High Temperature Mechanical Properties Research Articles

A novel oxide ceramic matrix composite with integrated high-temperature EMW absorption and mechanical performance

Oxide ceramic matrix composites (OCMC) are extensively used as thermal structural materials due to good high-temperature resistance, mechanical properties, and oxidation resistance, but they are rarely regarded as electromagnetic wave absorbing materials due to the high resistivity of oxide ceramics/fibers. However, high-performance Al2O3 fiber can be used to design and fabricate OCMC with integrated high-temperature microwave absorbing and mechanical properties by introducing dielectric material into Si-based ceramic matrix. The N610/Si3N4-SiOC composites have the prior mechanical performance because the uniformly distributed Si3N4 matrix made the fiber bear effectively. After that, the Sc2O3 modified SiOC matrix was filled into inter- and intra-fascicular pores forming interfacial polarization, dipole polarization, multiple reflection and scattering to attenuate the incident electromagnetic wave (EMW). The effective absorption bandwidth (EAB) and the minimum reflection coefficient (RCmin) of N610/Si3N4-SiOC(Sc) composites at the thickness of 2.75 mm reach 3.0 GHz and −42 dB in the X band, respectively. In addition, the EMW absorption performance of the composite is relatively stable with EABmax and RCmin of 3.5 GHz and −41 dB in 4–18 GHz at high temperatures. This work provides a strategy for designing and fabricating OCMC with excellent mechanical properties and EMW absorption properties.

The microcrack inhibition and mechanical properties of an in-situ synthesized refractory high-entropy alloy fabricated by additive manufacturing

A crack-free refractory high-entropy alloy (RHEA), composed of (Cr25Mo25Nb25V25)99C1 was synthesized in-situ from blended elemental powders using laser directed energy deposition (DED). This study revealed that a minor addition of carbon effectively enhances the grain-boundary cohesion of the DEDed RHEA, thereby mitigating microcrack propagation along the grain boundaries during the additive manufacturing process. Notably, the DEDed RHEA exhibited exceptional high-temperature mechanical properties, highlighted by a yield strength of 787 MPa at 1000 °C along with sustained strain-hardening capacity, surpassing other additively manufactured high-entropy alloys reported to date. The remarkable high-temperature strength of the DEDed RHEA can be attributed to the potent pinning effect exerted on dislocations within both dendritic and interdendritic regions at elevated temperature. This effect stems from severe lattice distortion, local chemical fluctuations (LCFs), solute atom pinning, and dislocation interactions. The method employed to inhibit microcrack formation, along with the elucidation of the high-temperature strengthening mechanism, opens avenues for the fabrication of RHEAs tailored for high-temperature structural applications.

Anti-oxidative quartz/carbon-braided fiber reinforced ceramizable composites modified with B4C and AlB2: Mechanical robustness, phase evolution and anti-oxidation mechanism

Carbon fiber reinforced ceramizable phenolic resin matrix composites (Cf/CPR) have been widely used in the field of thermal protection materials. However, the industrial application of the Cf/CPR is restricted by its easy oxidation failure under high temperature condition. In this work, a novel antioxidative ceramizable composites modified with AlB2 and B4C (Cf-Qf/CPR) were prepared by using quartz fiber-coated carbon fiber (Cf-Qf) as reinforcements via co-weaving technique. The thermal stability, mechanical properties, interfacial phase evolution and anti-oxidation mechanism of the composites were investigated. The residue yield and the flexural strength of the composites after treatment at 1200°C for 15 min were 93.4 % and 62.4 MPa, increased by 5.4 % and 67.3 % compared with those of Cf/CPR. Quartz fiber and ceramic protective layer showed synergistic effect to improve the oxidation resistance of carbon fiber and PyC, confirming good high temperature mechanical properties and oxidation resistance of as-received composites.

Study on Microstructure and High-Temperature Mechanical Properties of Al-Mg-Sc-Zr Alloy Processed by LPBF

Al-Mg-Sc-Zr alloy processed via laser powder bed fusion (LPBF) is poised for significant application in aerospace, where its high-temperature capabilities are paramount for the safety and longevity of engineered structures. This study offers a systematic examination of the alloy’s high-temperature tensile properties in relation to its microstructure and precipitate phases, utilizing experimental approaches. The LPBF-processed Al-Mg-Sc-Zr alloy features a bimodal microstructure, with columnar grains in the melt pool’s interior and equiaxed grains along its boundary, conferring exceptional properties. The application of well-calibrated processing parameters has yielded an alloy with an impressive relative density of 99.8%, nearly fully dense. Following a thermal treatment of 350 °C for 4 h, the specimens were subjected to tensile tests at both room and elevated temperatures. The data reveal that the specimens exhibit a tensile strength of 560.6 MPa and an elongation of 11.1% at room temperature. A predictable decline in tensile strength with rising temperature is observed: at 100 °C, 150 °C, 200 °C, and 250 °C; the respective strengths and elongations are 435.1 MPa and 25.8%, 269.4 MPa and 20.1%, 102.8 MPa and 47.9%, 54.0 MPa and 72.2%. These findings underpin the technical rationale for employing LPBF-processed Al-Mg-Sc-Zr alloy in aerospace applications.

