Longer Aging Times Research Articles

Mechanism of dissolution reactivity and reactions of various calcium borate glasses and glass-ceramics

This study investigated the effect of calcium addition on boron coordination and ion release in deionized water (DIW) of five melt-quench-derived compositions: xCaO-(100-x)B2O3 (x=25, 27, 33, 38 and 48 mol%) and one sol-gel-derived 48CaO-52B2O3 glass. The mechanism of hydration and hydrolysis reactions was elaborated based on ionic release and structural change pre- and post- ageing in DIW. For melt-quench-derived glasses, within 8 h ageing, the hydration reactions were more rapid than the hydrolysis reactions, resulting in solution pH increase, where ionic release was more dependent on composition than boron coordination. At longer ageing times, hydration and hydrolysis reactions occurred simultaneously, leading to solution pH stabilization, and calcium and boron release decreased with an increase in the boron tetrahedral fraction (N4) in glasses. Non-bridging-oxygens in the melt-quench-derived 48CaO-52B2O3 glass-ceramic and hydroxyl groups in the equivalent sol-gel-derived glass fostered calcium and boron release. All glass formulations demonstrated higher N4 post DIW exposure.

Low‐Cycle‐Fatigue Performance of Stress‐Aged EN AW‐7075 Alloy

The effect of a novel heat treatment, that is, aging under superimposed external stress, on the fatigue performance and microstructural evolution of a high‐strength aluminum alloy (EN AW‐7075) is presented. Stress aging, a combination of heat treatment and superimposed external stress, can enhance the mechanical properties of EN AW‐7075 under monotonic loading due to the acceleration of precipitation kinetics. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) reveal that a longer aging time and the presence of superimposed stress both promote the formation and growth of precipitates, that is, the precipitation of strengthening η´ precipitates. This is confirmed by differential scanning calorimetry (DSC) heating experiments of stressless and stress‐aged states. Furthermore, stress aging leads to a reduction of dimensions of precipitate‐free zones near grain boundaries. Cyclic deformation responses (CDRs) and half‐life hysteresis loops are evaluated focusing on the low‐cycle fatigue (LCF) performance of different conditions. A noticeable cyclic hardening seen in case of the specimens aged for a short time indicates the occurrence of dynamic strain aging (DSA). Eventually, stress aging allows for an enhancement of the monotonic mechanical properties of EN AW‐7075 without degrading the cyclic performance in the LCF regime.

Evolution of the microstructure and mechanical properties of Sanicro 25 austenitic stainless steel after long-term ageing

A newly developed heat-resistant austenitic steel, Sanicro 25 is currently considered the leading candidate material for an advanced ultra-supercritical installation. The test material was subjected to long-term ageing (up to 30,000 h) at 700 and 750 °C, after which investigations into the microstructure, identification of precipitates, and testing of mechanical properties were conducted. Sanicro 25 had an austenitic microstructure with annealed twins and numerous large primary NbX and Z-phase precipitates in the as-received condition. It was found that the long-term ageing of the steel resulted in numerous precipitation processes. For example, M23C6 carbides, Laves, σ and G phases occurred at the grain boundaries. However, Z-phase precipitates, ε_Cu particles, and Laves phase were observed inside the grains. At the same time, compound complexes of precipitates based on the primary Z-phase precipitates were revealed in the microstructure. The ageing process increased the particle size of M23C6 carbides and the σ phase. After longer ageing times, a precipitate-free zone (PFZ) near the grain boundaries was observed. The precipitation processes initially lead to an increase in the strength properties of the steel. However, after 5000 h, an over-ageing effect was observed at 750 °C, which was not observed at 700 °C.

