Rupture Life Research Articles

Co content dependence on the microstructure characteristic and creep performance at elevated temperature in Ru-containing single crystal superalloys

The composition design and optimization of Ru-containing fourth generation single crystal superalloys, which are currently considered as the most advanced and practical materials for manufacturing high-pressure turbine blades of aero-engines, have been in increasing and urgent demands. In this work, significant adjustments to the pivotal alloying element Co were made and the synthetic effect of Co on the microstructure as well as high-temperature creep property in Ru-containing alloys were systematically evaluated. The results illustrated that the size of γ′ phase was inversely proportional to the Co content but the volume fraction of γ′ precipitates stayed the same in different alloys. The TCP phases were more likely to precipitate in alloys with lower or extremely high Co addition during long-term aging, while the morphology of these particles transformed from long needle-like to short rod-like or blocky as Co content increased. It was noted that the element diffusion as well as the resultant coarsening and rafting of γ/γ′ structures were favorably facilitated by the increasing Co addition. The alloy with 12 wt% Co exhibited the longest stress rupture life at 1140 °C/137 MPa, regardless of the moderate precipitation of blocky TCP particles during creep. Moreover, the TCP phases in different alloys were analyzed to be typical μ phase but the specific element distributions varied with Co addition. Moreover, plenty of 2nd γ′ precipitates were observed after creep rupture, while their nucleation and precipitation could be affected by the Co addition in alloys. Ultimately, the comprehensive Co-effect was discussed and the optimum Co content was determined, in order to provide further guidance in the design and development of Ru-containing single crystal superalloys.

Roles definition of TiN particles in enhancing the creep properties of IN718C high temperature alloys

In this study, a TiN/IN718C composite was prepared by employing spark plasma sintering and induction melting methods via the addition of TiN particles to an IN718C superalloy. The creep behavior of IN718C and TiN/IN718C at 704 °C under 500 MPa was comprehensively investigated. The creep test showed that the rupture life of TiN/IN718C was 421 h, which is approximately 1.7 times longer than that of IN718C. The steady-state creep rate of TiN/IN718C (2.81 ×10−9 s−1) was one order of magnitude lower than that of IN718C. The enhanced creep behavior was attributed to the microstructure of TiN/IN718C. During casting, the TiN particles could act as an inoculant to slightly refine the Ni matrix grains. TiN particles reduced the formation of detrimental phases such as Laves and δ, facilitated the precipitation of carbides, and refined the γ′′ phases in the IN718C matrix. In addition, the transmission electron microscopy results indicated a strong adhesive interface between TiN and the Ni matrix. Therefore, the synergetic strengthening effect of the dispersed TiN reinforcements and the refined microstructure of the IN718C matrix significantly enhanced the creep behavior of the TiN/IN718C composite.

Influence of thermal exposure on microstructure and stress rupture property of a Re-containing Ni-based single crystal superalloy

In this study, the microstructural evolution and microstructure-property relationship of a Re-containing single crystal superalloy after long-term thermal exposure for up to 20,000 h at 900 °C have been investigated. The results showed that the microstructure of the experimental alloy significantly degraded after thermal exposure, including the coarsening of γʹ precipitates, the decrease of γʹ volume fraction and the morphology evolution of γʹ precipitates from cubic to spherical. The magnitude of γ/γʹ misfits decreased obviously and then remained stable. The rupture life of the alloy at 900 °C/330 MPa dropped dramatically after thermal exposure for 2000 h, which was mainly caused by the obvious reduction of γʹ volume fraction, the decreased magnitude of γ/γʹ misfits and the coarsening of γʹ precipitates. The rupture life of the alloy decreased slowly from 2000 h to 20,000 h, although γʹ size continued to increase from 680 nm to 1648 nm. The primary reason for this period was the unchanged γʹ volume fraction and the lower magnitude of γ/γʹ misfits.

Experiencing Change: Rhythms of Everyday Life Between Continuities and Disruptions


 
 
 Change is a constant condition of everyday life that we experience and transition through while often maintaining a sense of stability and continuity. But inevitably we come across disruptive changes that call into question the meanings we take for granted and thereby rupture life as we know it. How do those changes affect our rhythms of living? How do we make meaning of the changes and subsequently act upon them? How do individual, social, and environmental changes reciprocally influence one another? These are the guiding questions of this paper. The questions are explored by means of a sociocultural psychological approach to ruptures in the life-course coupled with Lefebvre’s rhythmanalysis. It is argued that those questions can be investigated within five interrelated analytical domains; time, space, the body, social others, and symbolic resources. Rather than primarily emphasizing adaptation to change, the analytical framework’s key focus is meaning-making, looking at how we integrate or resist new rhythms in our lives.
 
