Early Stages Of Creep Research Articles

Three-dimensional characteristic and evolution of creep cavity and microcrack of HR3C austenitic heat resistant steel after long-term creep at 650 °C

Three-dimensional characteristics of creep cavities and microcracks in HR3C austenitic steel after creep at 650 °C were investigated using high resolution X-ray computed tomography (CT) and three-dimensional atomic probe (3DAP). The variation of cavity volume fraction is related to the increasing fraction of cavities undergoing growth and coalescence. Irregular lamellar or “W” shaped microcrack along the grain boundary were observed by the CT and the number large sized microcracks decreased significantly with decreasing stress levels. 3DAP results show that the concentration of P elements was high at the M23C6/γ interface at the early stage of creep. With the increase of creep time, M23C6/γ interface have been found to not only contain high concentration of P element, but also be enriched with Si. The coarsening of the carbides at the grain boundary and the enrichment of P and Si elements at the interface creates the conditions for the nucleation of creep cavities and microcracks under stress. Thus, stress has an tremendous influence on the distribution of creep cavities and the propagation path of microcracks. Additionally, with the decrease of stress, the creep damage mode changes from intergranular/transgranular mixed cracks to creep cavity.

The Influence of Groove Geometry on the Creep Fracture Behavior of Dissimilar Metal Welds between Ferritic Heat-Resistant Steels and Nickel-Based Alloys

This study investigates the influence of groove geometry on the high-temperature creep life and fracture behavior of Dissimilar Metal Welds (DMWs) between low-alloy steel 2.25Cr1Mo and austenitic stainless steel 347H using Inconel 82 nickel-based filling metal. This research aims to reveal the effect of groove geometry, especially the stepped groove, on creep crack propagation path and creep life, through a combined approach of finite element simulation considering stress triaxiality and experimental validation. The study reveals that the stepped groove alters the creep crack propagation path, enhancing the endurance life by deflecting cracks away from the weld/heat-affected zone (HAZ) interface and directing them into regions with higher creep resistance. The experimental results verify the simulation findings, revealing that the stepped groove joints exhibited longer creep life with changes in failure location and mechanism compared to the V-groove joints. However, it was found that the stepped groove intensified the stress concentration at the early creep stage. Thus, a good balance should be achieved between the negative (stress concentration at interface) and positive (changing crack paths) effects of the stepped groove to extend the creep life of DMWs.

Evaluation of metallurgical risk factors in post-test, advanced 9% Cr creep strength enhanced ferritic (CSEF) steel

ABSTRACT About 9 wt.% Cr steels are widely used in the design and fabrication of thick section components in combined cycle or coal-fired applications for working temperatures of 600 ~ 650°C. This family of materials possesses a martensitic microstructure stabilised by precipitates. The presence of nitrides, inclusions or evolution of second-phase particles may increase the metallurgical risk to creep. The chemical composition and microstructural evolution of 9 wt.% Cr steels contribute to thermal stability and long-term performance. In some specialist alloys, Ta is added to the composition which causes the formation of fine MX precipitates which are only present at the nanometre scale in tempered martensite, which hinders the recovery of dislocations and the migration of laths to extend creep life. However, the presence of large Ta-containing particles or inclusions in the 9 wt.% Cr steels may have a detrimental effect on its creep performance, as they may act as preferred sites for cavity nucleation. To fully appreciate the development of damage in these steels, it is necessary to link the pre- and post-test conditions, evaluate damage in the parent metal, develop procedures that provide consistency of results, and obtain statistically relevant data. The evolution of the Ta-containing phase has been tracked and quantified using a variety of correlative characterisation approaches. Utilising focused ion beam microscopy and two-dimensional electron-based microscopic characterisation, three-dimensional tomography has identified a strong relationship between creep cavities and Ta-containing phases from the early stages of creep.

Evolution and criteria for early creep damage

ABSTRACT The life-limiting fitness for service of high-temperature components is of interest in design, fabrication and later assessments of remaining creep life. Of the associated indicators, strain reflects creep in design and in service, while the discontinuities like creep cavities are targeted in the in-service inspections. Microstructure and hardness can provide supporting information on the material condition. Here, we assess such indicators for the creep-associated damage, particularly at the early stages. Improvements appear possible, e.g. in microscopy to support metallographic inspections and in utilising the widening inspection experience on newer materials. The present work successfully integrated the Wilshire/LCSP creep strain and rupture models with FE analysis for predicting creep strain evolution. Since the model applies for the whole creep curve, it can be used for early stages of creep down to the limit of negligible creep and to the lower limit of the window where creep cavitation damage can be observed.

