Rupture Life Research Articles

Predicting creep rupture life of Ni-based single crystal superalloys using divide-and-conquer approach based machine learning

Creep rupture life is a key material parameter for service life and mechanical properties of Ni-based single crystal superalloy materials. Therefore, it is of much practical significance to accurately and efficiently predict creep life. Here, we develop a divide-and-conquer self-adaptive (DCSA) learning method incorporating multiple material descriptors for rational and accelerated prediction of the creep rupture life. We characterize a high-quality creep dataset of 266 alloy samples with such features as alloy composition, test temperature, test stress, and heat treatment process. In addition, five microstructural parameters related to creep process, including stacking fault energy, lattice parameter, mole fraction of the γ' phase, diffusion coefficient and shear modulus, are calculated and introduced by the CALPHAD (CALculation of PHAse Diagrams) method and basic materials structure-property relationships, that enables us to reveal the effect of microstructure on creep properties. The machine learning explorations conducted on the creep dataset demonstrate the potential of the approach to achieve higher prediction accuracy with RMSE, MAPE and R2 of 0.3839, 0.0003 and 0.9176 than five alternative state-of-the-art machine learning models. On the newly collected 8 alloy samples, the error between the predicted creep life value and the experimental measured value is within the acceptable range (6.4486 h–40.7159 h), further confirming the validity of our DCSA model. Essentially, our method can establish accurate structure-property relationship mapping for the creep rupture life in a faster and cheaper manner than experiments and is expected to serve for inverse design of alloys.

Asymptotic creep deformation behavior of modified HP steel after long-term service

Creep behavior of modified HP steel was investigated through the tensile creep tests under the stress conditions of 25–55 MPa at 950 °C with four reformer tubes of different service periods: Unused (virgin), 8.50, 9.67, and 16.17 year aged. The minimum creep strain rates, creep rupture life, as well as elongation and reduction of area of the ruptured specimen were measured. It was observed that all the measured properties did not vary with different service periods. The creep deformation behavior was modeled based on the experimentally measured creep data utilizing the secondary creep (SC) model, the tertiary creep (TC) model, and the combined model of two latter ones (SC-TC model). The results indicated the asymptotic creep deformation behavior of the secondary creep strain rate. The tertiary creep also demonstrated asymptotic behavior, however, its strain rate slightly increased, as the service period increased. The creep curves predicted by the SC-TC model were more accurate than those obtained using the SC or TC model. Microstructural observations supported the observed asymptotic behavior.

Investigation on microstructure and mechanical properties of a vacuum brazed joint of γ′-strengthened Co-based single crystal superalloy before and after the post-bond heat treatment

Transient liquid phase bonding of γ′-strengthened Co-based superalloy was carried out using Ni-based filler alloy. The effects of post-bond heat treatment (PBHT) on the microstructure and mechanical properties of the γ′-strengthened Co-based superalloy joint were systematically investigated. Two distinct regions were observed in the isothermally solidified zone: primary γ′ phase zone (PZ) and secondary γ′ phase zone (SZ). The diffusion of Al from the substrate to the liquid interlayer proves to be the main reason for the precipitation of γ′ phases, showing a significant growth trend after the PBHT. The joint after the PBHT shows higher microhardness in most areas of the bonding zone. However, some parts of the diffusion affected zone show the contrary result. This could attribute to the competition consequence of the solid solution strengthening and second phase strengthening. Furthermore, the post-bond heat treated joint exhibited higher yield strength and longer stress rupture life. Our experimental results help to better understand the microstructural characteristic of the new type Co-based superalloy and provide a direction for further optimizing the brazing joint.

The effect of Gd addition on the cyclic oxidation behavior and creep life of alumina-forming Ni-based superalloy

The oxidation resistance and creep property of alumina-forming Ni-based superalloys with varying amounts of Gd are studied at 1100 ℃ in ambient air. Regardless of the Gd content, from the outermost surface inwards, a typical oxide scale consists primarily of NiO, spinel, spinel with oxide particles containing refractory elements, and Al2O3. However, spallation and abnormal growth of the scale are observed in alloys without Gd, and Gd addition significantly improves oxidation resistance. The creep rupture life is almost identical for the alloys with 0.04 wt.% Gd and those without Gd. The addition of Gd improves alloy oxidation resistance without compromising creep property.

