Low Temperature Aging Research Articles

Improved creep behaviour for a high strength Al-Li alloy in creep age forming: Experimental studies and constitutive modelling

The creep deformation of 3rd generation 2xxx Al-Li alloys in creep age forming (CAF) is unsatisfactorily low due to its high strength and required low ageing temperature, especially for the commonly used T8 temper. Promoting creep deformation in such process will significantly reduce springback in CAF, making easier the springback compensation in tool design. In this study, two possible process routes of inducing increased creep deformation were explored, one by employing high applied stress to introduce small plastic strain in loading stage; and the other by conducting pre-deformation prior to creep-ageing. The creep behaviour with and without pre-deformation was investigated through creep-ageing tests under stresses ranging from 300 to 430 MPa at the ageing temperature of 143°C. Due to the decrease of threshold stress with introduced plastic strain in the loading stage and/or pre-stretching, a faster increase of creep strain with applied stress was observed for both methods when the applied stress surpassed 400 MPa. The strain promotion of the former was higher than the latter when above 415 MPa. A mechanism-based constitutive model was proposed, with additional work hardening equations to describe the relationship between threshold stress and dislocation density induced by plastic strain in the loading stage and pre-stretching, to model the microstructural evolution and reflect the nonlinear increase of creep strain with stress in both methods. The adequacy of this proposed unified model was demonstrated with good agreement with experimental data utilising both methods. The adaptability of the model in multiaxial case was verified using four-point bending CAF tests with the same Al-Li alloy.

Tuning the strength and ductility of near β titanium alloy Ti-5321 by ω and O′ intermediate phases via low-temperature aging

The formation of intermediate phases ω and O′ during low-temperature aging at 200 °C and its effect on plastic deformation in a new-developed near β-Ti alloy Ti–5Al–3Mo–3V–2Cr–2Zr–1Nb–1Fe (wt %) have been studied in this work. The results showed that the ω and O′ phases were continuously transformed upon aging, and larger amount of O′ precipitates was developed in air-cooled sample than water-quenched one. Upon early aging the yield strength and ductility of alloy were simultaneously improved until 60 min. The trade-off relationship of strength and ductility was overcome by the simultaneous precipitation of ω and O′ phases. It was suggested that the ω and O′ precipitation changed the deformation mechanisms, leading to this alloy exhibited different deformation behaviors and mechanical properties. With the quantities of ω and O′ increasing, the strain-induced α′′ martensite transformation was gradually suppressed, and the primary deformation mechanism changed into a mixture of strain-induced ω transformation, mechanical twining and dislocation slipping. These findings would provide an insight to achieve better combinations of strength and ductility in near β-Ti alloys through tailoring the ω and O′ phase transformations.

Chromizing of Additively Manufactured Maraging Steel; Microstructural Evolution and Corrosion Performance

Abstract While maraging steels are excellent candidates for additive manufacturing of high-strength materials, most of them provide little to no corrosion resistance. In this study, the surface of additively manufactured maraging steel is augmented with diffusion-based chromizing. The diffusion of chromium into the surface results in the formation of α-Cr, σ-phase and austenite depending on the chromium content at the respective distance from the surface. The bulk consists of a ductile bcc martensite hardened by precipitation of intermetallics during a low temperature ageing treatment. The σ-case provides a very high hardness, while the austenite phase is rather soft and unaffected by the ageing treatment. The chromizing treatment significantly reduced the corrosion rate of the maraging steel up to a factor of 14. Decomposition of the σ-case into a uniform case of austenite (austenitic stainless steel) further improved the corrosion resistance almost 24 times. This showed the strong potential of the chromizing treatment on a highstrength material to obtain a combination of their beneficial properties.

Effect of two-stage overaging on microstructure and corrosion properties of an Al–Zn–Mg–Cu alloy

A two-stage overaging process was proposed to improve the electrical conductivity and stress corrosion cracking resistance (SCCR) of an Al–Zn–Mg–Cu alloy. The overaging process include a low-temperature aging at 120 °C for 5 h and a subsequent aging at 177 °C for 8, 14 and 24 h. The SCCR of the samples aged at 120 °C/5 h + 177 °C/24 h was higher than that of the T6 temper, at which the ISSRT is 1.2% and the electrical conductivity is 39.0%IACS. The increase of electrical conductivity is caused by the depletion of solution atoms during overaging. Meanwhile, the improvement of SCCR resistance is caused by the coarsening of grain boundary precipitates with higher Cu content. The tensile strength decreased from 663.5 MPa to 519.7 MPa, which is caused by the coarsen of strengthening precipitates in grains during overaging process.

