Increasing Test Temperature Research Articles

Characterization of Fatigue Crack Growth Behaviour of Alloy 617M Base and Weld Material

Alloy 617M is a Ni-based super-alloy made in the form of tubes which are used in high-temperature boilers and super heaters in Advanced Ultra Super Critical (AUSC) power plants. The high-temperature oxidation and mechanical properties of this material have been derived based on chromium (Cr), cobalt (Co) and molybdenum (Mo) as prime alloying elements which cause solid solution strengthening, carbide precipitation (M23C6, M6C) and formation of intermetallic phase Ni3(Al, Ti) (γʹ-phase). The manufacturing route of these components involve bending and welding operations which are critical locations and potential sites for crack nucleation and growth. Also, the conjoint influence of high temperature and flow-induced vibrations in the tubes further aides to crack nucleation and growth. Thus, fatigue crack growth (FCG) properties are important. Therefore, in this work, detailed experimental studies on the FCG behaviour of Alloy 617M base and weld materials have been carried out at different test temperatures (27 °C, 650 °C, 710 °C and 750 °C). The effect of ageing (1000, 2000, 5000 and 20000 hours) at an operating temperature of 710 °C on FCG has also been evaluated. All the FCG tests have been conducted as per ASTM with a constant load ratio (R = 0.1) and frequency of 15 Hz. From the experimental results, a major trend is observed of decreasing threshold stress intensity factor (ΔKTH) with an increase in ageing time and test temperature. The variations in the ΔKTH with test temperature and ageing time have been discussed and the crack growth mechanism has been established through microstructure and fractographic studies validating from existing literatures.

A Study on the Role of Clay Nanoparticles and Temperature on the Creep Behavior of Polyethylene Matrix Nanocomposite

Objective: In the current research, the creep behavior of polyethylene (PE) matrix nanocomposites reinforced with different content of clay nano particles (i.e., 0, 1, 3, 5, 10wt%) at different temperatures (25 and 50℃) has been investigated. Methods: In order to produce composite samples of pure PE reinforced with different percentages of clay (0, 1, 3, 5 and 10% by weight), a twin extruder was used in the temperature range of 200-230℃. Also, to produce the standard samples, an injection machine was used using in the temperature range of 220-230℃. The samples were subjected to a constant load, typically around 0.7 to 0.8 times the yield strength, at 25 and 50℃ , and at constant strain rate of 0.1min-1. Results: The creep investigations showed that for PE and its nanocomposites the creep trend could be divided into primary, secondary, and tertiary stages. The length of each stage depended strongly on both temperature and the stress level at which the samples tested as well as the clay content. It was observed that adding nano particles in small amounts (1wt%) led to an increase in the length of the creep life of PE and it also improved this property in both temperatures. Conclusion: It was observed that increasing test temperature from causes to decrease the creep strength as well as a rapid onset of plastic deformation for all materials including PE and its nanocomposites. PE nanocomposites with higher than 1wt% clay exhibited a noticeable drop in creep strength particularly at 50℃.

Enhancing infrared emissivity of GdCoO3 with Ca doping: Potential for advanced thermal control materials

The high infrared emissivity material Gd1-xCaxCoO3 was synthesized using the sol-gel method. The structure and composition of Gd1-xCaxCoO3 were investigated through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), specific surface area analysis, Brunauer-Emmett-Teller (BET) method for void analysis, and Ultraviolet-Visible-Near-Infrared (UV-Vis-NIR) diffuse reflection spectrum were employed to analyze the surface morphology, elemental composition, specific surface area, pore size distribution, and band gap of the prepared powders. The average infrared emissivity of samples with different doping concentrations at various temperatures (8-14 μm and 3-5 μm) was determined using a dual-band emissivity meter. At room temperature, the infrared emissivity of the doped powders is generally higher, exceeding 0.75 in comparison to the undoped sample powder. As the test temperature increases, the infrared emissivity of all samples shows a significant rise. The infrared emissivity of all samples surpasses 0.91 at 500 °C, with the emissivity of the samples reaching as high as 0.988-14μm (0.993-5μm) with a Ca doping concentration of 5 %. These findings indicate that Ca-doped GdCoO3 powder represents a promising high-emissivity material with broad potential in spacecraft thermal control applications.