 

Use of a stepped preparation to improve the damage tolerance of grade 91 steel welds

ABSTRACT Damage equivalent to that seen over very long times in Grade 91 steel welded components was successfully formed in laboratory, feature sized samples tested under controlled conditions. This paper compares the creep behaviour of welds made using a typical preparation with results for novel step weld geometries. When well-designed the step geometry offsets the heat affected zone (HAZ) in the through thickness of the weld which would be equivalent to the pressure boundary in a pipe. The step in the weld profile thus provides benefit to in-service performance because it prevents the rapid link up of aligned creep cavitation that forms in the HAZ of welds made using typical preparations. The results demonstrate that a well-designed step weld geometry increases the creep rupture life and improves damage tolerance. In turn, the increased duration of stable crack growth promotes the opportunity for detection of creep damage using nondestructive testing methods.

Effects of vacuum on gas content, oxide inclusions and mechanical properties of Ni-based superalloy using electron beam button and synchrotron diffraction

The effects of vacuum induction melting (VIM) vacuum (<1 Pa–100 Pa) on gas content, oxide inclusions and mechanical properties of Ni-based superalloy K4648 has been investigated by electron beam (EB) button experiment under high vacuum (10-3 Pa) and high resolution synchrotron X-ray powder diffraction (SXPD). The results indicated that VIM remelting vacuum drop has obvious effect on the existing form of trace oxygen. The total amount of oxygen did not increase significantly but a dramatic increase in the amount of oxide inclusions by 1–2 orders of magnitude was found. The inclusions are mainly oxides including Al2O3, Cr2O3, NiAl2O4 and complex oxides or sulfides. Remelting under 100–110 Pa has no significant effect on mechanical properties such as stress rupture life and tensile strength but decreased ductility obviously. In comparison to the normal vacuum counterpart, the tensile elongation and impact ductility of the alloy remelted under lower vacuum level decreased by 67% and 39%, respectively. This study reveals the relationship between the vacuum level and mechanical properties of superalloys and highlights the trace amount of oxide inclusions which should be considered as one of the key issues for the cleanliness of virgin or revert superalloys apart from the typical measurement of the gas content only.

Creep behavior of nuclear grade 316LN austenitic stainless steel at 873 K and 923 K

Creep deformation and rupture behavior of nitrogen-alloyed (0.14 wt.%) nuclear grade 316LN austenitic stainless steel were investigated for the varying stress levels at 873 K and 923 K. The power-law dependency of creep properties such as steady-state creep rate and rupture life on applied stress was observed. For a given applied stress condition, a systematic increase in strain to failure was noticed with increasing temperature from 873 K to 923 K. Irrespective of test temperatures, creep rupture elongation of the steel increased with the increase in rupture lifetime ( $t_{r}$ ) for $t_{r} > 1000$ h. Analysis indicated that the interdependency between creep properties could be well described by the modified Monkman-Grant relationship. The predominance of inter-granular fracture arising from the triple point cracks and/or coalescence of cavities was observed at all the tested conditions for the steel. The enhanced tendency for wedge cracking was noticed for high stress levels at 873 K and 923 K. The evaluated damage tolerance factor $(\lambda ) < 5$ and the calculated ratio between time to reach the Monkman-Grant strain and creep rupture lifetime in the range of 0.69 to 0.80 indicated the accumulation of Monkman-Grant strain for the major fraction of lifetime during creep deformation of 316LN steel.

