Intergranular Cracking Research Articles

Insights into the Influence of Tensile and Compressive Strain on the Microstructure and Corrosion Performance of 304 L Stainless Steel

The effects of tensile and compressive strain, originating from U-bent deformation, on the corrosion behavior of 304 L stainless steel were studied via analyses of the material’s microstructure and electrochemistry in a 3.5% NaCl solution. In contrast with the as-received 304 L steel with the largest grain size, the deformed 304 L material with a small grain size had the lowest number of Σ3 grain boundaries and an overall low fraction, with special low-Σ values (≤29). Moreover, the dislocation density increased to 1.13 × 1016/m2 and 1.4 × 1016/m2 for the tensile and compressive 304 L steel testing, respectively. The decrease in Epit and increase in ipit suggested that there was a decrease in anti-corrosion properties due to tensile and compressive deformation. This might be attributed to the higher plastic strain found in deformed 304 L steel, which can induce the rupture of passive film and have a harmful influence on corrosion resistance. In particular, the compressive 304 L steel with the highest content of deformed grains (42.12%) promoted the formation of microgalvanic cells, thereby facilitating the nucleation of pits. Then, these pits grew to a large size through grain shedding. Subsequently, massive chloride ions were generated during metal dissolution and diffused along grain boundaries, which promoted the initiation and propagation of intergranular corrosion cracks.

Effect of grain size and grain boundary type on intergranular stress corrosion cracking of austenitic stainless steel: A phase-field study

A phase-field model coupled with polycrystalline microstructure is utilized to investigate the effect of grain size and grain boundary types on intergranular stress corrosion cracking in austenitic stainless steels. Considering the dilute solution environment, the free energy density of the two phases is hypothesized to be in parabolic form. The slower corrosion crack propagation rate in coarse-grained microstructures can be attributed to a higher frequency of transgranular cracking. Low-angle grain boundaries can effectively deflect intergranular corrosion cracks into grains with lower corrosion susceptibility, thereby impeding crack propagation. Twin boundaries mitigate corrosion crack propagation by reducing potential initiation sites.

Effect of Heat Input on Tin Bronze-Induced Intergranular Cracks During Arc Cladding Process

This work aimed to figure out the effect of heat input on the characteristics, formation, and elimination of liquid tin bronze-induced intergranular cracks in steel sheets with a thickness of 2 mm. Tin bronze cladding layers were prepared using an arc cladding technique on the steel. A statistical method was adopted to analyze the severity of intergranular cracks. Microstructures and intergranular cracks were characterized by SEM and TEM. The tensile experiments were carried out using an electronic universal testing machine. For the bare steel sheets, the intergranular cracks originated from the cladding layer and propagated into the interior of the steel along the grain boundaries. The intergranular cracks could evolve into macrocracks and lead to the failure of steel. With the increase in heat input, the maximum temperature, maximum stress, and contact time between steel and liquid tin bronze increased. The severity of intergranular cracks was also increased, and the longest crack reached 520 μm. The mechanical properties of the steel sheets decreased with the increase in heat input. For nickel-plated steel sheets, intergranular cracks were eliminated under low heat input, and a transition layer with a nickel content of 12.32 wt.% was generated. The intergranular cracks generated under high heat input and nickel content in the transition layer were only 1.34 wt.%. The strength of the nickel-plated steel also decreased drastically, and the ductility was almost zero.

Grain boundary sensitization kinetics of cold-rolled Al–Mg alloys

As super-saturated solid solutions of Al-Mg, 5XXX series aluminum alloys are susceptible to sensitization via intergranular precipitation of the anodic β-phase, which promotes intergranular corrosion, exfoliation and stress corrosion cracking under environmental conditions. This study presents important updates to a Johnson–Mehl–Avarami–Kolmogorov (JMAK) type model for low-temperature sensitization, correlating the intergranular corrosion response to impingement of locally sensitized regions surrounding discrete β-phase grain boundary precipitates. It is demonstrated that the sensitization response of these alloys can be approached as a combination of two independent contributions: the geometric configuration of grain boundaries passing through the microstructure that are most prone to sensitization, and the rate that these boundaries sensitize due to the formation of the β-phase. This allows for the large sensitization response variations found between nominally identical materials produced by different suppliers, which originate due to a lack of constraints within current cold-rolled plate tempers, to be removed as a sample-dependent linear scaling factor that is separate of the rate kinetics. The JMAK model describes the kinetics of 5xxx series sensitization with excellent accuracy across all data available in the literature. The results of the model imply that sensitization at environmental temperatures proceeds via a site-saturated process, with the β-phase forming on a set density of preferential nucleation sites. It is shown that site-saturation allows for extension of the JMAK model to non-isothermal aging profiles and supports a diffusion pathway dominated by pipe diffusion to the interface followed by precipitate growth via the collector plate mechanism.