Examination of long-time aging process on volatile organic compounds emitted from solid fuel combustion in a rural area of China

Volatile organic compounds (VOCs) emitted from solid fuels combustion (e.g., biomass and coal) are still the dominant precursors for the formation of tropospheric ozone (O3) and secondary organic aerosols (SOAs). Limited research focused on the evolution, as known as atmospheric aging, of VOCs emitted during long-timescale observations. Here, freshly emitted and aged VOCs from common residual solid fuel combustions were collected onto absorption tubes before and after passing through an oxidation flow reactor (OFR) system, respectively. The emission factor (EF) of freshly emitted total VOCs is in descending order of corn cob ≥ corn straw > firewood ≥ wheat straw > coals. Aromatic and oxygenated VOCs (OVOCs) are the two most abundant groups, accounting for >80% of the EF of total quantified VOCs (EFTVOCs). Briquette technology shows an effective reduction of the VOC emission, demonstrating a maximum 90.7% lower EFTVOCs in comparison to that of biomass fuels. In contrast, each VOC shows significantly different degradation in comparison to EF of freshly emitted and after 6- and 12-equivalent day aging (actual atmospheric aging days calculated from aging simulation). The largest degradations after 6-equivalent days of aging are observed on alkenes in the biomass group (60.9% on average) and aromatics in the coal group (50.6% on average), consistent with their relatively high reactivities toward oxidation with O3 and hydroxyl radical. The largest degraded compound is seen for acetone, followed by acrolein, benzene, and toluene. Furthermore, the results show that the distinction of VOC species based on long-timescale (12-equivalent day aging) observation is essential to further explore the effect of regional transport. The alkanes which have relatively lower reactivities but high EFs could be accumulated through long-distance transport. These results provide detailed data on fresh and aged VOCs emitted from residential fuels which could be used to explore the atmospheric reaction mechanism.

Influence of Precipitation Hardening on the Mechanical Properties of Co-Cr-Mo and Co-Cr-W-Mo Dental Alloys

Co-Cr alloys have good mechanical properties such as high hardness, excellent magnetic properties and good corrosion resistance. For this reason, they are most commonly used as dental and orthopaedic implants. Generally, cast Co-Cr-Mo alloys and forged Co-Ni-Cr-Mo alloys are used for the production of implants. In this study, we investigated two dental alloys, namely, Co-Cr-Mo and Co-Cr-W-Mo alloys. The aim of this study was to determine the effect of heat treatment on the development of the microstructure and to evaluate its influence on the alloys’ mechanical and tribological properties. The samples were first solution-annealed at 1200 °C in an argon atmosphere for 2 h, then quenched in water and subsequently aged at 900 °C in an argon atmosphere for 1, 3 and 12 h. A microstructural analysis was performed using SEM, with EDS for microchemical analysis and EBSD for phase identification. In addition, the Vickers hardness and wear resistance of the two alloys were analysed before and after heat treatment. The Co-Cr-Mo alloy showed better wear resistance and also a generally higher hardness than the Co-Cr-W-Mo alloy. Both alloys showed signs of abrasive and adhesive wear, with carbide particles detaching from the Co-Cr-W-Mo alloy due to the lower hardness of the matrix. The Co-Cr-Mo alloy showed the best abrasion resistance after the longest aging time (12 h), while the Co-Cr-W-Mo alloy showed the best as-cast abrasion resistance. With ageing, the wear resistance of both alloys increased.

Remarkable age-hardening response and microstructural evolution of Mg–7Sn alloy

The poor age-hardening response of Mg alloys limited their development and applications. In this study, the age-hardening response of Mg–7Sn alloy has been improved significantly by a 40 MPa compressive stress aging at 220 °C. Phase transition from metastable βˈ (Mg3Sn) phase to β (Mg2Sn) phase occurs during the stress aging. The βˈ with L12 structure has an orientation relationship with the Mg matrix of (11¯0)β＇//(112¯0)α and [1¯1¯0]β＇//⌈0001¯⌉α. The precipitated sequence in the stress-aged alloy is S.S.S. (supersaturated solid solution)→GP→βˈ(Mg3Sn)→β(Mg2Sn). The stress-aged alloy exhibits a high peak-aged hardness (∼71 HV) and short time to reach the peak hardness (12 h) as compared with the aged alloy without the stress. Interestingly, a long-time aging platform with 71 HV for 96 h is observed during stress aging. The formation of the platform is related to a balance of hardening and softening arisen from microstructure evolution during stress aging. The stress aging provides a promising heat treatment way to improve aging hardening response of Mg alloys.