Effects of Fe and Cr on the microstructure and mechanical properties of FexCr72−xNi14Al14 high-entropy alloys

Cobalt-free lightweight FexCr(72−x)Ni14Al14 (x = 34, 36; at%) high-entropy alloys were designed and prepared in this study to determine the effects of Fe and Cr content variations on the microstructure and mechanical properties of the alloys. Both alloys were prepared from pure metals using a vacuum arc melting furnace. The microstructures of the alloys were characterized using various analytical methods and the macroscopic compressive mechanical properties and microscopic creep properties were investigated. Both alloys were composed of a disordered α(Fe-Cr) body-centered cubic matrix and ordered (Ni-Al) B2 nanoparticle precipitates, with the differently sized nanoparticle precipitates and micron grains forming a four-stage layered structure. The alloys exhibited excellent compressive properties with a maximum yield strength of 1378 MPa, compression ratio of 34.1 %, and high specific yield strength of 210 kPa·m3/kg. Dense geometrically necessary dislocations (GNDs) were observed in the grains of the two as-cast alloys. GND was the main deformation mechanism in the early stages of creep, and the GNDs and B2 precipitated phase became obstacles to dislocation movement, which improved the creep resistance of the alloys. Excellent mechanical properties and good economy have great attraction in structural materials

Creep property and rafting kinetics of Co-based monocrystal superalloys with antiphase boundaries of γʹ phase

The precipitation strengthened Co-based superalloys show the great potential for the next generation superalloys of aerospace engine, their mechanical properties are associated with the morphology stability of L12-γʹ phase under high temperature and loading state. The antiphase boundaries (APBs) formed by different variants of the ordered fcc-L12-γʹ-Co3(Al, W) phase, affect creep morphologies and the high-temperature properties of Co-based superalloys. By coupling with the Kim-Kim-Suzuki (KKS) model and crystal plasticity model, the directional rafting and evolution kinetics of the γʹ phase are studied during the creep of Co–10Al–10W (at.%) superalloy. Creep rafting under high temperature and low stress is a process of stress-induced directional coarsening, rafting is affected by the APBs on hindering the directional element diffusion. In the early stage of creep, the increasing number of APBs leads to a lower creep strain. Higher temperature accelerates the rafting and reduces the γʹ area fraction. Furthermore, under tensile stress and compressive stress, the APBs show the hindering effect on both P-type rafting and N-type rafting. This study reveals the effects of APBs on the rafting behaviors and creep properties, the findings are theoretically important for the microstructure optimization of Co-based superalloys.

High-Temperature Creep Behavior of a Nickel-Based Single-Crystal Superalloy with Small-Angle Deviation from [111] Orientation

By means of tensile creep tests and microstructure analysis, the creep behavior and deformation mechanism of a nickel-based single-crystal alloy with small-angle deviations from the [111] orientation at 1040°C/137 MPa were investigated. The results suggested that in the early stage of creep, proliferating dislocations generated by different slip systems get accumulated in the γ phase. Dislocations mainly slipped in the γ phase and climbed over the γ′ phase, and they began to form a dislocation network at the interface. γ′ phases were connected to each other, and the coherence relationship was destroyed. In the steady-state stage of creep, the solid interfacial dislocation network was formed, and the γ′ phase transformed into a lamellar raft structure, which hinders the slip and climb of dislocations in the matrix channel. In the accelerated creep stage, the dislocation network was destroyed, and dislocations sheared into the γ′ phase in the way of dislocation pairs, which could react to form a superdislocation node and decompose to form APBs on the <111> plane or cross slip from the <111> plane to the <100> plane to form K-W locks.

Self healing of creep-induced damage in Fe-3Au-4W by multiple healing agents studied by synchrotron X-ray nano-tomography

Constant stress creep experiments at 550 °C were performed on a high-purity Fe-3Au-4W (wt.%) ternary alloy with about 1 at.% supersaturation for Au and W in order to study self healing of grain-boundary cavities by both Au-rich and W-rich precipitates. Using synchrotron X-ray nano-tomography, the development of the creep cavities and the healing precipitates at different stages of creep was visualised using two spatial resolutions (30 and 100 nm voxel size) for separate samples taken after different loading times. The healing kinetics was found to strongly depend on the nucleation time of the cavities. Cavities nucleated at an early stage of creep could be fully healed, while the healing of the late-nucleated cavities is much slower due to a decrease in the diffusional flux of the healing supersaturated solutes over time, as a result of (i) a decrease in inter-cavity spacing caused by cavity nucleation and (ii) a gradual depletion of the supersaturated solutes near the grain boundaries. The interaction between the competing healing mechanisms for creep cavities by Au-rich and W-rich precipitates is discussed. It was found that Au-rich precipitates are formed much faster than the W-rich precipitates, and thereby effectively provide creep damage healing on different time scales.

