Dynamic Aging Research Articles

Correlation between enthalpy relaxation and mechanical response on physical aging of polycarbonate in relation to the effect of molecular weight on ductile-brittle transition

A series of physical aging experiments at different annealing temperature and times was conducted on three kinds of polycarbonates (PCs) with different molecular weight using a differential scanning calorimetry and a tensile test. The enthalpy relaxation kinetics was well modeled by existing empirical equation using Kohlausch-Williams-Watts (KWW) function and effects of annealing temperature and times for the model parameters were clarified. A good correlation between the kinetics of enthalpy relaxation and that of yield stress was found. A dependence of the molecular weight on the physical aging kinetics could be attributed to its effect on the glass transition temperature.

Influence of Sn on the age hardening behavior of Al–Mg–Si alloys at different temperatures

Addition of minute amounts of Sn to Al–Mg–Si alloys is known to have a pronounced effect on their age hardening characteristics. In this study, the influence of Sn addition on the ageing behavior of lean and concentrated alloys at five different temperatures was studied. Hardness, positron annihilation lifetime spectroscopy and transmission electron microscopy measurements complemented by differential scanning calorimetry yield information that allow one to assess the microscopic mechanisms that govern ageing. It is found that Sn slows down the ageing kinetics at 100 °C and 140 °C but accelerates the kinetics and enhances the hardening response at 210 °C and 250 °C. At the standard artificial ageing temperature of 180 °C, the effect of Sn on ageing varies depending on the alloy composition. The observed different ageing kinetics can be explained by the different vacancy behaviors in the presence of Sn. Moreover, the activation energy analysis reveals that the ageing process in Sn-added alloys is controlled by both the separation of Sn-vacancy complexes and the migration of solute-vacancy complexes.

Predicting plasticity‐controlled failure of glassy polymers: Influence of stress‐accelerated progressive physical aging

ABSTRACTThis study focuses on the prediction of long‐term failure of glassy polymers under static or cyclic loading conditions, including the role of stress‐accelerated progressive aging. Progressive physical aging plays a dominant role in a polymer's performance under prolonged loading conditions, and to obtain accurate predictions of failure, its effect has to be considered. First, the aging kinetics, as influenced by temperature and stress history, are studied extensively. Similar to an elevated temperature, the application of a stress (below the yield stress) activates the aging process, and as a result, the yield stress will evolve faster in time. The activation by stress appears to be limited; at some stress level, the activation stagnates and is followed by rejuvenation. This evolution is captured in a model by introducing a state parameter, which describes the thermodynamic state of the material and is directly linked to the yield stress. With the aging kinetics included in the model, an accurate prediction of the failure time for cyclic loading conditions is obtained. For static loading conditions, however, the effect of physical aging is overestimated because of the stagnation of the activation by stress. It appears that there are marked differences in the stress level where stagnation and subsequent rejuvenation occur for a cyclic or static load. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1300–1314

Aging kinetics and mechanical properties of copper-bearing low-carbon HSLA-100 microalloyed steel

The precipitation kinetics of HSLA-100 steel and the correlation between tensile and impact properties were studied. According to the modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis and based on the analysis of the time corresponding to the transformed fraction of 0.5 (t0.5), the activation energy for the precipitation of copper during aging of martensite was determined as ˜111 and 105 kJ/mol, respectively. These values are much smaller than the activation energy for the diffusion of Cu in α-iron, which was related to the effect of high dislocation density of the quenched martensitic microstructure on the aging process. These results were verified based on the diffusional calculations. Based on the analysis of mechanical behavior, no reasonable correlation was found between strength of the material and the impact energy. However, the impact energy was found to be proportional to the UTS-YS, where the latter is an indicator of the work-hardening capability of the material. This revealed that the work-hardening capacity of the material is a much more important factor for determining the impact toughness compared to its strength.

Physical Ageing of Polystyrene: Does Tacticity Play a Role?