High-temperature tribological behavior of nanolayered TiAlN/TiSiN coating deposited on WC-Co cemented carbide

In this study, high-temperature tribological behavior of nanolayered TiAlN/TiSiN coating was evaluated against the Al2O3 counter-body using a pin-on-disk tribometer. The coating was deposited on the WC-Co substrate, in an industrial unbalanced magnetron sputtering system. Tribological tests were conducted at room temperature, 500, 600, 700, and 760 °C, in air and nitrogen atmospheres. After the tests, the coating was examined using confocal microscopy, tactile profilometry, scanning electron microscopy, focused ion beam, energy dispersive spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The coating retained its microstructure and mechanical properties after exposure to high temperatures. At lower temperatures coating exhibited abrasive and adhesive wear mechanisms, while at higher temperatures abrasive and oxidative wear mechanisms were observed. In high-temperature tests, AlO, TiO and SiO were detected inside of wear tracks, in both atmospheres. However, the oxide thickness was significantly lower in tests with nitrogen atmosphere. Additionally, the top of the oxide layer was enriched in AlO with respect to TiO and SiO. The enrichment of AlO was more pronounced in nitrogen atmosphere. The coating tested in air exhibited slightly higher and more unstable coefficient of friction (COF) values, than in nitrogen atmosphere. This is attributed to the increased oxidation in air atmosphere. At room temperature tests in nitrogen, the wear rate was approximately 3 times lower than in air. At higher temperatures the wear rate was lower than at room temperature, in both atmospheres, due to formation of protective oxides. However, with the increase in testing temperature the wear rate increased. The reason for such behavior is the loss of substrate's and coating's hardness at high temperatures, and thickening of the oxide layer which accelerated its removal. The latter effect is not so pronounced in nitrogen atmosphere due to thinner oxide layer, which agrees with the lower wear rate in nitrogen atmosphere at high temperatures than in air.

Dry and Lubrication Sliding Wear Characteristics and Wear Mechanism Mapping for In Situ Al–25Mg2Si Composites

This study investigates the wear characteristics of Al–25Mg2Si composite synthesized via gravity casting. The research aims to understand the influence of applied load, test temperature, contact condition (dry and lubrication), and sliding distance on wear characteristics. Results show that higher loads and temperatures lead to increased wear loss and coefficient of friction. The worn surface morphology for higher loads and temperature shows deeper scratches and grooves, indicating severity in wear. The debris size and volume increase with an increase in load and temperature. The dominant mechanism is abrasive, adhesive, and oxidative for both the room‐ and high‐temperature dry sliding conditions. However adhesive and oxidative wear increase with the increase in test temperature. Abrasive wear is the only wear mechanism present during the lubricating sliding condition. Wear transition is noticed during the wear map analysis subjected to load, temperature, and sliding distance. In the case of room‐temperature test, severe–moderate–mild wear transition occurs with the increase in sliding distance and mild–moderate–severe wear transition occurs with the increase in test temperature and applied load. The roughness of the worn surface increases with an increase in applied load and temperature and the use of lubricant hugely reduces the surface roughness.

Investigation on the high-temperature stability and fatigue behavior of cold mixed epoxy asphalt mixture with different gradations