Experimental investigation on the accelerated aging of silicone rubber insulators based on thermal stress

During recent decades, polymer insulators have become very popular due to their many advantages over other insulators. Since observing noticeable changes and signs of aging on insulators in the power grid requires a long time, therefore laboratory studies focusing on the accelerated aging of polymer insulators help to identify materials and study the trend of insulation changes during the aging process. In this paper, the accelerated aging of polymer insulators using thermal stress at 180 °C and 3000 h is investigated. The experience is done after investigating the effect of UV aging on insulators. During and after the accelerated thermal aging test, the electrical quality of the polymer insulator are analyzed using leakage current measurement tools, partial discharge and hydrophobicity. Analysis of surface changes are performed by Scanning Electron Microscopy, Energy Dispersive X-ray Analysis, Thermo Gravimetrical Analysis and Fourier Transform Infrared Spectroscopy in qualitative and quantitative modes. The results of different methods of the analysis show that in the thermal aging test, due to the high thermal endurance of polymer insulators, significant changes in parameters would be visible after longer aging times, so thermal aging is not practical on aging polymer insulators and is not recommended as an accelerated aging test independent of other aging tests, especially UV.

Evolution of precipitate and precipitate/matrix interface in Al-Zn-Mg-Cu (-Ag) alloys

• The phase evolution of Al-Zn-Mg-Cu alloys with and without Ag addition during ageing at 120 °C and over-ageing at 180 °C is revealed. • The mechanism controlling the early-stage solute clustering and the promoting effect of Ag are elucidated systematically. • The role of solute clusters on the formation of η’ precipitates is clarified by in-situ STEM heat treatment. • With longer ageing time, the precipitate/matrix interface shows an evolution from Zn dominating for the interface between early-stage η’ and Al matrix to Zn and Mg co-segregating for η’/Al and η 2 /Al interfaces. • The role of precipitate/matrix interface in the growth of η’ and η 2 is revealed. • The exact positions of Ag and Cu atoms are specified. Evolution of precipitate and precipitate/matrix interface in artificially aged Al-Zn-Mg-Cu (-Ag) alloys has been systematically studied. In the early stage of ageing, Ag, as a fast diffuser, can promote the formation of solute pairs and small clusters. Solute clusters are further demonstrated to be able to act as precursors for η’ precipitates by in-situ STEM heating. With prolonged ageing time, the precipitate/matrix interface evolves from the Zn-dominated interface between early-stage η’ and Al matrix to the Zn and Mg co-segregated η’/Al and η 2 /Al interfaces. The η’/Al interfacial layers are shown to precede the formation of η’, while the η 2 /Al interface is found to be closely related to the thickening process of η 2 and the involved particular atomic movements are specified. Experimental observations and DFT calculations reveal that for η’ and η 2 , Ag can dissolve into the precipitate as well as locate at the precipitate/matrix interface without showing preference. For Cu, its dissolution in the precipitate and segregation on the interface mainly occur for η 2 rather than η’. The incorporation of Ag and Cu does not change the defined precipitate structure.

Effects of Physical Hardening on Low-Temperature Properties of Asphalt Binders Containing Reclaimed Asphalt Pavement

Abstract Reclaimed asphalt pavement (RAP) materials are widely used in sustainable construction of the asphalt pavement, which has evident advantages in solving the problem of material costs and waste disposals. However, there are few studies on the influence of RAP materials on the physical hardening of asphalt binders. In this paper, the physical hardening rate from the dynamic mechanical analysis test and grade loss deduced by the extended bending beam rheometer experiment were used to characterize the physical hardening of the RAP-modified asphalt binder at low temperatures. The results show that the loading time–conditioning time superposition principle is suitable to determine the physical hardening rate of RAP-modified asphalt binder through a smooth master curve and verification of the typical 2S2P1D rheological model, and grade loss can be predicted by the physical hardening rate using their good correlation. The RAP binders can accelerate the degree of physical hardening and increase the thermal stresses of the asphalt binder, and these effects are more evident for the RAP binder with longer aging time and higher content. The base asphalt with less wax content will act as a regenerant, which can accommodate more content of the RAP materials to produce a new usable asphalt binder.