Chemo-mechanical instabilities in lithium cobalt oxide at higher state-of-charge in Li-Ion batteries

Transition metal oxide cathodes are widely used in commercial Li-ion batteries. However, their practical charge capacity is limited due to severe chemo-mechanical instabilities at higher charge voltage, and state-of-charge condition. Here, in situ stress and strain measurements were synchronized to probe mechanical deformations in the lithium cobalt oxide (LCO) cathode via a multi-beam stress sensor and digital image correlation, respectively. In situ mechanical measurements revealed how Li removal from the electrode structure induces deformations on the LCO composite cathodes during cycling. The structure and morphology of the LCO cathodes were further investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies. Two distinct electrochemical and mechanical behaviors were identified when the LCO was charged up to 4.65 V. The LCO undergoes a compressive stress generation when charged up to 4.2 V and surface fractures on the LCO particles were detected by SEM. LCO cathode experienced significantly large contractions (negative strains) when charged up to 4.65 V, where intergranular crack formation and phase transformation were detected on the LCO particles via SEM and XRD, respectively. Overall, the study bridges complicated structural deformations with in situ analysis of mechanical degradations in LCO cathodes charged at higher voltages. The correlation is vital to understanding instability mechanisms in transition metal oxides at high voltages for alkali metal ion batteries.

The adverse effect of grain refinement on hydrogen embrittlement in a high Mn austenitic steel

This study presents a novel observation wherein the reduction in grain size (GS) from 20 μm to 1 μm in austenitic steel adversely affects hydrogen embrittlement (HE). The findings indicate an increase in total absorbed hydrogen with grain refinement when subjected to cathodic hydrogen charging. The results demonstrate that even with grain refinement to 1 μm, the primary mode of hydrogen damage remains intergranular, with a significant increase in relative elongation loss (REL%). This phenomenon is attributed to the early nucleation of intergranular hydrogen-induced cracks (HICs) and the accelerated propagation rates of HICs, leading to local stress increments and rapid alloy failure. The accelerated development of HICs is ascribed to the impact of grain refinement on intensified normal stress exerted on grain boundary planes, as well as the suppression of ε-martensite and its positive effect on crack arresting. Besides the enhanced degradation rate of the hydrogen-affected area (HAA) due to grain refinement, the higher ratio of HAA to the entire cross-sectional area was identified as a crucial factor contributing to rapid failure and the inverse effect of grain refinement on HE resistance.

Gradient structured Al alloy with a synergistic improvement of strength-ductility obtained by friction stir processing

One Al 6061 with gradient microstructure is prepared by friction stir processing (FSP). Microscopic characterization indicates that the regulation of grain size gradient could be achieved by adjusting the process parameters of FSP. The mechanical property test indicates that one gradient Al 6061 combines an ultimate tensile strength of 266 MPa with a uniform elongation of 27 %, and the other comes 258 MPa with 29 %, which provides an excellent combination of strength and ductility. Moreover, the molecular dynamics (MD) method is employed to reconstruct and simulate the mechanical behavior of the gradient region and the dynamic evolution of the microstructure. When the stacking faults (SFs) with antisymbolic Burgers vector meet and detwinning occurs, grain refinement will induce localized strengthening. When the Shockley partials occur cross-slip, numerous sessile dislocations obstruct the dislocation motion more facilitate strain hardening. Meanwhile, a unique interaction mechanism between shear bands (SBs) and intergranular cracks in Al 6061 is discovered in plastic deformation. The coarse net-like SBs in the small-grain region are highly susceptible to catastrophic fracture of the material, and the large cracks formed within the severely band-like SBs in the large-grain region weaken the strength substantially. However, the formation of sessile dislocations in the gradient region effectively mitigates strain localization and inhibits the nucleation of large cracks. The results interpret the dynamic evolution process of gradient microstructure prepared by FSP and the mechanism of strengthening and toughening and provide process and theoretical references for the research and development of new engineering materials.