Effect of grit blasting and subsequent heat treatment on stress rupture property of a Ni-based single-crystal superalloy SGX3

In the present study, the effect of grit blasting and subsequent heat treatment on the stress rupture properties of a third-generation nickel-based single-crystal superalloy SGX3 sheet was studied. It was found that the stress rupture life of alloy SGX3 sheet at 980 °C/250 MPa was reduced by about 60% by only vacuum heat treatment at 1100 °C for 200 h and further reduced by 20% and 70% respectively with grit blasting of 0.3 MPa/1 min and 0.5 MPa/2 min before heat treatment. The microstructure analysis results indicated that the degradation of stress rupture life of alloy SGX3 sheet by vacuum heat treatment was mainly attributed to the variation of γ/γ′ microstructure, i.e., the decrease in γ′ volume fraction and the coarsening of γ′ precipitates. Furthermore, such degradation by grit blasting and subsequent vacuum heat treatment should be attributed to the formation of cellular recrystallization with different thicknesses at the surface of alloy SGX3 sheet, which not only acts as the vulnerable site for cracks to initiate and propagate but also reduces the effective loading area.

Numerical schemes based on the stress compensation method framework for creep rupture assessment

Evaluation of creep rupture limit and prediction of creep rupture life are two significant issues for high-temperature devices under the action of cyclic thermo-mechanical loadings. In this paper, the shakedown solution procedure proposed recently by the authors, so-called stress compensation method (SCM), is extended for creep rupture assessment via an extended shakedown theory including creep. Two distinct numerical schemes based on the SCM framework are presented, where Scheme 1 is utilised for evaluation of creep rupture limit and Scheme 2 is utilised for prediction of creep rupture life. Instead of using the detailed creep constitutive equations, the present methods just need to know several material parameters including the creep rupture data. A holed plate is provided as a typical example to validate the reliability of two numerical schemes. Detailed cycle-by-cycle analyses are carried out to illustrate the good accuracy of these calculated creep rupture limits and reveal the failure mechanisms of the structure under different combinations of loads. A numerical study on a pipe junction is also conducted to show the engineering application of the methods. As a result, the two numerical schemes are proved to be effective and reliable for solving practical industrial problems.

Recovery and recrystallisation during creep exposure of cold worked Ti-modified 14Cr–15Ni austenitic stainless steel

ABSTRACT Creep behaviour of Ti-modified 14Cr–15Ni austenitic stainless steel in solution annealed, 20%, 30%, and 40% cold-worked conditions, was studied at 973 K and 175 MPa stress level in the current investigation. Increase in cold-work level resulted in denser dislocation network resulting in prominent low-angle grain boundaries. Low-angle boundary fraction increased as the amount of cold working increased from 20% to 40% prior to creep loading. The variation in the fraction of low-angle boundaries impacted the creep rupture strength of the steel. During creep exposure of the cold-worked steels, softening occurred due to both recovery and recrystallisation. The extent to which recrystallisation and recovery occurred during creep was dependent on the level of cold work. Subgrain formation and recrystallisation resulted in enhanced recovery and decreased the creep strength of 40% cold-worked steel. The creep rupture life was highest for 30% cold-worked steel.

Effect of yttrium addition on creep properties of a Ni-base superalloy built up by selective laser melting

The effects of yttrium (Y) addition on the creep properties of a Ni-base superalloy fabricated by selective laser melting (SLM) were examined at 650 °C. The creep rupture life and ductility of a superalloy produced by SLM were improved by the addition of Y. In the Y-free superalloy, Al-rich oxide particles were observed around the δ phase which is detrimental to creep life and ductility. The adding of Y to SLM superalloy formed Y oxide (yttrria) instead of Al-rich oxide, and it brought about the suppression of δ-phase precipitate growth and improved creep properties.

Alloy Composition Screening for Ni-Base Turbine Disc Superalloys Using the Creep Property of Single Crystal

This paper proposes a method for alloy composition screening using the creep property of a single crystal (SC) for designing new Ni-base powder metallurgy (P/M) turbine disc superalloys. A commercial Ni-base disc alloy, U720Li®, a Ni-Co-base disc alloy, TMW-4M3, and a Ni-base conventionally cast alloy, TM-47, were selected for investigation. Their SC and P/M samples were prepared such that their γ/γ′ structures were similar. Moreover, tensile creep tests were performed on SC samples, and compressive creep tests were performed on SC and P/M samples. The tensile creep test results of SC samples at 1073 K (800 °C) and 735 MPa indicate that 0.2 and 1 pct creep times, as well as rupture life, are long in the order of TM-47 SC, TMW-4M3 SC, and U720Li SC. These results simulate the intragranular creep resistances of the corresponding P/M alloys. Furthermore, there is no significant difference in 0.2 and 1 pct compressive creep times between the SC and P/M samples of each alloy. Additionally, the 0.2 and 1 pct creep times of tensile and compressive creep tests of every alloy had an identical order. Therefore, alloy composition screening using SC creep property enables the design of new disc alloys with excellent creep resistance.