Investigation of the deleterious effects of low-temperature and hygrothermal aging conditions on the mixed-mode fracture resistance of epoxy resin using a short-beam bend specimen

The effect of different environmental aging conditions on the mixed-mode fracture behavior of epoxy resins was investigated using a sub-size specimen. The short-beam bend (SBB) specimens with different crack inclination angles were manufactured and subjected to low-temperature (-71 °C), hygrothermal (immersion in deionized water at 60 °C) and room-temperature conditions. The deleterious effect of low-temperature and hygrothermal aging conditions on the mode-I, mode-II and mixed-mode I/II fracture responses of the epoxy resin was studied by testing the aged and unaged SBB specimens under three-point bend loading. The experimental results indicated that the hygrothermal aging condition, with a reduction of 49% in the effective fracture toughness, imposed a more severe deleterious effect on the fracture response of the specimens in comparison with the low-temperature condition with a decrease of 35% in the effective fracture toughness. The generalized maximum tangential stress criterion was used to predict the fracture loads of the aged and unaged specimens. Moreover, optical microscopy and scanning electron microscopy techniques were employed to assess the fracture surfaces of the specimens aged under different conditions.

A Comprehensive Study on the Rejuvenation Efficiency of Compound Rejuvenators for the Characterization of the Bituminous Binder, Mortar, and Mixture.

This study aims to comprehensively investigate the rejuvenation efficiency of various self-developed compound rejuvenators on the physical, mechanical, and aging properties of aged bitumen, asphalt mortar, and mixture. The results revealed that the restoration capacity of vacuum distilled-oil rejuvenators on high-and-low temperature performance-grade of aged bitumen is more significant. In contrast, an aromatic-oil based rejuvenator is good at enhancing low-temperature grade and aging resistance. Moreover, the temperature and time of the curing conditions for mixing recycling of asphalt mixture were optimized as 150 °C and 120 min. Furthermore, the sufficient anti-rutting, structural stability, and moisture resistance of recycled asphalt mixture affirmed the rejuvenation efficiency of compound rejuvenators.

Influence of deposition strategy and heat treatment on mechanical properties and microstructure of 2319 aluminium WAAM components

There is an increasing interest in additive manufacturing of high strength aluminium alloys built with wire and arc additive manufacturing (WAAM) technologies. Al-Cu alloys are susceptible to hot cracking, however, pulse advanced cold metal transfer (CMT-PADV) welding process ensures a low heat input that avoids this defect while reducing the part porosity. This study has demonstrated that porosity below 1 % does not affect mechanical properties of 2319 WAAM walls, whereas the as-built microstructure and applied heat treatment highly influence the properties obtained. Without interlayer dwell time, a fine uniformly distributed equiaxed microstructure is obtained, however, if an interlayer arc stop period is applied, columnar dendritic grains in the interlayer zone is obtained. Heat treatment with low aging temperature and time demonstrated not to be suitable to ensure high mechanical properties and low anisotropy. Instead, if 190 °C and 26-hour aging heat treatment is used, properties up to 324 MPa for yield stress, 452 MPa tensile strength and 8 % elongation can be achieved perpendicularly to the building direction with anisotropy of 1 %, 2 % and 46 %, respectively, for the manufacturing conditions that avoided columnar grains. This fact was critical to ensure highest strength and ductility values and low anisotropy.

Microstructure and Mechanical Properties of an Al-Mg-Si-Zr Alloy Processed by L-PBF and Subsequent Heat Treatments.

The aim of this study was to develop a new Al–Mg–Si–Zr alloy with a high magnesium content to achieve a wide range of mechanical properties using heat treatment and at a lower cost. Additive manufacturing was conducted using a powder bed fusion process with various scan speeds to change the volumetric energy density and establish optimal process conditions. In addition, mechanical properties were evaluated using heat treatment under various conditions. The characterization of the microstructure was conducted by scanning electron microscopy with electron backscatter diffraction and transmission electron microscopy. The mechanical properties were determined by tensile tests. The as-built specimen showed a yield strength of 447.9 ± 3.6 MPa, a tensile strength of 493.4 ± 6.7 MPa, and an elongation of 9.6 ± 1.1%. Moreover, the mechanical properties could be adjusted according to various heat treatment conditions. Specifically, under the HT1 (low-temperature artificial aging) condition, the ultimate tensile strength increased to 503.2 ± 1.1 MPa, and under the HT2 (high-temperature artificial aging) condition, the yield strength increased to 467 ± 1.3 MPa. It was confirmed that the maximum elongation (14.3 ± 0.8%) was exhibited with the HT3 (soft annealing) heat treatment.

