Elevated Temperature Applications Research Articles

Investigation of Low-Cost FDM-Printed Polymers for Elevated-Temperature Applications.

While fused deposition modeling (FDM) and other relatively inexpensive 3D printing methods are nowadays used in many applications, the possible areas of using FDM-printed objects are still limited due to mechanical and thermal constraints. Applications for space, e.g., for microsatellites, are restricted by the usually insufficient heat resistance of the typical FDM printing materials. Printing high-temperature polymers, on the other hand, necessitates special FDM printers, which are not always available. Here, we show investigations of common polymers, processible on low-cost FDM printers, under elevated temperatures of up to 160 °C for single treatments. The polymers with the highest dimensional stability and mechanical properties after different temperature treatments were periodically heat-treated between -40 °C and +80 °C in cycles of 90 min, similar to the temperature cycles a microsatellite in the low Earth orbit (LEO) experiences. While none of the materials under investigation fully maintains its dimensions and mechanical properties, filled poly(lactic acid) (PLA) filaments were found most suitable for applications under these thermal conditions.

Modelling creep responses of plantain fibre reinforced HDPE (PFRHDPE) for elevated temperature applications

The growth in applications of Plantain Fibre Reinforced HDPE (PFRHDPE) has increased the importance of understanding the time-dependent viscoelastic properties such as creep resistance. This study focused on the determination of creep behaviour of a novel PFRHDPE designed for elevated temperatures applications. The creep responses of PFRHDPE were experimentally determined in line with ASTM D2290 and modelled for elevated temperature applications using the classical creep models, classical viscoelastic models and the time temperature superposition approach. Creep strain, creep stress, creep modulus and creep stress relaxation were modelled and analyzed. The PFRHDPE exhibited the characteristic of the unrelaxed and relaxed moduli of material to accommodate applications at elevated temperatures. The superposition method showed PFRHDPE to have moduli of 66 MPa and 2.26 MPa for one year and fifty years of operations respectively. The relaxation stresses were also evaluated as 9.24 MPa, 9.15 MPa and 12.96 MPa for the conditions investigated showing the new material as able to accommodate the constant loads of 25 MPa and 35 MPa under the elevated temperatures of 30 °C and 60 °C.

Intermediate temperature embrittlement in a precipitation-hardened high-entropy alloy: The role of heterogeneous strain distribution and environmentally assisted intergranular damage

Severe intergranular embrittlement has been found in a wide range of polycrystalline metallic materials in the intermediate temperature regime, setting severe limits on their engineering applications. In this study, we have systematically investigated the origin of such temperature-dependent premature tensile failure in a precipitation-hardened high-entropy alloy. We highlight the role of heterogeneous strain distribution and environmental attack in facilitating intergranular crack initiation and propagation at intermediate temperatures. The dislocation accumulation has been found in the vicinity of grain boundaries to accommodate thermally activated grain-boundary sliding. Grain boundaries with extensive dislocation pile-ups served as preferential sites for the initiation of voids and associated cracks. The growth and linkage of microcracks lead to the destructive tensile failure. More importantly, environmentally assisted grain-boundary damage plays a vital role in exacerbating the embrittlement. In contrast to the laboratory air atmosphere, tensile testing in an inert argon atmosphere protected the specimens from the environmental damage and helped to recover the tensile ductility at intermediate temperatures. Electron microscopy analyses have uncovered unique deformation substructures, in which both Orowan looping and particle shearing took place. These findings provide a fundamental understanding of the temperature-dependent deformation behaviors of polycrystalline high-entropy alloys, and this understanding is of a great significance for developing high-performance structural materials for elevated temperature applications.

Design and fabrication of a (6.4γ–Al2O3 + 18Al13Fe4)/Al (wt.%) composite utilizing fine grain strengthening and dispersion strengthening at elevated temperatures

In this work, a (6.4γ–Al2O3 + 18Al13Fe4)/Al (wt.%) composite (AFO) was in-situ synthesized through the chemical reaction between Al and oxide powders. After hot extrusion, the formed nanometric γ–Al2O3 particles exhibit networks, locating along matrix grain boundaries. The submicron Al13Fe4 particles have similar size with the α–Al grains and disperse uniformly among the matrix. It is supposed that the γ–Al2O3 particles are efficient for grain boundaries stabilization at elevated temperatures, while the Al13Fe4 particles generate further dispersion strengthening effect. As a result, the AFO composite exhibits attractive high-temperature intensity and heat stability. The yield strength of tensile test at 300 °C is 270 ± 6 MPa, while it can still remain to be 119 ± 10 MPa at 500 °C. It is regarded that this work may provide new insights for designing new Al based composites for elevated temperature applications.