Influence Mechanism of Silicon on Carbide Phase Precipitation of a Corrosion Resistance Nickel Based Superalloy.

The effect of silicon on diffusion behavior of the carbide forming elements in Ni-Mo-Cr-Fe based corrosion-resistant alloy is studied by diffusion couple experiment. One group of diffusion couples are made of the alloy with a different silicon content, another group of diffusion couples are made of pure nickel and the alloy with different silicon content (0Si, 2Si). Two groups of alloys with same silicon content and different carbon content are also prepared, the microstructure of solution and aging state of these two groups alloys are analyzed, and their stress rupture properties are tested. The effect of silicon on the diffusion of alloy elements and the interaction effect of carbon and silicon on the microstructure and stress rupture properties of the alloy are analyzed. The mechanism of Si on the precipitation behavior of carbide phase in Ni-Mo-Cr-Fe corrosion resistant alloy is discussed. The results show that silicon can promote the diffusion of carbide forming elements and the formation of carbide. The precipitation behavior of the secondary phase is the result of the interaction effect of silicon and carbon, and is related to the thermal history of the alloy. Combined with the characteristic of primary carbides, it is confirmed that the precipitation of M12C type secondary carbide is caused by the relative lack of carbon element and the relative enrichment of carbide forming elements such as molybdenum. The stress rupture properties of two silicon-containing alloys with different carbon contents in solution and aging state are tested. The stress rupture life of low carbon alloy is lower compared with high carbon alloy at solution state, but after aging treatment, the stress rupture life of low carbon alloy is significantly improved, and higher than that of high carbon alloy. The main aim of this research is to reveal the influence mechanism of silicon on carbide phase precipitation of a Ni-Mo-Cr-Fe based corrosion-resistant superalloy, which provides theoretical basis and reference for later alloy design and engineering application.

On new relation for theta projection approach for the prediction of creep behaviour of alloy D9I austenitic stainless steel

Creep behaviour of Ti modified 14Cr–15Ni austenitic stainless steel (designated as D9I SS), being considered for the cladding material in Prototype Fast Breeder Reactor (PFBR), is analysed using theta (θ) projection method and with a new relation. The material D9I SS, apart from fuel cladding tubes, is also being considered as a clad wrapper tube (long hexagonal tube) of FBRs that houses the fuel clad tubes. The new methodology is found to predict the creep behaviour much better than traditional 4-θ and 6-θ approaches. Further, the minimum creep rate, time to reach various strain levels and rupture life of the alloy D9I SS (with varying Titanium contents) have also been successfully predicted using this new relation.

Effect of carbon on microstructure and mechanical properties of HR3C type heat resistant steels

The modified HR3C austenitic heat-resistant steels for applications of ultra-supercritical (USC) power plants were developed and investigated. As the C content breaks through the limitation of the HR3C composition range (>0.1 wt%), the evolution of carbides and mechanical properties after isothermal aging at 700 °C have not been understood. In this study, two modified HR3C with different C content were studied by tensile test and Charpy impact test at room temperature after long-term aging up to 10,000 h. The M23C6 carbide is rapidly precipitated at the interface (GBs, TBs or NbC/γ), and these carbides at grain boundaries (GBs) are gradually changed from a continuous distribution to a semi-continuous distribution, then finally agglomerate and coarsen near the GBs. Moreover, a new morphology type lamellar M23C6 carbide forms in the grain. The effect of lamellar carbides on mechanical properties at room temperature (RT) is not obvious because the strength of GB is rapidly weakened. Besides, the Larson-Miller parameter was obtained and the creep strength of the modified HR3C was extrapolated, by conducting creep rupture experiments on un-aged sample. The creep tests reveal that the rupture lives decrease with increasing C content. Lamellar carbides are precipitated and weaken the strength of grain during high temperature creep.

