Intergranular Cracking Mechanism Research Articles

Acceleration Mechanism of Intergranular Cracking of Ni-Base Superalloy GH4169 under Creep Loading at Elevated Temperatures due to the Growth of δ-Phase

In order to quantitatively clarify the effect of δ-phase precipitation on the strength of grain boundary in Ni-based superalloy GH4169 at elevated temperatures, which is expected to be used in aircraft engines, intermittent creep tests at 800°C were conducted on specimens, and the microstructural changes were analyzed using EBSD analysis. The effects of δ-phase precipitation and creep damage were evaluated by micro tensile tests. It was confirmed that there is a large lattice mismatch between the δ-phase precipitated around the grain boundary and the matrix phase, and that dislocations and voids accumulate in the grain boundary. It was also suggested that the δ phase precipitation may be a stress-induced diffusion phenomenon. Furthermore, it was confirmed that the strength of this alloy decreased in the early period of creep, influenced by both creep damage and the precipitation of δ-phase.

Revealing two distinctive intergranular cracking mechanisms of Ni-rich layered cathode by cross-sectional scanning electron microscopy

Thorough understanding of intergranular cracking mechanism is essential and important for developing superior layered cathodes. As a chemomechanical failure, crack's nucleation and evolution are affected mainly by cycling protocols, but it is still lacking direct and accurate observations. Herein, we develop an experimental protocol to visualize the cracking evolution process by tracing the same secondary particles. Further combining large area cross-sectional observations, we identify two distinctive cracking mechanisms due to different state of charge (SOC). At low SOC, chronic fatigue cracking is dominant. At high SOC, particle bursting makes intergranular cracks quickly saturated during the initial cycles, causing battery rapid performance decay in the beginning. We further validate that cracks are nucleated at the end of charging process and it is the discharging that leads to high density of cracks. Managing individual secondary particle below the critical SOC to prevent particle bursting is essential for achieving high cycling stability of Ni-rich layered cathodes.

Fundamental Study on the Mechanism of Intergranular Cracking in Polycrystalline Ni-Based Superalloys by using Molecular Dynamics Analysis

Fundamental Study on the Mechanism of Intergranular Cracking in Polycrystalline Ni-Based Superalloys by using Molecular Dynamics Analysis

Acceleration mechanism of Intergranular Cracking of SUS316L under Creep-Fatigue Loading at Elevated Temperatures

Acceleration mechanism of Intergranular Cracking of SUS316L under Creep-Fatigue Loading at Elevated Temperatures

Clarification of the Initiation Mechanism of Intergranular Cracking of Ni-base Superalloy 617 under Creep Loading at Elevated Temperature and Proposal of its Prediction Method

Clarification of the Initiation Mechanism of Intergranular Cracking of Ni-base Superalloy 617 under Creep Loading at Elevated Temperature and Proposal of its Prediction Method

Dynamic Strain Aging and Embrittlement Behavior of IN718 During High-Temperature Deformation

The high-temperature deformation behavior of nickel-base superalloy IN718 was investigated in the solution-treated (ST) condition. High-temperature tensile tests were performed between 600 °C and 850 °C at strain rates of 1 × 10−3, 1 × 10−2, and 1 × 10−1 s−1. The deformation behavior of this material was analyzed using optical microscopy, scanning electron microscopy, and transmission electron microscopy. In the investigated temperature–strain rate regime, material undergoes partial precipitation, serrated yielding, and embrittlement. Serrated yielding was observed at 600 °C, 650 °C, and 700 °C and is attributed to dynamic strain aging. The appearance of serrated flow at high temperatures up to 700 °C in the ST condition can be attributed to the availability of excess Nb in the matrix. Beyond 700 °C, Nb concentration significantly decreases in the matrix due to the formation of Ni3Nb precipitate. Nb is responsible for the appearance and disappearance of serrations at high temperature. The alloy exhibits embrittlement phenomenon in the range of 750 °C to 850 °C when thermally exposed in air. The alloy shows a ductile mode of fracture when tested at 600 °C and 700 °C, whereas completely brittle fracture was observed at the 800 °C test temperature. Formation of brittle oxides at grain boundaries in the presence of atmospheric oxygen resulted in the embrittlement of the alloy at 750 °C to 850 °C. An oxidation-assisted intergranular cracking mechanism is responsible for embrittlement of this alloy, which was proved by scanning transmission electron microscopy-energy dispersive spectroscopy.

