High Temperature Mechanical Testing Research Articles

Temperature-dependent mechanical properties and crystal plasticity parameters for additively manufactured Haynes-214 alloy: Experiments and numerical modeling

Our experimental mechanical testing data demonstrated that the additively manufactured (AM) laser powder bed fusion (L-PBF) Haynes-214 alloy exhibits non-linear mechanical properties as the temperature rises from ambient to 870 °C. Crystal plasticity (CP) simulations provide an effective approach to gaining deeper insights into microstructure-property linkages under thermomechanical loading. This method can reduce the need for costly high-temperature mechanical testing while accounting for the effects of crystallographic texture and grain morphology on the mechanical behavior of AM materials. However, calibrating a CP model is time-consuming because individual simulations are computationally expensive and hundreds (or more) of iterations over parameter sets may be required. To address this issue, we have designed a machine learning-differential evolution (ML-DE) CP framework that can accurately interpolate the tensile properties of AM L-PBF Haynes-214 alloy across a wide temperature range from ambient to 870 °C, with minimal reliance on experimental data. The framework uses electron backscatter diffraction (EBSD) measurements to generate statistically equivalent microstructural volume elements to serve as inputs to the CP modeling framework. Stress–strain curves were generated from 1000 CP simulations, which serve as the training data set for the three ML regression algorithms explored: linear, extra-trees, and multi-layer perceptron. These three regression models were independently evaluated to compare their efficiency and identify the most suitable algorithm for the given problem. Results revealed that the extra-trees ML regressor outperforms the other models in both qualitative and quantitative aspects with an R2 of 0.98. Subsequently, the differential evolution optimization approach is employed to calibrate the ML-based CP material parameters with experimental results obtained at various temperatures. Finally, temperature-dependent CP material parameters are formulated. The effectiveness and efficiency of the designed framework are validated through comparison with experimental results, demonstrating a high degree of agreement. These calibrated parametric constitutive equations enable further use of the CP model to study the deformation behavior of this alloy under a wide range of thermo-mechanical loading conditions.

Laser powder bed fusion of oxide dispersion-strengthened IN718 alloys: A complementary study on microstructure and mechanical properties

In this study, two new grades of oxide dispersion strengthened (ODS) Inconel 718 (IN718) alloys were designed by the thermochemical CALPHAD method and produced by laser powder bed fusion (LPBF) technique. Alloys designated as IN718-YF and IN718-YFH, that consist Y2O3–FeO and Y2O3–FeO–Hf, respectively, were fabricated with >99.9 % densification using optimized process parameters. CALPHAD calculations were highly consistent with experimental findings, highlighting the formation of Al-containing Y–Ti–O and Y–Hf–O nano-oxides in both alloy types. Texture analyzes revealed no significant texture development in as-built (AB) or heat-treated (HT) alloys. Heat treatment was applied at 1050 °C for 1 h to enhance nano-oxide density. The nano-oxide number density remained similar in IN718-YF while it decreased in IN718-YFH alloy as a result of carbide formation after the heat treatment. Besides, formation of secondary γ′ particles was observed in the IN718-YFH/HT alloy. Even though the yield strengths of IN718-YF and IN718-YFH alloys in both AB and HT conditions were similar, the ductility of IN718-YFH was ∼50 % less in almost all conditions compared to the ductility of IN718-YF. This has been shown to be as a result of irregular shaped micron-sized Y-Hf-O oxides, martensite formation in AB condition, increased amount of carbides and existence of secondary γ′ particles in HT condition in IN718-YFH. High density of stacking faults (SF) forming at the interface of the nano-oxides have been detected in IN718-YF alloys. Besides dislocation/nanoparticle interactions, SFs which are responsible for the delocalization of the deformation improve the ductility of IN718-YF alloys. Overall, high temperature mechanical tests exhibit that both alloys have higher strength with improved ductility compared to the standard IN718 alloys, indicating the contribution of the nano-oxides.