The ageing kinetics of amorphous atactic (a-PS), isotactic (i-PS), and syndiotactic (s-PS) polystyrene were studied by means of flash-differential scanning calorimetry. The specimens were aged for up to 2 h at six different ageing temperatures: the optimum ageing temperature, that is, the temperature at which the enthalpy overshoot at the glass transition is maximal for the given elapsed time, and five ageing temperatures ranging from 20 to 80 K below the optimum ageing temperature. A logarithmic increase of the enthalpy overshoot with ageing time is observed for specimens at their optimum ageing temperatures. For temperatures significantly lower than the optimum, there is a range where the enthalpy overshoot is constant, but for higher temperatures (still below the optimum), a logarithmic increase is also observed. Moreover, the ageing kinetics appear to depend on tacticity, with s-PS and i-PS exhibiting the slowest and fastest ageing kinetics, respectively, and a-PS exhibiting ageing kinetics between these two extremes.

Aging and recrystallization behavior of quaternary Al-0.25Zr-0.03Y-0.10Si alloy

Aging behavior of Al-0.25Zr-0.03Y-0.10Si were investigated to evaluate the role of 0.10%Si usually contained in commercially pure Al. Compared with the Si-free ternary alloy (Al-Zr-Y), accelerated aging kinetics, enhanced hardening effect and higher conductivity were obtained. Aging precipitates with tripled number density were observed in alloy Al-0.25Zr-0.03Y-0.10Si, whereas size and coarsening resistance were not affected by Si content. Contribution of Si provides a large number of nuclei, rather than accelerating the diffusion of Zr. Formation of primary Al3Y during solidification resulted in a 50 °C drop in recrystallization temperature.

Impact of anti-ageing compounds on oxidation ageing kinetics of bitumen by infrared spectroscopy analysis

This paper investigates the effect of different anti-ageing compounds (AACs) on the oxidation kinetics of bitumen using Fourier Transformation Infrared (FTIR) spectroscopy. Twenty different AACs were examined, including new and existing AACs for bitumen and polymer products. The AACs were mixed with bitumen to fabricate thin film samples of AAC-modified bitumen which were subjected to laboratory oven ageing at 100 °C with different ageing periods up to 504 h. A Normalized Carbonyl Index (NCI) was proposed based on a selected reference peak (1377 cm−1) to eliminate the impact of the inherent carbonyl content from the bitumen or AACs and manifest the carbonyl growth rate for evaluating the AACs’ anti-ageing performance. It was found the activation energy of fast-term oxidation can be utilized to quantitatively screen the anti-ageing compounds and evaluate their anti-ageing effectiveness in terms of decreasing the formation of carbonyl groups in bitumen. AACs that exhibited high anti-ageing performance were those contained furfural, Irganox acid with sodium montmorillonite, furfural with DLTDP, and high concentrations (e.g., 15%) of Irganox acid. The proposed protocol should be followed by further laboratory rheological and mechanical tests on the AAC-modified bitumen with different binder sources.

Influence of Sn on the Age Hardening Behavior of Al-Mg-Si Alloys at Different Temperatures

Addition of minute amounts of Sn to Al-Mg-Si alloys is known to have a pronounced effect on their age-hardening characteristics. In this study, the influence of Sn addition on the ageing behavior of lean and concentrated alloys at five different temperatures was studied. Hardness, positron annihilation lifetime spectroscopy and transmission electron microscopy measurements complemented by differential scanning calorimetry yield information that allow one to assess the microscopic mechanisms that govern ageing. It is found that Sn slows down the ageing kinetics at 100 °C and 140 °C but accelerates the kinetics and enhances the hardening response at 210 °C and 250 °C. At the standard artificial ageing temperature of 180 °C, the effect of Sn on ageing varies depending on the alloy composition. The observed different ageing kinetics can be explained by the different vacancy behaviors in the presence of Sn. Moreover, the activation energy analysis reveals that the ageing process in Sn-added alloys is controlled by both the separation of Sn-vacancy complexes and the migration of solute-vacancy complexes.