Cold mixed epoxy asphalt mixture (CMEAM) is widely applied on steel deck bridge pavements due to its excellent high temperature and fatigue resistance. However, previous studies investigated above two properties through destructive test or without considering water-damage factor, respectively, which is far away from real servicing condition. To distinguish above two properties of EA-05 and EA-10 (mixture with nominal maximum particle size of 4.75 mm and 9.5 mm), uniaxial static creep tests, dynamic modulus test and four-point bending beam fatigue test were applied in this paper. At the same time, to comprehensively acquire elastic, viscous-elastic and plastic performance of CMEAM, four kinds of constitutive models were also put forward. Results showed that seven-element viscous-elastic-plastic creep model has the highest correlation coefficients which proves there exists plastic deformation in CMEAM. Meanwhile, EA-10 is likely to present viscous-elastic deformation while EA-05 is more likely to show plastic deformation. Dynamic modulus among different gradation type mixture at 40 °C is not so apparent. But with the increase of frequency and testing temperature, this difference would be clear. The middle gradation of EA-10 has the most excellent high temperature stability under dynamic load. In terms of the resistance to fatigue damage of control or freeze-thaw treated samples, EA-05-U(the upper limit of EA-05 gradation type) always takes the first place and has the lowest sensitivity to the change of stress level and water damage.

Dielectric and ferroelectric photovoltaic properties of epitaxial BiFeO3 film with La0.5Sr0.5CoO3 bottom electrode

In order to deeply investigate the dielectric and ferroelectric photovoltaic properties of BiFeO3 (BFO) thin films, the Pt/BFO/La[Formula: see text]Sr[Formula: see text]CoO3 (LSCO) heterostructure ferroelectric capacitor grown on (001) SrTiO3 (STO) substrate is successfully fabricated by off-axis magnetron sputtering. The X-ray diffraction (XRD) and Phi scanning results show that the STO-based BFO and LSCO films are both (001) epitaxial structure. Although the BFO ferroelectric film exhibits an obvious dielectric dispersion phenomenon, a remarkable ferroelectric photovoltaic performance is obtained with open circuit voltage ([Formula: see text]) of 0.38[Formula: see text]V and short-circuit current ([Formula: see text]) of 0.23[Formula: see text]mA/cm2, respectively. With increasing test temperatures, [Formula: see text] decreases slowly and then rapidly, while [Formula: see text] increases rapidly and then decreases. At a critical temperature of 80∘C, the BFO ferroelectric film exhibits a faster photovoltaic response, in which the values of [Formula: see text] and [Formula: see text] are about 0.19[Formula: see text]V and 0.28[Formula: see text]mA/cm2, respectively. Moreover, the energy band analysis indicates that a large work function difference ([Formula: see text][Formula: see text]eV) between LSCO and Pt leads to a strong built-in electric field in the BFO film, which is beneficial to separate the photo-generated carriers and improve ferroelectric photovoltaic effect. In all, this study not only shows that BFO is an excellent and environmentally friendly photovoltaic candidate material, but also provides a practical strategy for improving the performance of BFO ferroelectric photovoltaic devices.

Tensile deformation behavior of laser powder direct energy deposited Inconel 625: Cryogenic to elevated temperatures

This study investigated the tensile behavior of laser powder direct energy deposited (LP-DED) Inconel 625 at temperatures ranging from cryogenic (−195 °C) to an elevated temperature of 980 °C. The tensile properties and deformation mechanisms at different temperatures were revealed. The tensile strength of the material, i.e., yield strength (YS) and ultimate tensile strength (UTS), was observed to decrease with increasing test temperature. Sudden decreases in YS were observed from −195 °C to 20 °C and above 870 °C, and in UTS from −195 °C to 20 °C and above 650 °C. The sudden decline in tensile strengths from −195 °C to 20 °C was attributed to the reduction in deformation twin density in the material. In contrast, softening caused by the dynamic recrystallization (DRX) resulted in a rapid decline in YS and UTS above 870 °C and 650 °C, respectively. The ductility of the material was observed to decrease with increasing test temperature until 650 °C, where the tensile fractures were governed by the decohesion of carbides. Above 650 °C, DRX was observed in the microstructure and the grain boundary (GB) sliding of the DRX grains governed the tensile fracture. At 870 °C, a lower degree of GB sliding improved the ductility, while at 980 °C an excessive GB sliding induced fracture significantly deteriorated the ductility.

