Intergranular Crack Initiation Research Articles

Role of grain size and anisotropy of neighboring grains in hydrogen‐assisted intergranular fatigue crack initiation in austenitic stainless steel

AbstractThis study explores the impact of microstructural features on fatigue crack initiation in poly‐crystalline materials, emphasizing hydrogen‐induced complexities. Grain anisotropy, misorientations, grain size variations, and elastic–plastic inhomogeneities concentrate stress at grain boundaries, making them susceptible to crack initiation during fatigue loading. The presence of hydrogen compounds this process, due to complications of characterization of local hydrogen content and activating embrittling mechanisms. Building upon a model for nickel, this research investigates 316L austenitic stainless steel specimens with varying grain sizes, both uncharged and hydrogen‐charged. In situ low‐cycle fatigue loading experiments establish correlations between fatigue crack initiation and microstructural features. The study reveals specific combinations of features crucial in the initiation process, undergoing alterations in the presence of hydrogen. A proposed qualitative model links microstructural features with accumulated plastic shear strain during fatigue and prevalent hydrogen embrittlement mechanisms like hydrogen‐enhanced local plasticity and hydrogen‐enhanced decohesion.

Influence of microstructure evolution and element segregation on high temperature tensile behaviors in GH4706 alloy

This work systematically investigated the influence of microstructure evolution and element segregation on tensile properties and fracture behaviors of GH4706 superalloy under different temperatures (25 °C, 400 °C, 500 °C, 600 °C and 650 °C). The advanced methods of TEM, AES and SIMS were conducted to characterize the variation of microstructure and element distribution. Experimental results showed that the yield and tensile strength of the test alloy decreased while the corresponding elongation increased as the increment of test temperature. The dominated fracture mode was intergranular brittle fracture at the temperature of 25 °C, gradually transforming to ductile dimple fracture as the temperature increased. This phenomenon was determined to be closely related to the increment of average grain size and size of γ'/γ″ coprecipitates as temperature rose. It was worth noticing that the elongation performed abnormally low (16.3%) at the temperature of 500 °C, exhibiting intermediate temperature brittleness (ITB) phenomenon. The SIMS analysis showed that the segregation of S towards grain boundary was the most obvious at the temperature of 500 °C, which was considered as the main factor leading to ITB. In addition, the coarsening of carbides at grain boundaries and local strain non-uniformity caused by dislocations could also resulted in ITB. The main reason for maintaining good strength and plasticity at the temperature of 650 °C was contributed to the softening of γ' phases by repeated cutting of dislocations and deformation twins, reducing the hindrance to the movement of dislocations. Moreover, composite strengthening effect of P and B could improve bonding strength, stabilize grain boundaries, and effectively suppress the initiation of intergranular cracks.

High-temperature high cycle fatigue performance of laser powder bed fusion fabricated Hastelloy X: Study into the microstructure and oxidation effects

High-temperature fatigue properties are critical for laser powder bed fusion (LPBF) fabricated Nickel-based superalloy Hastelloy-X (HX) used in aero engines. In this study, superior high-temperature high-cycle fatigue properties of LPBF HX were achieved. The detailed investigations show that the grain boundary oxidation promoted by the different deformation modes between neighboring grains, despite the reduced oxygen-related damage in the coarser LPBF HX microstructures, are the potential causes for the inter-granular fatigue crack initiation and the subsequent crack coalescence. In the meantime, the crack propagation could be hindered by the refined carbides and annealing twins with Σ3 misorientation (60°/〈111〉) in LPBF HX. Furthermore, the recrystallized grains formed ahead of the crack tip due to the severe deformation within the plastic zone and high testing temperature have different crystallographic orientations, which leads to the crack propagation path changes and increases the fatigue crack propagation resistance.

