Intergranular Creep Fracture Research Articles

Beneficial role of the grain boundary precipitates for intergranular creep fracture of 25Cr–20Ni–Nb–N steel

Creep deformation and fracture behavior of pre-aged 25Cr–20Ni–Nb–N steel were examined to determine the effect of the M23C6 phase precipitated on the grain boundary in the initial microstructure. Aging at 873 K, 923 K, and 973 K for 500 h resulted in an area fraction of the precipitates on the grain boundary of 51%, 79%, and 89%, respectively, and the time to rupture at 873 K under 280 MPa increased with the area fraction. Additionally, the creep elongation was significantly improved by aging at 973 K, whereas that at 873 K and 923 K did not. Because the coherent matrix/M23C6 interface exhibited high crack propagation arrestability, the crack propagation path in the microstructure was the incoherent matrix/M23C6 interface and uncovered grain boundaries. The different creep deformation behaviors between the aging at 923 K or below and 973 K are discussed using the percolation theory of the crack propagation path. It was suggested that the enhanced creep properties due to the aging at 973 K were attributed to a sufficiently high area fraction of the precipitates on the grain boundary, which fragmented the crack propagation path and retarded the intergranular creep fracture.

Short-Term Creep-Rupture Behaviour of AISI 310S Austenitic Stainless Steel Sheets Manufactured in Salem Steel Plant, a Special Steels Unit of Steel Authority of India Limited, Ministry of Steel, Government of India

The creep behavior of AISI 310S stainless steel taken from SAIL’s Salem stainless steel plant has been investigated by constant load tensile creep test at the temperatures of 973, 1023, and 1073 K and loads of 66.6, 74.8, 86.6, and 94.8 MPa. It exhibits steadystate creep behavior in most test conditions. The double logarithm plot of rupture life and applied stress yielded straight lines at all the three test temperatures indicating that power-law creep due to dislocation climb is the operating mechanism of creep deformation. Linear relationship was obtained for plots of logarithm of rupture life against inverse temperature obeying Arrhenius type of temperature dependence with activation energy of 340 kJ/mol. The stress-rupture data yielded a master curve of Larson-Miller parameter. The plot of Monkman-Grant relationship is typical indicating that rupture is controlled by growth of grain boundary cavities. The metallographic examination of crept samples revealed formation of grain boundary voids and cracks leading to intergranular creep fracture. Deformation twins and carbide precipitates were also observed. Creep-rupture properties are compared with that of AISI 600 ironbased superalloy to analyze quantitatively its behavior

Comparison of failure analysis on reheater and waterwall tube power plant base on outer surface

Visual observation on waterwall tube failure showed crack at the outer surface and fish mouth on reheater tube. An analysis of this research is to compare the causes of failure of waterwall and reheater tube boiler. Method of this used descriptive qualitative with macro and micro visual observation. Crack appeared from the outside waterwall tube occurred at a place attached to the oxide layer, it indicated that failure on this tube achieved in long period. The small crack as a place for new oxide layer and corrosive media. Larger crystal grains at the edge of the oxide layer and tip of crack because the corrosion reaction between oxides and metal, it produces local heat on the area. Failure on reheater tube was presence of scale attached on the outer caused differences in the distribution of temperature between inner and outer surface tube.The dominant factor of this failure occurred intergranular creep fracture. The results shows that main mechanism on waterwall tube failure is physical and mechanical factor on reheater tube.

Tensile Creep Behavior and Microstructure Evolution of As‐Cast Mg–9.82Gd–0.38Zr Alloy at 250 °C

Herein the tensile creep behavior and microstructure evolution of an as‐cast Mg–9.82Gd–0.38Zr alloy under different stresses at 250 °C are investigated. The results of creep test show that the creep strain and steady creep rate increase with the creep stress at 250 °C. The fitted stress exponent n value (4.4) and transmission electron microscopy analysis suggest that the steady creep is dominated by the dislocation climb and glide mechanisms together. At the primary stage, due to the segregation of the Gd content, a fine‐strengthening β′ phase precipitates near the α‐Mg grain boundary. Then the transformation of β′ precipitates to a needle‐like β phase, coarsening, and growth of the β phase toward the intragranular direction gradually deteriorate the creep resistance of the experimental alloy. Moreover, two kinds of precipitate‐free zones (PFZs) appear at the end of the secondary creep stage and the PFZs nearly perpendicular to the applied stress direction widen gradually because of the directional diffusion. The nucleation and propagation of creep crack along the cracked eutectic phase and PFZs lead to the greatly rising creep rate at the end of the tertiary creep stage, which eventually results in the intergranular creep fracture of the experimental alloy.