Effects of Composition Variations on Mechanochemically Synthesized Lithium Metazirconate-Based Ceramics and Their Resistance to External Influences

The study examines the influence of variations in the compositions of components for the production of lithium-containing ceramics based on lithium metazirconate obtained by the method of mechanochemical grinding and subsequent thermal sintering. For component variation, two compositions were used, consisting of zirconium dioxide (ZrO2) and two distinct types of lithium-containing materials: lithium perchlorate (LiClO4·3H2O) and lithium carbonate (Li2CO3). Adjusting the concentration of these components allowed for the production of two-phase ceramics with varying levels of impurity phases. Using X-ray phase analysis methods, it was determined that the use of LiClO4·3H2O results in the formation of a monoclinic phase, Li2ZrO3, with impurity inclusions in the orthorhombic phase, LiO2. On the other hand, when Li2CO3 is used, the resulting ceramics comprise a mixture of two phases, Li2ZrO3 and Li6Zr2O7. During the studies, it was established that the formation of impurity inclusions in the composition of ceramics leads to an increase in the stability of strength properties with varying mechanical test conditions, as well as stabilization of thermophysical parameters and a decrease in thermal expansion during long-term high-temperature tests. It has been established that in the case of two-phase ceramics Li2ZrO3/Li6Zr2O7 in which the dominance of the Li6Zr2O7 phase is observed during high-temperature mechanical tests, a more pronounced decrease in resistance to cracking is observed, due to thermal expansion of the crystal lattice.

Influence of various fibers on the physico-mechanical properties of a sustainable geopolymer mortar-based on metakaolin and slag

Recently, studies on sustainability and ecology have become widespread in almost all sectors. One of the most important reasons for this spread is the rapid increase in industrialization and, thus, the increase in waste caused by industries. In this context, significant efforts are being made to evaluate some of these wastes. One of these efforts is the production of geopolymers. In this research, metakaolin and slag-based geopolymer mortar samples were manufactured, and polyvinyl Alcohol, basalt, and macro synthetic polypropylene fibers were used to enhance the physical, mechanical, and high-temperature resistance of the sample. Physical and mechanical tests of the produced samples were performed after 28 days. Then, elevated-temperature experiments were conducted to evaluate the behavior of the fibers under the influence of high temperature. Following the high-temperature test, physical, mechanical and microstructure tests of the samples were performed. As a result, basalt fiber enhanced the compressive strength of 800 °C-exposed samples by 7.72% compared to the fiber-free sample. Also, polyvinyl Alcohol fiber increased the energy absorption capacity of the samples by increasing Charpy impact values to 72.22% compared to fiber-free sample. Moreover, macro synthetic polypropylene fiber reduced capillary water absorption value up to 12.44% compared to fiber-free sample.

Mechanical data corrections for triaxial compression testing at high pressures and high temperatures with a Paterson gas-medium mechanical testing apparatus

Although restricted by a limited range of strain, the triaxial compression test is a mature and common technique for investigating the rheological properties of rock materials at high pressures and high temperatures, especially when establishing the constitutive equations for various flow laws. The Paterson gas-medium high-pressure and high-temperature mechanical testing apparatus (Paterson apparatus) is the best apparatus for triaxial compression testing due to its high stress resolution. However, to derive accurate mechanical information from the raw data recorded by the Paterson apparatus, some technical issues should be addressed, including the simultaneous distortion of the apparatus, the load force supported by the jacketing tube, and the change in the cross-sectional area of the specimen. In this paper, we introduce correction methods corresponding to these three technical issues for triaxial compression on a Paterson apparatus equipped with an internal load cell to significantly reduce experimental errors so that high-precision mechanical data for establishing the constitutive equations of flow laws, such as differential stress, strain, and strain rate, can be obtained. To facilitate corrections for the distortion of the apparatus and the load force supported by the jacketing tube, we determine the distortion of the Paterson apparatus as a function of axial load force by deforming tungsten steel specimens with a known Young’s modulus and the high-temperature flow laws of two common jacketing materials, iron and copper, by triaxial compression experiments at confining pressures of 200–300 MPa. Previous flow laws of iron and copper established by Frost and Ashby (1982) using ambient mechanical data are carefully compared with the flow laws obtained in this study to evaluate their effectiveness for correcting jacket tube strength. Finally, the errors eliminated by each correction step are analyzed and discussed to better understand the necessity of mechanical data corrections.