In-situ synthesis of TiC/Ti-6Al-4V-ELI composite by laser

Titanium-based alloys are reported to offer up to 50% strength to weight ratio. These lands titanium-based alloys as a preferred material over steel and nickel-base super-alloys, where strength and stiffness are required. Regardless titanium-based alloys are prone to losing strength and stiffness at elevated temperature applications such as jet engines. This prompted a need for titanium matrix to be braced with a material with superior properties, such as ceramic. In-situ synthesis with laser metal deposition (LMD) of Ti-6Al-4V-ELI braced with discontinuous particulates of TiC resulted in improved hardness and microstructure. It is reported that an increase in the feed rate of TiC during the LMD process directly increases the hardness of the TiC/Ti-6Al-4V-ELI composite and refine the grain size of Ti-6Al-4V-ELI. Various fabrication methods and properties of Ti-6Al-4V are well documented in the literature. This paper focuses on the effect of TiC on the microstructure and hardness of Ti-6Al-4V-ELI.

Hydrogen diffusivity and tensile properties degradation in SLMed Inconel 718

Besides elevated temperature applications, Inconel 718 is widely chosen for components subjected to heavy loads operating in aggressive environments, which can promote the production of hydrogen on the metal surface. Selective Laser Melting (SLM) is an emerging technology for the production of structural components in Inconel 718 or Inconel 625 alloys, allowing to create functionally optimized shapes and overcome the traditional manufacturing limitations. However, the SLM process introduces a peculiar microstructure and severe residual stresses that can affect hydrogen migration and accumulation. While the mechanical properties were deeply investigated in recent years, the resistance of SLMed Inconel 718 to the Hydrogen Embrittlement (HE) is still an open issue. The present work deals with a preliminary assessment of the hydrogen embrittlement susceptibility of the SLMed In718 in the as-built condition. Slow strain rate tests were carried out on unnotched specimens that were pre-charged by electrochemical cathodic charging. Fractographic analyses, carried out with a Scanning Electron Microscope (SEM), were used to identify the effects produced by the hydrogen intake on the material microstructure. The material resulted to be susceptible to hydrogen embrittlement, featuring a significant ductility reduction in presence of extremely elevated hydrogen concentrations. The hydrogen content was measured by employing the Hot Extraction Method (HEM), for each mechanical test. Due to a low diffusivity value, hydrogen penetrates only within a thin external layer of the specimen. Therefore, to identify the concentration profile, the effective hydrogen diffusion coefficient was identified and the hydrogen diffusion was simulated using Fick’s equation. The loss in mechanical resistance was then correlated to the hydrogen concentration near the fracture onset.

High Temperature Tensile and Fatigue Behaviors of Additively Manufactured IN625 and IN718

Recent advancement and research efforts in additive manufacturing (AM) have made it a promising technology to fabricate nickel-base superalloy parts in near net shapes. IN625 and IN718 are the commonly used superalloys in elevated-temperature applications in the aerospace and energy sectors. However, the fatigue performance of the additively manufactured (AM) components often depends on the defects like porosity, micro-cracks, and lack-of-fusions, etc. In addition, the presence of columnar grains and residual stresses also affects their fatigue performance. In this study, IN625 and IN718 specimens were fabricated via both laser powder bed fusion (L-PBF) and laser powder directed energy deposition (LP-DED) followed by stress-relief and standard heat treatments. Both alloys exhibited similar defect characteristics and grain morphologies. High-temperature fatigue properties of both IN625 and IN718 were tested at two strain amplitudes of 0.005 and 0.01 mm/mm and at two elevated temperatures of 427 and 625 °C. The fatigue lives of both alloys decreased with increasing testing temperatures. AM IN718 showed inferior fatigue performance at both test temperature and strain amplitudes in L-PBF conditions while in LP-DED condition both alloys have shown comparable fatigue performance.

