural heterogeneity of the Zr55Cu30Ni5Al10 bulk metallic glass could be tailored through minor addition of the iron element.

Modeling of the mechanical behavior of amorphous glassy polymer based on the quasi-point defect theory—Part II: 3D formulation and finite element modeling of polycarbonate

A three-dimensional formulation of the quasi-point defect theory has been developed and the corresponding constitutive equations have been implemented in the finite element package Abaqus, via the writing of an UMAT file. The proposed tool has been completed to support the large strain description and thus can perform a general structure calculation. Referring to a unique set of parameters identified in the part I of this paper for glassy amorphous bisphenol-A polycarbonate (PC-BPA), various commonly used tests were then considered as structural patterns and modeled. The whole mechanical response as well as localization phenomena measured via video technique is well predicted by the calculations. These results also highlight the need to consider the spatial dimension in a sense that the mechanical behavior up to large strain is related to structural modifications, especially for materials such as amorphous polymers that exhibit yield, softening and hardening intrinsic phenomena.

Modelling of the mechanical behaviour of amorphous glassy polymer based on the Quasi Point Defect theory—Part I: Uniaxial validation on polycarbonate

This paper is devoted to the modelling of the viscoelasto-viscoplastic behaviour of amorphous polymers based on the Quasi Point Defects (QPD) theory. A summarized and unified description of the main assumptions is presented in its scalar formulation to compute the complex viscoelasto-viscoplastic behaviour of an amorphous medium in the glassy state from linear to non-linear ranges. Based on an original scenario for the microstructural reorganization caused by an external thermomechanical stimulus and related to the two main relaxation processes, this theory allows a coherent description of the material response under large conditions of temperature and strain rate by using a single set of material parameters. A convincing validation is performed on a ductile bisphenol-A polycarbonate (PC-BPA) by comparison between computed results and experimental measurements.

Correlation between microstructure and internal friction in a Zr41.2–Ti13.8–Cu12.5–Ni8– Be22.5–Fe2 bulk metallic glass

Abstract The microstructural evolution in a Zr–Ti–Cu–Ni–Be–Fe bulk metallic glass (BMG) has been investigated by measurements of dynamical shear modulus and internal friction combined with other analytical methods such as differential scanning calorimetry (DSC), X-ray diffraction (XRD) and high resolution transmission electron microscopy (TEM). When heated from room temperature up to 873 K, the as-received BMG exhibits an exponential increase in internal friction accompanying the strong decrease of storage modulus and the presence of the first loss modulus peak during the dynamic glass transition, which can be well described using quasi-point defect model. The correlative changes of the mechanical response at higher temperature are associated with the crystallisation process of the supercooled liquid phase, which occurs in four different stages. It is shown that the main crystallisation process is completed in the first two stages. With further increasing temperature, the remaining amorphous phases crystallise and/or the metastable crystalline phases are transformed into the stable ones. Isothermal annealing were also performed at temperatures in the supercooled liquid region far below the onset temperature of the crystallisation process ( T x ). Their influence on microstucture and internal friction behaviour of the BMG is also presented in this paper. The most striking result is that the internal friction is very sensitive to the local atomic short range ordering induced by the preheating treatment.

Rheological behavior of cellulose/monohydrate of N-methylmorpholine N-oxide solutions. Part 2. Glass transition domain

Dynamic mechanical experiments performed with original conditions allowed the analysis of a solution containing 15% cellulose dissolved in a monohydrate of N-methylmorpholine N-oxide (NMMO) in the amorphous state. The glass transition zone is studied by dynamic tensile experiments, while dynamic torsion technique is used to determine the viscoelastic behavior in the glassy state. A master curve of the storage and loss modulus versus frequency can be deduced from the isochronal curves measured by both techniques. This work allows one to complete the corresponding master curve obtained for the ‘liquid’ state and presented in a previous work [Rheol. Acta 1998;37: 107]. The measurements below the glass transition temperature exhibit two secondary relaxations. A modeling of the overall viscoelastic behavior, using the Nowick and Berry approach and the quasi-point defect theory, is proposed. Physical parameters deduced from this modeling are then discussed.

Mechanical response of amorphous and semicrystalline poly(ethylene terephthalate) and modelling in frame of quasi point defect theory

The thermomechanical activation of deformation in amorphous and (38%crystalline) semicrystalline poly(ethylene terephthalate) (PET) has been investigated using dynamic mechanical tests and large strain experiments. Activation parameters for both materials are determined in the neighbourhood of the glass transition temperature using stress relaxation experiments. The complete mechanical behaviour of amorphous PET is then analysed in the frame of a molecular model or ‘quasi point defect’ theory. With this aim, a new method is proposed, based on only three isochronal measurements, and leading to the determination of the set parameters. This method makes it possible to reproduce the stress–strain curve over a range of temperatures, as well as the relevant activation parameters. Finally some qualitative explanations are given for the mechanical behaviour of semicrystalline PET.

Plasticized PVC reinforced with cellulose whiskers. II. Plastic behavior

In previous works, the processing of plasticized PVC reinforced by cellulose nanocrystalline whiskers has been presented, as well as its mechanical behavior in the linear viscoelastic domain. The purpose of this work is now to evaluate and model the plastic behavior of such material. Compression tests are performed in the glassy state. The “quasi-point defect” or “qpd” theory, developed by Perez et al. and previously used for the analysis of the composite mechanical behavior in the linear domain, is now applied to the nonlinear domain to account for the matrix behavior. Then, an incremental approach, using the relationship between the matrix and the composite modulus deduced from DMA experiments, is used to predict the composite behavior by calculating the instantaneous modulus of this composite along its load path. Moreover, a simple model is introduced to account for the stress concentration created by the whiskers in the matrix. It explains the disappearance of the stress peak on the stress–strain curve at increasing whisker contents. Finally, the damage phenomena are discussed, highlighted by successive compression tests. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 383–392, 2000

Plasticized PVC reinforced with cellulose whiskers. I. Linear viscoelastic behavior analyzed through the quasi-point defect theory

In a previous article, the processing of nanocomposite materials of plasticized poly(vinyl chloride) (pPVC) reinforced by cellulose crystalline whiskers was presented as well as preliminary dynamic mechanical measurements. The purpose of the present work is to evaluate the possible change of molecular dynamic of poly(vinyl chloride (PVC) at the interface with cellulose whiskers. The analysis, based on the quasi-point defect (qpd) theory for the anelastic deformation of amorphous polymer, confirms that PVC is heterogeneous. Thus, the matrix is described as a parallel assembly of phases with different plasticizer concentration (i.e., different glass transition temperature). It is shown that the whiskers do not lead to supplementary relaxation in the matrix, at least in the time-temperature window of the study, but, the satisfying modeling of the composite supports the assumption of a thin layer of immobilized phase around the whiskers.