Damage characterization of heat resistant stainless steel grate plates in an iron ore pelletizing plant

Grate plate is one of the consumable components of chain grate in pelletizing plants, which is made up of heat resistant stainless steels. Service life of grate plates has major effect on productivity and quality of pellets. Objective of this work is to study the mechanism of intergranular cracking in heat-resistant stainless steel, one of the major degradation mechanisms of grate plates, in order to suggest preventive methods. To reach this goal, plant samples were provided from used and as-cast grate plates. The samples were studied using a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectroscopy (EDS) probe and metallography techniques. The results revealed that no Cr-depletion zone was formed during solidification and also during using of the grate plates at elevated temperatures. It was also revealed that formation of interdendritic Mn, S and/or C concentrated precipitates is the reason of the intergranular crack nucleation and propagation during the service time. As a solution to prevent that defect, it is suggested to reduce the distance between the liquid impenetrable temperature (LIT) and solidus temperature during solidification (for example with decreasing the concentration of S content).

Acceleration mechanism of Intergranular Cracking of SUS316L under Creep-Fatigue Loading at Elevated Temperatures

Acceleration mechanism of Intergranular Cracking of SUS316L under Creep-Fatigue Loading at Elevated Temperatures

Molecular Dynamics Analysis on the Intergranular Cracking Mechanism of Ni-base Superalloy due to the Accumulation of Vacancies around a Grain Boundary

Molecular Dynamics Analysis on the Intergranular Cracking Mechanism of Ni-base Superalloy due to the Accumulation of Vacancies around a Grain Boundary

Intergranular Cracking Mechanism of the Stainless Steel SUS316LN under Creep Loading at Elevated Temperature

Intergranular Cracking Mechanism of the Stainless Steel SUS316LN under Creep Loading at Elevated Temperature

Grain Boundary Serration in Nickel-Based Superalloy Inconel 600: Generation and Effects on Mechanical Behavior

Grain boundary serration in the superalloy Inconel 600 was studied. Two microstructural variants, one with nonserrated and the other with serrated grain boundaries were generated by altering the heat-treatment conditions, while keeping other aspects of the microstructure unchanged. The effect on the creep response between 700 °C and 900 °C was measured, and the different failure modes and accumulated damage were quantified using high-angular resolution electron backscatter diffraction analysis in the scanning electron microscope and also by X-ray computed tomography. It is found that serration plays a more crucial role in the high-temperature/low-stress regime when an intergranular cracking mechanism involving cavitation is operative; here it plays a role in improving both creep life and creep ductility. Any effect of serration is less prevalent at low temperatures where transgranular failure is dominant.

Diffusion-coupled cohesive interface simulations of stress corrosion intergranular cracking in polycrystalline materials

To study the stress corrosion intergranular cracking mechanism, a diffusion-coupled cohesive zone model (CZM) is proposed for the simulation of the stress-assisted diffusional process along grain boundaries and the mechanical response of grain boundary sliding and separation. This simulation methodology considers the synergistic effects of impurity diffusion driven by pressure gradient and degradation of grain boundary strength by impurity concentration. The diffusion-coupled CZM is combined with crystal plasticity finite element model (CPFEM) to simulate intergranular fracture of polycrystalline material under corrosive environment. Significant heterogeneity of the stress field and extensive impurity accumulation is observed at grain boundaries and junction points. Deformation mechanism maps are constructed with respect to the grain boundary degradation factor and applied strain rate, which dictate the transition from internal to near-surface intergranular fracture modes under various strain amplitudes and grain sizes.