SLM process parameters effects on the fatigue strength of AMed Inconel 718

Selective Laser Melting (SLM) emerged as a technology suitable for the industrial production of structural components featuring complex geometries. In the field of elevated temperature applications, the possibilities offered by the SLM can be successfully used to produced complex geometries as internal cooling channels or lattice structures, as long as the process doesn’t jeopardize the mechanical properties, in particular the fatigue strength.The SLM process parameters play a fundamental role in determining the mechanical performances of the component but are also the key parameter for increasing the productivity of the process and thus the industrial spread of the technology.In the present work, it is presented an experimental assessment of the effects produced by different sets of productivity-oriented SLM process parameters on the Wöhler curves of cylindrical plain specimens. The adopted process parameters were defined on the basis of a previously developed thermal analytical model aimed to predict the melt pool dimensions and shape.HCF tests were carried out at room temperature in an axial load configuration with a stress ratio of 0.05 and a loading frequency of about 150 Hz, by using a resonant testing machine. Test frequency was monitored to detect the occurrence of crack nucleation and monitor the propagation phase.In order to understand the causes of the fatigue behavior, metallographic analyses were carried out to investigate the microstructural properties or the presence of internal defects, i.e. porosity and hot tearing cracks, produced by each set of process parameters. The surface quality was also investigated in detail through optical microscope analyses. Fractographic analyses were used to identify the nucleation and crack propagation region, as well as the presence of the defects in proximity to the fracture onset.

Development of Lightweight Al-Based Entropic Alloys for Elevated Temperature Applications

Development of Lightweight Al-Based Entropic Alloys for Elevated Temperature Applications

Chemical lattice strain in nonstoichiometric oxides: an overview

This article reviews the state of the art – from the experimental and computational data available to the models, origins and possible uses – in the field of chemical strain of oxide materials (primarily those for elevated-temperature applications).

Synergistic effects of Cd, Si and Cr additions on precipitation strengthening and thermal stability of dispersoids in AA3003 alloy

The thermally stable Mn-rich dispersoids in Al alloys were recently reported to have a significant dispersion strengthening effect. Further improvements in the dispersion strengthening and thermal stability of Mn-rich dispersoids are required to achieve satisfactory mechanical properties for elevated temperature applications. This study reports that, with minor additions of Cd, Cr and increased Si content, the yield strength of the modified AA3003 alloy is increased by 39% compared to the base alloy subjected to the same heat treatment. Moreover, the thermal stability of the modified alloy when exposed to a long-term thermal holding up to 1000 h at 400 °C is greatly improved. Microstructural characterizations reveal that the dispersoids in the modified alloy are greatly refined, and their distribution is more homogeneous compared to the base alloy. It is observed that the Cd-rich clusters/nanoparticles formed in the initial stage of heat treatment, can effectively “attract” dispersoid-forming atoms (e.g. Mn, Si and Cr) and subsequently lead to a heterogeneous nucleation of dispersoids on them, which is responsible for the enhanced dispersoid precipitation and dispersion strengthening in the modified alloy. The partitioning of the low-diffusivity peritectic element Cr into dispersoids can improve the thermal stability and increase the number density of dispersoids in the centre of dendrite arms, which also contributes to the promising mechanical properties of the modified alloy.

Influence of LRE (Ce, Y, and La) on microstructure and magnetic properties of (Nd0.8LRE0.2)–Fe–B hot-deformed magnets

To develop low-cost, high-performance light rare earth element (LRE)-substituted Nd–Fe–B permanent magnets, we systematically studied the effect of substituting Nd with 20% light rare earth elements (LRE = Ce, Y, and La) on the hard magnetic properties of (Nd0.8LRE0.2)–Fe–B hot-deformed magnets. The (Nd0.8Ce0.2)2Fe14B magnet showed a coercivity of μ0Hc = 1.41 T, which is comparable to that of an LRE-free magnet. Y substitution was found to decrease μ0Hc and μ0Mr to 1.22 and 1.32 T, respectively, as was expected from the intrinsic properties of (Nd0.8Y0.2)2Fe14B. Unlike our expectation, the thermal stability of coercivity is not degraded by the Y substitution, demonstrating its potential for elevated temperature applications. With La substitution, coercivity is substantially degraded to 0.67 T because of the low intrinsic properties and the lack of the intergranular phase. The Y and Ce co-substituted (Nd0.8Ce0.1Y0.1)2Fe14B hot-deformed magnets showed better thermal stability of coercivity compared to the (Nd0.8Ce0.2)2Fe14B sample.

