Rupture Lifetime Research Articles

X-ray Diffraction Analysis of Creep-deformed Nitrogen-added 316LN Stainless Steels

X-ray diffraction detailed analysis of nitrogen-added austenitic 316LN stainless steels with different nitrogen concentrations (0.07, 0.11, 0.14, 0.22 wt.%) was carried out in creep-tested (at 923 K, 200 MPa) and solution-annealed conditions. The effect of nitrogen on lattice parameter was studied before and after creep testing. Size and strain were estimated through Williamson–Hall analysis. High nitrogen-added 316LNSS showed lower dislocation density. Increase in creep rupture lifetime with nitrogen content correlated well with the lattice parameter, crystallite size, and strain.

A novel low-density and high-strength Fe-Ni-base high-entropy superalloy with stable γ/γ' coherent microstructure at 1023 K

The present work developed a novel low-density (7.71 g/cm3) and high-strength Fe-Ni-base high-entropy superalloy with a composition of Fe-33.2Ni-13.4Co-13.8Cr -3.1Al-2.2Ti-1.1Nb-1.7Mo-0.03C-0.015B-0.03Zr (wt. %), which contains spherical γ' nanoparticles coherently-precipitated into FCC-γ matrix. These γ' nanoprecipitates were coarsened slowly at 1023 K with the particle size being ∼40 nm after 1000 h-aging, showing a great microstructural stability due to lower lattice misfit between γ/γ'. Thus, this superalloy exhibits excellent mechanical properties at room and high temperatures, as evidenced by the yield strength being 743 MPa at 1023 K. The involved strengthening mechanisms were further discussed. Moreover, it has a superior creep-resistant property with the rupture lifetime of 61.5 h under 450 MPa/973 K. Also, it possesses a prominent strain-hardening capacity due to the presence of abundant stacking faults in γ' nanoprecipitates. The current work opens a new way to design low-density superalloys with stable coherent microstructure at high temperatures.

Synergistic Effects of Boron and Rare Earth Elements on the Microstructure and Stress Rupture Properties in a Ni-Based Superalloy.

The synergistic effects of boron (B) and rare earth (RE) elements on the microstructure and stress rupture properties were investigated in a Ni-based superalloy. The stress rupture lifetime at 650 °C/873 MPa significantly increased with the addition of B as a single element. Furthermore, the stress rupture lifetime reached its peak (303 h), with a certain amount of B and RE added together in test alloys. Although the grain size and morphology of the γ' phase varied a little with the change in B and RE addition, they were not considered to be the main reasons for stress rupture performance. The enhancement in stress rupture lifetime was mostly attributed to the segregation of the B and RE elements, which increased the binding force of the grain boundary and improved its strength and plasticity. In addition, the enrichment of B and RE inhabited the precipitation of carbides along grain boundaries. Furthermore, nano-scale RE precipitates containing sulfur (S) and phosphorus (P) were observed to be distributed along the grain boundaries. The purification of grain boundaries by B and RE elements was favorable to further improve the stress rupture properties.

Crystallographic texture and multiscale boundaries mediated creep anisotropy in additively manufactured Ni-based Hastelloy C276 superalloy

The microstructure and high-temperature creep mechanisms of Ni-based Hastelloy C276 superalloy fabricated using wire and arc-based directed energy deposition were investigated systematically and innovatively. The microstructural investigation revealed that the as-fabricated samples comprise γ-Ni matrix and topologically close-packed (TCP) P phase precipitates. The γ matrix subgrains and grains are spread over multiple highly textured columnar dendrites, with a majority of γ <001> crystallographic orientations closely aligned along the deposition direction. Moreover, the interdendritic regions exhibit severe Mo segregation, P phase particles, and dislocation bands. Creep tests were conducted on miniature samples under various temperature and stress conditions, loaded either in the deposition direction (DD) or travel direction (TD). DD samples exhibit lower minimum strain rates, greater strains-to-failure, and longer creep rupture lifetimes than TD samples, indicating significant creep anisotropy. Dislocation creep was identified as the primary creep mechanism for both DD and TD conditions. During creep, dynamic precipitation of TCP phases occurred in the interdendritic regions, resulting in varying creep resistance between interdendritic and dendritic core regions. Isostress and isostrain models, considering both crystallographic texture and precipitation strengthening, reasonably predicted the observed creep anisotropy during the secondary creep stage. Additionally, variations in the Schmid factor led to significant deformation incompatibility among dendrites in TD samples. Dislocation accumulation in TD sample interdendritic regions promoted new grain nucleation, triggering dynamic recrystallisation, facilitating grain boundary sliding, and accelerating tertiary creep. Furthermore, TCP phase particles in the interdendritic regions contributed to microcrack development, further accelerating creep fracture, especially in the TD condition.