A study on the influence of impurity content on fatigue endurance in a 6082 Al-alloy

The paper investigates the influence of different impurity/scrap elements on the high cycle fatigue (HCF) properties of 6082 Al-alloy. Accordingly, HCF tests are carried out on the different variants- GA1, GA2 and GA3 (designed as a function of varying impurity content)- of this alloy, which resulted in a lower fatigue limit in GA2/GA3 compared to GA1. To explain such behavior, detailed fractographic and microstructural investigations are conducted which bring out that although the crack initiation and propagation modes are mostly transgranular irrespective of alloy variant and stress levels, intergranular crack initiation is promoted in GA2/GA3 at 150 MPa in contrast to occurrence of run-out in GA1 at the same stress level. This phenomenon is found to be facilitated by intermetallic particles anchoring the grain boundaries. These observations point out the possibility of strong sensitivity of stress and alloy variant (varying impurity content) to fatigue life. The difference in fatigue properties as a function of alloy variant could be attributed to the variation in initial microstructure/particle size distribution as well as the slip character. In light of these, a fracture mechanism map is generated which underlines the different mechanisms responsible for fatigue crack initiation among different alloy variants.

The Role of Extrusions and Intrusions in the Initiation and Intergranular Growth of Fatigue Cracks

The role of extrusions and intrusions in the initiation and growth of intergranular fatigue cracks is studied by performing strain‐controlled fatigue experiments on polycrystalline copper and by analyzing the development of intergranular crack initiation and resulting fracture surfaces. Fatigue cracks initiate either in the persistent slipbands within a grain or on the grain boundaries. The grain boundary initiation mechanism is due to the production of extrusions and intrusions on the grain boundaries that decrease the grain boundary cohesion forming submicroscopic crack nuclei. The alternating extrusions and intrusions are found also on the grain boundary facets of the fracture surface of a growing fatigue crack. Up to three slip systems are identified on the facets of cracked grain boundaries. Their form and height are found using scanning electron microscope observations and focused ion beam sectioning. A mechanism similar to intergranular grain boundary crack initiation is considered to explain the formation of grain boundary facets. A novel mechanism of intergranular fatigue crack growth is proposed based on the damaging effect of the extrusions and intrusions produced in the cyclic plastic zone of the growing fatigue crack.

Effect of grain boundaries and rigid inclusions on plasticity in nickel bicrystals containing helium bubbles and radiation-induced self-interstitial atom clusters

We use molecular dynamics to assess the effect of grain boundaries and rigid intergranular inclusions on plastic deformation in nickel (Ni) containing helium (He) bubbles and self-interstitial atom clusters. Our simulations show that plasticity in Ni bicrystals is relatively uniform, with no localized slip bands or nano-twins. We attribute this behavior to grain boundaries, which block dislocations and favor activation of new sources over persistent slip along a single plane. While there is no initiation of intergranular cracks, He bubbles at matrix/inclusion interfaces elongate along the tensile axis and migrate towards regions of high tension, potentially setting the conditions for formation of crack-like flaws via bubble coalescence. We discuss the implications of our work for understanding degradation of mechanical properties in Ni-base alloys in nuclear reactors.

Creep rupture of a Ni-Fe based model alloy under simulated fireside corrosion in coal-fired boilers

The influence of fireside corrosion on creep rupture of a Ni-Fe based model alloy was investigated by comparing the rupture life, fractography and microstructure of specimens in fireside corrosion and air. Results showed that the rupture life at 140 MPa in fireside corrosion is only 40% of that in air. The internal sulfidation along grain boundaries promoted the initiation of intergranular cracks, and was responsible for the premature rupture. The surface collapse in fireside corrosion was attributed to the outward propagation of internal cracks, while surface cracks in air originated from the formation of recrystallized grains in the surface layer.

Effect of boron addition on the microstructure and cryogenic mechanical properties of N50 stainless steel after aging treatment

The effect of boron on the microstructure and cryogenic mechanical properties in N50 austenitic stainless steel after aging treatment was revealed by a comparative investigation of 0B and 25B (25 ppm) N50 steels. The results showed there is a large amount of irregular block and rod-like Cr2N precipitated along grain boundaries with intra-granular precipitation of lamellar Cr2N in 0B steel; nevertheless, the nucleation and growth of Cr2N were significantly suppressed in 25B steel. It is related to the retarded diffusion of Cr, Mo, and V atoms caused by the pre-occupied B atoms during the precipitation of nitride. Theoretical calculations verify that the high B segregation at grain boundaries is caused by both equilibrium and non-equilibrium segregation. The weakened precipitation of nitride and the enhanced cohesion of grain boundaries by segregated B synergistically suppress the initiation and extension of intergranular cracks. The impact toughness of 25B–N50 steel at both 77 K and 4.2 K can reach 200 J, which is about 2.5 times higher than that of 0B–N50 steel.