Microstructure and high-temperature mechanical properties of Cu-W composite prepared by hot isostatic pressing

In current work, Cu-20wt%W (Cu-20W) composite of near theoretical density (99.69 %) and high electrical conductivity (86.78 %IACS) was fabricated by spray drying technique and hot isostatic pressing (HIP), aiming to enhance the mechanical properties of Cu-W composite while maintaining high conductivity. The impacts of W nanoparticles on the microstructure and properties of Cu-20W samples were evaluated. Adding W nanoparticles can remarkably refine the grain of Cu and strengthen the Cu-W composite through a pinning effect. Room temperature tensile strength and compressive yield strength of Cu-20W composite were 380 MPa and 304.49 MPa, which are 117.14 % and 616.95 % higher than that of pure Cu (175 MPa and 42.47 MPa), respectively. Tungsten nanoparticles can promote the high-temperature reliability of Cu-W samples. Tensile strength of pure Cu and Cu-20W sample gradually decreased with increasing test temperature, but the percentage increment of tensile strength increased. The tungsten particles pinned at grain boundaries during hot deformation inhibit recrystallization grain growth and hinder dislocation movement, thereby contributing to the high-temperature properties of Cu-20W composite. The peak stress of Cu-20W sample is 146.60 % higher than pure Cu, even at close to the melting point of Cu (940 °C). The strength of Cu-20W composite is heightened by combining fine-grain strengthening and dispersion strengthening due to the introduced W nanoparticles.

Microstructural features and hot tensile behaviour of tungsten inert gas welded TP321 nuclear tubes at elevated temperatures

Austenitic stainless steel (ASS) tubes, specifically TP321, are widely used in nuclear reactors and heat exchangers due to their excellent strength, toughness, and corrosion at ambient and elevated temperatures. The microstructure observed in the TP321 weldment reveals the existence of equiaxed dendrites, along with delta ferrite and TiC precipitates within the austenitic matrix. Electron backscatter diffraction (EBSD) scans confirm differences in texture, grain size, and grain misorientation distribution in the weld metal (WM), heat-affected zone (HAZ), and base metal (BM) regions of the weldment. The EBSD texture at the WM zone is strong in the 〈0 0 1〉 fiber direction, consistent with the face-centered cubic structure of the TP321. Hot tensile tests were conducted on BM and WM samples at 450 °C, 550 °C, 650 °C, 700 °C, 850 °C, and 900 °C using a Gleeble 3500 simulator. The yield strength (YS), ultimate tensile strength (UTS), and percentage elongation (EL) exhibited a decreasing trend with the rise in test temperature. The strain hardening capacity (Hc) of both BM and WM decreases with increasing test temperatures due to enhanced dislocation movement. The strain hardening exponent, ‘n’ was determined for BM and WM through suitable fitting of stress–strain curves using the Hollomon equation and depicted a decreasing trend with rising test temperatures. The fractured samples are subjected to scanning electron microscope (SEM) analysis to investigate the failure mechanism. Fractography of the fractured samples reveal the presence of micro voids and dimples, supporting the ductile nature of the rupture.

Selection and thermal physical characteristics analysis of in-situ condition preserved coring lunar rock simulant in extreme environment