Machine learning assisted prediction of the Young\u2019s modulus of compositionally complex alloys

We identify compositionally complex alloys (CCAs) that offer exceptional mechanical properties for elevated temperature applications by employing machine learning (ML) in conjunction with rapid synthesis and testing of alloys for validation to accelerate alloy design. The advantages of this approach are scalability, rapidity, and reasonably accurate predictions. ML tools were implemented to predict Young’s modulus of refractory-based CCAs by employing different ML models. Our results, in conjunction with experimental validation, suggest that average valence electron concentration, the difference in atomic radius, a geometrical parameter λ and melting temperature of the alloys are the key features that determine the Young’s modulus of CCAs and refractory-based CCAs. The Gradient Boosting model provided the best predictive capabilities (mean absolute error of 6.15 GPa) among the models studied. Our approach integrates high-quality validation data from experiments, literature data for training machine-learning models, and feature selection based on physical insights. It opens a new avenue to optimize the desired materials property for different engineering applications.

High-throughput design of high-performance lightweight high-entropy alloys

Developing affordable and light high-temperature materials alternative to Ni-base superalloys has significantly increased the efforts in designing advanced ferritic superalloys. However, currently developed ferritic superalloys still exhibit low high-temperature strengths, which limits their usage. Here we use a CALPHAD-based high-throughput computational method to design light, strong, and low-cost high-entropy alloys for elevated-temperature applications. Through the high-throughput screening, precipitation-strengthened lightweight high-entropy alloys are discovered from thousands of initial compositions, which exhibit enhanced strengths compared to other counterparts at room and elevated temperatures. The experimental and theoretical understanding of both successful and failed cases in their strengthening mechanisms and order-disorder transitions further improves the accuracy of the thermodynamic database of the discovered alloy system. This study shows that integrating high-throughput screening, multiscale modeling, and experimental validation proves to be efficient and useful in accelerating the discovery of advanced precipitation-strengthened structural materials tuned by the high-entropy alloy concept.

Laser powder-bed fusion of Mo(Si,Al)2 – Based composite for elevated temperature applications

Mo(Si,Al)2-based composites are promising candidates to be used as heating elements, high temperature coatings and structural materials. In this work, powders mixture of 90 wt% MoSi2 and 10 wt% AlSi10Mg was subjected to a laser powder-bed fusion (LPBF) to produce Mo(Si,Al)2-based composites. The effect of process parameters on morphological features, phase composition, densification, hardness and oxidation resistance were examined in detail, and the optimal process window was defined. Thermodynamic calculations were performed to supplement the experimental data. The primary phase in the solidified material is the C40 hexagonal Mo(Si1−x,Alx)2 and the solubility of aluminum is governed by the applied volumetric energy density of the laser. The microstructure of the top surface of the samples is composed of fine and coarse cellular Mo(Si1−x,Alx)2, enveloped with Al and Si in inter-cellular regions. The materials exhibit hardness in 800–840 HV1 range depending on process conditions. The LPBF-prepared samples displayed a stable thin oxide layer formation after 120 h of oxidation. The glassy layer of oxides, which predominantly grow from inter-cellular regions, serves as a protective layer for further oxidation. The oxidation rate is two-orders of magnitude slower compared to the reference MoSi2 sample.

AK STEEL 309

Abstract AK Steel 309 is a chromium-nickel austenitic stainless steel. It is typically used for elevated temperature applications. Its high chromium and nickel contents provide comparable corrosion resistance, superior resistance to oxidation, and the retention of a larger fraction of room temperature strength than the common austenitic stainless steels, such as type 304. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties. It also includes information on corrosion resistance as well as forming, heat treating, and joining. Filing Code: SS-1333. Producer or source: AK Steel Corporation.

Generation of creep-fatigue interaction diagram for modified 9Cr–1Mo steel

The selection of structural materials for elevated temperature applications relies on the availability of proper design codes based on adequate data pertaining to the properties at service-relevant loading conditions. Two of the most widely used high temperature design codes, viz, the ASME Boiler and Pressure Vessel Code and the RCC-MRx code are at variance with each other on the bilinear interaction point in the creep-fatigue interaction diagram for modified 9Cr–1Mo ferritic steel. In view of this, a detailed investigation was carried out on the creep-fatigue interaction behavior of this alloy with special emphasis on the evaluation of the creep-fatigue damage and construction of the interaction diagram. Tests were performed on two chemical variants of the steel at two temperatures (773 K and 823 K) using the strain amplitudes of ±0.25%, ±0.4%, ±0.6% and ±0.8%. Hold times of different durations in the range, 1–60 min, were imposed in the tensile and compressive peak strains to introduce the creep damage. The ASME creep-fatigue interaction diagram was observed to be a more conservative design approach for the alloy.