Unraveling the nature of cerium on stabilizing Cu/SAPO-34 NH3-SCR catalysts under hydrothermal aging at low temperatures

To reveal how cerium stabilizes Cu/SAPO-34 at low-temperature hydrothermal aging, various amounts of cerium were introduced into Cu/SAPO-34 via impregnation method and treated at 70 °C with RH 80% for 96 h. Cerium as Ce3+ and CeO2 nanoparticle is located on the surface of Cu/SAPO-34, and Ce3+ plays a vital role on gradually decreasing surface acidity and blocking defect sites with an increase of Ce loading. After hydrothermal aging, Cu/SAPO-34 with high Ce loading shows the superior SCR activity comparable to fresh samples. It is proven that the surface acidity determines the stability of the structure during hydrothermal aging process, and lower surface acidity prevents the number of Cu(II) ions from decreasing significantly. Furthermore, the structure's stability helps the recovery of Cu(II) ions and renders an outstanding regeneration ability. Our finding paves the way for the design of new Cu/SAPO-34 catalysts with good SCR activity and long-term stability in real application.

Effective utilization of CuO derived from waste printed circuit boards as a peroxymonosulfate activator for the degradation of reactive blue 19

Waste printed circuit boards (PCBs), a common electronic waste, contain a high content of Cu. For environmental sustainability and economic benefit, it is necessary to recover the Cu as high value-added product instead of discarding it. In this work, Cu from PCBs was successfully recovered as CuO particles with high specific surface area and rich oxygen vacancies through cyclic leaching, chemical precipitation, low temperature aging, and calcination. Compared with CuO prepared from commercial Cu(NO3)2·5H2O, the PCBs-derived CuO (PCBs-CuO) exhibited comparable performance in activating peroxymonosulfate to degrade reactive blue 19, although its crystallinity and phase purity were relatively lower. Additionally, PCBs-CuO had an outstanding pH tolerance (2–12), confirming the practical application potential of PCBs-CuO in the removal of organic contaminants in various wastewater. Moreover, the quenching experiments, electrochemical measurements and electron paramagnetic resonance analysis suggested that 1O2 and ·OH played a significant role in PCBs-CuO/peroxymonosulfate system. The ≡Cu(II) and oxygen vacancies on the surface of PCBs-CuO was mainly responsible for the activation of peroxymonosulfate. This work provides a promising method for recycling metal from waste PCBs into high-effective peroxymonosulfate activator and simultaneously achieving environmental remediation.

Effect of cyclic ageing on the early-stage clustering in Al–Zn–Mg(-Cu) alloys

The cyclic application of mechanical stress to aluminum alloys at room temperature, referred to as cyclic ageing, continuously injects vacancies, enabling dynamic precipitation of a fine distribution of solute clusters. These are uniformly distributed throughout the bulk and are responsible for enhancing the mechanical properties to comparable values as T6 conditions obtained by conventional artificial aging. The atomic structure of the clusters in two Al–Zn–Mg (-Cu) alloys is studied using atomically resolved transmission electron microscopy, and their size, volume density and chemistry are investigated using atom probe tomography. It was found that cyclic ageing yields a high number density of solute clusters exhibiting structural similarities to the GPI zones commonly observed after low temperature ageing in such alloys. A subsequent 10-day natural ageing allows both cluster nucleation and further cluster growth, without altering their atomic structure. As compared to natural ageing, the strengthening caused by the solute clusters is accelerated during cyclic ageing, due to dynamic precipitation. The copper containing alloy also had higher amounts of Mg and Zn. Still, a slightly lower number density of smaller clusters with lower Zn/Mg ratio formed in this alloy, both after cyclic aging and after natural aging. The higher strength of this alloy is attributed to a higher strength contribution from each cluster.

