High Temperature Fracture Research Articles

Enhancing the High-Temperature Fracture Toughness of Ultrahigh-Performance Concrete through Optimization of Ternary Cement Matrix and Plastic Fiber Geometric Properties

Enhancing the High-Temperature Fracture Toughness of Ultrahigh-Performance Concrete through Optimization of Ternary Cement Matrix and Plastic Fiber Geometric Properties

Constitutive model of high-temperature rapid fracture of Si3N4 ceramics for engine turbine discs and their pore insensitivity parameters

ABSTRACT The metallic materials used to manufacture turbine components can potentially be replaced with ceramics such as Si3N4, but the stress on turbine discs is difficult to determine experimentally, which hinders the development and optimization of ceramics. The finite element method solves this problem, but it requires an accurate constitutive model and parameters. Existing methods for determining model parameters lack versatility and do not consider the internal pores of ceramics. This work proposes a method for determining the parameters of the high-temperature rapid fracture constitutive model of ceramics with internal pores based on the results of industrial computed tomography (CT) and high-temperature fracture toughness experiments. The constitutive model for a ceramic with internal pores was selected based on the typical service conditions of turbine discs. Industrial CT was used to measure pore information. A finite element preprocessor was developed to create simulated specimens with pores. The high-temperature fracture toughness of the ceramics was measured. Pore insensitivity was the parameter obtained from the high-temperature constitutive model of ceramics with pores using this method. The maximum relative error between the predicted and experimental flexural strengths was 6.8%. It provides a new idea for determining the parameters of constitutive models for materials with pores.

Investigation of mechanical properties and high temperature wear resistance of CoFeNi1.5VZr0.4Six high entropy alloys optimized by Si alloying

The current work studies the mechanical properties and high-temperature wear resistance of CoFeNi1.5VZr0.4Six high entropy alloys by Si alloying. The Si alloying transforms Ni7Zr2 into a more wear-resistant silicide phase, and the microstructure changes from lamellar eutectic structure to dendritic structure. The high temperature microhardness, compressive strength and fracture toughness of the HEAs increase first and then decrease with the increasing Si content. The Si alloying in HEAs brings about significant improvements in wear resistance at elevated temperature through the combined effects of phase structure modifications and compositional changes in the tribo-layer. The augmented mechanical properties of the worn subsurface layer contribute to improved wear resistance at elevated temperatures.

Analysis of the fracture behavior and mechanism of PIP-C/SiC composites at high temperatures

The high-temperature fracture toughness of C/SiC composites is of great significance for the tolerance assessment and safety application of components in service. In this work, combining the high-temperature experimental technique and phase field method, the fracture toughness of PIP-C/SiC composites at 25–1600 ℃ in inert atmosphere was tested, and the microcrack propagation at different temperatures was simulated. The fracture model considers the effects of residual stress, temperature-dependent properties of constituents and interfacial graphitization on the crack propagation process. The results show that the fracture toughness and modes of C/SiC composites have significant temperature dependence and difference in in-plane and out-of-plane orientations. As the increase of temperature, the cracks exhibit three typical modes of deflection, penetration and long-debonding at the fiber–matrix interphase. It is worth noting that C/SiC composites show a unique fracture mode at 1600 ℃ with the work of fracture increasing significantly. Overall, the work provides a guidance for the damage tolerance assessment of C/SiC composites in engineering.

Microstructural evolution and hot tensile deformation behaviors below δ-solvus temperature of IN718 alloy fabricated by plasma arc additive manufacturing

Plasma arc additive manufacturing (PAAM) stands out for its remarkable efficient utilization and good performance reliability, making it a highly favored additive manufacturing technique. This study delves into the tensile deformation below δ-solvus temperature of PAAM IN718 alloy with complete heat treatment. The results show that the peak stresses are decreased from 1069 to 1189 MPa at 550 °C to 620–1059 MPa at 750 °C. Through the orientation distribution function, the domain textures for hot deformation were identified as cube texture <100> and Rt-Goss texture {110}<110>. The results of the observations on the substructure and grain boundary components of the deformed alloy confirmed that the onset temperature of recrystallization in PAAM alloy is approximately 750 °C. The recrystallization nucleation mechanism of PAAM IN718 alloys was identified as discontinuous dynamic recrystallization. A deformation constitutive equation for hot deformation was established using the Zener-Hollomon parameter, with an average activation energy of 508.93 kJ/mol. A processing map for the hot working of PAAM alloy was developed, and the reasonable processing domains are located at a temperature 700–750 °C/strain rate 10−2–10−1 s−1 with a power dissipation efficiency range of 0.41–0.47. The high-temperature fracture mechanism of PAAM alloy was discussed, identifying the important inducements for high-temperature fracture failure including carbides, residual Laves phase, and δ phase in the PAAM alloy.