The rupture life prediction in cold dwell fatigue of Ti-6Al-4V based on the creep deformation

Titanium a lloys have been found that the fatigue strength of Ti alloys decreases due to cold dwell fatigue (CDF) at room temperature. Ti and Ti alloys generate creep deformation at room temperature (T/Tm = 0.15). Thus, it is considered that creep affects the reduction in fatigue life in CDF tests. This research intends to clarify the effects of long time dwell under tensile stress and rupture life prediction from the view of creep deformation in CDF characteristics of Ti-6Al-4V. Rupture cycle decreased with increase of dwell time. Additionally, lower limit of rupture life ratio “NCDF/NLCF” was defined from rupture in creep test if it was assumed that creep test was extremely long time dwell CDF test. When strain change in whole dwell time was extracted in CDF tests, strain change was like creep curves and minimum creep rate changed depending on dwell time. Minimum creep rate was calculated by the formula based on experimental results, and then rupture time was calculated by Monkman-grant relationship. All of rupture cycle predictions were in factor of 2. Therefore, rupture cycle and time can be calculated if dwell time is known in CDF tests.

Change of microstructure and properties of dual titanium alloy join interface in hot working history

In order to get workpiece with high tensile stresses in bore region and high temperature duration, creep strength in outskirts, the dual alloy samples made of high temperature titanium alloy Ti60 and high strength titanium alloy Ti6246 had been joined by inertia friction welding(IFW). Then these samples were isothermal forged at 9400c and 9750c, and different heat treatment followed. Changes of microstructure and properties of dual titanium alloy join interface in hot working history were examined at this article. The results show equiaxed α structure varied into basketweave structureat at as-welded join interface, especially a character of widmannstaten structure emerged from Ti6246 alloy side heat effect region, after gradient heat treatment. Immersed ultrasonic testing prove deformation can availably eliminate weld defect through metal on two side of weld line deeper embedding each other. The results of properties test also show the join strength of dual titanium alloy through isothermal deformation and gradient heat treatment are better than that of as-welded samples. Tensile strength, yield strength, elongating rate, reduction in area of sample at 5500c also increase 51 to145 MPa, 37 to 101 MPa, 1.6% to 5.3%, 15.3% to 3.3% than that of as-welded samples respectively. The rupture life of Ti60/Ti6246 dual titanium samples with join interface can sustain to go beyond 100 hours at 5500c and 320 PMa stress.

Tensile creep behaviour of unidirectional flax fibre reinforced bio-based epoxy composites

Abstract Composites made of natural fibres are promising in applications of structural components. Their creep resistance is a very important characteristic to be evaluated but is not well understood. This work investigates creep response of a unidirectional flax fibre reinforced bio-based epoxy under various tensile stress levels. Acoustic emission detection and fracture surface morphology analysis were utilized to analyse the creep mechanisms. In addition, a flax fibre pre-treatment using a green chemical (furfuryl alcohol) was applied in order to reduce to creep deformation. The results show that unidirectional flax fibre composites exhibit perceivable creep at low stress levels and have a relatively short creep rupture life at high stress levels. The creep deformation at low stress can be ascribed to the time-dependent shear deformation of matrices at interface area (hemicellulose/pectin matrix at fibrils interface and the epoxy at fibres interface). Creep at high stress are characterized with a development of extensive damage events and doesn't show typical three-stage creep deformation behaviour over creep life. Composites with furfuryl alcohol treated flax fibres displayed a reduced creep deformation of about 30% than their untreated counterparts at the same creep stress, proving that treating flax fibres with furfuryl alcohol is efficient to improve the creep resistance of flax fibre composites.

Effects of constant load on corrosion cracking of 304 ASS at 600°C in molten salts

AbstractThe effects of constant load (CL) on corrosion cracking of 304 austenitic stainless steel (ASS) in molten salts (binary nitrate salts, 60 wt% NaNO3, and 40 wt% KNO3) were investigated. A pair of novel clamping fixture was designed to fix the tensile sample, making the gauge section immersed in molten salts. The CL tensile tests were applied with the initial stress of 212 MPa and the temperature of 600°C. Optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were employed to characterize the deformation behavior and corrosion features. The results show that corrosion of 304 ASS in molten salts was aggravated by the continuous deformation due to the cracking of protective oxide scale. Grain boundaries were oxidized and weakened by the molten salt, leading to a crack propagating. Compared with the creep test result, a reduced rupture life of 304 ASS under constant loading in molten salts was observed.