Influence of cooling strategy on microstructure, deformation mechanism and mechanical properties of a nickel-based superalloy

In this study, the influence of cooling strategy after solution treatment on microstructure and mechanical properties of a novel cast and wrought nickel-based superalloy is studied. It was found that continuous oil cooling (CC) from the solution temperature led to formation of large primary γ' and ultrafine tertiary γ'. Interrupted cooling (IC), which involves furnace cooling from the solution treatment temperature (1080 °C) to a lower temperature (such as 1010 °C, 980 °C or 950 °C) first followed by an interrupted oil cooling, shows significant influence on microstructure. Owing to a short-period furnace cooling, secondary γ' started to form in these interrupted cooled samples and their volume fraction and size increased continuously with decreased IC temperature. In contrast, the volume fraction of tertiary γ' decreased with decreased IC temperature. The CC sample was found to deform mainly by dislocation glide/climb and extension of stacking faults at 650 °C/950 MPa. With increased volume fraction of large secondary γ' in the interrupted cooled samples, deformation became increasingly planar, i.e., the formation of stacking faults and nanotwins increasingly became the dominant deformation mechanisms. The cooling strategy shows no obvious influence on tensile properties and impact toughness but shows significant influence on stress rupture properties. With decreased IC temperature and coarsened microstructure, the stress rupture lifetime continues to decrease.

The Effect of Forging and Heat Treatment Variables on Microstructure and Mechanical Properties of a Re-Bearing Powder-Metallurgy Nickel Base Superalloy

In our previous works, the effects of forging and heat treatment variables on microstructure evolution and mechanical properties have been studied for an ingot-metallurgy Re-bearing nickel base superalloy. To overcome the issues associated with the production of large-scale ingots and fine-grained workpieces, in the present work, the effect of hot forging and heat treatment variables was studied in a Re-bearing nickel base superalloy prepared via powder metallurgy. The purpose of the study was to reach the properly balanced mechanical properties for the potential use of the superalloy as a disc material. The initial as-HIPed workpieces were subjected to different hot forging and post-forging heat treatment or only to heat treatment (no forging). For the processed workpieces, the recrystallization behavior, size, morphology and volume fraction of γ′ precipitates were evaluated by scanning electron microscopy followed by a study of mechanical properties. The most properly balanced mechanical properties (strength, ductility, creep resistance and creep rupture lifetime) were reached for the γ grain size of dγ ≈ 13.6 µm. A finer and coarser γ grain size (down to dγ ≈ 2.6 and up to dγ ≈ 37.5 µm) even when superimposed with a higher volume fraction of dispersed secondary γ′ precipitates (in the case of dγ = 27–37.5 µm) was associated with worse mechanical properties.

Creep rupture lifetime evaluation and creep deformation analysis using θ-projection and Blackburn-type methods for forged Ti43Al5V4Nb

ABSTRACT The high-temperature creep properties of Ti43Al5V4Nb were investigated experimentally and analyzed numerically. Creep testing of forged Ti43Al5V4Nb at 650°C and 760°C was conducted to investigate the creep rupture lifetime and high-temperature properties. Steady-state and accelerated creep regions were dominant in the high-temperature environment, indicating rupture ductility. The creep rupture lifetime in the range of 600°C−800°C was evaluated uniformly using the Larson – Miller and Orr – Sherby–Dorn parameters. The θ-projection method can be used to draw creep curves for experimental data up to a lifetime ratio of 0.9, and it was possible to represent the strain immediately before rupture.. The creep rupture lifetime could be predicted within a factor of two using the predicted minimum strain rate and the Monkman – Grant law. A Blackburn-type method was found to represents the transition and steady-state creep ranges accurately. Thesemethods were effective in predicting the 1% strain occurrence time

Effect of Heat-to-Heat Variability on Long-Term Creep Rupture Lifetime Predictions of Single Step Aged Wrought Haynes 282 Alloy

Abstract The impact of a weaker heat of the wrought form of Haynes 282 nickel superalloy on predicted 100,000 h creep rupture strength (CRS) and hence maximum allowable working stress (MAWS) was determined by correlating the creep rupture data of the two stronger heats of material that were used (along with the third weaker heat) to develop the ASME Code Case for this alloy. In comparison with the established MAWS values, estimated MAWS values were 3–7% higher in the temperature range of 700–800 °C and up to 30% higher above 800 °C when the weaker heat was removed from the data analysis. These results show the importance of minimizing heat to heat variability of properties that affect high temperature creep strength, including grain size, which can be affected by parameters that might not be included in the alloy specification. For nickel superalloys, such as Haynes 282, design optimization and manufacturing control of grain size, in addition to chemical composition and gamma prime content and size, can provide measurable improvements in MAWS values established by ASME in the time-dependent region, where these alloys are usually employed.