Anomalous increase of fracture toughness of TiAl-based alloys at high temperature

The anomalous increase of mechanical property of TiAl-based alloys at high temperature caters to the property requirements as the high-temperature structural materials, which is thus important and worth studying. However, people currently are not clear whether the fracture toughness of TiAl-based alloys has an anomalous increase at high temperature, and if so, how much it increases, and why it increases. In order to answer these questions, the forged Ti-44Al-8Nb model alloy with fine and uniform microstructure was used. The fracture toughness of the alloy at different temperatures was tested. The results indicated that the alloy did show an anomalous increase of fracture toughness at 700 °C–800 °C. The main crack deflected a large angle at 700 °C, and terminated at the upper part at 750 °C and 800 °C. The fracture morphology was fine, and the cleavage plane became smaller and less at high temperature. These characteristics corresponded to the anomalous increase of fracture toughness at high temperature. Through reverse thinking, we found that the high dislocation density was the root causing the anomalous increase of fracture toughness at high temperature. The forging-induced texture, such as B2 〈100〉//RD and γ-TiAl 〈011]//RD could also promote the dislocations slip on the main slip surfaces during the high temperature fracture toughness test, contributing to the final high dislocation density. The critical crack size at 700 °C–800 °C was about 4 times that of room temperature, corresponding to the higher fracture toughness of the alloy at high temperature.

High-temperature fracture behavior of an α/β Titanium alloy manufactured using laser powder bed fusion

The room and high temperature (up to 600 °C) mechanical properties, with emphasis on the fracture toughness and fatigue resistance, of an α/β titanium alloy that was additively manufactured using the laser powder bed fusion (LPBF) technique were evaluated in the as-printed (α’ lath martensite with negligible β phase content) and heat-treated (α/β structure with a significant volume fraction of the β phase) states for critically examining the β phase's role in the fracture and fatigue behavior (given the significantly higher diffusion rates of various species in β compared to α). Experimental results reveal that the heat-treated sample exhibits superior ductility and fracture initiation toughness at elevated temperatures, compared to the as-printed sample, due to the presence of β ligaments in the former. They also prevent strain localization along the prior β grain boundaries, and their absence in the as-printed state leads to cracking along those boundaries. While the fatigue crack growth (FCG) rates in both the as-printed and heat-treated samples deteriorated with increasing temperature, the threshold for FCG generally increased, which is attributed to creep-induced crack tip blunting. Moreover, a double Paris exponent phenomenon is observed for the heat-treated sample at 300 °C, attributed to the hydrogen-assisted crack growth, with β phase providing an accelerated diffusion pathway. Overall, the experimental results and their analyses illustrate the intricate relationship between creep, hydrogen diffusion, and the β phase in dictating the fracture behavior of LPBF α/β titanium at elevated temperatures.

Simultaneous improving high temperature oxidation resistance and room temperature toughness of Nb[sbnd]Si based alloy via appropriate Ta content addition strategy

NbSi based alloys are expected to become the application materials for high pressure turbine blades of aircraft engines. This paper investigates the impact of Ta addition on room temperature fracture toughness and isothermal oxidation behavior at 1250 °C of four NbSi based alloys with compositions of Nb-16Si-20Ti-8Zr-4Hf-1.5Al-xTa (x = 0, 1, 2, and 4 at.%) fabricated through vacuum non-consumable arc-melting. The results show that the room temperature fracture toughness and 1250 °C oxidation resistance of the alloys increase first and then decrease with the addition of Ta element. The room temperature fracture toughness of 1Ta alloy is the best, reaching 17.1 MPa·m1/2. Adding a moderate amount of Ta (1 at.%) significantly enhances the alloy's oxidation resistance, but an excessive Ta content diminishes this enhancement. This research has obtained a Nb-16Si-20Ti-8Zr-4Hf-1.5Al-1Ta alloy with high room temperature fracture toughness and excellent high temperature oxidation resistance simultaneously, which provide an ideal path for designing NbSi alloys with superior comprehensive properties.