Thickness dependency of creep crack propagation mechanisms in submicrometer-thick gold films investigated using in situ FESEM and EBSD analysis

To clarify the thickness dependency of creep crack propagation mechanisms, creep crack propagation experiments were carried out on submicrometer-thick metallic films; freestanding gold films with thicknesses of ~120 nm (thinner) and ~340 nm (thicker) were subjected to in situ field emission scanning electron microscopy (FESEM) and electron backscatter diffraction (EBSD) analysis. In both the thicker and thinner films, creep crack stably propagated at room temperature by repeating the processes of (i) stretch and local thinning ahead of the creep crack, (ii) void formation at grain boundaries in the creep-damaged region, and (iii) void growth and coalescence of voids or that between the main crack and voids. The creep-damaged region ahead of the creep crack tip was narrower, shorter and shallower in the thinner films compared to that of the thicker films. In the thicker films, the creep cracks propagated through transgranular creep fracture, whereas in the thinner films, the creep cracks propagated by transgranular and intergranular creep fracture, indicating that creep crack propagation mechanisms depend on film thickness.

Type IV failure in weldment of creep resistant ferritic alloys: II. Creep fracture and lifetime prediction

The creep rupture behavior (Type IV failure) for weldments of creep strength enhanced ferritic steel is numerically analyzed, using an integrated microstructure- and micromechanics-based finite element model. To account for the large microstructure gradients across weldments, a two-dimensional digital microstructure is constructed based on the actual observed microstructure of ferritic steel weldment by using the Voronoi-tessellation method. According to the fracture mechanism studies and literature experimental observations, the Type IV failure is identified as an intergranular creep fracture in the fine-grained or intercritical heated affected zone (FGHAZ or ICHAZ). In the present study, the following micromechanics model is employed to determine the micromechanical and microstructural origins for the failure process above, accounting for the underlying physical fracture mechanisms at different length scales, including nucleation of grain boundary cavities, their growth by competition of grain boundary diffusion and grain interior creep, viscous grain boundary sliding, and the emergence of microcracks by coalescence and their evolution to the ultimate failure. The methodology demonstrates the capabilities in modeling the Type IV failure and providing quantitative creep rupture lifetime prediction which shows an excellent agreement with long-term creep experimental data for creep strength enhanced ferritic steels and their weldments. In particular, the drop-off in time to rupture at high temperatures and low stress levels in the creep rupture curves is quantitatively predicted, and the transition of failure mechanisms from creep-controlled to diffusion-controlled creep fracture mechanism is illustrated.

Evolution of damage under combined low and high cycle fatigue loading in a type 316LN stainless steel at different temperatures

The present study investigates the effect of different damage modes like low cycle fatigue (LCF), high cycle fatigue (HCF), creep and ratcheting during combined cycling at various temperatures ranging from ambient to 923K, in a type 316LN austenitic stainless steel. The experiments were designed with multi-step load sequences where specific number of small amplitude HCF cycles (referred to as blocks) were introduced at the stabilized cyclic load under LCF for a given strain amplitude and repeated until failure. Cyclic life was found to decrease with increase in temperature as well as block-size. The decrease in cyclic life with block-size is more significant at 923K where multiple damage modes like creep and ratcheting are activated. Dynamic strain aging (DSA) was found to operate in the temperature range, 823–873K where the decrease in cyclic life with block-size gets saturated. Typically, transgranular fatigue fracture, intergranular creep fracture or dimpled rupture was identified when failure was dictated by LCF, creep and ratcheting respectively. However, synergistic interaction between the above damage modes leading to a mixed mode fracture carrying signatures of fatigue striations, intergranular facets and dimples occurred at specific combinations of block size and temperature. HCF damage played an important role for some specific loading conditions by acting as a link between intergranular (creep) cracks, thus facilitating the crack propagation and final failure. The regimes of dominant failure modes and interactions among them were suitably mapped as a function of temperature and block size.

Mechanisms of failure under low cycle fatigue, high cycle fatigue and creep interactions in combined cycling in a type 316LN stainless steel