Effect of Boron Content on the Transverse Stress Rupture Properties of a Directionally Solidified Superalloy

The effect of boron content on the transverse stress rupture properties of a directionally solidified columnar superalloy is studied in present research. The experimental results indicate that the transverse stress property of the alloy containing 0.015% boron reaches the peak value at 870°C /380MPa while that of the alloy with 0.005% boron is the best at 1100°C/40MPa. At 870°C, dislocation slip is the main deformation mode. Stress concentration is induced when dislocations are hindered by precipitates at grain boundaries and interdendritic regions. And then cracks nucleate due to the stress concentration. In the alloy containing 0.015% boron, no borides precipitate and boron strengthens the grain boundary cohesion by solid solution. At 1100°C, grain boundary movement becomes the main deformation mode. In the alloy with 0.005% boron, the strengthening of grain boundary cohesion decreases comparing with that of alloy with 0.015% boron, and the resistance to grain boundary movement reduces with decreasing B content. Therefore, it results in compatible deformation between adjacent grains, resulting in the longest stress rupture lifetime.

On the stress rupture behavior and deformation mechanism of an advanced hot-extruded nickel-based superalloy

The demand for increased aircraft engine efficiency requires continual development of high performance heavily-alloyed nickel-based superalloys. However, nickel-based superalloy castings with heavy alloying usually suffer from severe macro-segregation that results in poor hot workability or cracking during hot forging. To address this issue, hot extrusion is introduced as a transitional process between casting and forging to process a newly developed and highly alloyed nickel-based superalloy, GH4151. The results demonstrate that with hot extrusion, the GH4151 ingots can be successfully processed in the subsequent forging without cracking, thanks to the unique three-dimensional compressive stresses imposed on the billets and the development of chemical and microstructural homogeneity during hot extrusion. The refined microstructure endowed by further forging and heat treatment gives rise to superior stress rupture properties as compared with a number of the state-of-the-art disc superalloys. Moreover, it is shown that the stress rupture lifetime of the material decreases continuously with increased temperature and stress level while the elongation increases with increased temperature. The deformation at 650 °C mainly occurred by dislocation slipping in γ channels and formation of stacking faults in γ′. At 750 °C and 800 °C, microtwins were formed and cut through large regions. Paired dislocations associated with anti-phase boundaries were even found in the samples that were tested at 800 °C. In general, with increased temperature, the deformation becomes increasingly planar and easier, which accounts for the improved elongation. Large grain boundary MC carbides and primary γ′ were found to act as crack initiation sites and thus are harmful for stress rupture properties.

Verification of creep properties and rupture lifetime evaluation methods using small diameter Sn-3.0Ag-0.5Cu/Sn-58Bi/Sn-5Sb specimens

In this study, creep testing methods for three solder specimens with a diameter of 3 mm were investigated. The creep rupture lifetime of the 3 mm diameter Sn-3.0Ag-0.5Cu specimen was evaluated in comparison with reference data acquired from a 10 mm diameter sample, and it was determined that there was no significant effect of specimen size on the creep rupture lifetime. The creep curve for Sn-3.0Ag-0.5Cu revealed a significant increase in the strain ratio during the later stage of its lifetime; whereas, Sn-58Bi and Sn-5Sb showed substantial increases in the strain ratio during the early stages of their lifetimes. Strong correlations were observed between the minimum strain rate and applied stress for all solders, confirming the validity of the Norton rule. The activation energy of each solder was calculated from the creep tests performed on the small diameter specimens. The activation energies for Sn-3.0Ag-0.5Cu and Sn-58Bi were approximately 94 kJ/mol and 65 kJ/mol, respectively, whereas the activation energy for Sn-5Sb varied significantly from 91.0 to 169.9 kJ/mol.