Effects of standard heat treatment on micro-to nanostructure heterogeneities in a Rene 65 Ni-based superalloy billet

Advancements in the manufacturing of aeroengine components with superior mechanical properties rely on the microstructure homogeneity of industrially forged Ni-based superalloys. Billets often contain microstructural heterogeneities that can persist throughout the process, potentially compromising the integrity of the final product. Rene 65 is a relatively new superalloy with high-temperature mechanical properties superior to those of Alloy 718. However, systematic studies on its microstructure-property relationships are scarce. Here, we correlate the microstructural evolution with the hardness of a Rene 65 billet subjected to industrial processing and standard heat treatments. Multiscale microstructural heterogeneities, including grain size, geometrically necessary dislocations, twin boundary density, and the multimodal distribution of γʹ precipitates, are characterised. Grain growth occurs near the billet centre. Precipitation varies with radial position, showing up to a two-fold change in primary and secondary γʹ fractions. Tertiary γʹ precipitates exhibit variations in morphology and chemical composition, with Cr and Co enrichment near the billet surface. These heterogeneities arise from varying strain distributions and cooling rates during radial forging. Sub-solvus annealing and ageing are shown to mitigate these heterogeneities by promoting recrystallisation, recovery, and homogeneous precipitation of tertiary γʹ. A strengthening model elucidates the active mechanisms in as-received and heat-tread conditions, predicting hardness with a deviation of <8 %. Analyses of grain boundaries and γ/γʹ interfaces reveal strong segregation of Mo, W, Nb, C and B before and after heat treatment, with Mo and B showing the highest interfacial excess values. Insights into the microstructural evolution and its impact on properties could guide the development of next-generation aerospace parts.

A model for predicting grain-boundary energy of refractory high-entropy alloys based on local element concentration

Refractory high-entropy alloys (RHEAs) exhibit exceptional high-temperature mechanical properties, and their structural stability is primarily influenced by grain boundaries (GBs). However, it remains challenging to accurately quantify the grain boundary energy (GBE) in RHEAs. Herein, we introduce a quantitative descriptor for point-to-point prediction of GBEs in RHEAs based on local element concentration and valence-electron numbers. Our results demonstrate that the role of the constituent elements on GBEs is primarily determined by their valence-electron numbers in RHEAs. These findings are derived from the cohesive energy and atomic structure of the elements within the framework of the broken-bond model, as well as the differential contributions of s-electrons and d-electrons. The model establishes a structure-property relationship between local element concentration and GBEs, with the prediction consistent with experimental and theoretical results, which is crucial for evaluating the GB stability of RHEAs and facilitating the development of high-performance alloys.

Unusual secondary slip activity at room temperature in VC single crystals

Cubic transition-metal carbides are high-melting compounds with remarkable high-temperature mechanical properties but are generally considered to be brittle at low-temperatures. Here, we report on the activation of multiple slip systems and plasticity in 001, 110, and 111 oriented vanadium carbide (VC) single-crystals subjected to uniaxial compression at room-temperature. Using in situ scanning electron microscopy based mechanical testing, we observe plastic strains up to 21%, size-dependent yielding, and work hardening in VC(001) and VC(111). In comparison, VC(110) crystals are relatively brittle with limited localized plasticity. VC(111) crystals exhibit the highest yield strengths of up to ∼23 GPa while VC(001) crystals are some of the softest with yield strengths as low as ∼13 GPa. For loading along [110], we find that only the primary slip system, {111}〈11¯0〉, is active. In VC(001) and VC(111), we find the operation of two sets of slip systems, [{110}〈11¯0〉 and {111}〈11¯0〉] and [{111}〈11¯0〉 and {100}〈11¯0〉], respectively. Intriguingly, the estimated critical resolved shear stresses for all the three slip systems are nearly the same. Our results, which reveal orientation-dependent differences in the activity of the same slip system, provide new insights into plastic deformation in refractory compounds.