Effect of compositional homogenization on hot workability of Ni-based GH4061 superalloy

Complex solidification process, severe chemical segregation and large amounts of secondary phases with low melting point between dendrites deteriorate the hot workability of as-cast GH4061 superalloy. The effects of the homogenization temperature and time on the dendritic segregation, dissolution behavior of Laves phase with low melting point intermetallic phases and hot workability of the alloy were investigated using OM, SEM, EPMA and TEM. The results show that in the as-cast sample, severe compositional segregation is found among dendritic structures and the highest segregation coefficient of Nb is 2.435, followed by Ti of 1.346 and Mo of 1.200. After homogenization at 1160 °C for 20 h, the segregation among dendritic structures has been fully eliminated and the segregation coefficient of Nb declines to 1.098. Meanwhile, Laves and δ phases are mostly dissolved and no any localized grain boundary re-melting was observed. Increasing annealing temperature accelerates the speed of chemical homogenization and dissolution of intermetallic compounds. High-temperature mechanical properties testing results demonstrate that two-step homogenization treatment promotes the plasticity up to 1150 °C. Fracture analysis reveals that the dissolution of Laves and δ phases reduces the probability of crack initiation and fracture, resulting in a good hot workability.

Thermo-Mechanically Assisted Grain Growth in Ti6Al4V Fabricated Using the Powder Bed Additive Manufacturing During High-Temperature Mechanical Testing

Thermo-Mechanically Assisted Grain Growth in Ti6Al4V Fabricated Using the Powder Bed Additive Manufacturing During High-Temperature Mechanical Testing

Triaxial high temperature mechanical properties of Longmaxi shale at different depths

Based on the high temperature and confining pressure conditions at different depths, triaxial high temperature mechanical tests were carried out on Longmaxi shale with horizontal and vertical bedding, respectively. The results show that shale?s peak strength and deformation capacity increase with burial depth. The failure mode of shale is a typical brittle failure, and the brittle index can quantitatively describe the brittle mechanical behavior of shale. Because shale has an apparent thin bedding structure, shale with different bedding directions shows pro?nounced anisotropy in mechanical parameters and deformation characteristics. The burial depth and bedding direction significantly impact the energy evolution law of Longmaxi shale during the mechanical process.

Structure, crystallographic texture and anisotropy of properties of VZh159 alloy and how they are influenced by regimes of selective laser melting and final heat treatment

This paper examines the effect of scanning strategy (unidirectional scan, island scan and a scan with a 60о turn of the laser beam between layers) during selective laser melting and heat treatment on the crystallographic texture and mechanical properties of VZh159 alloy specimens. Application of different scanning strategies leads to two types of crystallographic texture formed in specimens: the axial component <100> gets reinforced in the unidirectional strategy, while it is the texture component {100}<001> in the other cases. After printing, the specimens were subjected to different regimes of heat treatment. They included quenching from 1,100 oC; quenching from 1,100 oC and ageing at 800 and 700 oC; quenching from 1,100 oC and ageing at 900, 800, 700 and 650 oC. When heat treatment (ageing) tests are conducted at room temperature, it leads to increased yield point and strength as a result of secondary phase precipitation at grain boundary and in grain core, while plasticity drops. During high temperature tests, no strength gain was found after heat treatment. In all heat treatment regimes, the authors observed anisotropy of mechanical properties in the specimens caused by the crystallographic texture that remains after heat treatment. The maximum strength and yield point were detected at 45о load application to growth direction during printing. At the same time, the anisotropy of yield point does not change during high temperature mechanical testing (at 850 oC) and has no dependence on the amount of additional precipitates. It suggests that crystallographic texture plays a defining role for anisotropy of properties.The research was conducted under financial support of Russian Federation presented by the RF Ministry of Science and Higher Education (Agreement No. 075-15-2021-1352).