Microstructural Evolution, Mechanical Properties, and Preosteoblast Cell Response of a Post-Processing-Treated TNT5Zr β Ti Alloy Manufactured via Selective Laser Melting.

A Ti–34Nb–13Ta–5Zr (TNT5Zr) β Ti alloy with a high strength-to-modulus ratio has been developed, showing its potential to become another candidate material in load-bearing implant applications. This work mainly investigates the microstructural evolution, mechanical properties, and biocompatibility of a post-processing-treated TNT5Zr alloy manufactured via selective laser melting (SLM). Transmission electron microscopy observation shows the existence of the single beta grain matrix and alpha precipitates along the grain boundary in the SLM + HIP manufactured TNT5Zr alloy (TNT5Zr-AF + HIP), and ellipsoidal nano-sized intragranular α″ precipitates (approx. 5–10 nm) were introduced after the subsequent low-temperature aging treatment. The precipitation strengthening enables the SLM + HIP + aging manufactured TNT5Zr (TNT5Zr-AF + HIPA) alloy to show a comparable ultimate tensile strength (853 ± 9 MPa) to that of the reference material (Ti64-AF + HIP, 926 ± 23 MPa). Including the inferior notch-like surface of the test pieces, the slip-band cracking that occurs in this ductile TNT5Zr-AF + HIPA alloy is regarded as the main factor in determining its fatigue strength (170 MPa). In vitro short-term biocompatibility evaluation reveals almost no significant difference in the preosteoblast viability, differentiation, and mineralization between TNT5Zr-AF + HIPA and the reference biomaterial (Ti64-AF + HIP).

Effect of Post-Fabricated Aging on Microstructure and Mechanical Properties in Underwater Friction Stir Additive Manufacturing of Al-Zn-Mg-Cu Alloy.

The fabricated Al–Zn–Mg–Cu alloy build has low mechanical properties due to the dissolution of strengthening precipitates back into the matrix during friction stir additive manufacturing (FSAM). Post-fabricated aging was considered an effective approach to improve the mechanical performance of the build. In this study, various post-fabricated aging treatments were applied in the underwater FSAM of Al–7.5 Zn–1.85 Mg–1.3 Cu–0.135 Zr alloy. The effect of the post-fabricated aging on the microstructure, microhardness, and local tensile properties of the build was investigated. The results indicated that over-aging occurred in the low hardness zone (LHZ) of the build after artificial aging at 120 °C for 24 h as the high density of grain boundaries, subgrain boundaries, dislocations, and Al3Zr particles facilitated the precipitation. Low-temperature aging treatment can effectively avoid the over-aging problem. After aging at 100 °C for 48 h, the average microhardness value of the build reached 178 HV; the yield strength of the LHZ and high hardness zone (HHZ) was 453 MPa and 463 MPa, respectively; and the ultimate tensile strength of the LHZ and HHZ increased to 504 MPa and 523 MPa, respectively.

Superb strengthening behavior in a precipitation strengthened Co-rich CoCrNiAlTi medium entropy alloy with acceptable ductility

For metallic materials, it is challenged to enhance the strain hardening rate under uniaxial tension when high flow stress is achieved. Here, we present a strategy enabling to obtain superior strain hardening ability via fabricating an incompletely recrystallized structure in a precipitation-strengthened Co-rich CoCrNiAlTi medium-entropy alloy. It involves to introduce a low-temperature ageing process in the CoCrNiAlTi alloy experienced high-strain cold rolling process. Accompanied with the formation of the partially recrystallized microstructure, high-density band-like L12-type ordered phase has formed via discontinuous precipitation. During the plastic straining process, deformation twins have been triggered by the high-stress field owing to its metastable structure. As expected, the alloy demonstrates synergistically substantial strengthening and toughening effects, thus, a superb mechanical property combination with ultimate tensile stress of 2.20 GPa and ductility of over 5.0% has been achieved at ambient temperature.