A study of intergranular fracture in an aluminium alloy due to hydrogen embrittlement

This work concerns a study of the effects of plasticity on the mechanism of intergranular cracking assisted by hydrogen induced embrittlement in an aluminium alloy. Here, tensile specimens charged with hydrogen were used to investigate quantitatively the effect of plastic deformation on the mechanism of intergranular crack initiation at the scale of the individual grains. An experimental procedure was set up to monitor the evolution of surface strain fields on in situ tested SEM notched specimens using digital image correlation techniques. In addition, measurements of the associated crystal orientation evolution at the micron scale were carried out using electron backscatter diffraction (EBSD). These measurements were then compared with finite element predictions of the local strain fields on the observed regions of the in situ specimen. The numerical predictions were obtained using a dislocation mechanics-based crystal plasticity model to describe the constitutive behaviour of each individual grain. The crystallographic grain orientations of the region of interest were discretised for the finite element analyses from EBSD maps. From this study, it was found that intergranular cracking due to hydrogen embrittlement in the Al alloy is locally triggered by high tensile grain boundary tractions, here estimated to be 170±35MPa. As importantly, the results also revealed that the conditions needed for grain boundary microcracks to initiate are greatly affected by the deformation of neighbouring grains: i.e. it was established that boundaries between two “hard” grains, inside a neighbourhood of “softer” deformed grains, are the first to fail.

Cracking behaviour of PVD tungsten coatings deposited on steel substrates

Tungsten coatings have been deposited on steel substrates by magnetron sputtering. For the same processing conditions, the increase of the coating thickness enhances the (111) component of the crystallographic texture whereas the residual stress level decreases. Tensile and four-point bending tests, associated with an acoustic emission analysis, have been performed inside a SEM chamber in order to study the cracking mechanisms. When the residual stresses are taken into account, an intrinsic critical cracking stress and the associated energy release rate can be determined; the obtained values suggest an intergranular cracking mechanism. No debonding has been observed at the interface despite the large plastic deformation of the substrate at the crack tips. The observed strain localisation modes in the substrate near the interface have been shown to have a major influence on the limit crack density.

Fracture toughness of alloy 690 and EN52 welds in air and water

The effect of low- and high-temperature water with high hydrogen on the fracture toughness of alloy 690 and its weld, EN52, was characterized using elastic-plastic JIC methodology. While both materials display excellent fracture resistance in air and elevated-temperature (>93 °C) water, a dramatic degradation in toughness is observed in 54 °C water. The loss of toughness is associated with a hydrogen-induced intergranular cracking mechanism, where hydrogen is picked up from the water. Comparison of the cracking behavior in low-temperature water with that for hydrogen-precharged specimens tested in air indicates that the critical local hydrogen content required to cause low-temperature embrittlement is on the order of 120 to 160 ppm. Loading-rate studies show that cracking resistance is improved at rates above ∼ 1000 MPa √m/h, because there is insufficient time to produce grain-boundary embrittlement. Electron fractographic examinations were performed to correlate cracking behavior with microstructural features and operative fracture mechanisms.

Fracture toughness of alloy 600 and an EN82H weld in air and water

The fracture toughness of alloy 600 and its weld, EN82H, was characterized in 54 °C to 338 °C air and hydrogenated water. Elastic-plastic J IC testing was performed due to the inherent high toughness of these materials. Alloy 600 exhibited excellent fracture toughness under all test conditions. While EN82H welds displayed excellent toughness in air and high-temperature water, a dramatic toughness degradation occurred in water at temperatures below 149 °C. Comparison of the cracking response in low-temperature water with that for hydrogen-precharged specimens tested in air demonstrated that the loss in toughness is due to a hydrogen-induced intergranular cracking mechanism. At loading rates above ∼1000 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9vqpe0x% c9q8qqaqFn0dXdir-xcvk9pIe9q8qqaq-dir-f0-yqaqVe0xe9Fve9% Fve9qapdbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaie% aacaWFTbaaleqaaOGaai4laiaa-Hgaaaa!3BD6! $$\sqrt m /h$$ , the toughness in low-temperature water is improved because there is insufficient time for hydrogen to embrittle grain boundaries. Electron fractographic examinations were performed to correlate macroscopic properties with key microstructural features and operative fracture mechanisms.