The application of in situ TEM on the grain boundary brittleness of precipitation-strengthened Ni-based superalloys: Recent progress and perspective

Due to their superior strength and oxidation resistance at high temperatures, precipitation-strengthened Ni-based superalloys are widely used in load-bearing hot components in energy generation systems, such as gas/steam engines in aircraft or power plants. However, brittleness originating from a grain boundary (GB) in a certain temperature range is one of the greatest deadlocks, which desperately restricts their thermal-mechanical processing capabilities and also industrial applications. Experimental and theoretical investigations of the origin of GB brittleness with aims to overcome it still attract many research efforts in the high-temperature material field. It is desirable to understand the GB embrittlement mechanism by dynamically investigating the entire GB cracking process in real time under stress/temperature combination, which might be hardly revealed by a traditional experiment on precipitation strength/theoretical technologies. Current advances in high-temperature mechanical testing systems, which can be operated in a transmission electron microscope (TEM), provide unique opportunities for in situ exploration of the mechanistic origins of GB brittleness of superalloys with the resolution up to an atomic scale. Here, we first briefly give an overview of the phenomenon and current understanding on GB brittleness, followed by introducing the state-of-art techniques in an in situ TEM/mechanical testing system (MTS). In the end, we will also discuss the potential application of the in situ TEM/MTS on GB brittleness and a perspective overlook.

High-temperature compressive creep tests of U3Si2 with spark plasma sintering: Experiments and finite element modeling

This paper reports a systematic high-temperature creep behavior of dense U3Si2 pellets using a spark plasma sintering (SPS) apparatus at elevated temperatures and pressures under vacuum conditions. The stress exponent was subsequently derived to be 3.21 at 1173 K and 2.17 at 1223 K, respectively, indicating a grain boundary sliding creep mechanism. The creep activation energy was determined to be 203.6 ± 19.0 kJ/mol, which agrees well with the literature. Finite element modeling was performed using the creep parameters fitted from the strain-time plot. The results suggest an excellent match with the experimental data, confirming the validity of the experiments. Microstructure characterizations indicate that the main phase of the specimens after creep tests remains to be U3Si2, with a 4 μm thick layer of nano-sized particles induced from the diffusion between U3Si2 and alumina disc used to avoid electric current passing through the sample. The successful conduct of creep experiments demonstrates the great potential of SPS to perform high-temperature mechanical testing of nuclear fuels under vacuum conditions. The subsequent finite element modeling exhibits excellent capabilities for accurately predicting material performance in the creep tests and provides a practical tool in evaluating nuclear fuels’ performance for a much-extended time scale.

High-temperature digital image correlation techniques for full-field strain and crack length measurement on ceramics at 1200°C: Optimization of speckle pattern and uncertainty assessment

High-temperature mechanical tests coupled with Digital Image Correlation (DIC) on ceramics which exhibit rather low level of strain require to overcome extreme experimental conditions that usually can reduce significantly measurement accuracy. Thermal resistance of speckle pattern, black body radiation and heat haze are three main concerns, which should thus be taken into account while designing a high-temperature image acquisition setup. In this aim, an experimental procedure has been specifically designed in order to minimize the three above-mentioned disturbances. The main objective of this study is to select a suitable high-temperature resistant speckle pattern for mechanical characterization at 1200 °C (or above) on refractory ceramics. Most of tested speckle patterns were performed with white alumina adhesive and dark ceramic grains (silicon carbide or brown fused alumina). Different grain sizes of silicon carbide were tested. At first, different speckle patterns are compared in terms of DIC strain measurement uncertainty by discussing speckle features, some main DIC parameters and two image pre-treatments (low pass filter, image size reduction). Then, these speckle patterns are tested to analyse fracture behaviour of refractories through a Brazilian test. An enhanced digital image correlation technique (2P-DIC), dedicated to monitor the fracture behaviour, is applied to study the evolution of crack length. The best representation of crack progression has been achieved for sample surface covered with a fine SiC powder ranging from 50 to 100 µm. It is then possible to compare the fracture behaviour between 1200 °C and 20 °C and to show that the refractory exhibits more crack branching at 1200 °C in comparison with behaviour at room temperature.

High-temperature deformation followed in situ by X-ray microtomography: a methodology to track features under large strain.