Carbon diffusion in bcc- and bct-Fe: Influence of short-range C–C pair interactions studied from first-principles calculations

Identifying the mechanisms of interstitial diffusion in iron is important to understanding the low-temperature ageing of Fe–C ferritic and martensitic alloys. In spite of the low solubility of carbon in ferrite at equilibrium, carbon-rich areas are often found at segregated grain boundaries and in Cottrell atmospheres around dislocations. Carbon-rich areas also form by spinodal decomposition in martensite. In those cases, carbon atoms experience short-range interactions, susceptible to modify their migration behaviour. We performed first-principles calculations to study the influence of these C–C interactions on the migration of interstitial carbon in body-centered iron. The ab initio energy database was introduced in kinetic Monte Carlo simulations to compute the thermodynamic and kinetic parameters. We found that the migration energies of carbon are largely affected by the presence of a neighbouring carbon atom. We explain the evolution of these energies by the relative stability of the C–C configurations corresponding to stable and transition-state positions. The C–C pair interactions slightly modify the ferrite/martensite transition conditions and significantly change the carbon atomic migration path. The latter leads to an increase of the diffusivity up to 10 times and an important kinetic correlation at low temperature (<300 K) and high carbon contents (>1 at.%).

Mechanical-electrical-thermal responses of lithium-ion pouch cells under dynamic loading: A comparative study between fresh cells and aged ones

Mechanical integrity is one of the study focuses of lithium-ion batteries, which is greatly influenced by the loading rate and the cycling ageing level. This paper investigates how dynamic loading and low-temperature ageing collectively affect the safety performance of lithium-ion batteries under indentation loadings, by monitoring and comparing the in-situ mechanical-electrical-thermal responses. For the mechanical response, results show that dynamic loading increases the battery stiffness while low-temperature ageing alleviates that. During the intrusion process, the electrical response is mostly influenced by the loading rate. Fast voltage dropping and recovering occur under dynamic loading scenarios, leaving a downward trench in the voltage profile. The higher the loading rate, the earlier the short circuit and the faster the voltage dropping. After the termination of the loading, the voltage dropping is mostly affected by the ageing level. Low-temperature ageing accelerates the long-term voltage dropping rate. As for the thermal response, it is highly related to the short circuit behavior. Upward temperature peaks are formed right after the short circuit under dynamic loadings, corresponding to the voltage trenches. Besides, aged cells have higher long-term temperature profiles, corresponding to the accelerated long-term voltage dropping rates.

A comprehensive review of bio-oil, bio-binder and bio-asphalt materials: Their source, composition, preparation and performance

To promote the application of bio-materials and provide a direction for their further researches, this paper comprehensively summarizes the research progress of biomaterials in recent years. The review results show that bio-oil is a mixture obtained from different biomasses through pyrolysis, alcoholysis, acidolysis, high liquefaction, etc., and those biomasses mainly include wood fiber type, waste oil type and animal manure type. Bio-binder refers to the product of bio-oil processed by distillation, extraction oxidation and polymer modification, and it can be served as a modifier, diluent or substitute material of asphalt; the main chemical components of bio-oil include ethers, esters, aldehydes, ketones, phenols, organic acids, alcohols and sugars. Bio-asphalt is obtained by adding bio-binder into the petroleum asphalt for modification or blending, and the shear temperature and shear rate of bio-asphalt derived from wood fiber type and waste oil type are usually higher than that of bio-asphalt derived from swine manure. Compared with petroleum asphalt, bio-asphalt binder usually shows lower high-temperature performance as well as higher low-temperature performance and aging resistance. Also, bio-asphalt mixture generally exhibits lower high-temperature stability, higher low-temperature crack resistance and water stability than petroleum asphalt mixture. Future studies should be conducted combining with the source, composition, preparation, properties and oil production rate of bio-oil. First, how to raise the bio-binder content in bio-asphalt as much as possible while ensuring the sufficient performance becomes the focus of future researches. Second, the micro reaction mechanism between bio-binder and petroleum asphalt should be illustrated in depth. Moreover, developing a complete and unifying technical standard and application specification of bio-asphalt technology is necessary for future researches. Furthermore, determining the optimum bio-binder potentially used as the substitute of petroleum asphalt is also an interesting topic.