Effect of the withdrawal rate on the microstructure and thermal durability of a third-generation single crystal superalloy

Single crystal Ni-based superalloys are the typical structural materials for high-pressure turbine blades, and their microstructure is critical in determining their mechanical properties. The withdrawal rate is a key parameter affecting the microstructure during the single crystal growth process. In the present work the effect of the withdrawal rate on the microstructure of a third-generation single crystal superalloy containing 6.8 ​wt% Re has been investigated, and the creep resistance of the alloy determined. The results showed that increased withdrawal rate refined the dendritic structure, reduced dendritic arm spacing, promoted the growth of secondary tertiary dendrites and decreased solidification segregation with a reduced size of γ′ phase. The porosity density of the as-cast alloy first decreased and then increased with the withdrawal rate, while the minimum porosity densityoccurred when the alloy was under the solidification condition of withdrawal rate of 4.5 ​mm/min. The maximum creep rupture life of 326.4 ​h of the heat-treated alloys under the test condition of 1100 ​°C/140 ​MPa also appeared at the alloys under the withdrawal rates of 4.5 ​mm/min. It is believed that the minimum porosity density and reduced size of the γ′ phase may be the main reasons for the enhanced creep rupture life of the alloys with withdrawal rates of 4.5 ​mm/min. This investigation provides theoretical support and a practical basis for the development of third-generation single crystal superalloys.

The creep strength of aberrant grade 91 steel

ABSTRACT The high-temperature steel grade 91 is intended to operate in service with a fully martensitic microstructure, achieved by a well-defined heat treatment. Unfortunately, lack of control, either during the initial material manufacturing or later during post-weld heat treatment, can lead to components entering service in a substandard condition. In the worst cases, this can be a fully ferritic microstructure with a complete absence of martensite. In this aberrant condition, the steel can have a creep strength significantly lower than that of normal martensitic grade 91 with an associated risk of early failure in plant life. This paper reports on investigations into the creep strength of aberrant grade 91,,using both conventional uniaxial rupture testing and small-scale impression creep testing. The creep strength has been found to fall into the range mean-35% to mean-50% in terms of stress relative to the most recent (2019) ECCC rupture life assessment of grade 91.

Inconsistent creep between dendrite core and interdendritic region under different degrees of elemental inhomogeneity in nickel-based single crystal superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa. Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared. For specimen with higher inhomogeneity, stronger segregation of refractory elements reinforces the local strength in dendrite core, but damages the strength in interdendritic region. Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing in γ′ phase, faster degradation of dislocation networks and facilitated topological inversion of rated structures. Although the segregation of refractory elements produces a high density of topologically close-packed (TCP) phase in dendrite core, faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen. In another specimen, increased solid solution time gives rise to overall reduced inhomogeneity. Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region. The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity. Importantly, the balance of local strength between dendrite core and interdendritic region results in over 40% increase of creep rupture life of the second specimen.

Microstructural Evolution and Creep Behavior of the Weld Interface between 10% Cr Steel and Haynes 282 Filler Metal

This study investigated the microstructural evolution of the weld interface and creep fracture behavior of 10% Cr martensitic steel welds using Haynes 282 filler metal. The welded joints were subjected to post-weld heat treatment (PWHT) at temperatures of 738 °C for 4, 8, and 15 h. Creep tests were carried out at 600 °C under stress of 200 MPa. The creep rupture life increased with an increase in holding time for PWHT up to 8 h compared to the as-welded condition. However, when the holding time for PWHT was further increased to 15 h, creep properties (i.e., rupture life and creep strain) decreased considerably, and the failure location was found to shift from the ICHAZ to the weld interface. The microstructural investigation revealed that Type I carbides precipitated at the weld interface and recrystallized grains with Cr depletion were formed near Type I carbides in the partially mixed zone during creep exposure. The creep failure at the weld interface began on the surface and propagated inward through the recrystallized grains of the weld-interface region.

Characterization of thermal aging effects on the creep performance of T91 using small punch creep testing

AbstractThis paper is concerned with the development and application of a small punch creep testing method for assessment of the effects of thermal aging and associated precipitate coarsening on the high temperature creep life and deformation behavior of a 9Cr ferritic–martensitic steel. The tests are conducted on P91 steel, for both aged (up to 1 year) and unaged specimens, at temperatures of 600°C. The unaged test results are shown to be consistent with previously published creep tensile test rupture life data in terms of Chakrabarty membrane stress. Aging is shown to have a significant, detrimental effect on creep life and deformation rate. The steady‐state displacement rate, time to failure, and displacement at failure data, for both aged and unaged, were successfully correlated via the modified Monkman–Grant relationship.

