High Temperature Creep Properties Research Articles

Investigation on high-temperature creep property and mechanism of AA6111 matrix composites reinforced by in-situ ZrB2 nanoparticles

Investigation on high-temperature creep property and mechanism of AA6111 matrix composites reinforced by in-situ ZrB2 nanoparticles

Enhancing the high-temperature creep properties of Mo alloys via nanosized La2O3 particle addition

Molybdenum (Mo) alloys with different La 2 O 3 particle additions (0.6, 0.9, 1.5 wt.%) were prepared by powder metallurgy to investigate the effect of La 2 O 3 particles on microstructural evolution and creep behavior of the alloy. Pure Mo, annealed at 1500 °C for 1 h, presented a fully recrystallized microstructure characterized by equiaxed grains. The alloys doped with La 2 O 3 particles (Mo-La 2 O 3 alloys), on the other hand, exhibited fibrous grains elongated in the rolling direction of the plate. In contrast to the shape of the grains, the average grain size of the alloys was found to be insensitive to the addition of La 2 O 3 particles. Nanosized La 2 O 3 particles with diameters ranging from 65 nm to 75 nm were distributed within the grain interior. Tensile creep tests showed that dislocation creep was the predominant deformation mode at intermediate creep rate (10 −7 s −1 -10 −4 s −1 ) in the present alloys. The creep stress exponent and activation energy were found to decrease with increasing temperature, particularly within the low creep rate regime (<10 −7 s −1 ). The Mo-La 2 O 3 alloys exhibited remarkably greater apparent stress exponent and activation energy than pure Mo. A creep constitutive model based on the interaction between particles and dislocations was utilized to rationalize the nanoparticle-improved creep behavior. It was demonstrated that low relaxed efficiency of dislocation line energy, which is responsible for an enhanced climb resistance of dislocations, is the major creep strengthening mechanism in the Mo-La 2 O 3 alloys. In addition, the area reduction and creep fracture mode of the Mo-La 2 O 3 alloys were found to be a function of the creep rate and temperature, which can be explained by the effect of the two parameters on the creep and fracture mechanisms.

Directional recrystallization of an additively manufactured Ni-base superalloy

Metal additive manufacturing processes can create intricate components that are difficult to form with conventional processing methods; however, the as-printed materials often have fine grain structures that result in poor high-temperature creep properties, especially compared to directionally solidified materials. Here, we address this limitation in an exemplary additively manufactured Ni-base superalloy, AM IN738LC, by converting the fine as-printed grain structure to a coarse columnar one via directional recrystallization. The directional recrystallization behaviors of AM IN738LC were characterized through a parameter study in which the peak temperature and draw rate were each independently varied. Recrystallization began when the peak temperature was higher than the γ′ solvus of 1183 °C. Varying the draw rate from 1 to 100 mm/hr while maintaining a fixed peak temperature of 1235 °C and a thermal gradient of order 10 5 °C/m ahead of the hot zone showed that a draw rate of 2.5 mm/hr maximized the grain size, giving a mean longitudinal grain size of 650 µm. Specimens processed under these optimal conditions also inherited the 〈100〉 fiber texture of the as-printed material. Close inspection of a quenched specimen revealed Zener pinning of the longitudinal grain boundaries by MC carbides and a discrete primary recrystallization front whose position followed the γ′ solvus isotherm. The present results demonstrate for the first time how directional recrystallization of additively manufactured Ni-base superalloys can achieve large columnar grains, manipulate crystallographic texture to minimize thermal stresses expected in service, and functionally grade the grain structure to selectively enhance fatigue or creep performance.