Effect of Desulfurization Process Variables on the Properties of Crumb Rubber Modified Asphalt

A large number of waste tires are in urgent need of effective treatment, and breaking waste tires into crumb rubber powder for modifying asphalt has been proved as a good idea to solve waste tires. Crumb rubber modified asphalt not only has good high and low temperature performance, durability, and aging resistance but can also reduce pavement noise and diseases, which has wide application prospects. In this study, crumb rubber powder was desulfurized by mechanochemical method to prepare desulfurized crumb rubber modified asphalt. During the desulfurization process of crumb rubber, the effects of desulfurization process variables including desulfurizer type, desulfurizer content, and desulfurization mixing temperature and time were considered, and then the physical properties of modified asphalt were tested. The test results showed that after mixing crumb rubber powder with desulfurizer, the viscosity of crumb rubber powder modified asphalt can be reduced. Moreover, the storage stability of crumb rubber powder modified asphalt could also be improved by mixing crumb rubber with desulfurizer. Based on the physical properties of crumb rubber powder modified asphalt, the desulfurization process of selected organic disulfide (OD) desulfurizer was optimized as follows: the OD desulfurizer content was 3%, the desulfurization mixing temperature was 160 °C, and the mixing time was 30 min. In addition, Fourier infrared spectroscopy analysis was carried out to explore the modification mechanism of desulfurized crumb rubber powder modified asphalt. There is no fracture and formation of chemical bonds, and the modification of asphalt by crumb rubber powder is mainly physical modification.

EFFECT OF GRAIN-BOUNDARY SEGREGATION ON LOW-TEMPERATURE DEGRADATION OF ORAL 3Y-TZP CERAMICS CO-DOPED WITH GEO2AND TIO2

Purpose: To explore the improvement of its low temperature aging resistance by co-doping oral 3Y-TZP with TiO2and GeO2. Methods: After the samples of 3Y-TZP co-doped with GeO2and TiO2were prepared by mechanical ball milling, drying, and pressureless sintering, the microstructure was studied by scanning transmission electron microscope, energy spectrometer, and high angle annular dark field detector. After treatment in the autoclave, the conversion of all samples was measured by XRD. Results: After mechanical ball milling for 24 hours, the particle size of the composite powder of 3Y-TZP co-doped with GeO2and TiO2is uniform, distributed at about 50[Formula: see text]nm, the particle shape is basically spherical, the dispersion is good, but there is a small amount of agglomeration; Tetravalent stabilizers (Ge[Formula: see text], Ti[Formula: see text]) can inhibit the low-temperature aging effect of zirconia materials. With the increase of germanium oxide doping, the hydrothermal degradation resistance of zirconia can be improved. GeO2doped 3Y-TZP has better low temperature aging resistance than TiO2doped 3Y-TZP. With the increase of dopant content, the degree of grain boundary segregation increases, but even if the doping amount of Ge[Formula: see text] is small, the degree of grain boundary segregation can be comparable to that of Ti[Formula: see text] with a large doping amount. Conclusion: In this paper, the composite powder of 3Y-TZP co-doped with GeO2and TiO2with uniform particle size and good dispersion was prepared by mechanical ball milling. The preparation process is simple, the energy consumption is low, and the cumbersome processes of the co-precipitation method and liquid-phase method are avoided. On the other hand, adding GeO2and TiO2can enhance the low-temperature aging resistance of 3Y-TZP ceramics. GeO2is superior to TiO2in low-temperature degradation resistance, which can be used as the first choice. This study contributes to the research on doping cations of dental zirconia and provides a reference for dentists to choose a reasonable zirconia material.

Dielectric, structural and mechanical properties of thermally aged biaxially oriented polymeric substrates for flexible electronics

Two materials, polyethylene naphthalate (PEN) and polyethylene terephthalate (PET), which are often used as flexible substrates for electronics, were exposed to accelerated thermal aging at temperatures of 190 °C and 210 °C for 5000 and 2000 h, respectively. Changes in their thermal, dielectric, and mechanical properties were investigated. The results revealed that dielectric losses increase slower for PET than for PEN at lower aging temperatures. In contrast, at higher aging temperatures, similar behavior was observed for both materials. During aging, the dissipation factors of PET and PEN increased by one order of magnitude over those of the original states. The dielectric frequency-temperature characteristics obtained by broadband dielectric spectroscopy (BDS) shifted and indicated changes in the individual dielectric relaxation mechanisms (i.e., α, β, β*). Discoloration induced by thermal aging and quantified by ultraviolet–visible spectroscopy by the peak at 453 nm was detected primarily in the spectrum of PEN. The mechanical tensile strength of PET deteriorated considerably (10 times) after only a few hundred hours, whereas PEN was stable after even longer exposure times. It was found that all of the changes mentioned above are primarily related to the change in the crystalline-to-amorphous phase ratio, as demonstrated by differential scanning calorimetry (DSC) and supported by density measurements.