In-situ high-temperature micromechanical behavior of flash-sintered strontium titanate

Strontium titanate (SrTiO3) is a versatile material with various applications but understanding its mechanical properties, especially in polycrystalline form prepared via field-assisted sintering methods, is limited. This study investigates the high-temperature mechanical properties of flash-sintered SrTiO3 through in-situ microcompression tests comparing the behaviors near the positive and negative electrodes. Due to a significant irregularity in densification, the negative electrode exhibited superior fracture strength and strain across all temperatures when compared to its positive counterpart. Micropillars near the positive electrode contained multiple pre-existing pores that became crack initiation sites, and thus inducing catastrophic failure. Micropillars near the negative electrode exhibited prominent intergranular cracks accompanied by high-density dislocations in the vicinity of the fracture surfaces. This study elucidates the effect of flash sintering-induced defects on the mechanical properties of polycrystalline SrTiO3 at various temperatures.

Novel Chromium–Silicon Slurry Coatings for Hot Corrosion Environments

AbstractNi-based superalloys are commonly used in gas turbines because of their exceptional high-temperature mechanical properties. To secure a long service life, the materials must also have sufficient corrosion resistance. Therefore, diffusion coatings are widely used to enrich the surface in protective oxide scale-forming elements. For temperatures between 650 and 950 °C, where hot corrosion occurs, Cr-based coatings are advantageous. These are commonly applied via the laborious pack cementation process. Recently, a novel cost-effective Cr/Si slurry coating process has been developed which demonstrated resistance to oxidative high-temperature environments. Here, the protection of the slurry coatings against hot corrosion type I at 900 °C on the Ni-based superalloy Rene 80 is investigated and compared to coatings produced by pack cementation. Prior to the 300-h exposures in air containing 0.1% SO2 at 900 °C, 4 mg/cm2 of Na2SO4 was deposited on the material surfaces. The uncoated Rene 80 exhibited rapid dissolution of the initial oxide scale followed by catastrophic break away oxidation. In comparison, the slurry coatings showed significantly improved hot corrosion resistance compared to the uncoated alloy and a better protection than a Cr pack cementation coating. The Cr pack cemented Rene 80 showed improved hot corrosion resistance, but Cr depletion in the subsurface zone occurred with increasing exposure time, associated with the propagation of Al internal oxidation and increasing sulfidation. In contrast, the slurry coatings formed an external Cr2O3 scale coupled with an agglomeration of SiO2 underneath and a continuous Al2O3 subscale which offered a better diffusion barrier and leading to superior long-term protection against hot corrosion.

Microstructure and High-Temperature Mechanical Properties of CrMnFeCoNi/TiAl Composites Prepared by Powder Metallurgy

Microstructure and High-Temperature Mechanical Properties of CrMnFeCoNi/TiAl Composites Prepared by Powder Metallurgy

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.

Excellent high temperature mechanical properties of Fe2Ni2CrAlx (x= 0.9 to 1.3) multi-principal elements alloys

The high temperature mechanical properties of a family of Fe2Ni2CrAlx (x=0.9, 1.1, 1.2, 1.3 in molar ratio) multi-principal element alloys (MPEAs) were determined. The initial microstructure and grain size of Fe2Ni2CrAlx MPEAs were characterized. Effect of Al on the density, melting point and mechanical properties at elevated temperature were investigated. Fe2Ni2CrAl1.3 MPEA exhibit the lowest density of 6.78 g/cm3 and highest melting point of (1337.7 ∼ 1392.6 °C). The as-cast Fe2Ni2CrAl1.3 MPEA showed an excellent combination of strength and plasticity at elevated temperatures. Fe2Ni2CrAl1.3 MPEA showed high strength of 2119 MPa and 1722 MPa at 200 °C and 400 °C, respectively. The Fe2Ni2CrAlx alloys exhibited exceptional high-temperature plasticity, no fracture occurred even at a compressive strain above 70%. Recrystallization softening appeared at above 600 °C, the transformation of coarse equiaxed grains into subgrains were captured during plastic deformation. Significantly, the specific yield strength of the Fe2Ni2CrAl1.3 MPEA was comparable with traditional Ti based alloys and Ni-based superalloy. The fracture mechanism of Fe2Ni2CrAlx MPEAs was a combination of cleavage and slip separation. Deformation behavior and work hardening ability at elevated temperature were discussed. Finally, finite element method (FEM) was performed to capture the evolution of stress and strain under high-temperature compression.