With the increasing scarcity of Earth’s resources and the development of space science and technology, the exploration, development, and utilization of deep space-specific material resources (minerals, water ice, volatile compounds, etc.) are not only important to supplement the resources and reserves on Earth but also provide a material foundation for establishing extraterrestrial research bases. To achieve large depth in-situ condition-preserved coring (ICP-Coring) in the extreme lunar environment, first, lunar rock simulant was selected (SZU-1), which has a material composition, element distribution, and physical and mechanical properties that are approximately equivalent to those of lunar mare basalt. Second, the influence of the lunar-based in-situ environment on the phase, microstructure, and thermal physical properties (specific heat capacity, thermal conductivity, thermal diffusivity, and thermal expansion coefficient) of SZU-1 was explored and compared with the measured lunar rock data. It was found that in an air atmosphere, low temperature has a more pronounced effect on the relative content of olivine than other temperatures, while in a vacuum atmosphere, the relative contents of olivine and anorthite are significantly affected only at temperatures of approximately −20 and 200 °C. When the vacuum level is less than 100 Pa, the contribution of air conduction can be almost neglected, whereas it becomes dominant above this threshold. Additionally, as the testing temperature increases, the surface of SZU-1 exhibits increased microcracking, fracture opening, and unevenness, while the specific heat capacity, thermal conductivity, and thermal expansion coefficient show nonlinear increases. Conversely, the thermal diffusivity exhibits a nonlinear decreasing trend. The relationship between thermal conductivity, thermal diffusivity, and temperature can be effectively described by an exponential function (R2>0.98). The research results are consistent with previous studies on real lunar rocks. These research findings are expected to be applied in the development of the test and analysis systems of ICP-Coring in a lunar environment and the exploration of the mechanism of machine-rock interaction in the in-situ drilling and coring process.

Effect of Temperature on Interface Characteristics between Compacted Recycled Asphalt Pavement and Geogrid

This study presents an experimental investigation of the interface behaviors of recycled asphalt pavement (RAP) reinforced with biaxial geogrid. A series of direct shear tests were conducted on compacted RAP specimens with or without geogrid under different normal stresses (50, 75, and 100 kPa) and test temperatures (0, 20, and 35°C) using an improved temperature-controlled direct shear apparatus. The effect of test temperature on interface shear strength and strength parameters was systematically examined. Test results showed that shear stress versus shear displacement curves of RAP specimens with or without geogrid show strain-softening characteristics. Under the same normal stress, the curve of the reinforced RAP sample with a high temperature is always below that with a low temperature, which indicates the test temperature has an adverse effect on the strength of the reinforced RAP sample. The shear strength consistently increases linearly with the increase of applied normal stress for all RAP samples. The shear strength with geogrid is greater than pure RAP under a given test temperature and normal stress. The shear strength of RAP samples at lower test temperatures is higher than that at a higher temperature. Both apparent adhesion intercept of RAP-geogrid and cohesion of RAP show a decreasing trend with the increase of the test temperature and tend to be stable with increasing temperature. Both the interface friction angle of RAP-geogrid and the internal friction angle of RAP slightly decrease with the increase of test temperature. For RAP-geogrid, the value of the apparent adhesion intercept is higher than that for sand and gravel with geosynthetics. The interface friction angle is close to that of sand and gravel with geosynthetics. Therefore, RAP material can be well used as alternative backfill materials in lieu of natural aggregates such as gravel and sand in geotechnical applications.

The Properties of Modified Bagasse Fiber/Nano-TiO2 Composite Asphalt in a High-Temperature and High-Humidity Salt Environment.

The southern tropical coastal areas of China are high-temperature and high-humidity salt environments, which hinder the durability and service life of ordinary asphalt pavement. To enhance the durability of asphalt pavement in these areas, modified bagasse fiber combined with nano-TiO2 was used to improve the corrosion resistance of asphalt pavement in high-temperature and high-humidity salt environments. The micro-morphology, high-temperature oil absorption, high-temperature heat resistance, and hygroscopicity of bagasse fiber modified using three silane coupling agents combined with NaOH were compared, and the best silane coupling agent/NaOH modification scheme for bagasse fiber was found. Based on conventional physical tests (penetration, softening point, ductility), rheological property tests (rotational viscosity, dynamic shear rheological test, multi-stress creep recovery test, linear amplitude scanning test), and a four-point bending fatigue test of the asphalt mixture, the properties of modified bagasse fiber asphalt binder and mixture after cyclic dry-wet erosion under pure water and salt solution (NaCl, Na2SO4) were determined, and the effects of the erosion environment and fiber ratio on the basic physical and rheological properties of the asphalt were clarified. Compared with the silane coupling agents KH550 and KH590, the bagasse fiber modified with KH570/NaOH had a better high-temperature oil absorption capacity, heat stability capacity, and matrix asphalt compatibility. The worst erosion environment was Na2SO4, but the increase in test temperature and fiber content weakened the sensitivity of the asphalt binder performance in different erosion environments. The erosion capacity order was as follows: Na2SO4 > NaCl > pure water. In the worst erosion environment, 0.5% modified bagasse fiber/Nano-TiO2 asphalt binder (Bn-570-0.5) had the best corrosion resistance in a high-temperature and high-humidity salt environment. The penetration, softening point, creep recovery rate R3.2, non-recoverable creep compliance Jnr3.2, and fatigue life after long-term aging (with 5% strain) of Bn-570-0.5 were, respectively, increased by -16.9%, 37.5%, 37.95%, -27.86%, and 38.30% compared with unblended base asphalt binder (B). In addition, the four-point flexural fatigue life of Bn-570-0.5 was 169.2% higher than that of the unblended base mixture.