Unidirectional columnar microstructure and its effect on the enhanced creep resistance of selective electron beam melted Inconel 718

In this study, the selective electron beam melting (SEBM) process was used to fabricate Inconel 718 superalloy, and their microstructures and creep resistances were investigated. The proper SEBM condition ensuring a relative density higher than 99.9% was found in electron beam current range of 24–30 mA at a scanning speed of 4500 mm/s and a preheat temperature of 1025 °C, from which the SEBMed Inconel 718 exhibited an unidirectional columnar grain microstructure with a strong <100> texture along building direction. It was suggested that the local thermal condition during solidification after SEBM facilitates a continuation of epitaxial growth of crystals on layer beneath. The γ″ precipitates formed in the as-built condition regardless of focus offset, and their size increased with increasing focus offset from 1 to 12 mA. The creep rupture life of the as-built sample fabricated with focus offset of 12 mA showed 2 times longer than that of conventional forged Inconel 718 subjected to full heat treatment under creep condition of 650 °C/700 MPa. EBSD analysis on the crept samples indicated that strain accumulation was significant at the porosities and the high-angle grain boundaries. Additionally, the grain boundaries inclined to loading axis were more damaged during creep. The γ″ particles were occasionally sheared by matrix dislocations with Burger vector of a/2<110>, without leaving stacking faults behind. The enhanced creep resistance of SEBMed Inconel 718 superalloy can be attributed to the columnar microstructure with a strong <100> texture and the high density of dislocations resulted from strong interaction with the γ″ particles.

Influence of temperature on multiaxial creep behaviour of 304HCu austenitic stainless steel

ABSTRACTEffect of temperature on multiaxial creep behaviour of 304HCu austenitic stainless steel has been investigated. The multiaxiality was introduced by incorporating notches in smooth specimens. Creep rupture life increased with notch acuity ratio having a saturation/decline tendency. Notch strengthening increased with temperature, stress and notch sharpness. Multiaxial ductility decreased rapidly with notch sharpness and tended towards saturation. Fracture mode was found to change from transgranular ductile to intergranular creep depending on the stress, temperature and notch sharpness. Finite element analysis of notched specimens along with orientation imaging microscopic study was carried out to assess the deformation and damage at different normalised stress ratio. A temperature independent unique master plot for multiaxial rupture life as a function of stress has been established.

Creep Behavior of As-Cast Mg-10 wt.%Sn and Mg-10 wt.%Sn-3 wt.%Al-1 wt.%Zn Alloys: A Comparative Study

The present investigation highlights the creep deformation and rupture behavior of indigenously produced as-cast Mg-10 wt.%Sn-3 wt.%Al-1 wt.%Zn (TAZ1031) and Mg-10 wt.%Sn (T10) alloys. Conventional creep tests were conducted at 423 K (150 °C) in the stress range of 30-45 MPa and at 45 MPa in the temperature range of 423-453 K (150-180 °C) for both the alloys. The nature of creep curves was found to be identical for both the alloys with short primary and secondary creep regime, followed by extended tertiary creep region. The creep resistance of the TAZ1031 alloy was better than the T10 alloy. The former exhibited reduced minimum creep rate and enhanced creep rupture life under identical creep test conditions with respect to the latter. The change of minimum creep rate (έmin) followed a power-law relationship with a change in applied stress (σ) for both the alloys with a significant change in stress exponent (n) value. Activation energy for creep deformation of TAZ1031 alloy (~ 117 kJ/mol) was found to be higher than T10 alloy (~ 105 kJ/mol). Fine secondary Mg2Sn precipitates within the α-Mg matrix and primary Mg2Sn, Mg17Al12 and MgZn2 precipitates along the grain boundaries improved the creep properties of TAZ1031 alloy. On the contrary, the threshold stress required for creep deformation was found to be significantly low in TAZ1031 alloy due to the presence of Mg17Al12. Both creep damage analysis and dimpled fracture surfaces revealed necking dominated creep deformation for the alloys. SEM studies revealed the absence of creep cavities along grain/dendritic boundaries for the investigated alloys, which substantiated the formation of dimpled fracture surfaces during creep deformation.

Creep behaviour of a novel CoNi-base single-crystal superalloy at high temperature and low stress

The tensile creep behaviour of a novel CoNi-base single-crystal superalloy was investigated at 1093 °C/62 MPa. The rupture life of this alloy reached 140 h, which was mainly attributed to a high fraction of γ' phase. The interrupted creep test at 0.5% strain indicated that no rafting structure formed and γ' phase remained a spherical morphology during creep, due to the small lattice misfit, 0.024% at 1093 °C. Shearing γ' phase by multiple dislocations was the predominant substructure configuration in the steady-state stage, instead of planar defects which are commonly reported during relatively low-temperature creep of Co and CoNi-base single-crystal superalloys.