Overview on performance degradation behavior of 20Cr25NiNb steel for gas cooled reactor cladding during service

In this paper, the performance degradation behavior of fuel cladding material 20Cr25NiNb for the British Advanced Gas Reactor (AGR) is reviewed in detail, which is a strong guideline for the material selection of supercritical carbon dioxide cooled reactors. The degradation behavior during in-core service mainly includes high-temperature creep, thermal aging and mechanical property degradation caused by neutron irradiation (fission gas products, helium embrittlement and irradiation sensitization) and CO2 (oxidation and carburizing). Long-term service in AGR leads to coarsening of the second phase and precipitation of harmful phases such as σ, leading to performance degradation of the cladding. A point that should require special attention is that intergranular stress corrosion cracking (IGSCC) and intergranular attack (IGA) problems occur during wet storage of spent fuel.

Thermally Stable Nanotwins: New Heights for Cu Mechanics.

Nanocrystalline and nanotwinned materials achieve exceptional strengths through small grain sizes. Due to large areas of crystal interfaces, they are highly susceptible to grain growth and creep deformation, even at ambient temperatures. Here, ultrahigh strength nanotwinned copper microstructures have been stabilized against high temperature exposure while largely retaining electrical conductivity. By incorporating less than 1vol% insoluble tungsten nanoparticles by a novel hybrid deposition method, both the ease of formation and the high temperature stability of nanotwins are dramatically enhanced up to at least 400°C. By avoiding grain coarsening, improved high temperature creep properties arise as the coherent twin boundaries are poor diffusion paths, while some size-based nanotwin strengthening is retained. Such microstructures hold promise for more robust microchip interconnects and stronger electric motor components.

Improved mechanical performances of dynamically solidified Mg97.7Y1.4Al0.9 alloy by three dimensional ultrasounds

Three dimensional (3D) ultrasounds with 20 kHz frequency and 14–22 μm amplitudes were applied to the solidification process of Mg97.7Y1.4Al0.9 alloy. With the increase of ultrasonic dimension and amplitude, both the primary Al2Y particles and the principal α-Mg phase were significantly refined in combination with an increased proportion of Al2Y/α-Mg parallel growth interfaces, while the eutectic Al2Y phase changed from long strip to fine granular shape. The dynamic solidification mechanism was further explained on the basis of in-situ acoustic spectrum measurements. It was found that once tiny primary Al2Y and α-Mg nuclei formed, they immediately strengthened acoustic field by facilitating bubble formation at their solid/liquid interfaces, and the subsequent bubble oscillation and collapsing in turn promoted their further nucleation by improving wettability and increasing local undercooling. The ultrasonically solidified alloy showed superior mechanical performances as compared with static solidification structures. The maximum tensile strength and plasticity achieved 271 MPa and 12.8% which were 1.41 and 1.36 times higher than static values, and the high temperature creep properties were enhanced as well.

Anti-corrosion effect of insulating firebrick coated with CA6 in the calcination of lithium-ion cathode materials

The anti-corrosion properties of calcium hexaluminate (CA6) during the roasting of Li(NixCoyMnz)O2 (LNCM) materials have been investigated by means of specific experimental setups and test methods to verify its feasibility as an adiabatic insulation material for the roasting kiln of LNCM materials. Mullite insulating firebrick currently in industrial use was coated with CA6 (labelled M-CA6) and corrosion tests were carried out. The corrosion tests were carried out at different additions of corrosion sources, roasting temperatures, number of cycles of corrosion experiments. And the materials before and after corrosion characterized by the X-ray diffraction and scanning electron microscopy to determine the influence of different corrosion conditions on the interactions between M-CA6 and LNCM materials. In addition, The corrosion resistance of calcium CA6 and the feasibility of its application in practical industry was identified by characterising and comparing the high-temperature creep properties of the materials. The results of the laboratory scale experiments confirmed that M-CA6 was more resistant to LNCM than mullite insulating firebrick, and confirmed the feasibility and superiority of using CA6 as a lining insulation for LNCN kilns.