Metallic materials processing such as rolling, extrusion or forging often involves high-temperature deformation. Usually under such conditions the samples are characterized post mortem, under pseudo in situ conditions with interrupted tests, or in situ with a limited strain rate. A full in situ 3D characterization, directly during high-temperature deformation with a prescribed strain-rate scheme, requires a dedicated sample environment and a dedicated image-analysis workflow. A specific sample environment has been developed to enable highly controlled (temperature and strain rate) high-temperature deformation mechanical testing to be conducted while performing in situ tomography on a synchrotron beamline. A dedicated digital volume correlation algorithm is used to estimate the strain field and track pores while the material endures large deformations. The algorithm is particularly suitable for materials with few internal features when the deformation steps between two images are large. An example of an application is provided: a high-temperature compression test on a porous aluminium alloy with individual pore tracking with a specific strain-rate scheme representative of rolling conditions.

Mechanics and fracturing techniques of deep shale from the Sichuan Basin, SW China

Deep shale gas exploration and production in Fuling (Sichuan Basin, SW China) are confronted with hydraulic fracturing challenges owing to high stress, high fracture pressure, low pump rate and proppant concentration, as well as high closing pressure in deep strata. This study focused on the mechanical properties of shale core samples from Fuling through high-temperature triaxial rock mechanical tests and in-situ stress tests based on the Kessel effect of acoustic emission. Their mechanical property variations with depth were delineated using brittleness index calculated via simulating different depths and different confining pressures for the samples. The results showed that several parameters of deep shale reservoirs, i.e. brittleness index, fracture density, performance of self-propping, and flow conductivity, are lower than that of shale reservoirs with moderate burial depth. Thus, the current operating pressure in deep shale reservoir stimulation should be taken full advantage of, rather than channeling the focus on the propagation of fracture length. The objective is to increase the complexity of the near-hole fracture network for enhancing self-propping and flow conductivity of the fractures. This can be achieved by reducing the number of perforation clusters and cluster spacing, adopting variable-rate fracturing, decreasing proppant size, increasing sand volume, and optimizing the fracturing parameters. A field application showed that, compared with the neighboring wells, the test well had larger drainage area, doubling the gas yield.

Efficient production of a high-performance dispersion strengthened, multi-principal element alloy

Additive manufacturing currently facilitates new avenues for materials discovery that have not been fully explored. In this study we reveal how additive manufacturing can be leveraged to produce dispersion strengthened (DS), multi-principal element alloys (MPEA) without the use of traditional mechanical alloying or chemical reactions. This new processing technique employed resonant acoustic mixing to coat an equiatomic NiCoCr powder with nano-scale yttrium oxides. Then, through laser powder bed fusion (L-PBF), the coated powder was successfully consolidated into 99.9% dense parts. Microstructural analysis confirmed the successful incorporation and dispersion of nano-scale oxides throughout the build volume. Furthermore, high temperature mechanical testing of the DS alloys showed significant improvements in strength and ductility over the baseline NiCoCr. As a result, this recently discovered processing route opens a new alloy design and production path that is synergistic between additive manufacturing and dispersion strengthening, possibly enabling a new generation of high-performance alloys.

A novel insight into the primary creep regeneration behaviour of a polycrystalline material at high-temperature using in-situ neutron diffraction

Primary creep regeneration (PCR) is observed during cyclic creep deformation in many engineering alloys. PCR is a phenomenon in which reverse inelastic strain fully, or partially, resets the early creep strain hardening memory of the material. The current understanding regarding the origin of the PCR behaviour in engineering alloys is limited to the phenomenological observations from the changes in dislocation structures whereas a good mechanistic understanding of the PCR behaviour is crucial for developing robust plasticity-creep predictive models. In this study, we investigated the micromechanical origin of PCR behaviour in type 316H stainless steel at 650 °C using high-temperature mechanical testing and neutron diffraction. A cyclic creep experiment was conducted in-situ at a neutron diffraction beamline, during which various degrees of unloading and reverse loading were applied to the specimen, followed by creep deformation under a load above the material's yield strength. Partial PCR was observed after reverse plastic loading for all the creep dwells, which is contrary to current high-temperature lifetime assessment's procedures advice which is to account for full recovery of primary creep after any reverse plastic loading. The extent of PCR is observed to be proportional to the magnitude of reverse plastic strain up to a level of 0.5% reverse plastic strain. From the measured neutron diffraction data, a strong correlation was observed between the changes in magnitude of the accumulated intergranular residual lattice strains and the macroscopic primary creep strain. Moreover, the increases of lattice strain to saturation and transition from primary creep to secondary regime occur at the same time. Based on these correlations it can be postulated that the PCR behaviour observed due to cyclic loading in type 316H stainless steel at elevated temperature originates from the accumulation of intergranular residual lattice strains during the reverse plastic loading and those time-dependent changes during the creep dwells.