Microstructural stability and mechanical properties of GH742 Ni-based wrought superalloy for turbine disk applications

Microstructural stability and mechanical properties were investigated in a Ni-based wrought superalloy during long-term thermal exposure at 650 °C and 700 °C for up to 7500h. The results demonstrate that the morphology of primary γ′ precipitate has no significant change after long-term thermal exposure, and it exhibits excellent thermal stability both at 650 °C and 700 °C. However, the morphology of secondary γ′ precipitates transformed from complex flower-like to rounded cubic during long-term exposure at 700 °C. The coarsening of tertiary spherical γ′ precipitates occurs with increasing thermal exposure time, and its coarsening behavior is consistent with Ostwald ripening theory. M23C6 carbides coarsen at grain boundaries and form continuous film with increasing time, resulting in the embrittlement of grain boundary. The multimodal distribution of γ′ precipitates results in high strength of the alloy after long-term thermal exposure. Both the strength of the alloy after exposure at 650 °C and 700 °C for 7500 h are not less than that of the alloy after standard heat treatment. The stress rupture lifetime tests at 650 °C/873 MPa increases firstly and then decreases with increasing exposure time at 650 °C and 700 °C. The increase of stress rupture lifetime is attributed to the supplementary precipitation and growth of finest tertiary γ′ precipitates, while the decrease of it is attributed to the γ′ evolution and the coarsen of M23C6 carbides.

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 Si addition on the creep performance of a Ni-based superalloy

In this research, the effect of two different per centages of silicon, i.e. 0.08 and 0.3 wt-%, on microstructure and stress rupture life of a Ni-based superalloy was investigated. The results showed that the fraction of eutectic increased from 1.86 to 3.55% with increasing Si content and a higher fraction of eutectic was found on grain boundaries in high Si-bearing alloy. Upon stress rupture at 975°C/225 MPa, cracking initiates and propagates along Laves phases in 0.3 wt-% Si alloy, causing shorter lifetime comparing to that of 0.08 wt-% Si. The microscopic examination uncovered that Si prefers to segregate towards liquid and γ phase, which can explain different rupture lifetimes of two groups of Si-bearing alloys.

Stress rupture of fiber-reinforced ceramic-matrix composites subjected to different stochastic loading spectrums at intermediate temperatures

In this paper, stress rupture of fiber-reinforced ceramic-matrix composites (CMCs) subjected to different stochastic loading spectrums at intermediate temperatures (600 to 1000 °C) is investigated. Under stress rupture at stress level higher than the first matrix cracking stress, multiple damage mechanisms of matrix cracking, fiber/matrix interface debonding, interphase and fiber oxidation, and fiber fracture are considered to analyze evolution of composite strain. Four different stochastic loading spectrums are considered in the analysis of damage evolution and lifetime of fiber-reinforced CMCs under stress rupture loading. Relationships between stochastic loading stress, frequency, time, interface debonding and oxidation length, fiber failure probability, and stress rupture lifetime are established. Effects of stochastic loading stress and time, fiber volume, matrix crack spacing, interface debonding energy, interface shear stress, and temperature on composite strain, interface debonding and oxidation length, and the fiber failure probability versus time are analyzed. Experimental stress rupture strain and lifetime of SiC/SiC composite under constant stress and stochastic loading spectrums are predicted. When the stochastic loading stress, frequency, and temperature increase, the stress rupture lifetime and the time for the interface complete debonding and oxidation all decrease.

Composition rules of Ni-base single crystal superalloys and its influence on creep properties via a cluster formula approach

The present work investigated the composition evolution of the TMS series of Ni-base single crystal (SC) superalloys in light of the cluster formula approach systematically. The cluster formula of SC superalloys could be expressed with {[}overline{{{text{Al}}}} {-} overline{{{text{Ni}}}} 12{](}overline{{{text{Al}}}} {, }overline{{{text{Cr}}}} {)}m, in which all the alloying elements were classified into three groups, Al series (overline{{{text{Al}}}}), Cr series (overline{{{text{Cr}}}}), and Ni series (overline{{{text{Ni}}}}). It was found that the total atom number (Z) of the cluster formula units for TMS series of superalloys varies from Z ~ 17 to Z ~ 15.5, and then to Z ~ 16 with the alloy development from the 1st to the 6th generation, in which the superalloys with prominent creep resistance possess an ideal cluster formula of {[}overline{{{text{Al}}}} {-} overline{{{text{Ni}}}} 12{](}overline{{{text{Al}}}} 1.5overline{{{text{Cr}}}} 1.5{)} with Z = 16. Similar tendency of composition evolution also appears in the PWA and CMSX series of SC superalloys. Typical TMS series of superalloys with prominent creep properties generally exhibit a moderate lattice misfit of γ/γ′ which could render alloys with appropriate particle size of cuboidal γ′ precipitates to acquire a maximum strength increment by precipitation strengthening mechanism. More importantly, the relationship between the lattice misfit (δ) of γ/γ′ and the creep rupture lifetime (tr) of superalloys was then established, showing a linear correlation in the form of lgtr–lg|δ|3/2 at both conditions of 900 °C/392 MPa and 1100 °C/137 MPa. Combined with the lattice misfit, the cluster formula approach would provide a new way to modify or optimize the compositions of Ni-base superalloys for further improvement of creep property.