Enhanced high temperature tensile and creep properties of a β-solidified γ-TiAl alloy with the hybrid addition of C and Y2O3

β-solidified γ-TiAl alloys are popular lightweight structural materials well suited for high temperature applications, however, whose service temperature limit is greatly restricted due to the insufficient high temperature strength and creep resistance. In present work, a hybrid reinforced β-solidified γ-TiAl alloy was successfully developed with the hybrid addition of C and Y2O3 by induction skull melting (ISM) technique, accordingly, the microstructure, high temperature tensile and creep properties of the alloy are detailly discussed. The results indicate that the hybrid addition of C and Y2O3 can improve the high temperature tensile strength of this nearly lamellar TiAl alloy with the increase of about 13.7%. More importantly, creep resistance of this alloy is significantly enhanced, which is attributed to multiple strengthening mechanisms: (ⅰ) the second phase strengthening from multi-scale Y2O3 particles and dynamically precipitated H–Ti2AlC carbides; (ⅱ) mechanical twins and twin intersections can act as special obstacles to dislocation movement, which is of great significance in decreasing the steady-state creep rate. Particularly, this alloy exhibits excellent microstructure stability during creep. On the one hand, nano-scale Y2O3 and dynamically precipitated H–Ti2AlC particles could pin the B2/γ interface to strengthen lamellar colony boundary. On the other hand, dispersed B2 phase particles could precipitate along α2 lamellae and grow up with the consumption of the α2 phase, which will inhibit the degradation of α2/γ lamellae. This study offers a new composition design approach to optimize high temperature performance of β-solidified γ-TiAl alloys.

Effects of hybrid reinforcements to the high temperature tensile and creep properties of a (TiB + TiC + Y2O3)/α-Ti composite

A (TiB + TiC + Y2O3)/α-Ti composite and the corresponding matrix alloy were prepared by induction skull melting. The tensile properties and creep behaviors of the matrix alloy and the composite at 700 °C were systematically studied. The results showed that the reinforcements provided the nucleation position for the α phase, which changed the microstructure from the widmanstatten structure to the basket-weave structure. Thanks to the weakening of grain boundary α phase and the load transfer effect of reinforcements, the UTS of the composite at 700 °C increased from 502 MPa to 696 MPa. High temperature and stress in creep process promoted the dynamic recrystallization and dissolution behavior. Spheroidized β-Ti phases made the α/β interfaces lose the limiting effect on dislocation, which was the reason for the low creep life of the matrix alloy. In the process of creep, a large number of dislocations accumulated around the reinforcements through reinforcement-dislocation interactions. The reinforcements with good bonding with the matrix contributed to a good strengthening effect and the creep life of the composite at 700 °C/150 MPa was increased from 9.9 h to 74.9 h. Spheroidized TiC particles separated from the TiB whiskers and promoted the formation of equiaxed-like α phases.

High-temperature creep properties of 9Cr-ODS tempered martensitic steel and quantitative correlation with its nanometer-scale structure

ABSTRACT The Japan Atomic Energy Agency (JAEA) has been developing 9Cr-oxide dispersion strengthened (ODS) tempered martensitic steel (TMS) as a candidate material for use in the fuel cladding tubes of sodium-cooled fast reactors (SFRs). The creep property is essential for the fuel cladding tube of SFR; the reliable prediction of in-reactor creep-rupture strength is critical for implementing the 9Cr-ODS TMS cladding tube in the SFR. This study investigated the quantitative correlation between the creep properties of 9Cr-ODS TMS at 700°C and the dispersions of nanosized oxides by analyzing the creep data and the material’s nanostructure. The possibility of deriving a formula for estimating the in-reactor creep properties of 9Cr-ODS TMSs based on an analysis of the nanostructure of neutron-irradiated 9Cr-ODS TMSs was also discussed. The creep properties of 9Cr-ODS TMS at 700°C closely correlated with the dispersion of nanosized oxide particles. The correlation between creep-rupture lives and nanosized oxide particle dispersion in 9Cr-ODS TMS was determined using existing creep models. The elucidation of correlation between the stress exponent of secondary creep rate and the nanostructure is essential to enhance future modeling reliability and formulation.