Crystal plasticity phase-field simulation of slip system anisotropy during creep of Co-Al-V monocrystal alloy under multidirectional strain

The preferable thermodynamic stability of L12 γ'-Co3(Al, V) phase and wide γ+γ' two-phase region endow the Co-Al-V alloy with great application potential. By introducing the digitized image of SEM morphology into phase-field simulation, utilizing the sublattice free energy model and the crystal plasticity strain model to simulate the anisotropic slip system, the rafting behaviors of the L12 γ'-Co3(Al, V) phase and the creep properties of Co-5Al-14V (at.%) alloy under tensile, compressive, shear, and monoclinic strains are studied. The partitioning ratio of element and orientation degree of γ' phase indicate that the uniaxial compressive strain improves the microstructural stability and the creep resistance at early creep stage. The creep resistance of alloy in state of equilibrium volume fraction of γ' phase is superior to that in the nonequilibrium volume fraction, and the creep resistance increases in order of tensile strain, compressive strain, and shear strain. Additionally, the dominant octahedral slip systems in fcc structure are (1¯1¯1)[101] under tensile strain, (1¯11)[01¯1] under compressive and 15° monoclinic strains, and (1¯1¯1)[1¯10] under 45° monoclinic strain. The hardening effect is prominent in γ matrix, and the high average critical resolved shear stress leads to the low creep strain. This study reveals the correlations between the slip system anisotropy and creep behaviors of Co-Al-V alloy under external strain, and also provides a method to combine the phase-field simulation with the experimental image for predicting the morphology evolution.

A new Cr25Ni35Nb alloy critical failure time prediction method based on coercive force magnetic signature

High-temperature damage in Cr25Ni35Nb alloys can be differentiated into four stages: the almost perfect stage, Carbide grain boundary coarsening stage, early creep stage and critical failure stage. In this paper, a novel approach based on a magnetic signature to monitor the alloy transitions between the four stages is proposed. Since the difference (ΔNi/Cr) between the content ratio of Ni to Cr in grains and the average value of the ratio of Ni to Cr of the alloy vary at different stages, these variations are highly correlated to the magnetic signature, coercive force (Hc). Thus, the curve of Hc vs ΔNi/Cr, which is “M” shaped over the whole life of the alloy, can be used to estimate the remaining life of the alloy tube using the easy measurement of coercive force. The first “U” inflection point indicates that the early creep stage has started and the second inverted “U” inflection point indicates that the critical failure stage is coming up. By identifying the inflection points, the alloy critical failure time can be predicted.

The effect of deviations from precise [001] tensile direction on creep of Ni-base single crystal superalloys

Low temperature (1023 K) high stress (800 MPa) tensile creep behavior of the superalloy single crystal ERBO-1 (CMSX-4 type) is investigated. Three loading directions are compared: precise [001] and 15 ° deviations from [001] towards [111] and [011]. It is found that creep rates ε˙ scale as ε˙[001]→[111]>ε˙[001]>ε˙[001]→[011]already in the early stages of creep (ε≤1%), where dislocation network formation and planar fault intersections cannot rationalize the observed rate effects. An analysis based on Peach-Köhler force calculations suggests, that fast creep rates are observed, when dislocations from two octahedral systems, which are required to react and form the leading part of a planar fault ribbon in the γ’-phase, experience similar driving forces. Creep data, micromechanical calculations and TEM results are in good qualitative agreement. From a technological point of view, the results show that while 15 ° deviations from [001] towards [011] can be tolerated, deviations towards [111] must be avoided.

Understanding creep of a single-crystalline Co-Al-W-Ta superalloy by studying the deformation mechanism, segregation tendency and stacking fault energy

A systematic study of the compression creep properties of a single-crystalline Co-base superalloy (Co-9Al-7.5W-2Ta) was conducted at 950, 975 and 1000°C to reveal the influence of temperature and the resulting diffusion velocity of solutes like Al, W and Ta on the deformation mechanisms. Two creep rate minima are observed at all temperatures indicating that the deformation mechanisms causing these minima are quite similar. Atom-probe tomography analysis reveals elemental segregation to stacking faults, which had formed in the γ′ phase during creep. Density-functional-theory calculations indicate segregation of W and Ta to the stacking fault and an associated considerable reduction of the stacking fault energy. Since solutes diffuse faster at a higher temperature, segregation can take place more quickly. This results in a significantly faster softening of the alloy, since cutting of the γ′ precipitate phase by partial dislocations is facilitated through segregation already during the early stages of creep. This is confirmed by transmission electron microscopy analysis. Therefore, not only the smaller precipitate fraction at higher temperatures is responsible for the worse creep properties, but also faster diffusion-assisted shearing of the γ′ phase by partial dislocations. The understanding of these mechanisms will help in future alloy development by offering new design criteria.