Ultrafine grained Al 7075 alloy fabricated by cryogenic temperature large strain extrusion machining combined with aging treatment

In this study, cryogenic temperature large strain extrusion machining (CT-LSEM) as a severe plastic deformation (SPD) method is applied to fabricate chips with ultra-fine grained (UFG) structure. The chip morphology, microstructure evolution, aging behavior, as well as micro-hardness characteristics of a solution treated (ST) Al 7075 alloy subjected to CT-LSEM and room temperature (RT) LSEM are investigated. It is observed that the processing temperature has a great influence on the chip morphology, i.e., the chips produced by CT-LSEM have better surface quality than that by RT-LSEM. The SPD process of LSEM, especially under CT, can accelerate the aging kinetics of the alloy. Much more deformation energy stored in the CT-LSEM sample could promote the dynamic recrystallization. A higher dislocation density exists in the CT-LSEM samples due to effective suppression of dynamic recovery. The optimized hardness for all samples is obtained at an aging temperature of 120 °C. The hardness of CT and RT LSEM samples after peak aging (PA) have increased from 102 Hv to 201 Hv and 192 Hv respectively. A large number of fine and closely spaced ηʹ and η precipitates are present in the LSEM samples after PA treatment, accounting for the further enhancement of hardness. The ST Al 7075 samples subjected to LSEM are severely deformed. The mean grain/sub-grain size of PA samples has hardly changed as compared with the initial LSEM samples. It is feasible to realize simultaneous hardening including grain refinement, dislocation and precipitation in the Al 7075 alloy via application of CT-LSEM and subsequent aging.

DMA, EIS and thermal fatigue of organic coatings

The importance of temperature variations on coatings aging is well known. The aim of this paper is to evidence how aging kinetic, monitored by EIS, can be complemented by dynamic mechanical analyses to access susceptibility of high performance corrosion protective composite coatings, designed for harsh conditions, when subjected to temperature variations, proposing methodology to aid in the selection criteria. Five coatings provided by recognized brands were submitted to corrosion tests with immersion and temperature cycling or shocking. The range of testing temperatures included the so-called humid Tg obtained by Arrhenius-type plot of low frequency impedance modulus. Different aging kinetics were evidenced by EIS for the coatings. Pull-off adhesion measurements complemented by SEM indicated loss of cohesion with cracks and voids nucleation at filler-matrix interfaces as one important feature of the coatings with more accelerated thermal aging kinetics. The irreversibility of impedance on Arrhenius-type plot, follows the thermal stress, but does not predict performance of the coatings. On the other hand, associating EIS with mechanical dynamic analyses allows understanding susceptibility to thermal aging, by considering the coatings viscoelastic properties measured within the range of testing temperatures. The input given by DMA testifies to the importance of having this technique included in coatings assessment protocols whenever temperature variations are in context. With this, one can not only better understand the behavior of the coatings, but also get clues to develop products with even better performance.

Surface treatment methods for mitigation of hydrothermal ageing of zirconia

Hydrothermal ageing of zirconia ceramics (tetragonal-to monoclinic phase transformation in the presence of humidity) is highly sensitive to the surface characteristics. Thus chemical modifications of the surfaces over a few microns may be a good way to mitigate ageing without affecting bulk mechanical properties. Here, the efficiency of different post-sintering thermal treatments in powder beds of different compositions to prevent or slow down hydrothermal ageing of 3Y-TZP is tested. The microstructure, ageing kinetics at different temperatures and mechanical properties of the treated samples are then evaluated.Treatments with 12Ce-TZP proved to be the most efficient to limit ageing at 134 °C while keeping good mechanical properties. However, this is accompanied by a decrease of the activation energy of ageing, thus by an acceleration of ageing at lower temperature. These results also show that the activation energy of ageing is material-dependant, thus should be ascertained for every developed zirconia-based ceramic.

State-of-the-art review on the performance of cellulosic dielectric materials in power transformers: Mechanical response and ageing

Power transformers are essential elements in electrical systems and the end of their useful life is frequently conditioned by the state of the cellulosic insulation materials inside them. These materials are subjected to high temperatures and mechanical stresses, primarily generated by short-circuits or inrush currents. Moreover, both the cellulosic materials and the dielectric oil in which they are impregnated are affected by chemical reactions. In order to achieve a better understanding of ageing kinetics and improve the management and planning in electrical systems from a technical and economical point of view, it is important to better understand the behaviour of cellulosic insulation. The purpose of this review is to analyse the current state-of-the-art with reference to the modelling of the mechanical behaviour of paper materials used as insulation in power transformers, as this topic has been previously analysed in many different publications but only fragmentarily. A suitable model for transformer cellulosic insulation, in combination with engineering measurements, could help to optimise the design of the insulation and manufacturing processes, as well as to accurately predict the end-of-life of power transformers.