High-Temperature Rheological Properties of Asphalt Mortar Modified with Spent FCC Catalysts

Spent fluid catalytic cracking catalysts (S-FCC-Cs) constitutes a fraction of the hazardous solid waste generated in the petrochemical industry. The resource application of S-FCC-Cs remains a challenge. This study aims to explore utilizing S-FCC-Cs in asphalt mortar as a means to enhance resource utilization. Five different S-FCC catalysts were used as substitutes for mineral powder in the asphalt slurry at varying proportions. The high-temperature rheology of the resulting spent FCC catalyst-modified asphalt slurry was analyzed using temperature scanning tests and multiple stress creep recovery (MSCR) tests conducted at different temperatures and substitution doping levels. As the proportion of alternative doping increased, both the phase angle and irrecoverable creep flexibility decreased, while the absolute values of the rutting factor, deformation recovery rate, and irrecoverable creep flexibility difference increased. Moreover, as the temperature rose, the phase angle increased while the rutting factor decreased. The inclusion of an alternative admixture significantly improved the high-temperature performance of the asphalt mastic. This improvement was attributed to several factors, including the increase in the elastic component, enhanced deformation resistance, and improved deformation recovery. While the high-temperature performance of spent FCC catalyst-modified asphalt mastic gradually declined with increasing test temperature, all performance indices remained superior to those of limestone mineral powder asphalt mastic. In addition, the asphalt mortar modified by S-FCC-C JX with a surface area and hydrophilic coefficient of 105 m2/g and 1.026, respectively, exhibited the best rutting resistance and resilience performances among the five mortars, suggesting that the two factors co-affected the high-temperature rheological properties of S-FCC-C asphalt mortar. Considering stress sensitivity, it is more advantageous in improving the high-temperature deformation resistance of asphalt slurry at the JX dosage of 20%. These research findings offer valuable guidance for the application of S-FCC catalysts in asphalt pavement.

Evaluation of rutting resistance and moisture susceptibility of high RAP content asphalt mixtures containing bio-based rejuvenators

Hot Mix Asphalt (HMA) incorporating high Reclaimed Asphalt Pavement (RAP) contents can potentially have good rutting resistance due to the higher stiffness of the aged RAP-binder. The use of rejuvenators to enhance the workability and performance of high RAP mixes is becoming a common practice. However, the impact of rejuvenators on the rutting resistance and moisture susceptibility of high RAP mixes has not been studied exhaustively. The Hamburg Wheel-Tracking (HWT) test was used in this study to evaluate the asphalt mixture performance in terms of their resistance to permanent deformation and moisture damage at two conditioning temperatures of 44℃ and 50℃. Highly recycled asphalt mixes containing 50% RAP were prepared with various bio-based rejuvenators and a soft binder to compare with a control mix with 20% RAP content. Different HWT data analysis methods were used to quantify the mix performance. The results indicated that the rejuvenated high RAP mixes showed increased rut depth and higher possibilities of stripping compared to the control mix and the 50% RAP mix without rejuvenators. Regardless of the rejuvenator type and dosage, both the control and the rejuvenated high RAP mixes experienced severe stripping due to the increase in testing temperature. The 50% RAP mix performed best at both test temperatures. Therefore, caution is warranted concerning the adverse effects of rejuvenators or soft binders on the rutting performance and moisture susceptibility of high RAP mixes. Furthermore, traditional HWT parameters could be misleading due to the confounding effects of post-compaction and moisture-induced deformation. Parameters developed by the novel HWT data analysis methods could separately evaluate the rutting resistance and moisture susceptibility.