Delta processing effects on the creep behavior of a typical Nb-bearing nickel-based superalloy

Since the δ phase is stable at temperatures higher than the normal service conditions but the γ" phase looses its strengthening effects, the formation of large amount of δ phase might be advantageous. In this work, it was revealed that such a delta processed microstructure can be obtained via overaging at 810 °C in Inconel 718 superalloy and its modified version known at Allvac 718Plus. While the peak aged samples (720 °C for 8 h) with the strengthening γ" phase lost their creep resistance at temperatures higher than 649 °C, the delta processed samples showed lower steady state creep rates and higher creep lives (rupture lifetime). Therefore, this study might shed light on the applicability of well-designed delta processing for creep resistance at higher temperatures in similar alloys.

Correlation between failure mechanism and rupture lifetime of 2D-C/SiC under stress oxidation condition based on acoustic emission pattern recognition

Creep tests of 2D-C/SiC in a wet oxidizing atmosphere were implemented for six samples. The loading process was monitored by acoustic emission (AE). Principal component analysis and a fuzzy clustering algorithm were used to perform pattern recognition of the AE data. All of the AE events were divided into four clusters and labelled as matrix cracking, interfacial damage, fiber breakage and fiber-bundle breakage respectively, according to their physical origin. It was found C/SiC has very scattered rupture lifetimes even under the same test conditions, and the evolution of AE events corresponding to fiber failure is quite different. With increasing rupture lifetime, the AE energy of fiber-bundle breakage is higher, while the number of these events is less. Thus, it is concluded that local oxidation and damage development is the controlling failure mechanism for short-lived specimens and uniform oxidation and damage development is the controlling failure mechanism for long-lived specimens.

Correlation of creep fracture lifetime with microstructure evolution and cavity behaviors in G115 martensitic heat-resistant steel

In order to deepen the understanding towards the creep rupture lifetime of G115 martensitic heat-resistant steel, a series of creep tests using 130–200MPa loading stresses at 923K were carried out to investigate the as-crept microstructural characteristics and cavity behaviors. The significant microstructure deformation and a dramatic increase in the areal density of high-angle grain boundaries occur when the stress loading is over 160MPa. Apart from the strain concentration at elongated grain boundaries and refined grains, the elevated stress also leads to the appearance of a strong α-fiber texture with {001}<110>//RD. The nucleation and growth of cavities can be effectively accelerated by the stress elevation, and ductile fracture is determined as the dominate mechanism within the test stresses. However, a transition towards brittle intergranular fracture is observed due to the rapid coarsening of Laves phase. Furthermore, a good prediction for creep lifetimes of G115 steel has been obtained based on the constrained diffusional cavity growth model.

Data Assessment Method to Support the Development of Creep-Resistant Alloys

This work introduces a methodology to assess data quality for the tensile, creep/stress relaxation, and fatigue properties of alloys (as well as metadata associated with manufacture) as a part of a project to develop new materials for extreme environments. The extreme environments in question deal with those found in the power generation sector. Data quality assessment is needed to ensure the reliability of data used in analytics to develop new materials for the power generation sector and to predict the performance of established materials in current use. As data quality metrics have not been standardized for material properties data, quality rating guidelines are developed here for the aspects of data completeness, accuracy, usability, and standardization. The specific design requirements for heat-resistant alloy development were considered in creating each metric. Establishing the quality of a dataset in these areas will enable robust analysis. High-quality data can be set aside to develop predictive models. Lower-quality data need not be discarded but can be used for experimental design. Determining the quality of a materials dataset will also provide additional metadata with the data resource and will promote data reusability. A sample high-quality dataset is presented to indicate the typical data attributes collected from relevant mechanical property testing results, which were considered when generating the data quality metrics. A data template of these attributes was created as a tool for data generators and collectors to promote uniformity and reusability of alloy data. The sparsity of the sample dataset was calculated in order to highlight the areas where data gaps pose a challenge for reliable prediction of creep rupture lifetime.