Study of the creep cavitation behavior of P91 steel under different stress states and its effect on high-temperature creep properties

In this study, circumferential U-notches of different depths were machined on a plain cylindrical creep specimen to simulate different stress states during steam pipe operation. The effects of stress concentration on the creep cavitation behavior and high-temperature creep properties of P91 steel were investigated through creep experiments and microstructure analysis, as well as measurement and analysis of scanning electron microscopy (SEM) micrographs using ImageJ to obtain the equivalent diameter, circularity, area and distribution of the voids after high-temperature creep of P91 steel. In the analysis of 98 notched samples of 14 bars, it was found that a large number of voids with diameters of 0.78–3.56 μm existed at the notch root, the voids were distributed in a fan shape near the notch root, the size of the distribution area was related to the stress, and the larger the stress was, the larger the area. The number, density and average area of voids increased with stress and time. The stress level at the notch root was found to be positively correlated with the area of damage caused by the voids in the region and negatively correlated with the number of voids based on the FE analysis. By analyzing the experimental data, it is found that the circularity is closely related to the void diameter, and the void coalescence occurs when the void diameter reaches a critical value, which varies with temperature, stress and time.

Towards creep property improvement of selective laser melted Ni-based superalloy IN738LC

• SLM IN738LC superalloy was treated with unconventional two-step heating schemes. • The 950 °C first step was found the most effective in redistributing the carbides. • Carbide redistribution significantly reduced grain boundary pinning and promoted grain growth. • Larger grain size led to 56% to 339% increments in creep life depending on creep strength. Nickel-based superalloy IN738LC produced by selective laser melting (SLM) exhibits inferior high-temperature creep properties than its cast counterparts due to relatively smaller grain size, particularly for the plane normal to the building direction. This work studied effects of post heating strategy on the microstructure and especially the grain size to improve the high temperature creep resistance. The as-built microstructure exhibited a fine grain size and large quantities of M C carbides that could effectively hinder grain growth. It was found that unconventional two-step heat treatments could lead to substantial grain growth, and the effect is particularly prominent at a specific temperature. The ease of grain growth was explained after classifying the microstructural evolution (boundary carbide transformation) during each heating step and related to the reduced grain boundary pinning force from M C carbides. Creep tests validated the effect of the new heat treatment scheme on the SLM-processed IN738LC at 850 °C. An extended creep fracture life (1.5 to 4 times improvement) and lower secondary creep rates were achieved with samples subjected to the newly optimized two-step heat treatment. The complete creep curves are also firstly presented for SLM-IN738LC, confirming the effectiveness of grain growth and highlighting the importance of dedicated heat treatment for SLM superalloys.

High-temperature creep property deterioration of the alumina-forming austenitic steel: Effect of σ phase

The microstructural evolution of the alumina-forming austenitic (AFA) steel subjected to different cold rolling reductions during creep test at 973 K and 120 MPa was investigated by multiple characterization methods, and the high temperature creep property was also examined. It was found that the creep property of CR10 sample is not satisfactory although cold rolling truly improved it. The creep cavities were observed to nucleate at the interface between B2–NiAl phase and σ phase in the δ-ferrite. The serious coarsening of B2–NiAl phase and σ phase in δ-ferrite was considered as the main factors for the decline of creep property. In addition, the results also suggested that the shape of B2–NiAl phase changes from round to plate with increasing creep time, which resulted from the increase of elastic strain energy. Simultaneously, it was noteworthy that σ phase always grows adjacent to plate-shape B2–NiAl phase, which indicates the coarsening of σ phase is inseparable from that of B2–NiAl phase.