The high temperature creep and fracture behavior of Inconel 718 produced by additive manufacturing

High-temperature creep tests of additively manufactured (AM) and wrought nickel alloy 718 (IN718) were conducted at 650 °C and 704 °C at stresses ranging from 316 to 819 MPa. The AM alloy had a greater creep strength than the wrought alloy at nearly all testing conditions due to increased volume fractions of γ” in the matrix and carbides at grain boundaries; however, the creep rupture ductility of the AM material was significantly reduced at all testing conditions. Secondary ion mass spectrometry (SIMS) revealed that sulfur segregation, resulting from a lack of Mg in the AM alloy, is the most plausible explanation for the observed ductility loss in AM IN718. Overall, this work focuses on understanding and defining the mechanism of embrittlement in AM IN718.

High temperature fracture behavior of 316L stainless steel-Inconel 718 functionally graded materials manufactured by directed energy deposition: Role of interface orientation and heat treatment

Investigation of functionally graded component consisting of stainless steel 316L (SS316L) and Inconel 718 superalloy (IN718) deposited by laser directed energy deposition (LDED) is presented here. Ambient as well as high temperature mechanical behavior is observed for as-deposited (AD) and for heat-treated (HT) states. The gradient sample was manufactured with the interface orientation parallel and perpendicular to the loading direction to understand the impact of this factor on the materials properties. The evolution of chemical composition, grain structure, phase transformations, and tensile properties at room and elevated temperatures, as well as creep behavior before and after heat treatment was determined. With use of digital image correlation (DIC) analysis, the effects of interface orientation and heat treatment on deformation and fracture mechanism was revealed. Results show that heat treatment smooths the chemical composition and mechanical deformation process, as characterized by the formation of an enhanced gradient diffusion zone in the IN718-rich region near the interface. Heat treatment impacts mechanical properties based on the orientation of the interface. Specimens with interfaces parallel to the load direction combine the strengths of both materials effectively. After HT, they exhibit enhanced mechanical performance, achieving an ultimate tensile strength of 973 MPa and 14% elongation at room temperature. Additionally, at elevated temperatures, the specimen demonstrates exceptional creep resistance, sustaining structural integrity for over 1700 h at 650 °C and 225 MPa. Sample orientation is shown a decisive effect on deformation and fracture mechanism, with the interface not consistently being the weakest point across various gradient samples.

Initiation mechanism of micro-defects in polycrystalline iron under high-temperature stress coupling effect

To investigate the failure mechanism of high-temperature fracture in polycrystalline iron, this study conducted in-situ observation experiments to study the dynamic damage evolution process of iron under tensile loading at high temperatures. Meanwhile, through molecular dynamics simulation, the micro-defect initiation mechanism of polycrystalline iron under the coupling effect of high temperature and stress was revealed at the microscopic scale. The results indicate that during the high-temperature deformation process of polycrystalline iron at 1400 K, grain boundary slip is the main deformation mechanism. When the grain boundary slip is obstructed, stress concentration can easily occur at the grain boundaries (especially the triple junctions), leading to crack initiation. Furthermore, molecular dynamics simulations have shown that in the initial stage of deformation, with increasing strain, both HCP phase transformation atoms and dislocation length exhibit a trend of initially increasing followed by decreasing, peaking at around 5 % strain. Afterward, the system undergoes dynamic recrystallization, with dislocation movement and grain boundary migration leading to dislocation elimination. When the strain reaches 12.3 %, microvoids initiate at the triple junctions and grow up in an O shape. At a strain level of 12.45 %, there is a sudden drop in system stress, indicating fracture initiation.

Investigation of pore structure and high-temperature fracture behavior of lamellar hydrates bonded alumina-spinel castables

Lamellar hydrates of CAC were designed with the introduction of Mg-Al hydrotalcite (M-A-H), and the effects on the early setting behavior, demolding strength, pore structures, mechanical properties, and fracture behavior of alumina-spinel castables were investigated. The results showed that Mg-Al hydrotalcite stimulated rapidly the hydration of CAC and the formation of lamellar C2AH8 and C4AcH11 when curing at 25 and 40 °C. In comparison, CAH10 and C2AH8 were detected without M-A-H and were transformed completely into C3AH6 at 40 °C. The formation of lamellar C2AH8 and C4AcH11 would contribute to a more complicated pore structure, especially in the range of 1–10 µm. Meanwhile, the incorporation of MgO from M-A-H also regulates the distribution of CA6 and spinel (pre-formed and in-situ). Consequently, the optimized microstructure and complicated pore structure can induce the deflection and bridging of cracks, thus facilitating the consumption of fracture energy when testing at 1400 °C.