Improving creep age formability of an Al-Cu-Li alloy by electropulsing

Electrically assisted creep age (ECA) forming of an Al-Cu-Li alloy, when applied electropulsing during the different stages, has been experimentally investigated under the applied stress of 160 MPa at 160 °C for 20 h. Creep strain of the sample assisted by electropulsing when introduced during the initial creep stage increased by ~27.2% comparing with that of conventional creep ageing test (CCA) at the end of 20 h. However, there is almost no effect on creep strain when applied electropulsing at the beginning of 4, 8 and 19 h. Tensile tests indicate that strength of the aged specimens assisted by electropulsing when applied during the different creep stages is almost the same, but a slight reduction in elongation for the ECA specimen when introduced electropulsing during the initial stage and an increase by ~27.4% in elongation of the ECA specimen when introduced electropulsing during the last stage, compared to that of the CCA specimen. The fractograph of aged samples assisted by electropulsing when applied during the various creep stages, was observed to an obvious difference. Transmission electron microscopy (TEM) observations show that applying electropulsing during the early creep stage can accelerate dislocation motion, which for the first time proven by the observed larger dislocation structure, and promote Li solutes diffusion to grain-boundary, which is responsible for an increased creep strain and a decreased elongation, respectively. And dissolution of the Cu-rich at grain-boundary and GPI zone/θ´ precipitates within the grain, induced by electropulsing when applied during the last creep stage, is considered as the cause of an increased elongation. Thus, applying electropulsing during both the initial and last creep age forming stages is a promising method to control the microstructure to attain a sound sheet metal forming/property synergy.

Effect of Different Precipitation Routes of Fe2Hf Laves Phase on the Creep Rate of 9Cr-Based Ferritic Alloys

We performed creep tests for three types of Fe-9Cr-Hf alloys with a ferritic matrix w/o Fe2Hf Laves phase particles formed by two precipitation routes: (1) with fine Fe2Hf particles formed by the conventional precipitation route (hereafter the particles are called CP particles), namely formed in the α-ferrite matrix after γ-austenite → α-ferrite phase transformation; (2) with fine Fe2Hf particles formed by interphase precipitation (hereafter called IP particles) during δ-ferrite → γ-austenite phase transformation before γ → α phase transformation and (3) without Laves phase particles. CP particles were found to be effective in reducing the creep rates from the transient creep regime to the early stage of a slowly accelerating creep regime but were coarsened after the creep tests. IP particles were less effective in reducing the creep rate in the early creep stages but showed a higher stability against particle coarsening than CP particles in the creep tests, suggesting their effectiveness in delaying the recovery and recrystallization processes in the matrix and thereby retarding the onset of a rapid creep acceleration and creep rupture. The effects of the different precipitation routes are discussed based on the results obtained.

Different roles of stacking fault energy and diffusivity in the creep performance of nickel-based single-crystal superalloys

Two nickel-based single-crystal (SC) superalloys (designated T6 and T13) were investigated in order to reveal the effects of stacking fault (SF) energy and diffusion on their high-temperature creep behavior. In this study, the effect of dislocation spacing on creep was the same between T6 and T13. The microstructure and deformation rate attained prior to 1% creep were investigated in detail. Several differences were detected in the two superalloys, the reasons for these differences were discussed. The results suggested that the SF energy of the γ matrix was the key factor affecting development of the dislocation network. Because the matrix of the T6 superalloy has lower SF energy than T13, the cross-slipping of dislocation was more difficult in the early stages of creep, which resulted in lower levels of dislocation propagation and of formation of the dislocation network. When creep had entered the steady stage, the key process controlling high-temperature creep was atomic migration in the γ matrix, and the creep rupture life was lengthened by reducing the penetration of dislocations into the γ′ raft and slowing down its topological inversion.

Effects of pre-creep on dislocation and tensile property of Cr-Ni steel

Pre-creep experiments were performed on chromium-nickel (Cr-Ni) stainless steel in the early stage of creep. The temperature was held for 500–2000 h under high-temperature load conditions (873 K and 150 MPa), and various analysis methods, including optical microscopy, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction, were used to determine the pattern of dislocation evolution under different temperature-holding times. The results showed that the slip bands intersected at the initial stage of pre-creep, and a quadrilateral network structure was formed by the dislocation pinning. As the temperature-holding time increased, the dislocation network began to climb to form dislocation walls, dislocation cells, and other substructures. At 2000 h, the grain boundaries widened considerably; creep holes were found at the grain boundaries; and dislocation pairs with oscillating contrast occurred, which indicates nitrogen diffusion. The yield and tensile strengths of the Cr-Ni steel samples subjected to pre-creep at holding times of above 1000 h decreased after they were subjected to room-temperature tensile tests.