Dynamics and microrheology of colloidal clay-polymer glasses and gels: Size-dependent phenomena and re-entrant behavior at early aging times.

Colloidal clay Laponite forms a variety of arrested states that display interesting aging behavior. Microrheology has been applied to Laponite-based glasses and gels, but few studies evaluate the influence of probe particle size. In this work, we report the dynamics and microrheology of Laponite-polymer dispersions during aging using passive microrheology with three different probe particle sizes. At early aging times, the neat Laponite dispersion forms an arrested state; the nature of this state (e.g., a repulsive glass or gel) has remained the subject of debate. The addition of polymer retards gelation and melts the arrested state. While this melting has been observed at the macroscale and has been attributed to a re-entrant transition of a repulsive glass to a liquid state, to our knowledge, it has not been observed at the microscale. The delay of the gelation time needed to form an arrested state was found to depend on the polymer concentration and could vary from ∼24 h for neat Laponite to seven days for some Laponite-polymer samples. Significant effects of probe particle sizes are observed from the mean-squared displacement (MSD) curves as small and intermediate probe particles show diffusive motion, while the motion of large particles is restricted. By examining the factor of ⟨Δr2 (τ)⟩a, structural heterogeneity can be confirmed through the strong size-dependence displayed. Different MSD trends of probe particles are obtained at longer aging times, but no significant changes occur after 30 days of aging. Our microrheology results also reveal significant effects of probe particle size.

Assessing rheometry for measuring the viscosity-average degree of polymerisation of cellulose in paper degradation studies

In paper degradation studies, the viscosity-average degree of polymerisation (DPv) is often used as a key indicator of the extent of degradation of cellulosic paper. DPv can be deduced from the viscosity of dilute paper solutions, as typically measured through glass capillary viscometry. The current study proposes an efficient, alternative method to evaluate DPv of cellulosic paper, which is based on rotational rheometry. The proposed methodology relies on the application of a shear flow in a thin film of cellulose solution to measure its dynamic viscosity, from which DPv can be subsequently derived in a straightforward fashion. Rheometry allows to measure the viscosity for a range of shear rates, which results in multiple DPv evaluations per sample, and thus in statistically representative data from an individual test. Further, rheometry typically requires considerably less paper mass per test than glass capillary viscometry, which makes the method attractive for paper degradation studies with limited sample availability. Also, rheometry measurements are less work-intensive than glass capillary viscometry measurements. The rheometry method has been applied to 4 hygrothermally aged cellulose paper samples and the unaged counterpart. The measurement results regarding the age-dependency of DPv and the number of cellulose chain scissions are compared to those obtained by glass capillary viscometry, showing a very good agreement. At a longer ageing time, both experimental methods reveal a non-linear decrease in time of DPv, and a non-linear increase in time of the number of cellulose chain scissions, which indicate that the cellulose ageing process is realistically captured. The agreement in measurement results further demonstrates that rheometry is an easy-to-use, accurate and efficient alternative for DPv measurements by glass capillary viscometry.

Surprising Structural and Functional Properties of Favism Erythrocytes Are Linked to Special Metabolic Regulation: A Cell Aging Study.

Favism uniquely arises from a genetic defect of the Glucose-6 Phosphate Dehydrogenase (G6PD) enzyme and results in a severe reduction of erythrocytes' (RBCs) reducing power that impairs the cells' ability to respond to oxidative stresses. After exposure to fava beans or a few other drugs, the patients experience acute hemolytic anemia due to RBCs' lysis both intra and extra-vascularly. In the present paper, we compared selected biochemical, biophysical, and ultra-morphological properties of normal RBCs and cells from favism patients measured along cellular aging. Along the aging path, the cells' characteristics change, and their structural and functional properties degrade for both samples, but with different patterns and effectors that have been characterized in biophysical and biochemical terms. In particular, the analysis revealed distinct metabolic regulation in G6DP-deficient cells that determines important peculiarities in the cell properties during aging. Remarkably, the initial higher fragility and occurrence of structural/morphological alterations of favism cells develop, with longer aging times, into a stronger resistance to external stresses and higher general resilience. This surprisingly higher endurance against cell aging has been related to a special mechanism of metabolic regulation that permits lower energy consumption in environmental stress conditions. Our results provided a direct and coherent link between the RBCs' metabolic regulation and the cell properties that would not have been possible to establish without an investigation performed during aging. The consequences of this new knowledge, in particular, can be discussed in a more general context, such as understanding the role of the present findings in determining the characteristics of the favism pathology as a whole.