Ratcheting-fatigue behavior and fracture mechanism of 316H ASS under cyclic random loading block

In this study, a set of programmed random factors with non-zero mean were designed. Then various stress levels (15, 18 and 22 KN) were multiple superimposed to factors to form one random loading block (RLB), the blocks were repeated to failure to investigate the synergistic damage of 316H ASS under low-cycle fatigue (LCF), high-cycle fatigue (HCF) and ratcheting effect. The lifetime of cyclic RLB tests decreased with the increase of block-mean stresses σmBlock (208、255 and 311.5 MPa). The normalized strain amplitudes indicate that when the σmBlock amplitude below the yield strength (208 and 255 MPa), a stable ratchet accumulation phase allows the specimens to exhibit cyclic hardening behavior. When σmBlock exceeds the yield strength (311.5 MPa), the ratcheting strain increases significantly and the specimens exhibit cyclic softening behavior. Especially, the transgranular cleavage fracture, quasi-cleavage fracture and intergranular secondary cracks were identified when the failure of cyclic RLB tests were induced by LCF, HCF and ratcheting. With the increase of σmBlock amplitude, the decrease of LAGB proportion and the increase of dislocation density further reduce the fatigue resistance. In addition to dislocation motion, the α’-martensite phase transformation induced by ratcheting-fatigue has been further demonstrated as a mechanism for coordinated deformation. The percentage of stresses (within one block) that exceeds the diverge critical stress (375.6 MPa) of stacking faults (SFs) determines the α’-martensite nucleation mechanism.

High-Volumetric-Energy-Density Cathode Material from Single-Sized Precursor via Precise Additive Control

To overcome the energy density limitations of current layered cathode materials, high-nickel cathode materials with a bimodal distribution composed of ∼ 15um-sized secondary particles and micron-sized single crystals are synthesized from a single-sized high-nickel precursor by incorporating the grain-growth promoting/inhibiting additive. Synthesized cathode material effectively reduces the pores within the electrode and shows an excellent electrode density of > 3.9 g/cm3, exceeding ∼ 80 % of the theoretical density. In addition, due to efficient contact between large secondary particles and small single crystals, bimodal particles with additives show higher electrical conductivity compared to additive-free monomodal secondary particles, and a capacity of ∼ 220 mAh/g is achieved even at a high loading level of ∼ 30 mg/cm2 and 96 % active-material fraction. Based on remarkable electrode density and high capacity, it is possible to realize a volumetric energy density of more than 700 mAh/cm3. To the best of our knowledge, this is the highest energy density result for electrodes made from layered cathode materials. Moreover, through the synergistic effect of morphology control by selective additives and surface coating, the synthesized bimodal cathode simultaneously exhibits excellent capacity as well as improved cycle-life performance compared to a bare cathode without additives. The enhanced electrochemical performance is attributed to increased connectivity between individual cathode particles, decreased bulk/surface resistance, and reduced intergranular cracks after cycling through structure design by additives, which is confirmed through various electrochemical analyses and electrode observations.

Stress Corrosion Cracking of Manganese Bronze in Water

AbstractContinuous and centrifugal cast manganese bronze parts (C86300) experienced intergranular cracking in as few as 24 h after being stored outside. C-ring tests according to ASTM G38 were conducted to assess the material’s susceptibility to stress corrosion cracking (SCC) susceptibility and found that water was the most likely agent. The failure mechanism has been identified as intergranular SCC due to a combination of residual stress from a rough machining operation and water from rain or dew. To prevent cracking, a stress relief heat treatment immediately after the rough machining operation is recommended.

Microstructure evolution and mechanical response of hetero-induced SiCp addition in Al-6061 under high strain rate compressive loading

Al/SiC composites having a high-volume fraction of SiC are eye-catching materials in the automobile and aerospace industries. However, their behavior under stringent environments i.e., under high strain rate compression is much different than quasi-static compression. For such conditions, the morphology, interfacial bonding, and the fraction of the SiC play a decisive role in achieving the high performance of the Al/SiC composites. Here, in this study, we reported that a transitional effect of de-bonding and intergranular crack in SiC has occurred in Al–SiC composites having SiC particle size >0.7 μm. Below the 0.7 μm, the diffusion of the Al dislocations is characterized on the boundary of SiC along with the strong interfacial bonding. To prove it, we have fabricated an Al6061/30 vol% SiC composite through powder metallurgy route and subsequently subjected to compression in the range of strain rate 0.001–2100 s−1. In support of our observations, the microstructure was characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, electron backscattered diffraction, and transmission electron microscopy analysis. The result also revealed a positive strain rate sensitivity up to a strain rate of 1800 s−1, while for 2100 s−1, the flow stress decreased, which is the cause of adiabatic temperature rise, dynamic recrystallization in Al, and massive cracks and deboning in coarse SiC particles. We also characterized that the SiC having particle size 1–4 μm showed stacking fault and dislocation induced in the SiC due to strong interfacial bonding. A quantitate analysis of dislocation was also established by using the Williamson-Hall technique which further confirmed the dislocation activity and dynamic recrystallization in 2100 s−1, characterized by EBSD and TEM. Thus, this study enables our understating that using the fine SiC particles is promising for achieving the high performance of the Al/SiC composites for use in stringent environments.