Dynamic recrystallization under hot deformation of additively manufactured 316 L stainless steel

The high-temperature deformation behavior of additively manufactured 316 L stainless steels prepared via the laser powder bed fusion (LPBF) technology was investigated using Gleeble compression testing in a wide temperature range of 473–1273 K under a constant strain rate of 0.001 s−1, perpendicular to the building direction (BD). The softening flow property upon continuous concurrent isothermal heating deformation displayed an excellent thermal stability performance up to the temperature of 873 K, preserving the material strength higher than 600 MPa compared to the room-temperature property of the as-built material. By the gradual increase of testing temperature, the compressive strength of the material was continuously decreased to even <50 MPa at a temperature of 1273 K. After hot compression; all tested specimens were characterized across different regions in terms of deformation substructure evolution and crystallographic texture using electron backscattering diffraction (EBSD) and constitutive analyses. Accordingly, the estimated activation energies based on established constitutive modeling proposed the operation of dynamic recovery (DRV) and discontinuous dynamic recrystallization (DDRX) mechanisms for the hot deformation of additively manufactured 316 L stainless steel below (~9.77kJmol) and higher (~553.23kJmol) than the critical temperature of 873 K, respectively. The competition between strain accumulation and thermal heating induced two different Brass deformation and Goss recrystallization textures below and above this critical temperature by altering the operative dominant dynamic restoration mechanism, with various texture severity depending on the testing temperature.

The microstructure and strength of a tantalum alloy: Influence of temperature

Tantalum-tungsten alloys are considered as potential materials used in high temperature applications due to their exceptional mechanical properties. To evaluate the mechanical properties of tantalum-tungsten alloys at elevated temperatures, a binary tantalum alloy containing 4 wt% tungsten (Ta–4%W) was chosen and their mechanical properties were tested at 1400–2000 °C, respectively, with the aid of a homemade coupled thermal-mechanical material testing system. The results show that the ultimate tensile stress, the yield stress and the uniform elongation gradually decrease, while the total tensile elongation keeps constant as the test temperature increases. Ta–4%W alloy have continuous work hardening capability at ultrahigh temperature even up to 2000 °C. The microstructural observations show that grain size increase and dislocation density decrease rapidly when the test temperature was increased to 1800 °C and 2000 °C. Furthermore, the dislocation substructure and dislocation types at elevated temperatures were analyzed and found to be distinctly different from that at room temperature. Base on the model between quantitative microstructural parameters and strength, the flow stress was calculated utilizing additive law from the microstructures with the structural morphology and related quantitative parameters. Mechanical strength loss of Ta–4%W at elevated temperature was evaluated after comparing the calculated and experimentally measured values.