Achieve high plasticity and strength in 6016 alloy by high-pressure torsion combined with post-aging treatments

High-pressure torsion (HPT) processing currently receives much attention mainly because it can significantly improve the strength of alloys compare to other sever plastic deformation. However, a common problem for these HPT processed alloys is that they suffer from low plasticity. Here we proposed an optimized post-aging treatment that achieves HPT processed 6016 alloy with tensile elongation as large as ∼36% and ultimate tensile strength over 500 MPa. Through systematically comparing the effects of high and low temperature aging treatments on the microstructure and properties of alloys, it was found that low-temperature long-time aging treatment can make HPT processed 6016 alloy exhibit higher plasticity while maintaining higher strength. The higher strength mainly originates from finer grains and higher strengthening effect of L precipitates, while the higher plasticity is attributed to the formation of nanoscale Si particles with smaller size and distributed in the grain interior after low-temperature long-time aging treatment. In addition, thermo-kinetic calculation demonstrates that the low-temperature long-time aging treatment provides higher thermodynamic driving force, higher kinetic activation energy and higher generalized stability for L phase precipitation, which is conducive to achieve a good strength–plasticity combination of HPT processed alloys.

An investigation into the aging of disposable face masks in landfill leachate

Due to the excessive use of disposable face masks during the COVID-19 pandemic, their accumulation has posed a great threat to the environment. In this study, we explored the fate of masks after being disposed in landfill. We simulated the possible process that masks would experience, including the exposure to sunlight before being covered and the contact with landfill leachate. After exposure to UV radiation, all three mask layers exhibited abrasions and fractures on the surface and became unstable with the increased UV radiation duration showed aging process. The alterations in chemical groups of masks as well as the lower mechanical strength of masks after UV weathering were detected to prove the happened aging process. Then it was found that the aging of masks in landfill leachate was further accelerated compared to these processes occurring in deionized water. Furthermore, the carbonyl index and isotacticity of the mask samples after aging for 30 days in leachate were higher than those of pristine materials, especially for those endured longer UV radiation. Similarly, the weight and tensile strength of the aged masks were also found lower than the original samples. Masks were likely to release more microparticles and high concentration of metal elements into leachate than deionized water after UV radiation and aging. After being exposed to UV radiation for 48 h, the concentration of released particles in leachate was 39.45 μL/L after 1 day and then grew to 309.45 μL/L after 30 days of aging. Seven elements (Al, Cr, Cu, Zn, Cd, Sb and Pb) were detected in leachate and the concentration of this metal elements increased with the longer aging time. The findings of this study can advance our understanding of the fate of disposable masks in the landfill and develop the strategy to address this challenge in waste management.

Long-time strain aging responses of commercial dual-phase steels at room temperature

ABSTRACT In this study, the effects of room temperature static strain aging period on the tensile properties of DP600 dual-phase steel were investigated. 6% and 12% of pre-strained DP600 steels were carried out static strain aging treatment at room temperature for up to 4 months. It was observed that with increasing both pre-strain rate and room temperature static aging period, the tensile strength values increased and ductility values decreased significantly. Also, dual-phase steels maintain the continuous yield behaviour which is general characteristic. The fracture surfaces exhibited a lower depth dimple type mode with increasing room temperature static strain aging period.

Spatially Confined Fabrication of Polar Poly(Vinylidene Fluoride) Nanotubes.