Laser additive manufacturing and post-heat treatment on microstructure and mechanical properties of 9Cr steel

9Cr steel is a material that has been widely used in pressure vessel parts in thermal power plants and nuclear power plants, and has good high-temperature creep properties. Laser Melting Deposition (LMD) is a promising method for preparing complex 9Cr steel components. It provides a rare opportunity to improve existing designs and produce fine features and complex geometries with higher efficiency. The LMD-9Cr steel sample has high density, the maximum tensile strength of the sample is 1057.75 MPa, which is much higher than the standard cast 9Cr steel of 650 MPa. We use 760 °C tempering heat treatment, after heat treatment, the average grain size of the material is reduced, the Charpy impact performance is improved, and the tensile strength and microhardness are slightly reduced. Although the tempering heat treatment greatly reduces the average grain size of the sample by 35.59%, but at the same time the tempering heat treatment greatly reduces the high dislocation density of lath martensite, and the supersaturation behavior of Cr, W and C elements weakens the effect of solid solution strengthening. In addition, through the nanoindentation test, we found that although the M23C6 precipitated phase can harden the material, at the micro level, the elastic modulus and nano-hardness of the precipitated phase are lower than that of the homogeneous phase.

The microstructure and tensile property changes of S31042 steel after long time service

Abstract Ultra-supercritical power generation technology is one of the main solutions to the many contradictory problems of carbon emissions and environmental protection in coal-fired power plants. S31042 austenitic heat-resistant steel has become a typical material for high-temperature components used in ultra-supercritical units because of its good high-temperature creep properties and high-temperature corrosion resistance. In this study, typical characterization techniques such as SEM (Scanning Electron Microscopy), TEM (transmission electron microscopy), and EBSD (Electron Backscattered Diffraction) were used to observe the microstructure of S31042 austenitic heat-resistant steel, while the room-temperature tensile and high-temperature tensile properties of the material were tested under the corresponding conditions. The results show that the grain of S31042 heat-resistant steel after service is the same as that of the non-service samples, which is still twinned austenite. The room-temperature tensile properties and high-temperature tensile properties of S31042 after long service were almost unchanged.

Corrosion of Li-ion battery cathode materials on mullite insulation materials during calcination

A specially designed experimental device was used in laboratory to investigate the corrosion of mullite during the calcination of Li(NixCoyMnz)O2 (LNCM) materials. The anti-corrosion tests were carried out at 1000, 1100, 1200 and 1300 °C, and characterized with X-ray diffraction and scanning electron microscopy. The influence of temperature on the interactions between mullite insulation materials and LNCM materials was determined. In addition, the high-temperature creep properties of the mullite insulation materials before and after corrosion were tested. The laboratory scale tests, thermodynamic and kinetic calculations allowed a more comprehensive understanding of the evolution of the mullite insulation materials during serving for the roasting process of LNCM materials. Through this research, it is suggested that the upgrading of the kiln lining in the lithium battery industry should select materials with excellent resistance to alkali corrosion, especially excellent resistance to Li+ corrosion.

High temperature tensile and creep properties of CrMnFeCoNi and CrFeCoNi high-entropy alloys

High temperature tensile and creep properties of face-centered cubic (FCC) high-entropy alloys (HEAs), CrMnFeCoNi and CrFeCoNi, were evaluated at 500–700 and 650–725 °C, respectively, to evaluate high temperature structural integrity of the most well-known FCC HEAs with special attention on (i) sigma phase formation in CrMnFeCoNi and (ii) difference in solid solution strengthening between the two alloys in the temperature range where commercial heat-resistant wrought alloys including austenitic heat-resistant steels and Ni-based superalloys are used. No remarkable difference was detected in tensile behavior measured both at room and high temperatures between the two alloys. However, creep rupture life turned out to be significantly longer for the CrFeCoNi quaternary alloy. On top of the longer creep rupture life, CrFeCoNi alloy is characterized by lower minimum creep rate and larger creep activation barrier energy which could be attributed to higher level of lattice distortion of CrFeCoNi than CrMnFeCoNi resulting in higher solid solution strengthening. Also, grain boundary strength of CrMnFeCoNi was compromised by the formation of sigma phase during creep deformation which led to lower elongation especially for long-term creep conditions. Therefore, the longer creep life of CrFeCoNi is mainly attributed to the combination of enhanced solid solution strengthening and stronger grain boundary which is free from the deleterious sigma phase due to the reduced thermodynamic driving force for formation of intermetallic phase.