The present study investigates the effect of different damage modes like low cycle fatigue (LCF), high cycle fatigue (HCF), creep and their interactions during combined cycling at high temperature (923K) in a 316LN stainless steel. The experiments were designed with multi-step load sequences where specific number of small amplitude HCF cycles (block) were introduced at the stabilized cyclic load under LCF for a given strain amplitude and repeated until failure. Cyclic life was found to decrease with the increase in HCF block size. However, the extent of decrease in cyclic life also depends on the LCF strain amplitude, which is attributed to the additional damage incurred by creep and ratcheting. Creep damage was found to be opposed by a strong compressive ratcheting originating from Bauschinger effect, resulting in net strain accumulation in tensile or compressive direction depending on whichever damage process of the two is dominant. Typically, fatigue fracture, intergranular creep fracture or creep rupture can be identified when failure was dictated by LCF or creep. However, failure life was actually found to be governed by multiple damage interactions between LCF, HCF and creep for specific loading conditions, as emphasized through detailed fracture surface investigations. HCF damage was found to act as a catalyst by joining small transgranular (LCF) or intergranular (creep) cracks, thus facilitating the crack propagation and final failure by the respective modes. This leads to strong synergistic LCF-HCF-creep or HCF-creep interaction, the regimes of which were suitably mapped as a function of LCF strain amplitude and block size.

Creep Behavior and Microstructure Evolution of Sand–Cast Mg–4Y–2.3Nd–1Gd–0.6Zr Alloy Crept at 523–573 K

High temperature tensile–creep behavior of Mg–4Y–2.3Nd–1Gd–0.6Zr (wt%, WE43(T6)) alloy at 523–573 K was investigated. The creep stress exponent is equal to 4.6, suggesting the underlying dislocation creep mechanism. The activation energy is (199 ± 23) kJ/mol, which is higher than that for self-diffusion in Mg and is believed to be associated with precipitates coarsening or cross slip. The creep mechanism is further suggested to be dislocation climb at 523 K, while a cross slip at 573 K is possible. The metastable β′ and β1 phases in the WE43(T6) alloy were relatively thermal stable at 523 K and could be effective to hinder the dislocation climb, which contributed to its excellent creep resistance. However, at 573 K it readily transforms into equilibrium βe phase and coarsens within two hours, thereby causing a decrease of creep resistance. In addition, precipitate free zones approximately normal to applied stress direction (directional PFZs) developed during the creep deformation, especially at 573 K. Those zones became preferential sites to nucleate, extend and connect microcracks and cavities, which lead to the intergranular creep fracture. Improving the thermal stability of precipitates or introducing thermally stable fine plate-shaped precipitates on the basal planes of Mg matrix could enhance the high temperature creep resistance.

On High-Temperature Materials: A Case on Creep and Oxidation of a Fully Austenitic Heat-Resistant Superalloy Stainless Steel Sheet

The creep behavior of AISI 310S stainless steel taken from SAIL’s Salem stainless steel plant has been investigated by constant load tensile creep test at the temperatures of 973, 1023, and 1073 K and loads of 66.6, 74.8, 86.6, and 94.8 MPa. It exhibits steady-state creep behavior in most test conditions. The double logarithm plot of rupture life and applied stress yielded straight lines at all the three test temperatures indicating that power-law creep due to dislocation climb is the operating mechanism of creep deformation. Linear relationship was obtained for plots of logarithm of rupture life against inverse temperature obeying Arrhenius type of temperature dependence with activation energy of 340 kJ/mol. The stress-rupture data yielded a master curve of Larson-Miller parameter. The plot of Monkman-Grant relationship is typical indicating that rupture is controlled by growth of grain boundary cavities. The metallographic examination of crept samples revealed formation of grain boundary voids and cracks leading to intergranular creep fracture. Deformation twins and carbide precipitates were also observed. Oxidation tests were also carried out isothermally at 973 K, 1023 K, and 1073 K in dry air. The plots of mass gain versus square root time were linear at all the three test temperatures obeying parabolic kinetics of oxidation. It was found that scales are well adherent to the substrate. The plot of parabolic rate constant and inverse temperature was linear giving an activation energy value of 210 kJ/mol. The metallographic examination of an oxidized sample reveals duplex types of scales. Finally, rupture properties are compared with that of AISI 600 iron-based superalloy and oxidation weight gain analysis with surface nanocrystalline AISI 310S stainless steel to analyze quantitatively its behavior.

Stress Rupture Fracture Model and Microstructure Evolution for Waspaloy

Stress rupture behavior and microstructure evolution of nickel-based superalloy Waspaloy specimens from tenon teeth of an as-received 60,000-hour service-exposed gas turbine disk were studied between 923 K and 1088 K (650 °C and 815 °C) under initial applied stresses varying from 150 to 840 MPa. Good microstructure stability and performance were verified for this turbine disk prior to stress rupture testing. Microstructure instability, such as the coarsening and dissolution of γ′ precipitates at the varying test conditions, was observed to be increased with temperature and reduced stress. Little microstructure variation was observed at 923 K (650 °C). Only secondary γ′ instability occurred at 973 K (700 °C). Four fracture mechanisms were obtained. Transgranular creep fracture was exhibited up to 923 K (650 °C) and at high stress. A mixed mode of transgranular and intergranular creep fracture occurred with reduced stress as a transition to intergranular creep fracture (ICF) at low stress. ICF was dominated by grain boundary sliding at low temperature and by the nucleation and growth of grain boundary cavities due to microstructure instability at high temperature. The fracture mechanism map and microstructure-related fracture model were constructed. Residual lifetime was also evaluated by the Larson–Miller parameter method.