An analytical model for the high temperature fracture strength of SiC fiber reinforced ceramic matrix composites considering oxidation and residual thermal stresses

An analytical model for the high temperature fracture strength of SiC fiber reinforced ceramic matrix composites considering oxidation and residual thermal stresses

High-temperature fracture behavior of carbon fiber reinforced PEEK composites fabricated via fused filament fabrication

This study investigates the influence of temperature on the mechanical and fracture properties of carbon fiber reinforced (CF) polyetheretherketone (PEEK) composites, with a specific focus on different layer heights used in the additive manufacturing process. Tensile and compact tension fracture tests were conducted at varying temperatures to evaluate the orthotropic mechanical properties and crack growth resistance curves, respectively. The key findings underscore the pronounced orthotropic mechanical behavior in additively manufactured CF-PEEK composites, showing substantial variations of up to 50% in Young's modulus and yield strength across orientations. Additionally, as test temperatures increase, crack initiation becomes easier, indicating reduced crack resistance at elevated temperatures. Notably, the effect of layer height on initiation toughness is relatively minor compared to temperature. Moreover, the study reveals that fracture energy for crack growth is less temperature-sensitive but depends on layer height, with larger defects in taller layers resulting in decreased damage resistance. These findings stress the importance of considering the distinct sensitivities of initiation and steady-state fracture energies to temperature, emphasizing the critical role of effective fracture toughness in evaluating material resistance to high-temperature fractures. These observations contribute to our understanding of the mechanical and fracture behavior of CF-PEEK composites under high operating temperatures. This understanding facilitates the optimization of their manufacturing processes and their application in various industries such as aerospace.

Investigation of the influence of impurities typical in secondary raw materials on the behavior of high alumina castables – Part 1: Design of the castables, setting properties and high temperature fracture behavior

The research was carried out using a primary high alumina raw material that was selectively spiked with typical elements found in secondary raw materials. These artificially spiked raw materials were processed into cement-free high alumina monolithics by 1:1 substitution of the corresponding high alumina matrix fraction in the formulation. Regardless of the impurity, the rheology and setting behavior of the developed model refractory castables showed a very high degree of comparability with the reference material. The same workability allowed the production of comparable test pieces for hot wedge splitting tests. The effects on high temperature fracture behavior observed in this study can therefore be attributed solely to the impurities.

Role of Nb (C, N) and Cr carbides on the fracture behaviour of Super304H steel using in-situ tensile studies

The present work investigates the role of Nb (C, N) precipitates and Cr-carbides on the high-temperature fracture behavior of Super304H steel. To accomplish this, both as-received and sensitised steels were subjected to in-situ tensile testing at 650 °C. Both samples exhibit the formation of primary cracks along the coarse (>3 µm) Nb (C, N) precipitates and their decohesion. Additionally, in sensitised samples, Cr-carbides embrittle the grain boundaries resulting in reduced ductility (36 %) and toughness (39 %). Moreover, fine cracks initiation/propagation along the grain boundary Cr-carbides (typically a weaker region in sensitised steels) is observed.

High-temperature fracture behaviour of layered alumina ceramics with textured microstructure

Mimicking the damage tolerance of biological materials such as nacre has been realised in textured layered alumina ceramics, showing improved reliability as well as fracture resistance at room temperature. In this work, the fracture behaviour of alumina ceramics with textured microstructure and laminates with embedded textured layers are investigated under uniaxial bending tests at elevated temperatures (up to 1200°C). At temperatures higher than 800°C monolithic textured alumina favours crack deflection along the basal grain boundaries, corresponding to the transition from brittle to more ductile behaviour. In the case of laminates, the loss of compressive residual stresses is counterbalanced by the textured microstructure, effective up to 1200°C. This study demonstrates the potential of tailoring microstructure and architecture in ceramics to enhance damage tolerance within a wide range of temperatures.

Microstructure Evolution and Fracture Mechanism of 55NiCrMoV7 Hot-Working Die Steel during High-Temperature Tensile

In this paper, through high-temperature tensile tests of 55NiCrMoV7 steel, high-temperature fracture behavior, microstructure evolution, and carbide distribution characteristics of both the thermal–mechanical coupling zone (fracture zone) and thermal stress zone (clamping zone) at different temperatures were studied. Intrinsic relationships between high-temperature fractures and carbide types, distribution and size were revealed, and evolution mechanisms of microstructure near cracks in 55NiCrMoV7 hot-working die steel during high-temperature deformation was clarified. Samples were stretched at different temperatures from 25 °C to 700 °C, and microscopic examinations were carried out using SEM and TEM. The results showed the following. With the increase in temperature, tensile strength and yield strength decreased, elongation and reduction of area increased, and fracture mode changed from brittle fracture to ductile fracture by transition temperature at about 400 °C. During high-temperature deformation, the grain dislocation density decreased and the tempered martensite decomposed, recovered, recrystallized, and then grain grew. M7C3 and M23C6 carbides precipitated and grew along the grain boundary, and a small amount of fine granular MC carbides was dispersed in the grain. The work done by the external force on the deformation zone would cause the temperature of it to be higher than the tensile temperature, which provides thermodynamic conditions for the redissolution of small carbides near the fracture zone and the grain growth of large carbides, resulting in a decrease in small carbides and increase in large carbides in thermal–mechanical coupling zones.