Processing of Al–Zn–Mg–Cu/SiC composite prepared by mechanical alloying

In the present work, using powder metallurgy method, mechanical alloying and extrusion, the metal matrix composite material Al–Zn–Mg–Cu/10% SiC was obtained. Phase transformations occurring during heat treatment were studied. After deformation aluminum solid solution, MgZn2 and SiC are present in the composite material. The X-ray phase analysis and TEM results showed that when the composite material is heated for quenching, the MgZn2 phase is completely dissolved. Study of the aging kinetics showed that the peak of aging of the composite material was achieved in 15 h, which is much faster than in the matrix alloy (24 h). After aging, the solid solution decomposition products appear in the form of a metastable η'- phase (MgZn2), looking like rods located in three mutually perpendicular directions <100> Al.

Effects of Machining Velocity on Ultra-Fine Grained Al 7075 Alloy Produced by Cryogenic Temperature Large Strain Extrusion Machining.

In this study, cryogenic temperature large strain extrusion machining (CT-LSEM) as a novel severe plastic deformation (SPD) method for producing ultra-fine grained (UFG) microstructure is investigated. Solution treated Al 7075 alloy was subjected to CT-LSEM, room temperature (RT) LSEM, as well as CT free machining (CT-FM) with different machining velocities to study their comparative effects. The microstructure evolution and mechanical properties were characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Vickers hardness measurements. It is observed that the hardness of the sample has increased from 105 HV to 169 HV and the chip can be fully extruded under CT-LSEM at the velocity of 5.4 m/min. The chip thickness and hardness decrease with velocity except for RT-LSEM at the machining velocity of 21.6 m/min, under which the precipitation hardening exceeds the softening effect. The constraining tool and processing temperature play a significant role in chip morphology. DSC analysis suggests that the LSEM process can accelerate the aging kinetics of the alloy. A higher dislocation density, which is due to the suppression of dynamic recovery, contributes to the CT-LSEM samples, resulting in greater hardness than the RT-LSEM samples.

Combined wear and ageing of ceramic-on-ceramic bearings in total hip replacement under edge loading conditions

Ceramic-on-ceramic bearings in total hip replacement have shown the potential to provide low wear solutions in hip replacement. Assessing the tribological performance of these materials is important to predict their long-term performance in patients. In this study, a methodology was devised to assess the tribological in vitro behaviour of composite ceramics under combined adverse edge loading conditions and accelerated ageing. Two commercial ceramic composites were considered, namely Alumina-Toughened Zirconia (ATZ, ceramys®) and Zirconia-Toughened Alumina (ZTA, symarec®). The bearing couples were studied using the Leeds Mark II hip joint simulator for a total of eight million cycles, the first two million under normal gait (no edge loading) and the following six million cycles with the addition of edge loading conditions driven by medial-lateral separation. The bearing couples underwent hydrothermal ageing using an accelerated protocol in autoclave every million cycles.The influence of edge loading combined with ageing was significant for ATZ bearings, resulting in a slower overall ageing kinetics over the wear stripe than on the control heads. During the autoclave ageing steps, the monoclinic fraction increased more over the wear stripe area than over the unworn area. Both results thus indicated that the monoclinic phase was removed during shocks induced by edge loading. The wear performance of the two materials were similar exhibiting relatively low wear rates and low level of microstructural damage for these clinically relevant adverse conditions.

On the linking of the rheological properties of asphalt binders exposed to oven aging and PAV aging

ABSTRACT This study proposed a rapid method to predict rheological characteristics of asphalt binders due to oxidative aging based on pressure aging vessel (PAV) as the accelerating aging tool. Six types of asphalt binders were aged at various temperatures for different durations with two aging methods, which were oven aging under standard atmosphere, and PAV aging under a pressure of 2.1MPa. The crossover modulus was determined to characterize the rheological property changes with respect to aging. The oxidation aging kinetics model in terms of crossover modulus under oven aging method was developed, the model parameters showed universal correlations of each other. The aging kinetics model parameters using PAV aging method also obtained. Relationship between the aging kinetics model parameter in oven and PAV aging was established. The activation energy as one of the aging kinetics model parameters in oven aging calculated from PAV data using the relationship, along with the universal correlations of aging kinetics model parameter in oven aging, the crossover modulus predicted rapidly and accurately. Such a rapid method is capable of predicting the aging behavior and rheological properties of asphalt binder subjected to oxidation, and is significantly save a great deal of time and effort in the laboratory.

