Defects activation in CoFe-based metallic glasses during creep deformation

Maxwell-Voigt model with two Kelvin units and one Maxwell unit was utilized to analyze the microalloying effect of Cu on the creep behavior of CoFe-based metallic glasses at different loading rates. The defect activation during creep deformation was detected by the relaxation time spectrum based on this model. The defect, with respect to a short relaxation time in relaxation spectra, intends to be activated at a quasi-static loading mode in the alloy with 0.5 at.% Cu addition. With further increasing loading rates,more defects with a large size were provoked activated at both hard and soft regions in the Cu-containing sample. A softening with the reduction of elastic modulus and hardness about 10 % and 15 %, respectively,was also observed in the Cu-doped sample. It is consistent with the pronounced viscoplastic deformation of this alloy along with the decrease of viscosity. Our work provides a microscopic insight into structural evolution during creep deformation in a Cu-doped metallic glass, which might help for understanding the plastic deformation of metallic glasses upon a minor addition.     

Dislocation network models for recovery creep deformation

The development of dislocation network models for recovery creep and the important results which arise from the application of the models are discussed. These models are basically aimed at describing the two simultaneous processes, namely strain hardening and recovery, which occur during high-temperature creep deformation. These processes are modelled using detailed dislocation mechanisms which occur in the deforming crystalline materials. The present models, although still being approximations, are reasonably well able to describe high-temperature recovery creep deformation of crystalline materials.     

The activation areas for grain boundary sliding

The activation areas for grain boundary sliding in Al, Pb, Sn, Zn, and Cu are compared with those for creep in the same materials. It is found that the activation area-stress relation for grain boundary sliding is similar to that for creep. This observation is consistent with a dislocation or ledge mechanism of grain boundary sliding.     

Short-term flexural creep deformation in synroc-C

The flexural creep behaviour of synroc-C in an inert atmosphere was studied at temperatures of 860°C, 900°C and 940°C under constant-load conditions in four-point bending. Applied stresses ranged from 100 to 160 MPa. Individual creep curves show primary and secondary creep but little or no tertiary creep stage. The log of the creep rate was found to increase linearly with log of the applied stress at each temperature over the entire stress range. Analysis of the creep data using the Norton power-law function revealed that the stress exponent decreased from 3.3 ± 0.6 for the 860°C and 900°C data to 2.0 ± 0.2 for the 940°C data, and an activation energy of 440 ± 40 kJ/mol was obtained over the entire temperature and stress range. Comparative analysis with the theta-projection equation was found to adequately represent the data yielding an activation energy of 464 kJ/mol while also showing a trend for the stress exponent to decrease with increasing temperature. Microstructural examination revealed extensive cavitation on the tensile surface of the creep specimens subjected to higher stresses at 900°C and 940°C. Dynamic high temperature X-ray diffraction analysis indicated little change in the phase assemblage apart from a slight reduction in the amount of the hollandite phase at higher temperatures which was attributed to a minor amount of oxidation. The possible creep damage mechanism was explored with reference to creep test results and microstructural modifications and the implications of the observations are discussed.     

Characteristics of creep deformation in ceramics

Abstract

The creep mechanisms in ceramics are reviewed briefly and then compared with experimental data. It is shown that there are three major types of creep behaviour: a stress exponent close to 5 due to control by dislocation climb and fully ductile behaviour; a stress exponent close to 3 due to control by climb from Bardeen–Herring sources and less than jive interpenetrating independent slip systems; and a stress exponent close to 1 due to diffusion creep. The role of interface reaction control and the transitions from diffusion to power law creep are also examined.

MST/1389     

Estimating creep deformation of glass-fiber-reinforced polycarbonate

Thermoplastic resin and fiber-reinforced thermo-plastics (FRTPs) were used without post-cure treatment as “molded material.” For such materials, creep behavior and physical aging occur simultaneously. This study examined the creep behavior of polycarbonate (PC) and glass-fiber-reinforced polycarbonate (GFRPC) injection moldings, including the effect of physical aging and fiber content, and determined that the time–temperature superposition principle could be applied to the creep behavior for different fiber contents. The effects of physical aging on creep behavior were evaluated quantitatively on pure resin and with various fiber contents without heat treatment. We found that the effect of physical aging could be evaluated with the proposed factor, “aging shift rate.” To discuss the linearity of viscoelasticity in FRTPs, this study used two shift factors: time and modulus shift factors. The fiber content affected creep behavior by both retarding and restraining it through changing the elastic modulus. This was shown by generating a grand master curve of creep compliance, which included the effects of time, temperature, and fiber content. Using the grand master curve of creep compliance and shift factors, it was possible to estimate the creep deformation of molded materials under varying conditions and fiber contents. The estimated creep deformation gave a very good fit to the experimental creep deformation.     