High-Temperature-Resistant Thermal Shape Memory Polymers as Lost Circulation Materials for Fracture Formations

Summary Lost circulation during the drilling of fractured formations is one of the most challenging engineering problems. Shape memory polymers (SMPs) have been used as lost circulation materials, but most of them are not resistant to high temperatures. In this study, a high-temperature-resistant thermal shape memory epoxy resin (SME) was synthesized by conducting an orthogonal experiment using the glass transition temperature (Tg) as an index. The Tg of the SME synthesized by using the optimum formula was 124℃. This SME had good thermal stability, and its compression and tension stresses were 94.2 and 58.8 MPa, respectively. In addition, the thickness swelling ratio (Rrc) of the SME was optimized by performing another orthogonal experiment, and the Rrc of the SME prepared by using the optimized formulation (OSME) was 78.8%. The OSME did not swell at 25–150℃ in water, brine, or base fluid. The total size reduction percentage of the OSME was 1.7% after aging at 150℃, whereas that of a nutshell was 10.7%, indicating that OSME particles had better compression and temperature-resistance performance. The shape memory ratio (Rc) of the OSME was 6, 70, and 100% at 80, 120, and 125℃ after being heated for 50 minutes, respectively, and it was fully activated in 5 minutes at 150℃. The breakthrough pressure of the plugging mud with or without the OSME was 15 MPa at 25, 80, 120, and 150℃ when plugging the wedge fracture model with an inlet/outlet width of 3/1 mm. However, when plugging the wedge fracture model with an inlet/outlet width of 5/2 mm, the plugging slurry with the OSME could withstand a pressure of 3, 5, and 15 MPa at 80, 120, and 150℃, respectively, and the plugging mud with conventional lost circulation materials could bear a pressure of below 3 MPa at 80, 120, and 150℃. These results indicated that the OSME had good plugging and thermosensitive performance. OSME particles matched better with the fracture size, owing to their elastic and shape memory performance at above Tg. They migrated and bridged in fractures, aggregated and filled the pore space with other lost circulation materials, and formed a dense plugging layer at above Tg. Thus, the synthesized SME is a promising material for plugging high-temperature fracture formations while drilling.

Influence of Si segregates on the structural evolution, mechanical properties, and high-temperature fracture toughness of Cr-Si-B2±z coatings

The impact of Si-segregates and varying deposition conditions on the structural and mechanical properties of sputter deposited, high-temperature oxidation-resistant Cr-Si-B2±z coatings is studied from ambient, to elevated temperatures. Overstoichiometric, AlB2-structured Cr-Si-B2±z thin films with Si-content up to 15 at.% were synthesized on Ti-6Al-4V by magnetron-sputtering using a substrate bias of −120 V. The enhanced surface diffusion promotes mechanically superior, (001)-oriented coatings with hardness of H∼30 GPa up to a Si-content of 3 at.%. Higher Si-concentrations result in significant hardness loss to H∼20 GPa, related to a bias-independent solubility-limit in the CrB2-structure and the formation of mechanically-weak Si grain-boundary segregates. The as-deposited hardness of all Cr-Si-B2±z compositions is maintained after annealing to 800 °C, despite the initiation of material recovery. A B/Cr-ratio-independent oxidation resistance up to 1400 °C is demonstrated, underlining a minimum Si-content of 8 at.% to form a stable SiO2-based scale. In line with the room-temperature hardness, increasing Si-contents are accompanied by decreasing fracture toughness, reducing from KIC∼2.9 (Cr0.28B0.72) to ∼1.7 MPa√m (Cr0.24Si0.10B0.66). High-temperature cantilever bending up to 800 °C revealed a brittle-to-ductile-like transition for Cr0.28B0.72, resulting in an increased fracture toughness of KIC∼3.3 MPa√m. Si-alloyed coatings show decreasing fracture resistance up to 400 °C, whereas beyond, Si-segregates enable high-temperature plasticity and thus a significantly increased damage tolerance.