Statistical aspects of grain-level strain evolution and reorientation during the heating and elastic-plastic loading of a Ni-base superalloy at elevated temperature

The advancement of microstructure-sensitive models for elevated temperature applications requires experimental data at the relevant length scales and usage environments. Towards this goal, we studied the evolution of grain-level strains in a Ni-based superalloy during heating and subsequent loading at elevated temperature. The specimen was heated using two halogen lamps in a furnace designed explicitly for the special load frame used for the synchrotron-based experiment. High energy X-ray diffraction was used to non-destructively characterize the initial 3D microstructure, confirm the temperature of the specimen, and monitor the evolution of grain-level strains, stresses, and orientations through the elastic-plastic transition. The distributions of grain-level normal strains were observed to broaden with elastic loading, whereas elastic-plastic loading led to development of local intergranular “hotspots”, or activation of plastic slip within select grains, that both broaden and skew the grain-level strain distributions. These grain-level strain distributions were fit to a generalized extreme value (GEV) distribution function. The GEV shape parameter is observed to change significantly at the onset of macroscopic plasticity. At the grain scale, more grain reorientation (plasticity) occurred for grains oriented for lower strength-to-stiffness ratios. During the onset of plastic deformation for LSHR at 660∘C load shedding from lower strength-to-stiffness-oriented grains to higher strength-to-stiffness-oriented grains was observed. Additionally, based on the grain-averaged reorientation axis, it was found that LSHR has a preference for activation of multiple slip systems at 660∘C. We conclude with a discussion on the critical need for statistical analysis of such mesoscale data obtained in situ at elevated temperature for the verification/validation of the next generation of microstructure-sensitive models.

Insight into elevated temperature and thin membrane application for high efficiency in polymer electrolyte water electrolysis

We present a study of a liquid water fed polymer electrolyte water electrolyzer (PEWE) at cell temperatures of up to 120 °C using thin membranes of 50 µm thickness. Further, we show that under these conditions conversion efficiency increases by up to 14% at 3 A cm−2 in comparison to today's state of the art (60 °C, 180 μm membrane). Alternatively, an increase of the current density by a factor of 3 at an efficiency of 75% is possible. A detailed voltage loss analysis is provided that helps to understand to which extent the overpotential contributions are reduced. From hydrogen permeation measurements, we determine faradic efficiency and the safety limits of cell operation revealing that operating temperatures higher than 100 °C with thin membranes are not possible at current densities lower than 0.8 A cm−2 due to the safety limit of 2% hydrogen in oxygen in the anode compartment. The decrease in overpotential allows to significantly reduce the energy requirement or to increase the production rate of hydrogen. The results can help to benchmark future efficiency targets and points towards next generation PEWE materials and components to further reduce loss contributions.

Synthesis of therminol‐graphite nanofluids and photo‐thermal conversion properties

Therminol-based nanofluids with graphite nanopowders (GNs) were proposed as an efficient working fluid in direct-absorption solar collectors (DASCs), especially for elevated-temperature applications. In this work, Therminol VP-1/GN nanofluids (TG nanofluids) were prepared in two steps with surfactant (Triton X-100). In the synthesis procedure, the vortex generator for coating GN with sticky surfactant (μ = 240 mPa⋅s) and the ultra-sonicator for diluting GN/Triton X-100 mixture with Therminol VP-1 are used. We have demonstrated excellent dispersion stability up to 4 weeks and even after heating over 100°C without any surface modification of nanoparticles. The feasibility of using the synthesized TG nanofluid as a working fluid in DASCs was experimentally demonstrated by measuring its absorption coefficient as well as by characterizing photo-thermal conversion performance. The absorption coefficient of the TG nanofluid at 0.004 wt.% of GNs was measured to be 1.12 cm−1, which is more than eight times enhanced from that of bare Therminol VP-1 (ie, 0.13 cm−1) in the wavelengths from 350 to 1600 nm. Finally, the temperature increment of 43°C was achieved after t = 3600 s under the 5-sun condition in the static photo-thermal conversion experiment. Moreover, the low viscosity of TG nanofluids (only 2.2% higher than that of the bare Therminol VP-1) supports the superiority of TG nanofluids to be applied as working fluid in DASCs.