Polar poly(vinylidene fluoride) (PVDF) nanotubes have attracted significant attention due to their excellent piezoelectric and ferroelectric properties, yet a tunable fabrication of homogeneous polar PVDF nanotubes remains a challenge. Here, a simple method is reported to fabricate polar PVDF nanotubes using anodize aluminum oxide (AAO) membranes as templates that are removed by etching in a potassium hydroxide (KOH) solution and then ageing at room temperature. PVDF nanotubes originally crystallized in the AAO membrane are pure α-crystals with very low crystallinity, yet after being released from the templates, the crystallinity of the nanotubes markedly increases with ageing at room temperature, leading to the formation of β-PVDF crystals in a very short time, with the formation of γ crystals after longer ageing times. A large amount of γ crystals formed when the released PVDF nanotubes are heated to ≈130°C. The formation of polar PVDF nanotubes released from the AAO templates treated with higher concentrations of alkaline solution results from the reaction of the surface of the PVDF nanotubes with the alkaline solution and structure reorganization under confined conditions. This large-scale preparation of β- and γ-PVDF opens a new pathway to produce polar PVDF nanomaterials.

Unveiling the effect of Hf-(Mo,W) addition on the coarsening, agglomeration, and dissolution behaviour of γ'-Ni3Al precipitates in model Ni-based superalloys

In compositional design of Ni-based superalloys, atomic size of alloying elements appears as the primary factor affecting alloy strengthening. Depending on atomic size, the phase partitioning preferences of alloying elements not only control the coarsening and dissolution characteristics of γ' precipitates, but also govern their morphological development upon aging. The current study, therefore, investigates the influence of larger atomic radius X = Hf element addition and its co-additions with smaller atomic radii refractory X = Mo(W) elements on the microstructural and mechanical properties of model Ni80Al15Hf5 and Ni80Al15Hf2.5Mo(W)2.5 superalloys. Accordingly, it is concluded that the replacement of 2.5 at% X = Hf addition with the same amount of X = Mo(W) additions reduces and increases γ' participation tendency of X = Hf and X = Mo(W) elements, respectively. Agreeing with classical Lifshitz-Slyozov-Wagner (LSW) kinetics, γ'-Ni3Al-Hf-W precipitates coarsen at a slower rate compared to their γ'-Ni3Al-Hf-Mo counterparts, both begin to agglomerate at longer aging times, i.e., 256 h. On the other hand, readily coarsened, aligned, and agglomerated γ'-Ni3Al-Hf precipitates start to dissolve in γ matrix after 16 h, which hence disobey LSW theory. Most probably, higher γ'/γ interfacial energy contribution of faster-diffusivity X = Hf element leads to this three-step temporal evolution of γ'-Ni3Al-Hf precipitates, i.e., (i) coarsening, (ii) alignment/agglomeration, and (iii) dissolution, to occur much earlier and accelerates the strength degradation. Conversely, X = Hf-W co-addition, which causes better mechanical strength than both X = Hf and X = Hf-Mo additions, reduces the degree of strength worsening possibly due to the beneficial contributions of X = W element at γ'/γ interfaces.

Microstructure evolution, IMC growth, and microhardness of Cu, Ni, Ag-microalloyed Sn-5Sb/Cu solder joints under isothermal aging.

In this work, various Cu, Ni, Ag-microalloyed Sn–5Sb/Cu joints, ordinary Sn–5Sb/Cu joints, and low-melting-point Sn–3Ag–0.5Cu (SAC305)/Cu (used for comparison) were prepared, focusing on the influence of Cu, Ni, and Ag on the microstructure evolution, interfacial IMC growth, and microhardness of Sn–5Sb/Cu joint under long-time isothermal aging process. Results showed that the microstructure of microalloyed joints consisted of β-Sn matrix, SbSn, and (Cu, Ni)6Sn5 and Ag3Sn compounds. (Cu, Ni)6Sn5 compounds generated a coarsening effect in the aging microalloyed joints, yet its coarsening speed is significantly lower than the ordinary Sn–5Sb/Cu. Meanwhile, the total IMC layer thickness increased with the rising aging time. A single fine dendritic (Cu, Ni)6Sn5 IMC at the interface of microalloyed joint was observed and evolved into a larger scallop or layer-like duplex IMCs ((Cu, Ni)6Sn5 + Cu3Sn) after the aging. Considering the combined effect of Cu, Ni, and Ag, the microalloyed joints exhibited the improved microstructure relative to ordinary counterparts and low-melting-point SAC305 materials, significantly inhibiting the interfacial IMC growth, especially Cu3Sn. The Cu3Sn IMC thickness and diffusion coefficient in the Sn–5Sb–0.5Cu–0.1Ni–0.5Ag/Cu joint were 0.71–2.81 μm and 0.96 × 10–6 μm·s−2, respectively. Besides, the precipitation strengthening mechanism triggered by the microalloyed elements was extremely obvious and the soldering and aging joints revealed superior microhardness values of 20–35 HV. This could effectively improve the application range of Sn–5Sb-based materials in higher-temperature package conditions such as third-generation semiconductors.