Coupled cellular automata-crystal plasticity modeling of microstructure-sensitive damage and fracture behaviors in deformation of α-titanium sheets affected by grain size

Concerning the micro-scale deformation of titanium metal sheets, the number of grains in the sheet thickness direction decreases, and their formability exhibits a strong grain size sensitivity. Meanwhile, the twinning-induced dynamic recrystallization (TDRX) associated with grain size significantly affects the fracture behavior in the microforming of titanium sheets. Therefore, an accurate prediction of formability to improve manufacturing reliability remains challenging in the microforming of miniaturized titanium components. To address this issue, an in-depth understanding of the grain size-dependent TDRX behavior and its role in damage and fracture development in the microforming of α-titanium sheets is critical, and a coupled cellular automata-crystal plasticity (CA-CP) modeling framework was thus developed as an approach providing efficient solutions and insightful comprehensions of the issue. For the proposed modeling framework, a kinematic model for TDRX was established and integrated into the CP model by the CA algorithm. As a result, the microstructure evolution caused by TDRX was regarded as an intrinsic part of the constitutive behavior to connect heterogeneous plastic deformation and damage evolution through data transmission between the CP model and the CA algorithm. Additionally, the coupled CA-CP modeling framework was validated with the internal defect morphologies and deformation microstructures characterized by X-ray computed tomography (X-CT) and electron backscattered diffraction (EBSD). Experiment and simulation results demonstrated that the fine recrystallized (DRXed) grains were generated after the twin fragmentation when the dislocation density at twin boundaries reached a threshold of 9.2 × 1013 /m2. After TDRX, the dislocation density and the stress concentration intensity in recrystallization regions were revealed to decrease, accounting for the ductility improvement. Nevertheless, the dislocation density at twin boundaries was determined to decrease with the increase of grain size, leading to less twin fragmentation and the absence of TDRX. The uncoordinated deformation between fine DRXed grains motivated defects to grow spherically into microvoids, thereby preventing premature intergranular cracks along twins/grain boundaries. Ultimately, the deformation microstructures resulting from TDRX with the decrease of grain size were confirmed to control the brittle to ductile fracture transition of α-titanium sheets. The presented modeling framework and simulation procedure were validated to be able to predict the material integrity affected by crystalline microstructure in the deformation of titanium metal sheets.

Insights from tensile fracture properties and full-field strain evolution of deep coral reef limestone under dynamic loads

Coral reef limestone (CRL) commonly undergoes dynamic tension when underground structures of island reefs encounter impacts, explosions, or seismic activities. Given the complexity of biological pores, the dynamic tensile fracture characteristics of CRL are poorly understood. Therefore, the dynamic tensile fracture behaviors of deep CRL were systematically observed by Split Hopkinson Pressure Bar tests and digital image techniques. Comparing to traditional rocks, the macro-pores near failure band would significantly change cracking path. The failure patterns are dominated by loading rate. The strongly dependence of dynamic tensile strength and dynamic crack initiation toughness on loading rate suggests the two indices overcome the effect of CRL macro-pores under dynamic impacts. At low loading rates, tensile fractures predominantly follow intergranular cracks, whereas transgranular cracks dominate at higher rates. The fractal dimension of fracture surface decreases with increasing crack propagation velocity, loading rate, and dynamic crack initiation toughness. Due to the unique marine sedimentary environment, the mechanical heterogeneity in multiple scales distinguishes CRL from terrestrial rock materials. The insights into underlying mechanisms of dynamic tension provide support to optimization of blasting scheme and stability assessments for island underground engineering.