High-temperature creep behavior of Al–Cu–Mg alloy

Abstract The high-temperature creep behavior of Al–Cu–Mg alloy was studied by the constant-stress uniaxial tensile creep experiments under the temperatures of 473 K, 503 K and 533 K. Constitutive models for describing the high-temperature creep behavior of Al–Cu–Mg were established based on the θ projection method. The material parameters of the established constitutive models are associated with applied creep stress and temperature. Results show that a good agreement between the experimental and predicted results is obtained, which confirms that the established creep constitutive models can give an accurate and precise estimate of the high-temperature creep behavior for the studied Al–Cu–Mg alloy. Additionally, the specimens at 533 K failed by the intergranular creep fracture.

Intermediate temperature embrittlement of copper alloys

<title/>Copper and its alloys generally display a severe reduction in ductility between roughly 300 and 600°C, a phenomenon variously called ‘intermediate temperature embrittlement’ or ‘ductility trough behaviour’. This review of the phenomenon begins by placing it in the wider context of the high-temperature fracture of metals, showing how its occurrence can be rationalised in simple terms on the basis of what is known of intergranular creep fracture and dynamic recrystallisation. Data in the literature are reviewed to identify main causes and mechanisms for embrittlement, first for pure copper, and then for monophase and multiphase copper alloys. Coverage then turns to the ‘grain boundary embrittlement’ phenomenon, caused by the intergranular segregation of even minute quantities of alloying additions or impurities, which appears to worsen dramatically the intermediate temperature embrittlement of copper alloys. Finally, metal-induced embrittlement, including in particular liquid metal embrittlement, is presented as a second mechanism leading to an exacerbation of the intermediate temperature embrittlement of copper and its alloys.

Fracture mechanism maps in unirradiated and irradiated metals and alloys

This paper presents a methodology for computing a fracture mechanism map in two-dimensional space of tensile stress and temperature using physically-based constitutive equations. Four principal fracture mechanisms were considered: cleavage fracture, low temperature ductile fracture, transgranular creep fracture, and intergranular creep fracture. The methodology was applied to calculate fracture mechanism maps for several selected reactor materials, CuCrZr, 316 type stainless steel, F82H ferritic–martensitic steel, V4Cr4Ti and Mo. The calculated fracture maps are in good agreement with empirical maps obtained from experimental observations. The fracture mechanism maps of unirradiated metals and alloys were modified to include radiation hardening effects on cleavage fracture and high temperature helium embrittlement. Future refinement of fracture mechanism maps is discussed.

Creep Fracture Mechanism Map and Creep Damage of Cr–Mo–V Turbine Rotor Steel

Specimens of a Cr–M–V rotor steel with creep rupture lives of up to 100000 h were examined metallographically to clarify their creep fracture mechanisms and to construct creep fracture mechanism maps showing the dominant regions of each mechanism. The creep fracture mechanism maps were constructed with stress on the y-axis and time to rupture on the x-axis, and also with stress on the y-axis and testing temperature on the x-axis. The maps included three different creep fracture mechanism fields: transgranular creep fracture, intergranular creep fracture with cavitation, and rupture with dynamic recrystallization. The maps indicate that the steel in the service of power plants is being used under conditions within the field of intergranular creep fracture with cavitation. This means that long term use of this steel under the conditions could cause extensive cavitation and low ductility fracture due to the cavitation.

Evaluation of the mechanical properties of a ternary Sn-20In-2.8Ag solder

The Sn-20In-2.8Ag solder alloy is a potential lead-free solder for replacing the traditional Sn-Pb solders. In this study, the mechanical properties of the bulk material are reported by tensile test at various strain rates and temperatures. The Sn-20In-2.8Ag solder possessed a solidus and liquidus between 170.8°C and 195.5°C. The ultimate tensile strength (UTS) and elongation were 59.3 MPa and 50.2% at a strain rate of 10-3 s-1 at room temperature. Moreover, the UTS of this alloy decreased, but its elongation increased, with increasing testing temperature. Stress exponents of Sn-20In-2.8Ag alloy varied from 6.5 at room temperature to 4 at 100°C, and the activation energy for creep was 51.0 kJ/mol at the higher temperature range from 50°C to 100°C. The typical intergranular creep fracture mode was observed in Sn-20In-2.8Ag solder during tensile creep deformation.