Assessment of Natural Radioactivity in Cements Used as Building Materials in Poland.

It has been analyzed in this article the radioactivity concentrations of 226Ra, 232Th, 40K and radiological hazard parameters in different types of cements commonly used in Poland and available on the Polish market. The radiological hazard parameters are, in particular, absorbed gamma dose rate, annual effective dose, radium equivalent activity, the external hazard index, and the gamma and alpha indices. The radionuclide activities of the most important radionuclides 226Ra, 232Th, 40K have been determined by gamma-ray spectrometry with the use of two kinds of spectrometers of different operational parameters. One performed also measurements on 30-day and 45-day aged samples as to verify if there is a statistically significant difference in radioactivity concentration for shorter and longer aging time. The radioactivity concentrations in the cement samples ranged from 21.7–75.7 Bq·kg−1 for 226Ra, 12.3–47.3 Bq·kg−1 for 232Th to 123–430 Bq·kg−1 for 40K. The radiological parameters in cement samples were calculated as follows: mean radium equivalent activity Raeq = 127 Bq·kg−1, mean absorbed gamma dose rate D = 115 nGy·h−1, mean annual effective dose E = 570 µSv·y−1, external hazard index Hex = 0.32, internal hazard index Hin = 0.51, mean activity concentration index Iγ = 0.47 and mean alpha index Iα = 0.28. The results were compared with the reported data from other countries and the international standard values given by European Commission (EC) and United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2000). Finally, thorough statistical analysis has been performed.

Effect of artificial aging on the strength, hardness, and residual stress of SLM AlSi10Mg parts prepared from the recycled powder

This study investigated the effect of artificial aging on the hardness, strength, and residual stress of SLM (Selective Laser Melting) AlSi10Mg parts prepared from recycled powder used for 30 SLM printing cycles. First, an analysis was conducted for a comparison of recycled and virgin powders; it revealed that the powders have the same shape and phase and chemical composition. The only significant differences were the slightly increased average particle size and higher oxygen content in the recycled powder, especially on the powder surface. Typical molten pools were found in the microstructure, and a fine cellular type of structure was observed. The peripheral areas of the molten pools contained coarser cells with an average size of 0.6 μm2, and the middle part of the molten pools contained finer cells with an average size of 0.17 μm2. Fine Si precipitates with an average size of 25 nm were observed inside the cells. In terms of microstructure, no significant differences were observed when compared to the published data relating to SLM using virgin powders. This suggests that recycled powder after 30 printing cycles can be utilized for SLM component preparation to achieve cost-effectiveness. However, comparison of products with the use of virgin and recycled powders showed that the use of recycled powder affects the submicron characteristics (precipitate size increase), density and the resulting mechanical properties. Artificial aging of samples from the recycled powder at 170 °C caused an increase in hardness, Young's modulus and strength, while ductility remained essentially the same after aging. Peak values of hardness and strength were determined after 6 h of artificial aging. The increase in strength of the prepared samples can be explained by the precipitation of fine Si particles and their coarsening inside the cells. However, after longer aging times, the precipitates continued to coarsen, coalesced and displayed a plate-like shape, which led to the reduction in strength. The residual stress measured by the bridge curvature method (BCM) showed a significant decrease with increasing artificial aging time. The lowest residual stress was determined for an aging time of 100 h; however, the difference between residual stress for 6–7 h and 100 h was not significant. In order to obtain higher strengths and remove internal stresses after SLM during one heat treatment, the most suitable time for artificial aging at 170 °C seems to be 6 h.