Microscopic characterisation of brittle fracture initiation in irradiated and thermally aged low-alloy steel welds of a decommissioned reactor pressure vessel

Microstructure has a significant effect on material's integrity and in a heterogeneous weld microstructure the discontinuities affect the brittle fracture initiation and propagation and determine the fracture toughness. The knowledge of brittle fracture initiation mechanisms in high-Mn/high-Ni welds is limited. The brittle fracture initiation behaviour of the decommissioned Barsebäck Unit 2 reactor pressure vessel (RPV) welds of high-Mn/high-Ni weld metal from three different locations, the RPV head and the beltline regions, were investigated and compared with specimens from the surveillance program with high fluence. Systematic fractography has been performed on impact and fracture toughness specimens and the main features of the brittle fracture initiation in the component weld are presented and discussed. Two main types of initiators are identified as the weakest links to initiate the cleavage fracture and the initiation mechanism is found independent from the operation condition. The high-fluence surveillance specimens have a larger amount of intergranular cracking. The cleavage fracture initiation appears to be independent of the operation conditions but dependent on the welding process and metallurgical features. The findings aid in the development of improved material-property correlations which will result in better computational tools for predicting aging of welds based on microstructure.

Effect of test temperature on deformation microstructure and tensile property of a novel Ni-Co-based superalloy

The tensile behavior, deformation microstructures and failure mechanisms of a novel Ni-Co base precipitation-hardened superalloy, have been investigated at room temperature (RT), 450, 550, 650 and 750 °C. Deformation of the alloy at RT, 450, 550 °C is primarily mediated by dislocation slip and stacking faults while nano-sized deformation twinning is also involved at 650 and 750 °C. The yield and tensile strength decrease with increasing test temperature. In the temperature range of 450–650 °C, however, no obvious reduction in the yield strength and work-hardening rate at low strains is observed, which could be correlated to a high density of stacking faults or deformation twins. Low yield strength and work-hardening at 750 °C, could be related to the softening by thermal activation and the softening of γ′ particles caused by the repeated cutting via stacking faults or twins. However, as the testing temperature is over 450 °C, elongation of the alloy decreases significantly with increasing test temperature, which could be associated with intergranular cracking induced by stress-assisted grain boundary oxidation.

Concurrent dramatic enhancement of high-temperature strength and ductility in a high-entropy alloy via chain-like dual-carbides at grain boundaries

Grain boundaries (GBs) are often known as intergranular cracking sources in alloys at high temperatures, resulting in limited high-temperature strength and ductility. Here, we propose a GB-dual-carbide (denoted as GB-DC) strengthening strategy and have developed a high-performance (NiCoFeCr)99Nb0.5C0.5 high-entropy alloy (HEA) with exceptional strength-ductility synergy at 1073 K. Chain-like coherent M23C6 carbides have been successfully introduced at GBs and remain a cube parallel crystallographic orientation with the face-centered cubic (FCC) matrix during deformation. Nano-scale NbC particles are distributed alternatively between M23C6 carbides and inhibit their coarsening. Both strength and ductility of the GB-DC HEA increase dramatically at strain rates ranging from 10−4 to 10−2 s−1 at 1073 K, compared with those of the single-phase NiCoFeCr HEA. Specifically, yield strength of 142 MPa, ultimate tensile strength of 283 MPa, and elongation of 34 % were obtained, which are twice that of the reference NiCoFeCr HEA (82 MPa, 172 MPa, and 18 %, respectively). EBSD investigations demonstrated that chain-like carbides enhance the GB cohesion at high temperature, and TEM analysis revealed that dislocations can go through the coherent phase boundaries (CPBs) and activate dipoles inner M23C6 carbides, which weakened the stress concentration in GBs. This substantially reduces the critical stress for dislocation generation and transmission to a stress level lower than that required for intergranular fracture. Theoretical estimation suggests that carbides result in a much higher activation energy (∼510 kJ/mol) for GB sliding and a rather low interface energy (∼101 mJ/m2) compared with the GB energy (1000 mJ/m2), which rationalizes the enhanced GB cohesion by carbides.