The importance of multiaxial stress in creep deformation and rupture

This paper investigates the importance of multiaxial stress states by considering several distinct testing techniques used in assessing both creep deformation and creep damage accumulation. The requirements of testing programmes to determine the necessary data are discussed in respect of sensitivity and interdependence of the principal and hydrostatic stress ratios.     

The correlation between creep deformation and stress relaxation in concrete

Summary The functional relation between creep deformation and stress relaxation can be represented by the Volterra integral equation. By substituting the creep function and the relaxation function into this equation and comparing coefficients it is possible to obtain an expression which relates the ultimate creep to the relaxed stress at time t=∞. The rate at which the stress decreases at the begenning of the experiment is proportional to the creep velocity, and the constant of proportionality is the modulus of elasticity. It is shown the half-life time of the relaxation process is always smaller than the corresponding half-life time of the creep process. The derived equations are compared with experimental results described in the literature.

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Résumé On peut traduire la relation entre fluage et relaxation par l'équation intégrale de Volterra. La substitution des fonctions ?fluage? et ?relaxation? dans cette équation et la comparaison des coefficients permet d'obtenir une expression qui relie le fluage final et la contrainte relachée au tempst=∞. Le taux de décroissance au début de l'expérience est proportionnel à la vitesse de fluage; la constante de proportionnalité est le module d'élasticité. On montre que la mi-temps du processus de relaxation est toujours plus petite que la mi-temps correspondance du processus de fluage. On compare les équations dérivées avec les résultats expérimentaux décrits dans la littérature.

     

Generalized viscoelastic model for creep analysis coupled with plastic deformation

A creep analysis capability, using a generalized viscoelastic model, is introduced to process the creep behaviour coupled with elastoplastic deformation. The formulation is based on the step-by-step time integration of the Kelvin-Maxwell rheological model with non-constant parameters. The concept of a rheological model is extended to the multiaxial stresses by the Prandtl-Reuss stress-strain relationship, from which the tangential stiffness matrix is formed for Newton's iteration. If the plastic deformation is coupled with creep, the algorithm will seek a solution in two distinct steps. Various choices of empirical creep laws are available and small variations in temperature are allowed as implemented in the general purpose finite element analysis program.     

Reference stress approach for predicting creep deformation of metal matrix composites

Abstract

A sound, mechanics based approach, using the reference stress concept, has been provided to allow the effects of volume ratio, fibre aspect ratio, and fibre spacing on the creep behaviour of uniaxial metal matrix composites to be quickly assessed. It is shown that fibres are much more effective than particles in reducing creep deformations. In addition, volume ratio and fibre aspect ratio have a large effect on creep properties, while fibre spacing has a relatively small effect. The existence of cracks at the ends of fibres is shown to reduce seriously the effectiveness of the reinforcement. The creep properties for loading in transverse directions are much lower than for loading in longitudinal directions.

MST/2059     

The distribution of creep activation energies in oriented polyethylene films

The “flow” creep in oriented polyethylene was studied using a Doppler velocity meter. The activation energy of the creep process was evaluated and it was established that there are three activation barriers for creep in the material studied. The most probable values, the scatter, and the relative concentration of these barriers in polyethylene samples were determined.     

Study on creep deformation mechanism of 316LN stainless steel from impression creep tests

Abstract

Impression creep tests were carried out on 316LN stainless steel (SS) at various temperatures in the range 898–973 K. The stress dependence of the steady state impression velocity followed the power-law with stress exponent n?=?6. The temperature dependence of the steady state impression velocity obeyed Arrhenius type rate equation. The apparent activation energy for creep deformation (Q_c) was estimated to be 500 kJ mol^?1. Based on the n and Q_c values, it is concluded that the rate controlling mechanism is dislocation creep.     

Impression creep deformation behaviour of 316LN stainless steel

The impression creep deformation behaviour of 316LN SS was investigated from microstructure, substructure, microhardness and profilometry studies of the creep deformed region. Impression creep tests were conducted on 316LN SS in the temperature range of 923–973?K, at different punching stresses in the range of 472–760?MPa. The impression creep deformation was characterised by a hemispherically shaped plastic zone which developed around the indentation. The study revealed the distinct regions under the punch undergoing deformation to different extents. The deformation was found to occur predominently on (111) planes. The dislocations in the highly deformed region were well dispersed in the matrix. The size of the plastic zone was estimated to be ～1·5 times the diameter of the indenter based on the microhardness and profilometry studies. The critical spacing to be maintained between the adjacent indentations was estimated to be >5 times the diameter of indenter.