Prediction of inter-granular crack initiation susceptibility in post irradiated grade 300 steels

Grade 300 austenitic steel components are subjected to irradiation-assisted stress corrosion cracking hazard, in the context of nuclear pressurized water reactor operation. In this paper, it is assumed that inter-granular cracking susceptibility primarily depends on sub grain plasticity mechanisms, controlling the slip band thickness and spacing achieved for a plastic strain level imposed. A computational model evaluating the related grain-boundary stress evolutions is developed and evaluated, using actual post-irradiation observations. In practice, we calculate a quantitative damage susceptibility factor R depending on the characteristic local slip band arrangements and the grain to grain misorientation. The statistical R evolutions obtained are consistent with the strain localization and related intergranular crack susceptibility trends, under corresponding irradiation and deformation conditions.

Impact of Fireside Corrosion on the Creep Rupture Life and Oxide Scale Structure of a Super304H Boiler Tube

The creep rupture life of commercial Super304H in fireside corrosion (coal ash and flue gas) and static air environment at 650°C was investigated. Results showed that the creep rupture life under fireside corrosion conditions decreases by 26.5% to 83.3% at different stress levels, compared with that in air. The corrosion products, including their composition, morphology, and distribution, and the microstructure of substrate were characterized to research the impact of the fireside corrosion on the creep rupture life. The exfoliation of oxide scales and serious surface cracking resulted from fireside corrosion were detrimental for the creep properties of the alloy. The formation of internal sulfides promoted the initiation and propagation of intergranular cracks in the substrate during creep tests. These degradations of Super304H in fireside corrosion contributed to premature creep rupture.

Microstructural effects on fatigue crack initiation mechanisms in a near-alpha titanium alloy

The role of different microstructural constituents on crack initiation in two-phase titanium alloys is still an area of great controversy. The present study investigates the effects of primary α volume fraction and concomitant macrozones on two different fatigue crack initiation modes concurrently observed in a near-α titanium alloy. Statistically representative regions were monitored in quasi-in-situ studies by interrupting fatigue testing to detect slip trace formation leading to crack initiation. In addition, high-resolution 2D strain maps were generated to quantify in-plane shear strains of slip traces related to microstructural features. The detailed analysis demonstrates that at 90% of the proof stress basal 〈a〉 slip plays a crucial role in crack initiation, regardless of whether cracking is transgranular or intergranular. In addition, a distinct shift from transgranular to intergranular crack initiation was observed with increasing αp fraction driven by the increased number of αp/αp grain boundaries. The high-resolution 2D strain mapping suggest that these intergranular cracks initiated from a burst of basal 〈a〉 slip starting from (0001) twist grain boundaries but penetrating one side of the grain pair in case of a partial (0001) twist grain boundary. From these observations, a new criterion for intergranular cracking is proposed based on a geometrical grain boundary parameter and the preference of neighbouring αp grain pair well aligned for basal 〈a〉 slip. It was found that while (0001) twist grain boundaries increase with increasing αp fraction, macrozones do not further enhance their frequency. However, macrozones of hard orientated grains did enhance transgranular crack formation by potentially higher local stress and increasing slip length over clustered αp grains with low misorientation reducing the requirement for out-of-plane shear.

Grain boundary stresses in elastic materials

A simple analytical model of intergranular normal stresses is proposed for a general elastic polycrystalline material with arbitrary shaped and randomly oriented grains under uniform loading. The model provides algebraic expressions for the local grain-boundary-normal stress and the corresponding uncertainties, as a function of the grain-boundary type, its inclination with respect to the direction of external loading and material-elasticity parameters. The knowledge of intergranular normal stresses is a necessary prerequisite in any local damage modeling approach, e.g., to predict the probability for intergranular stress-corrosion cracking or fatigue-crack initiation in structural materials.The model is derived in a perturbative manner, starting with the exact solution of a simple setup and later successively refining it to account for higher order complexities of realistic polycrystalline materials. In the simplest scenario, a bicrystal model is embedded in an isotropic elastic medium and solved for uniaxial loading conditions, assuming 1D Reuss and Voigt approximations on different length scales. In the final iteration, the grain boundary becomes a part of a 3D structure consisting of five 1D chains with arbitrary number of grains and surrounded by an anisotropic elastic medium. Constitutive equations can be solved for arbitrary uniform loading, for any grain-boundary type and choice of elastic polycrystalline material. At each iteration, the algebraic expressions for the local grain-boundary-normal stress, along with the corresponding statistical distributions, are derived and their accuracy systematically verified and validated against the finite element simulation results of different Voronoi microstructures.