Micromechanical analysis and theoretical predictions towards thermal shock resistance of HfO2-Si environmental barrier coatings

As HfO2-doped Si bond coat acts a pivotal part in application of progressive environmental barrier coating (EBC) system owing to improving the high-temperature capability and creep properties of bond coat, it is essential to control the doping content and to balance the mechanical and thermal properties of doped coatings challenge in terms of upgrading service life and operating temperature of EBC system. In this study, the physical properties and thermal shock performance at 1723 K of HfO2-Si composite coatings with three different compositions were investigated via first-principle calculations and micromechanical model. Evidently, the composite coatings deposited by magnetron sputtering exhibit a better resistance towards crack initiation and propagation than pure Si bond coat up to 100 h thermal cycling time. Such improvement is attributed to the dispersion toughening effect caused by the characteristic structure of dispersed HfSiO4 particle within cristobalite “mortar”. With thermal cycling, the continuous Ostwald ripening of HfSiO4 particles decreased the elastic modulus and effective thermal expansion coefficient gap at room and high temperature of composite coatings, which was advantageous to thermal shock resistance. Unfortunately, as a consequence of excessive volume expansion during particle ripening, the composite coatings encountered varying degrees of interface separation after 100 cycles and hence not protective anymore. The results suggested that the further modification for low-frequency vibration patterns of HfSiO4 and realizing the interfacial metallurgical bonding can reduce the thermal expansion coefficient of HfO2-doped Si composite coatings and strengthen interface adhesion.

Ultra-High Temperature Creep of Ni-Based SX Superalloys at 1250 °C

Very high temperature creep properties of twelve different Ni-based single crystal superalloys have been investigated at 1250 °C and under different initial applied stresses. The creep strength at this temperature is mainly controlled by the remaining γ′ volume fraction. Other parameters such as the γ′ precipitate after microstructure evolution and the γ/γ′ lattice parameter mismatch seem to affect the creep strength to a lesser degree in these conditions. The Norton Law creep exponent lies in the range 6–9 for most of the alloys studied, suggesting that dislocation glide and climb are the rate limiting deformation mechanisms. Damage mechanisms in these extreme conditions comprise creep strain accumulation leading to pronounced necking and to recrystallization in the most severely deformed sections of the specimens.

Microstructural features and mechanical properties of similar and dissimilar ferritic welded joints for ultra-supercritical power plants

The grade P91 and P92 steel are commonly used materials for the components in ultra-supercritical power plants in the temperature range of 590–650 °C. For welded joints, the notch toughness and microstructure stability played an essential role in meeting the requirement of high temperature creep properties. The microstructural stability and notch toughness were strongly affected by the composition of base and filler materials, welding processes and pre and post-weld heat treatments (PWHTs). In the present research, four different combinations of similar/dissimilar welded joints of P91 and P92 steel are prepared using shielded metal arc welding (SMAW). The microstructure of different welded joints has been characterized in the as-welded condition. For the as-welded state, the room temperature tensile properties, flexural strength, microhardness and notch toughness have been studied. The δ-ferrite patches were observed for P92 filler welded joints. It has been concluded that in the weld fusion zone, the higher amount of ferrite stabilizers promotes the formation of the δ ferrite. The tensile strength and flexural strength were increased in P92 filler welds due to the presence of solid solution hardening elements in the P92 filler. The detrimental effect of δ-ferrite patches has been observed on the Charpy impact toughness and microhardness values of the weld fusion zone. The Charpy toughness values in the weld fusion zone were 12 ± 2, 7 ± 2, 8 ± 2 and 5 ± 3 J for P91-P91-P91, P92-P92-P92, P91-P91-P92 and P91-P92-P92 weld joints, respectively, which were lower than the minimum recommended Charpy toughness value (47 J) for ferritic steel welded joints.