Understanding raft formation and precipitate shearing during double minimum creep in a γ′-strengthened single crystalline Co-base superalloy

ABSTRACT The compressive creep deformation behavior of a Co-base single-crystalline superalloy is studied in the high temperature / low stress regime at 950°C/150 MPa. Emphasis is placed on the mechanisms causing the double minimum creep behavior consisting of a local and a global creep rate minimum. The local minimum occurs at ∼0.2% strain with dislocation accumulation at γ/γ′ interfaces after bowing out of dislocations in horizontal matrix channels. Plate-like rafting occurring normal to the applied stress axis in the early creep stages is regarded as the primary reason for the decreasing creep rate until the global minimum takes place at ∼0.8% strain. In the final creep stage following the global minimum the creep rate accelerates due to coarsening and extensive precipitate shearing. The shearing by partial dislocations leads to the formation of numerous stacking faults, nano/microtwins, and occasionally to the formation of small amounts of ordered D019-Co3W precipitates. The reasons for twinning and second phase formation as well as differences of the deformation mechanisms between Co- and Ni-base superalloys during double minimum creep are discussed in detail in this work.

Effect of interface dislocations on mass flow during high temperature and low stress creep of single crystal Ni-base superalloys

In this work, the nanometer-scale mass flow coupled to dislocation processes near the γ/γ′-interface during high temperature and low stress creep of a model Ni-base single crystal superalloy is investigated. In the early creep stages, the dislocation networks in the γ-phase at γ/γ′-interfaces attract γ-stabilizing elements like Cr, Co and in particular Re, resulting in compositional gradients close to the interface. At larger strains, where dislocations frequently cut into the γ′-phase, this local interfacial enrichment in these elements is no longer observed. The cutting dislocations take part of the segregated atoms away, whilst the remaining atoms are released and diffuse back into the γ-channels.

Phase-field simulation of γ' precipitates rafting and creep property of Co-base superalloys

Co-base superalloys show attractive properties for the strengthening of γ'-Co3(Al,W) precipitates. Directional coarsening of γ' precipitates under external stress forms the rafting morphology, which plays a crucial role in the creep property. The microelastic theory, coupled with the crystal plasticity model, is utilized to study the rafting behavior during the early stage of creep in Co-Al-W superalloys. High W content and large eigenstrain can produce a lower creep strain of early stage, for the subdued plastic deformation in the γ matrix. Driving forces of element redistribution during rafting are compared among the chemical, interfacial and elastic contribution, in which the elastic contribution is dominant in rafting. With the change in creep stress from 197 MPa to 297 MPa, the element redistribution is strengthened, and the rafting is accelerated in Co-10 Al-9 W (at.%) alloy. Elements redistribution in the rafting process widens the γ/γ' interface. In contrast, the width of γ/γ' interface along the rafting direction begins to decrease when the coalescence of γ' precipitates happens. The straight-forward insight on the element diffusion during rafting and the related creep properties guides the composition design and rafting behavior prediction of this novel Co-Al-W superalloy.

In Situ 3D-µ-Tomography on Particle-Reinforced Light Metal Matrix Composite Materials under Creep Conditions

In transportation light metal matrix composites (L-MMCs) are used increasingly due to their improved creep resistance even at higher application temperatures. Therefore, the creep behavior and failure mechanisms of creep loaded particle reinforced L-MMCs have been investigated intensively. Until now, creep damage analyses are usually performed ex situ by means of interrupted creep experiments. However, ex situ methods do not provide sufficient information about the evolution of creep damage. Hence, in situ synchrotron X-ray 3D-µ-tomography investigations were carried out enabling time and space resolved studies of the damage mechanisms in particle-reinforced titanium- and aluminum-based metal matrix composites (MMCs) during creep. The 3D-data were visualized and existing models were applied, specifying the phenomenology of the damage in the early and late creep stages. During the early stages of creep, the damage is determined by surface diffusion in the matrix or reinforcement fracture, both evolving proportionally to the macroscopic creep curve. In the late creep stages the damage mechanisms are quite different: In the Al-MMC, the identified mechanisms persist proportional to creep strain. In contrast, in the titanium-MMC, a changeover to the mechanism of dislocation creep evolving super-proportionally to creep strain occurs.