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.

Mechanisms of intergranular fatigue crack initiation in polycrystalline materials

Fatigue failure in polycrystalline materials occurs by initiation and propagation of fatigue cracks. Fatigue cracks can initiate transgranularly or intergranularly. While considerable progress has been achieved in the explanation of the mechanism of transgranular fatigue crack initiation intergranular initiation is still the subject of controversy. A recent study of the crack initiation in superaustenitic Sanicro 25 steel [1] revealed the important role of persistent slip markings (PSMs) in the grains adjoining the grain boundary. A novel mechanism of intergranular fatigue crack initiation has been proposed. Provided the cyclic slip cannot proceed from one grain to the neighbour one, extrusions and intrusions arise not only on the surface but also on the grain boundary. Growing intrusions represent void-like defects and growing intrusions push neighbour grain apart. A systematic study of intergranular cracking has been performed on polycrystalline copper cycled with high and low strain amplitudes. SEM observations of the early stages of fatigue crack initiation on the grain boundaries, EBSD study of grains orientation, FIB cutting of the grain boundaries and simultaneous documentation of the effect of extrusions and intrusions on the grain boundary cohesion are reported simultaneously with the conditions favourable for the initiation of the fatigue cracks.

Creep anisotropy behavior, deformation mechanism, and its efficient suppression method in Inconel 625 superalloy

Mill products, such as sheets, usually show obvious anisotropy in their mechanical properties, which greatly affects both their applications and workability. In this study, the orientation-dependent tensile and creep behaviors of Inconel 625 alloy sheets with weak local textures were systematically investigated at 650℃. The results showed that Inconel 625 superalloy exhibits nearly isotropic tensile properties; however, obvious creep anisotropy appears when loading along different directions. Creep life in the rolling direction (150 ± 5 h) was approximately 4.5 times longer than in the transverse direction (33 ± 1 h). Severe creep anisotropy was found to be determined by two aspects: (i) the change in deformation mechanisms along the rolling direction (a combination of boundary sliding, dislocation slipping, and twinning) and the transverse direction (dislocation slipping). Stronger impingement of slip bands on grain boundaries accelerated intergranular crack initiation and propagation during loading along the transverse direction, which resulted in a short creep life. (ii) The differences in strain compatibility at grain boundaries (i.e., creep loading along the transverse direction resulted in the initial grains forming strong Brass, Cu, and S textures). Creep cracks preferentially nucleated at the junctions between Cu/Brass texture grains, as these interfaces exhibit the worst strain compatibility (Luster–Morris parameter m′ ≤ 0.22). Furthermore, pre-stress aging (PSA) treatment is proposed as an efficient method to suppress creep anisotropy. The discrepancy rate of creep life was found to be reduced by nearly 50% after PSA treatment.

Microstructural and oxidation effects on fatigue crack initiation mechanisms in a turbine disc alloy

Effects of microstructure and oxidation on fatigue crack initiation and early propagation processes were investigated in RR1000 turbine disc alloy with different γ′ distributions and carbide distributions on the grain boundary. Fatigue tests were carried out under three-point bending and trapezoidal waveform loading (with a 90 s dwell) at 650 °C in air. The failure mode in both γ′ variants is clearly characterised by intergranular features. A number of fatigue cracks are seen to initiate at grain boundaries with bulged Co-rich oxides at the surface and/or interfaces between carbides and grain boundaries, resulting from oxidation damage assisted by applied loading. Reduced lifetime is closely linked to significant intergranular crack initiation and frequent consequent crack coalescence events, which results in enhanced fatigue crack growth (FCG) rates. The extent of intergranular features and enhanced FCG are more marked where more continuous carbides exist at the grain boundary.