Effect of Stress States on Creep Fracture Modes of an Austenitic Steel with High Ductility.

Tensile creep rupture tests using smooth specimens and circumferential U-notched specimens having 4 kinds of notch root radius, and torsional creep rupture tests using smooth and circumferential 60° V-notched specimens of SUS310S austenitic steel with high ductility and high metallographical stability, were conducted at 700°C. In tensile creep rupture tests, it was found that as turning into lower stress level the creep fracture mode of smooth specimens changed from ductile transgranular fracture to brittle intergranular fracture. And irrespective of applied stress and notch geometry, the creep fracture mode of all notched specimens showed brittle intergranular fracture mixed with ductile transgranular fracture. While in torsional creep rupture tests, all smooth and notched specimens in the present test range exhibited a ductile fracture of shear type. These creep fracture modes were discussed on the basis of stress states and steady-state creep stress distributions in notch cross sections calculated by finite element method. The results suggested that mean stress promotes the intergranular creep fracture strongly, and that the equivalent stress promotes the nucleation of intergranular cavities but contributes less to their growth.

Environmental sensitivity and mechanical behavior of boron-doped Fe–45at.%Al intermetallic in the temperature range from 77 to 1000 K

Abstract The tensile properties and fracture behavior of a coarse-grained (grain size ∼420 μm) Fe–45at.%Al intermetallic doped with 0.05 at.% boron were examined at ambient temperature in air, argon and vacuum as well as in the 77–1000 K temperature range in liquid nitrogen, dry ice and air. Before testing the alloy was low temperature annealed (vacancy annealed) in order to remove all the retained vacancies. At ambient temperature ductility increases accordingly to decreasing water vapor (moisture) content in each environment. The mixed transgranular cleavage (TGC)+intergranular failure (IGF) mode in vacuum, which is associated with the highest elongation (∼6%), exhibits around 40% of IGF and the mixed fracture mode in argon, which is associated with the second highest elongation (∼3.2%), exhibits ∼15% of IGF. The TGC fracture mode in air is associated with the lowest elongation (∼1%). Elongation in the cryogenic temperature range from 77 to 213 K is very low being in the range from 0.6 to 2.8%, and is associated with a mixed transgranular+intergranular fracture mode. Elongation increases gradually from 300 to 800 K attaining a ductility peak at 800 K and then decreases rapidly with increasing temperature. At 600–800 K, the yield strength of Fe–45Al–0.05B exhibits anomalous temperature dependence with the yield strength peak at 800 K. The mode of fracture from 300 to 700 K is predominantly TGC and that at the ductility peak is ductile rupture with very deep dimples. At temperatures above 800 K the mode of fracture changes to a typical intergranular creep (fibrous) failure with numerous flat dimples (voids/cavities) at the grain boundary facets, which is associated with a tensile ductility drop. Fine particles (borides) are observed at the grain boundary facets, which assist the development of intergranular creep fracture.

Microstructurally-based modelling of intergranular creep fracture using grain elements

Abstract A new numerical method is proposed to simulate intergranular creep fracture in large polycrystalline aggregates. The method utilizes so-called grain elements to represent the polycrystal. These grain elements take care of the average elastic and creep deformation of individual grains. Grain boundary processes, like cavitation and sliding, are accounted for by grain boundary elements connecting the grains. Results are compared with full-field finite element calculations. The method is demonstrated to capture the essential features of creep fracture, like creep constrained cavitation and the interlinkage of microcracks. Also the performance in polycrystals with random variations in microstructure, in terms of grain shape, is shown to be reasonably well. For the size of the unit-cell considered, a factor of around 600 is gained in computer time as compared with the full-field calculations.

Micromechanical Studies of Cyclic Creep Fracture Under Stress-Controlled Loading

This paper deals with a study of intergranular failure by creep cavitation under stress-controlled cyclic loading conditions. Loading is assumed to be slow enough that diffusion and creep mechanisms (including grain boundary sliding) dominate, leading to intergranular creep fracture. This study is based on numerical unit cell analyses for a planar polycrystal model with the grains and grain boundaries modeled individually, in order to investigate the interactions between the mechanisms involved and to account for the build-up of residual stress fields during cycling. The behaviour of a limiting case with a facet-size microcrack yields important insight for damage accumulation under balanced loading. Under unbalanced loading, the time-average accumulation of creep cavitation gives rise to macroscopic ratchetting, while its rate is demonstrated to depend subtly on material and loading conditions.