A synergistic model of stress and oxidation induced damage and failure in silicon carbide‐based ceramic matrix composites

AbstractA micromechanics‐based modeling approach that allows for the simultaneous consideration of deformation, damage, and oxidation associated with each constituent of silicon carbide (SiC)‐based ceramic matrix composites (CMC), including the fiber, fiber coating, and matrix, is described. Chemical kinetics models from the literature are combined with a progressive damage model. Rupture predictions of unnotched and notched stress‐hold (creep) specimens are compared with experimental measurements from a SiC/SiC CMC to assess the efficacy of the modeling approach. Techniques of improving creep rupture life are explored using the model.

Effect of grain boundary precipitates on the stress rupture properties of K4750 alloy after long-term aging at 750 °C for 8000 h

In K4750 alloy, the evolution of grain boundary (GB) precipitates, including the degradation of blocky MC carbide particles and the precipitation of granular/needle-like η phase particles, were observed after long-term aging (LA) at 750 °C for 8000 h. During MC degradation, the Ti and C released from the MC carbide combined with Ni and Cr, respectively, in the γ matrix to form η-Ni3Ti phase and Cr-rich M23C6 carbide. Large amounts of granular η phase precipitated at GBs and the needle-like η phase grew gradually from GBs toward the grain interior. Because of the growth of the η phase through absorbing γ′ phase, γ′-depleted zones were formed around the η phase. The evolution of the MC carbide and η phase was primarily responsible for the decrease of the stress rupture life and the increase of elongation. After an LA sample was tested at 750 °C and 360 MPa, the residual strain distribution was investigated by electron backscatter diffraction (EBSD). The results showed that the residual strain mainly distributed at GBs, especially in the region of MC degradation and at the edges of η phases, which was closely related to the appearance of phase interfaces. Microvoids/cracks easily initiated at phase interfaces, then easily extended along the γ′-depleted zones, thus the stress rupture life of LA samples was substantially shorter than that of samples subjected to the standard treatment. In particular, because of large amounts of fine degraded MC, granular M23C6 and granular η phase particles distributed at GBs after 750 °C /8000 h LA and microvoid/crack formation could be hindered by the formation of dimples, which led to an increase of elongation.

Machine learning augmented predictive and generative model for rupture life in ferritic and austenitic steels

The Larson–Miller parameter (LMP) offers an efficient and fast scheme to estimate the creep rupture life of alloy materials for high-temperature applications; however, poor generalizability and dependence on the constant C often result in sub-optimal performance. In this work, we show that the direct rupture life parameterization without intermediate LMP parameterization, using a gradient boosting algorithm, can be used to train ML models for very accurate prediction of rupture life in a variety of alloys (Pearson correlation coefficient >0.9 for 9–12% Cr and >0.8 for austenitic stainless steels). In addition, the Shapley value was used to quantify feature importance, making the model interpretable by identifying the effect of various features on the model performance. Finally, a variational autoencoder-based generative model was built by conditioning on the experimental dataset to sample hypothetical synthetic candidate alloys from the learnt joint distribution not existing in both 9–12% Cr ferritic–martensitic alloys and austenitic stainless steel datasets.

Creep rupture mechanisms and life prediction of IN617 for VHTR applications

The creep rupture mechanisms of IN617 were systematically investigated by establishing the relationship between microstructural characteristics and creep performance, including rupture time, percentage elongation, microhardness, and minimum creep rate. The Wilshire equation was employed to extrapolate long-term creep life by normalizing the stress, σ , using the ultimate tensile strength, σ TS . The results indicated that at temperatures ≤950 °C, when σ / σ YS values were >0.2 ( σ YS = yield strength), the creep rupture mechanism involved the sliding and migration of recrystallized grain boundaries, leading to matrix softening and necking fractures. When applied stresses ranged from 0.12 σ TS to 0.2 σ YS , creep voids along the grain boundaries became a pronounced feature, implying the rupture mechanism of the weakening of the grain boundaries without sufficient secondary-phase pinning. At a low stress level ( σ / σ TS < 0.12), creep ruptures were governed by a composite mechanism of microstructural degradation and internal oxidization, which was reflected in the brittle fractures caused by the connection of cracks along carbide networks and internal oxidized voids. A temperature–stress–rupture mechanism map was constructed by combining a comparison and summary from the literature. The Wilshire extrapolation curve described the creep rupture times well with temperatures and stresses, and the turning point of the curve suggested that microstructural degradation and internal oxidization increased the minimum creep rates and accelerated creep rupture.