Prediction of Aging Kinetics and Yield Strength of 6063 Alloy

Bulk hardness and tensile properties of an AA6063 Al-Mg-Si alloy specimens subjected to artificial age-hardening treatment over a wide range of temperatures (373-523 K) and time durations (5 min to 60 days) have been determined and analyzed to predict complete aging kinetics and yield strength. The predictions have been made using simplified hardness-based models. The experimental results are used to generate kinetic models for under- and overaging regimes following the Johnson–Mehl–Avarami and the Lifshitz–Slyozov–Wagner equations, respectively. The obtained kinetic parameters are found to satisfactorily describe the overall aging kinetics of the Al-Mg-Si alloy. Additionally, relationship between the estimated hardness and the yield strength values of the AA6063 alloy has been established and its utility has been validated. The developed hardness-based models can be used as convenient tool to determine the state of aging and to predict the yield strength of Al-Mg-Si alloy.

Effect of aggregates containing iron sulphide on asphalt ageing

The impact of asphalt cement ageing on pavement durability is a well-known phenomenon for pavement engineers, yet due to complexity in the chemical structure of asphalt cement, not understood thoroughly. Accelerated laboratory ageing procedures usually consider aggregates as inert material, which are insensitive to environmental effects. While in most cases this assumption is not far from reality, there are some cases where the aggregates could play a significant role. Iron Sulfide exists in many aggregate sources in different forms such as pyrite and pyrrhotite. Products of the reactions from such aggregates may have a catalytic effect on the ageing procedure of asphalt cement. In this research, three different sources of Iron Sulfide-containing aggregates were selected to produce laboratory mixes, which were conditioned under moisture and heat. Complex modulus tests were performed on unconditioned and conditioned samples, and the parameters of the rheological behaviour of each mix were determined using the 2S2P1D model. After each step of conditioning, recovered asphalt was subjected to chemical and rheological analysis. The 2S2P1D model was also employed to determine the rheological parameters for recovered asphalt cement from different mixes. Shift factors determined as part of developing master curves were also used to investigate the temperature susceptibility of samples. Carbonyl and sulfoxide indices and the ratio between them were also used to study the chemical path of reactions. The results of this study showed that for samples that were subjected to both heat and moisture, mixes containing Iron Sulfide minerals seem to follow somehow different ageing kinetics resulting in an important change in the behaviour, which is reflected through different rheological parameters and chemical indices.

Effect of equal-channel angular pressing on the precipitation kinetics in Cu-Cr-Hf alloys

An analysis of phases precipitation during the supersaturated solid solution (SSSS) decomposition in binary Cu–Cr, Cu-Hf and ternary Cu–Cr–Hf alloys after equal-channel angular pressing (ECAP) was carried out. ECAP affects the ageing kinetics by changing the sequence of phase precipitation, and accelerates the decomposition of SSSS.

Description of physical aging kinetics of glassy polymers by interpretation of parameters of the Kohlrausch-Williams-Watts relaxation function via simulation.

The empirical Kohlrausch-Williams-Watts (KWW) relaxation function has been widely used to describe the nonexponential relaxation behavior of glassy polymers during their physical aging. The characteristic relaxation time (τ_{KWW}) and the stretching parameter (β_{KWW}) of the KWW equation are obtained from experimental relaxation data by a curve-fitting procedure. However, knowing the values of KWW parameters is not sufficient to completely characterize the physical aging kinetics of glassy materials in order to compare, for example, the aging rate of different systems. To be able to overcome such problems, at least, an exact interpretation of these parameters is absolutely vital and then it would be quite desirable to convert them into a single relaxation-time constant. This approach, indeed, provides the possibility to directly evaluate and compare the kinetics and aging rate of different systems as well as different methods used for monitoring the physical aging of the same materials. To do these, a method based on the notion of the relaxation spectrum is employed which is modeled via the well-known statistical distribution of log-normal. By detailed numerical simulations, some explicit expressions between KWW parameters with descriptor parameters of the distribution are established. From this point of view, it is tried to interpret τ_{KWW} and β_{KWW} and convert them into a single relaxation-time constant. Finally, the results of suggested simulations are compared with those obtained in the literature.