High temperature softening mechanism of powder metallurgy TA15 alloy

The understanding of softening mechanisms for high-temperature titanium alloys at service temperature is essential to guarantee their service stability and safety. High-temperature softening is mainly manifested as grain boundary softening for metallic materials. To clarify the softening mechanism and failure mode of high-temperature titanium alloy TA15 (Ti-6.5Al–2Zr–1Mo–1V, wt.%) at temperature around service temperatures, coarse prior β grains were obtained by spark plasma sintering at 1300 °C, other than 1000 °C which is the traditional power metallurgy (P/M) consolidation temperature for TA15. Meanwhile, the effect of grain boundary softening was amplified. The high-temperature tensile test was carried out at 500–650 °C at intervals of 50 °C. The experimental results showed the maximum prior β grain size of P/M TA15 alloy is 2.9 μm, and its tensile strength decreases from 579 to 389 MPa with the increase of tensile temperature from 500 °C to 650 °C. Combined with calculation according to the Read-Shockley formula and analysis by macro-fracture, which show that the grain boundary strength is higher than the grain strength at 500 and 550 °C, and there are obvious grain and phase boundary torsion. The fracture mode demonstrated as a trans-granular fracture. The specimens soften rapidly, grain boundary strength decreases to less than grain strength, which led to fracture mode changed to inter-granular fracture at tensile temperatures up to 600 °C. If the crack extends to the critical length, the stress at the crack tip reaches easily the fracture strength of P/M TA15 alloy, and crack propagates rapidly at the temperature above 600 °C. Moreover, which corresponds to the elongation of true stress-strain decreasing with the increase of test temperature. OM and EBSD result demonstrated that the equi-cohesive temperature of TA15 alloy between 550 °C and 600 °C. Our findings provide a promising route to improve service temperature of high temperature TA15 alloy by raising the equi-cohesive temperature area in zone Ⅱ to a higher temperature.

Analysis of Modulus Properties of High-Modulus Asphalt Mixture and Its New Evaluation Index of Rutting Resistance

High-modulus asphalt mixture (HMAM) is one of the most effective materials to enhance the rutting resistance of asphalt pavement and upgrade pavement sustainability. The objectives of this study are to investigate the modulus properties of different HMAMs and their correlation with the rutting resistance, to propose reasonable modulus evaluation indicators, and to analyze the rutting resistance mechanisms of different materials (hard asphalt, polyethylene, dissolved polyolefin). The effect of three HMAMs and two styrene-butadiene-styrene (SBS) modifiers on asphalt mixtures’ rutting resistance were evaluated by dynamic modulus test and wheel track test, and the results were simulated and further analyzed via ABAQUS. The results indicate that the dynamic modulus of the mixtures showed a gradual increase and decrease with the increase of loading frequency and testing temperature, respectively. The ratio of dynamic modulus in low frequency to that in high frequency correlates well with dynamic stability under high-temperature conditions, and the wider the frequency coverage, the higher the correlation between this ratio and dynamic stability. The rutting resistance of asphalt pavements can be improved by reducing the frequency sensitivity of HMAMs under high temperatures or by increasing the modulus’ absolute value of the pavement structural layer. Therefore, two indicators, the absolute value of the modulus and the ratio of 0.1 Hz dynamic modulus to 25 Hz dynamic modulus at 55 °C, are recommended for the evaluation of rutting resistance of HMAMs. Based on the evaluation indexes proposed in this paper, a comparative analysis of the rutting resistance mechanism of HMAMs prepared with different materials was carried out, and it was concluded that the mixture with high-modulus agents had the best rutting resistance, which is consistent with the test road observations, thus verifying the feasibility of the modulus evaluation indexes recommended in this paper for the evaluation of the rutting resistance of different types of HMAMs.

Microstructural evolution and high-temperature deformation mechanism of 4Cr5MoSiV1Ti steel

Hot-working die steel usually withstands complex loading conditions and higher service temperatures, and its mechanical properties will inevitably deteriorate because of dynamic recovery, recrystallization, and plastic deformation of tempered martensite during service. A systematic study was carried out on a newly developed 4Cr5MoSiV1Ti hot working die steel to clarify the related microstructure evolution and deformation mechanism by stretching deformation at different temperatures ranging from room temperature to 640 °C. Results obtained show that the ultimate and yield tensile strength values at room temperature are about 1904 MPa and 1691 MPa, respectively. With increasing testing temperature, the strength decreases, the plasticity increases, and the fracture mode changes from brittle to ductile. All of these are closely related to a combined effect of tempered martensitic dynamic recovery, recrystallization, and secondary phase re-dissolution. Deformation mechanisms change from dislocation slip to rotation or sliding of ferrite with increasing stretching temperature.