Rejuvenation of Directionally Solidified and Single-Crystal Nickel-Base Superalloys

During service, superalloy turbine components degrade over time by creep and fatigue deformation mechanisms due to a complex combination of stresses at high temperatures. The high cost of fabricating Ni-base superalloy components and, consequently, replacement components has encouraged the development of rejuvenation (restoration) procedures to extend useful service life. The processes that limit repeated rejuvenation of directionally solidified GTD444 and single-crystal Rene N5(SX) alloys have been studied in detail. A rejuvenation cycle includes a rejuvenation heat treatment and/or small-scale material removal, followed by creep or fatigue testing. With the application of multiple rejuvenation cycles, the total creep rupture life of Rene N5(SX) tested at 982 °C and 206 MPa was extended by a factor of 2.8 over the baseline rupture life. To produce this increase in rupture life, creep strains were limited to 2 or 3 pct prior to application of a rejuvenation cycle, involving solutioning at 28 °C below the $$\gamma ^{\prime }$$ solvus temperature for 2 hours and aging at 1079 °C for 4 hours. Rejuvenation of compressive hold-time fatigue damage was also successful with the use of small-scale material removal. Due to the tortuosity of the grain boundaries in GTD444(DS), some boundaries are initially oriented transverse to the growth direction. The enhanced plasticity near these grain boundaries may be the primary reason why the initially single-crystal Rene N5(SX) specimens were more amenable to rejuvenation than GTD444(DS). For both alloys, recrystallization after multiple rejuvenation cycles was responsible for early failure during the subsequent creep test. Resonant ultrasound spectroscopy was successfully implemented to detect the presence of recrystallized grains.

An orientation-dependent creep life evaluation method for nickel-based single crystal superalloys

This paper aims to propose a creep life evaluation method considering the effect of crystallographic orientation. First, the maximum Schmid factor of {111} 〈112〉 and the corresponding lattice rotation angle were introduced to form an “orientation factor”. Then the equivalent stress was calculated by multiplying this factor and the nominal stress. Latterly, the Larson-Miller Parameter (LMP) method was adopted as the rupture life evaluation criteria, in which the input variable was the equivalent stress instead of the nominal stress. All the predictions showed high accuracy when the proposed method was applied to Mar-M247, SC7-14, CMSX-2, Alloy454, CMSX-4 and DD6. Finally, the applicable temperature ranges of the orientation-dependent method (using equivalent stress) and the traditional LMP method (using nominal stress) were discussed. The results show that only the Orientation-Dependent (OD) method is reliable at intermediate temperatures (760–850 °C) because the orientation has significant effect on the stress rupture life, while the influence of orientation is considerably reduced at high temperatures. Both methods provide precise predictions in this situation, and the LMP method should be favored since it is much easier to implement.

Effects of microstructure and lattice misfit on creep life of Ni-based single crystal superalloy during long-term thermal exposure

Abstract According to the microstructural evolution during longterm thermal exposure at 1100 °C, the creep rupture life of Ni-based single crystal superalloys at 980 °C/270 MPa was evaluated. The microstructure was characterized by means of scanning electron microscopy, X-ray diffraction and related image processing methods. The size of γ’ precipitates and the precipitation amount of topologically close-packed increased with the increase in thermal exposure time, and coarsening of the γ’ precipitates led to the simultaneous increase of the matrix channel width. The relationship between the creep rupture life and the lattice misfit of γ/γ’, the coarsening of γ’ precipitate and the precipitation of TCP phase are systematically discussed. In addition, according to the correlation between γ’ phase evolution and creep characteristics during thermal exposure, a physical model is established to predict the remaining creep life.

On the microstructural origin of premature failure of creep strength enhanced martensitic steels

The creep strength enhanced martensitic steels are key material for the main power generating units in ultra-supercritical plants. Studies on the evaluation of their creep rupture life show there is an overestimation of rupture life after long-term creep, which is known as premature failure. However, the microstructural origin of the premature failure remains unclear. Here in this study, we have carefully investigated the microstructural transformations and their influences on creep rupture behavior, showing that the evolution of martensite and M23C6 carbides as well as Laves phase are responsible for the premature failure. By using multi-step TTP-LMP method, we confirmed a three-stage creep rupture behavior under different stress regions. Further quantitative analysis showed that the coarsening of M23C6 carbides and recovery of martensite exert equal and dominant effects on the premature failure in the medium stress region, while precipitation and coarsening of Laves phase are responsible for the premature failure in the low stress region.