Microstructure and High-Temperature Mechanical Properties of a Superalloy Joint Deposited with CoCrMo and CoCrW Welding Wires

Microstructure and High-Temperature Mechanical Properties of a Superalloy Joint Deposited with CoCrMo and CoCrW Welding Wires

High-temperature structural evolution and mechanical properties of oxygen-containing NiCrAlY coatings

NiCrAlY is utilized as a protective coating for gas turbine components at high temperatures. Enhancing the erosion resistance of NiCrAlY coatings can be achieved by optimizing their mechanical properties. In this study, a NiCrAlY coating containing oxygen was deposited by magnetron sputtering and investigated concerning microstructural evolution at 900° C. Oxygen inclusion does not increase the hardness of the NiCrAlY coating in its initial deposited form. When annealing at 900 °C for 1 h, the NiCrAlYO coating develops into a nanocrystalline structure, where Y2O3 is evenly distributed inside the γ/γ′ matrix. This leads to the phenomenon of age-hardening, causing the hardness to increase to 12.4 GPa. Conversely, the hardness of the O-free NiCrAlY coating sharply declines to 9.3 GPa. Moreover, the wear resistance of the NiCrAlYO coating has been enhanced at a temperature of 700 °C, owing to its exceptional mechanical stability.

Study of Aging Temperature on the Thermal Compression Behaviors and Microstructure of a Novel Ni-Cr-Co-Based Superalloy.

Nickel-based superalloys have been widely used in the aerospace industry, and regulating the reinforcing phases is the key to improving the high-temperature strength of the alloy. In this study, a series of aging treatments (650 °C, 750 °C, 850 °C and 950 °C for 8 h) were designed to study different thermal deformation behaviors and microstructure evolutions for a novel nickel-based superalloy. Among the aged samples, the 950 °C aged sample achieved the peak stress of ~323 MPa during the thermal deformation and the highest microhardness of ~315 HV after thermal compression, which were the greatest differences compared to before deformation. In addition, the grains of the 950 °C sample exhibit deformed fibrous shapes, and the grain orientation is isotropic, while the other samples exhibited isotropy. In the 850 °C and 950 °C high-temperature aging samples, the γ' precipitate (about 20 nm in size) is gradually precipitated, which inhibits the movement of dislocation in the grain during compression, thus inhibiting the occurrence of dynamic recrystallization and improving the high-temperature mechanical properties of the alloy.

Research about the microcrack mechanisms in molybdenum and corresponding suppression strategies during laser additive manufacturing process

In this study, the mechanism of microcracks in the laser powder bed fusion of molybdenum was analyzed, and the corresponding solutions were proposed. Through specially designed single-track experiment and finite element simulation analyses, we found that thermal stress was the main cause of cracking during the laser powder bed fusion process. The cracks were generated almost synchronously with the laser scanning process, and thus optimizing the process parameters alone could not inhibit the occurrence of cracks. Herein, alloying strategy was proposed and proved to be an effective way to solve the cracking problems. Specifically, by adding Cr and Ti elements, the high-temperature mechanical properties of Mo were improved through solid solution strengthening mechanism, hence the thermal stress induced cracking was suppressed effectively. In addition, according to the thermodynamic calculations, it was found that Cr and Ti had an inhibitory effect on the segregation of impurity at the grain boundaries and this conduce to improve the strength as well.

Achieving back-stress strengthening at high temperature via heterogeneous distribution of nano TiBw in titanium alloy by electron beam powder bed fusion

Back-stress strengthening has been proven to be an effective strategy in breaking the room temperature strength–ductility dilemma in hetero-structural metals matrix composite materials (MMC). However, back-stress is difficult to achieve at high temperature because of grain boundary softening, grain boundary sliding, dislocation annihilation and grain rotation. Here, back-stress strengthening is exploited to improve the high temperature tensile properties of Ti-6Al-4 V (TC4) alloy by precipitating a heterogeneous distribution of nano TiB whiskers (TiBw). In this work, the rapid solidification associated with electron beam powder bed fusion (EB-PBF) generates heterogeneous in-situ nano TiBw at both grain boundaries and grain interiors. The nano-TiBw on the grain boundaries (GB nano-TiBw) not only refines the grains but can also pin the grain boundaries. The intragranular nano TiBw (IG nano-TiBw) significantly reduces the dislocation mean free path, inhibit dislocation movement, and dramatically enhances the dislocation storage density. Meanwhile, specimens are strengthened by high-density < a + c > dislocations induced by the precipitated nano TiBw. The heterogeneous distribution of nano TiBw achieved a 50 °C increase in the service temperature of TC4 alloy while also increasing stability. This strategy provides a new perspective for further improving the high-temperature mechanical properties of titanium alloys.