Achieving high mechanical and corrosion properties of AA2050 Al-Li alloy: The creep aging under plastic loading

The influences of elastic/plastic loading (100–220 MPa) on the creep behavior, mechanical properties, and corrosion behavior of creep-aged AA2050 alloys were investigated. The results show that the creep rate increased from 0.35 % to 0.61 % with the increase of stress from 100 MPa to 220 MPa. The creep rate was increased rapidly under plastic loading (220 MPa) due to the increased dislocation density. Meanwhile, the plastic loading shortened the peak-aged time of creep-aged alloys and achieved outstanding strength (UTS=534 MPa, YS=496 MPa, peak aged), which increased by 33 MPa and 32 MPa compared with elastic loading, respectively. The strength enhancement was attributed to the increase in dislocation density, weak oriented precipitation effect, and dense precipitation of T1 phases. Additionally, compared with elastic loading, GBPs under plastic loading coarsened and distributed discretely, their elements content distributed evenly, and the Cu content increased. Therefore, the intergranular corrosion (IGC) depth and stress corrosion cracking (SCC) susceptibility index (ISSRT) decreased from 174 μm, and 8.7 % to 121 μm, and 5.9 %, respectively. These findings pave a way in breaking curvature limit of creep aging technology.

Nano-micrometer scale characterization of PWSCC crack tips in the transition zone of 52M overlay and the implication to intergranular cracking

This study examines the microstructural characteristics of the primary water stress corrosion cracking (PWSCC) crack tip in the transition zone (TZ) of the 52 M overlay and its implications for intergranular cracking. In the TZ, the Cr content at grain boundaries near the fusion boundary (FB) is lower than those farther from the FB. Additionally, residual strain at grain boundaries near the FB is higher than that farther away, with the fine-grained zone around the FB showing higher local deformation than the surrounding columnar grains. Stress corrosion cracking (SCC) crack growth rate (CGR) test results indicate that the TZ showed certain SCC sensitivity, and the closer to the FB, the lower the SCC resistance. Analysis of the SCC crack tip found that the distinctive composition distribution of the 52 M overlay TZ, characterized by low Cr and high Fe near the FB, is prone to intergranular oxidation, thereby reducing SCC resistance. Conversely, higher Cr and lower Fe content at grain boundaries in the TZ farther from the FB form dense, Cr-rich oxides ahead of the crack tip that slow SCC crack growth, resulting in the diffusion of oxidation along the dislocation structure into the grains and forming a fibrous oxidation zone.

Grain-based coupled thermo-mechanical modeling for stressed heterogeneous granite under thermal shock

Microscopic damage and macroscopic mechanical properties of granite under the coupling effect of thermal load and initial stress are crucial considerations for the safe construction of underground geo-energy engineering. However, visualizing real-time micro-crack processes in rocks under high-temperature and high-pressure conditions using the current experimental techniques remains challenging. In this study, a numerical method is developed to analyze the thermally induced damage in heterogeneous granite under the coupled influence of initial stress and thermal loading. A biaxial thermo-mechanical grain-based model considering real mineral distribution is established based on digital image processing technology, the grain-based modeling method, and heat conduction theory. The microscopic parameters are calibrated and the effectiveness of the model is verified based on thermal shock and uniaxial compression experiments. The thermal destruction mechanism of granite under initial stress from a microscopic perspective was unveiled for the first time. During the thermal shock process, the stress within the rock does not remain constant at the initial stress value. Instead, it changes continuously with the progression of heat conduction. The impact of the initial stress on the thermally induced cracks is relatively minor. Cooling causes more damage to the rock than heating during thermal shock. The intragranular cracks of quartz consistently outnumber other intragranular or intergranular cracks during thermal shock. The initial stress and thermal shock damage enhance and weaken the biaxial peak strength of granite, respectively. The weakening effect of thermal shock on the peak strength becomes more pronounced at a higher initial stress. These research findings and proposed research techniques contribute to the management and optimization of underground geo-energy engineering.

Effects of irradiation and stress on crack initiation behavior of 316SS flux thimble tubes in PWR environment

Irradiation assisted stress corrosion cracking (IASCC) behavior and irradiated microstructural features of cold-worked 316 stainless steel flux thimble tube specimens were investigated in 340 °C simulated PWR primary water. It covers three dose levels (∼40, ∼60 and ∼100 dpa) and five stress levels (35%, 40%, 50%, 60% and 70% of irradiated yield strength at high temperature. Crack initiation in highly irradiated materials occurred rapidly in 33 failed specimens at high-stress level. A clear dose dependence for IASCC initiation was observed. The susceptibility to IASCC initiation reached saturation around 40–60 dpa. Fracture surface of different dose level O-ring specimens showed that intergranular cracking was the main fracture mode. Discrete slip steps were observed on intergranular fractured regions of each dose level specimens, which is associated as evidence of dislocation channeling. A threshold stress level which was close to 30% yield strength was determined on various testing results from literature and this study. Data scatter was observed for those results from different studies but all agreed on that the failure rate increases with stress level and neutron dose.