Influence of stress state on high temperature fatigue crack growth in Inconel 718

The influence of stress state on fatigue crack growth in nickel‐base superalloys at high temperature is considered, based on studies in corner crack specimens of Inconel 718 at 600 °C. At high frequency and low R, cycle‐dependent trans‐granular crack growth occurs along the whole crack front, and growth rates are similar at the surface and within the interior of specimens, maintaining the original quarter‐circular shape. For conditions of low frequency and high R, increased crack growth rate per cycle is observed with the crack tunnelling ahead at the centre. A time‐dependent intergranular crack propagation mode occurs in the plane strain interior, attributed to an oxidation mechanism, whereas near the surfaces under plane stress, a trans‐granular cyclic plasticity mechanism is observed. It is proposed that in addition to frequency and R, that stress state influences the competition between the mechanisms controlling crack growth and the transition between them: plane strain in the interior favouring an oxidation‐controlled intergranular cracking mechanism as compared with the plane stress surfaces where cyclic plasticity dominates. An FEM study suggests that this influence of stress state is not associated with variation of ΔK along the crack front.

Environmentally-induced cracking of zirconium alloys — A review

The general field of environmentally-induced cracking of zirconium alloys has been reviewed and the phenomena that are observed and the progress in understanding the mechanisms are summarized. The details of the industrially important pellet-clad interaction failures of nuclear reactor fuel have been left for a companion review and only observations on the mechanism are summarized briefly here. It is concluded that in the zirconium alloy system, by virtue of the physical peculiarities of the System, it is easier to reach unambiguous conclusions about the environmental cracking mechanisms that are operating than with other systems. Thus, chemical dissolution in either liquid or vapour phase is thought to be the principal mechanism for intergranular cracking, while adsorption-induced embrittlement is thought to be the most common transgranular quasi-cleavage process. Hydrogen embrittlement in this system can be identified because it requires precipitated hydride that gives characteristic fractography when cracked. Only in a few instances does stress-corrosion cracking appear to proceed by a hydride cracking mechanism.

The mechanism of intergranular cracking of Ni-Cr-Fe alloys in sodium tetrathionate

A series of electrochemical, immersion, and constant extension rate tests was conducted on samples of Ni-16Cr-9Fe in sodium tetrathionate at room temperature. Samples were heat treated to produce severe chromium depletion at the grain boundaries. Titrimetric analysis of the tetrathionate solution, before and after exposure to a sensitized alloy, under an applied cathodic current shows that the tetrathionate ion is reduced. The species primarily responsible for the observed IGA in immersion tests and IG cracking in constant extension rate tests is the tetrathionate ion, S4O6 -, although elemental S also causes shallow IGA. The mechanism responsible for the observed IGA and IG cracking in sensitized Ni-Cr-Fe alloys is stress assisted intergranular attack with the effect of stress being purely mechanical in nature. The degree of IGA and IG cracking is directly related to the grain boundary chromium content. Samples with less than 5 wt pct Cr at the grain boundary are rapidly attacked while those with 8 wt pct Cr are less susceptible and 12 wt pct Cr renders the grain boundary immune to attack. Lower extension rates and higher Na2S4O6 concentrations represent more aggressive conditions for attack.

The mechanism of stress-corrosion cracking in the brass-ammonia system

The effect of corrosive environments, compositions of alloying elements and degree of pre-strain on the stress-corrosion cracking of the brass-ammonia system, has been investigated and the mechanism of intergranular cracking and of the transition from intergranular to transgranular cracking have been discussed. Intergranular cracking arises from the formation of thick and large cuprous oxide grains over slip steps and the resulting locking of the movement of dislocations. The transition from intergranular to transgranular cracking depends on the mechanical properties of the surface film produced in a corrosive environment. The mechanism of the stress-corrosion cracking has been explained on the basis of the film theory.