High Temperature Mechanical Properties Research Articles

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.

High temperature tensile properties and deformation behavior of CoCrFeNiW0.2 high entropy alloy

High entropy alloys (HEAs) have been a new-type of structural materials, which show good performance in various extreme conditions. In the present study, we systematically investigated the high temperature deformation behavior of as-cast CoCrFeNiW0.2 HEA under different temperatures from 298 to 1173 K and various strain rate of 1 × 10−4 s−1 to 5 × 10−3 s−1. An obvious temperature dependence of yield strength can be found with a remarkable decrease from 362 MPa at 298 K to 119 MPa at 1173 K. The normal Portevin–Le Chatelier (PLC) effect was presented in the intermediate temperature range of 673 K to 1073 K, the serrated flow transformed in the sequence of A → A + B → A + C → B + C → C with the increasing temperature, which is caused by dynamic strain aging (DSA). W atoms played an important role in the interaction between solute atoms and dislocations. Fractography analysis suggests the failure mode transition from transgranular fracture to intergranular fracture. Electron backscatter diffraction (EBSD) demonstrates that the high temperature deformation mechanism of the HEA is dominated by dynamic recovery (DRV). Additionally, transmission electron microscopy (TEM) was performed to observe the dislocation configurations and the interaction between dislocations and μ phase precipitates at various temperatures, and these results further identify the high temperature deformation mechanism. To summarize, this study indicates the proposed HEA has excellent high temperature mechanical properties and a wide range of potential applications in many fields.

Boron and Nitrogen Doping in Fused Silica Ceramics: Structural, High-Temperature Mechanical and Long-Term Ablation Resistance Properties of Si-B-O-N Ceramics

Boron and Nitrogen Doping in Fused Silica Ceramics: Structural, High-Temperature Mechanical and Long-Term Ablation Resistance Properties of Si-B-O-N Ceramics

High-temperature mechanical properties and microstructure of 2.5D C/C–SiC composites applied for the brake disc of high-speed train

This paper uses chemical vapor infiltration method to prepare 2.5D C/C-SiC composites, and conducts various tests to reveal its high-temperature mechanical properties and failure mechanism. The results show that the test temperature significantly affects the mechanical properties. As the test temperature increased from room temperature to 800 °C, the tensile strength of the 2.5D C/C-SiC composite dropped from 127.67 ± 20.87 MPa to 94.74 ± 28.15 MPa, and the performance dropped by 25.79 %. The compressive strength dropped from 326.92±17.37 MPa to 192.31 ± 29.15 MPa, resulting in a performance decrease of 41.18 %. The bending strength dropped from 355.74 ± 40.80 MPa to 235.88 ± 23.52 MPa, and the performance dropped by 33.69 %; the interlaminar shear strength dropped from 9.77 ± 2.79 MPa to 7.80 ± 3.26 MPa, and the performance dropped by 20.16 %. High-temperature oxidation will lead to interface degradation, destroy the composite matrix and carbon fiber structure, change the interface bonding strength, and ultimately lead to a decline in composite performance.

Effect of deep cryogenic treatment on improving the high-temperature impact resistance of Al–Zn–Mg–Cu alloy

The aging Al–Zn–Mg–Cu alloy exhibits a significant decrease in performance after undergoing thermal shock. In order to improve the resistance of this type of alloy to thermal shocks, the influence of deep cryogenic treatment (DCT) on the high temperature mechanical properties and microstructure of thermal shocked Al–Zn–Mg–Cu alloy is investigated in this paper. It was found that the mechanical properties of Al–Zn–Mg–Cu alloys decrease significantly with the increase of electric heating temperature. The deep cryogenic treatment reduced the grain size of Al–Zn–Mg–Cu alloy, increased the density of the precipitates, and also inhibited the coarsening of the precipitates. Therefore, the deep cryogenic treatment can slow down the weakening effect of Al–Zn–Mg–Cu alloy at high temperatures.

Effects of normalizing and tempering temperature on the bainite microstructure and properties of low alloy fire-resistant steel bars

Abstract Different combinations of normalizing and tempering were carried out to optimize the microstructure and enhance the high-temperature mechanical properties of HRB400FR fire-resistant steel bars. The results showed that with the increasing of the tempering temperature from 400 to 600°C, the steel bar’s hardness decreases linearly, mainly due to the formation of quasi-polygon ferrite and granular bainite. Besides, the reduced width and the dissolution of the lath bainite also undermine the performance of the tempered steel bars. The highest Vickers hardness of 380 HV is achieved when the steel is normalized at 950°C and then tempered at 400°C, mainly due to precipitation strengthening and bainite strengthening. The hardness of the test steel tempered at 600°C gives the lowest value, only 230 HV since the least amount of bainite is obtained. When the tempering temperature reaches 650°C, the hardness rises to 260 HV due to the formation of the lath bainite. The emergence of needle bainite generally reduces the matrix grain size, and the appearance of lath martensite refines the precipitated carbides.

Effect of Nd on Microstructure and High-Temperature Mechanical Properties of As-cast Mg–Y–Nd–Zn–Zr Alloy

Effect of Nd on Microstructure and High-Temperature Mechanical Properties of As-cast Mg–Y–Nd–Zn–Zr Alloy