Microstructures evolution, texture characteristics and mechanical properties of extruded Mg-9Gd-3Nd-1Zn-1Sn-0.5Zr alloy

In this study, a comprehensive investigation was conducted to explore the microstructural evolution, texture characteristics, and mechanical properties of extruded Mg-9Gd-3Nd-1Zn-1Sn-0.5Zr alloys with varying extrusion ratios (ERs). Moreover, the high-temperature mechanical properties of the alloy and analyzed the corresponding fracture mechanisms were investigated. The findings unveiled a refinement in grain size and an improvement in homogeneity with increasing ER. Such refinement coincided with the disruption of coarse Mg5(Gd, Nd, Zn) eutectic phases, facilitating dynamic recrystallization. As a result, the extruded alloy manifested small grains, significant orientation difference in grain boundaries, and a weakened texture. In particular, the average grain size in the extrusion direction (ED) diminished to 13.29 μm (ER4), 9.62 μm (ER16), and 4.05 μm (ER25). And the ED texture of the ER4 alloy featured a strong (0001) basal bimodal pattern, whereas the ER16 and ER25 alloys showcased a random texture. Furthermore, the ER25 alloy displayed notable work hardening at temperatures of 200 °C or below, exerting a significant influence on its stress–strain curve. The ultimate tensile strength (UTS) of the alloy remained consistently above 275.09 MPa, demonstrating ductile fracture characteristics characterized by numerous dimples at temperatures of 200 °C or higher. The study identified the second phase as the primary contributor to enhancing the high-temperature tensile characteristics of the alloy. In summary, this research developed a unique high-strength Mg-9Gd-3Nd-1Zn-1Sn-0.5Zr alloy characterized by fine grain and texture strengthening. The alloy exhibited remarkable stability at temperatures of 250 °C or lower, making it a promising candidate for applications in high-temperature applications.

Influence of Oxide Inclusions on the Creep Properties of Electron Beam Welds in Ti-added Reduced Activation Ferritic/Martensitic Steel

In this study, the effects of reducing oxide inclusions on the microstructure, tensile properties at 550 ℃, and creep resistance of electron beam welded (EBW) joints in Ti-added reduced activation ferritic/martensitic (RAFM) steel were investigated. Two RAFM steels, designated as Steel A and Steel B, were prepared by adjusting the Al and Si contents to contain different fractions of oxide inclusions. The microstructures of the base metal and heat-affected zone (HAZ) were analyzed, and the mechanical properties of the welds were evaluated. Reducing the Al and Si content in Steel B resulted in a decrease in both the fraction and size of oxide inclusions compared to Steel A. Both steels exhibited tempered martensitic microstructures with M23C6 and MX precipitates. Hardness tests revealed that the over-tempered HAZ (OTHAZ) was the weakest region. However, both tensile test and creep tests at 550 ℃ showed fractures occurring in the base metal, indicating that the low heat input of EBW effectively minimized the weak HAZ regions. Steel B demonstrated improved tensile properties and creep resistance due to the reduced oxide inclusions. It was found that Steel B had a significantly longer creep life than Steel A at 550 ℃. SEM analysis revealed ductile fracture characterized by dimples, where oxide inclusions containing Al, Si, Ta, and Ti were predominant. The presence of these inclusions reduced the effectiveness of solid solution and precipitation strengthening by consuming Ta and Ti. Additionally, microvoids could easily nucleate at the interface between the hard oxide inclusions and the soft metal matrix during creep. Therefore, the reduction of oxide inclusions improved deformation resistance and high-temperature stability, resulting in better creep resistance in Steel B. In conclusion, the reduction of oxide inclusions in Ti-added RAFM steel enhances the high-temperature mechanical properties and creep resistance of EBW joints, underscoring the importance of oxide control in the development of RAFM steels for fusion reactor applications.

Extraordinary ductility in the 75W–25Cu composite at elevated temperatures

The high-temperature mechanical properties are an essential indicator in the application of W–Cu composites. In this study, a 75W–25Cu composite with high relative density is prepared by infiltration. Its tensile properties are investigated from 25 °C to 1000 °C, and the fracture strain shows a non-linear trend with a peak value of 20 % at 200 °C. The fracture mode is the ductile fracture of Cu and the combination of the transgranular and intergranular fracture of W at 200 °C. Microstructure characterization of the post-deformed tensile specimens at 25 °C, 200 °C, and 1000 °C is conducted in conjunction with the finite element analysis. The atypical increase of the fracture strain at 200 °C is rationalized through the perspective of dislocation slip in Cu and the stress partitioning between the two phases. The high-temperature mechanical properties of W–Cu composites are summarized at the end for specimens with a range of compositions and fabrication processes. The current work provides a solid basis for any subsequent optimization and potential application of W–Cu composites.