An experimental study of ultra-high temperature ceramics under tension subject to an environment with elevated temperature, mechanical stress and oxygen

Ultra-high temperature ceramic (UHTC) composites are widely used in high-temperature environments in aerospace applications. They experience extremely complex environmental conditions during service, including thermal, mechanical and chemical loading. Therefore, it is critical to evaluate the mechanical properties of UHTCs subject to an environment with elevated temperature, mechanical stress and oxygen. In this paper, an experimental investigation of the uniaxial tensile properties of a ZrB2-SiC-graphite subject to an environment with a simultaneously elevated temperature, mechanical stress and oxygen is conducted based on a high-temperature mechanical testing system. To improve efficiency, an orthogonal experimental design is used. It is suggested that the temperature has the most important effect on the properties, and the oxidation time and stress have an almost equal effect. Finally, the fracture morphology is characterized using scanning electron microscopy (SEM), and the mechanism is investigated. It was concluded that the main fracture mode involved graphite flakes pulling out of the matrix and crystalline fracture, which indicates the presence of a weak interface in the composites.

Threaded Sample for High-Temperature Mechanical Tests of Carbon–Carbon Composites

A sample is proposed for high-temperature testing of carbon–carbon composites on a machine with threaded clamps. A general approach to assessing the carrying capacity of composite threaded joints is employed.

Efficient oil/water separation by a durable underwater superoleophobic mesh membrane with TiO2 coating via biomineralization

In this work, a novel and facile method was used to coat a uniform and conformal TiO2 film on a stainless steel mesh via polyethylenimine-catalyzed biomineralization of titanium bis(ammonium lactato)dihydroxide at neutral pH, ambient temperature and pressure. The fabricated mesh showed excellent superhydrophilicity and underwater superoleophobicity. It separated immiscible oil/water mixtures by gravity at a high flux (∼3 × 104 L m−2 h−1), high separation efficiency (∼99.999%), and exhibited excellent anti-oil-fouling ability. The mesh also demonstrated great durability: (1) it had outstanding stability in 1 M NaCl, 0.1 M HCl, 0.1 M NaOH solutions, and absolute ethanol; (2) it maintained underwater superoleophobicity in solutions of pH from 1 to 13 and saturated NaCl solutions; and (3) the separation efficiency was retained after high temperature treatment and mechanical wear tests (sonication, abrasion and crumpling). The biomineralization coating method was also applied on other substrates (nonwoven mesh, filter paper and PVDF membrane), which also showed good capability for oil/water separation. Therefore, this new environmentally benign approach of preparing superwetting surfaces provides a durable and sustainable solution to the treatment of oily wastewater.

Experimental study and multiscale modelling of the high temperature deformation of tempered martensite under multiaxial loading

The microstructural deformation of ex-service 9Cr-1Mo steel, with a tempered martensitic microstructure, has been examined in this study, through the combined use of electron backscatter diffraction (EBSD) and multiscale modelling techniques. Both the experimental and predicted deformation of the material at a notch root on a range of scales from the specimen level down to the microstructural block level are compared. A tension loaded notch specimen of the material which was extracted from an ex-service power plant pipe was used for this analysis. The deformation at the specimen level was quantified by analysis of the load displacement curves and notch opening displacement, which showed excellent agreement with the predicted results from the experimentally calibrated elastic-plastic finite-element model of the specimen geometry. The microstructural deformation was experimentally measured through the use of EBSD carried out at the notch root before and after high temperature mechanical testing. The initial orientation of the microstructure as well as the displacement around the boundary of the area of interest in the macroscale model were applied to a representative volume element (RVE) and a slip based crystal plasticity modelling framework was implemented to model the in-elastic deformation of the material under high temperature loading.