Advances in understanding of damage formation during laser-assisted milling of ZnO-based varistor ceramics

Zinc oxide (ZnO)-based varistor ceramic is a hard-to-machine material. Its microstructure, developed for its electrical properties, results in extremely low fracture toughness. Therefore, the current state-of-the-art in machining this material is lapping. In the present work, laser-assisted milling (LAMill) of ZnO ceramics was evaluated with a systematic approach to allow for new insights and advance the understanding of damage formation during machining of this material. A machining strategy to machine ceramic workpieces along the edge was studied. Additionally, a new transient analytical thermal model was developed to accurately predict laser-induced temperatures near the edge of the workpiece without excessive use of computational power, and the model predictions were experimentally verified. A novel approach to evidence machining-induced damage with the mathematical description of the ideal machined surface was also designed and tested. It was found that at higher temperatures the propagation of intergranular fractures is inhibited. Another mechanism based on the material porosity and contributing to more instances of intergranular crack initiation has been identified. To reliably predict the extent of damage on the machined surface, the analysis has been extended by covering the effects of the two identified influential process parameters: feed per tooth, fz, and the temperature of the to-be-cut material, T. By adjusting these parameters, surface damage was successfully controlled and the extent of damage was minimized at fz = 0.18 mm and T = 380 °C, achieving a 44 % reduction compared to the conventional process with the same machining parameters.

Simultaneous enhancement of strength and ductility for AZ31 magnesium alloy by pre-twinning induced heterostructure

Pre-twinning was used to tailor the strength and ductility of AZ31 magnesium alloy. Simultaneous enhancements in both ductility and strength were achieved in the AZ31 after the introducing of a high fraction of twin-matrix pairs. The slip trace analysis indicated that although the area-weighted average global Schmid factors (SFs) were not significantly changed, much more non-basal slip was activated in the pre-twinned sample compared with that in the as-received sample, which was beneficial to achieving higher work hardening and obtaining higher uniform elongation. The high-resolution digital image correlation (DIC) combined with ex-situ electron backscatter diffraction (EBSD) characterization revealed that the uniformity of the grain-size scale strain distribution was improved and the local strain concentration at the boundary was reduced during the loading as result of the extra activated non-basal slip, which delayed intergranular crack initiation and increased ductility. The activation of much non-basal slip with high critical resolved shear stress, the hetero-deformation, and the fine equivalent grain size caused the strengthening of the AZ31 sample with a high fraction of twin-matrix pair interfaces. These results imply that tailoring the twin-matrix interface density is an effective strategy to achieve excellent strength-ductility combinations in HCP metals.

Microcrystallographical Characterization of Initiation of Intergranular Stress Corrosion Cracking on Alloy 600 in Terms of Local Stress at Grain Boundary Induced by Slip Deformation

The initiation of intergranular stress corrosion cracking (IGSCC) on Alloy 600 in a simulated pressurized water reactor primary water environment was investigated by the characterization using an electron back scattering diffraction (EBSD) on flat tensile specimens subjected to a slow strain rate testing. The IGSCC initiation was evaluated for many grain boundaries in terms of the grain boundary characteristics and the local stress generated at each grain boundary; the latter is estimated by considering the specific slip deformation determined by the Schmid factor of two grains adjacent to a grain boundary. This simplified local stress evaluation for IGSCC susceptibility based on EBSD analysis is introduced for the first time in this study. IGSCC tends to occur as a result of induced tensile stress rather than shear stress at the grain boundary, whereas no IGSCC occurred when compressive stress was applied at the grain boundary. Similar results were observed for both 10% and 20% cold worked (CW) specimens in the stress analysis. It was noted that crack initiation depends not only on the stress at the grain boundary but also on the strain concentrated around the grain boundary for the 20% CW specimen.

Molecular dynamics study on the effect of Ni atoms on the crack arrest performance of Fe–Ni alloy

AbstractMolecular dynamic simulations are applied to test the nickel's modification mechanism of Fe–Ni alloy. Mono displacement loading is applied to a perfect single crystal model, a single crystal model with vacancies, and a model with transgranular crack. Moreover, constant strain load is applied to the polycrystal model to test the Ni effect on intergranular crack initiation. The results elucidate that Ni atoms could decrease the free surface energy and the stacking fault energy simultaneously. However, Ni atoms have a more significant effect on the reduction of stacking fault energy. If the Ni concentration is above 0.03, the transgranular crack constantly emits dislocations under loading, thus, postponing the cleavage cracking. Particularly, as the Ni concentration is above 0.05, the recrystallization process could be a favorable energy‐releasing behavior compared with the intergranular cracking. The findings suggest that a low concentration of Ni might degrade the physical property of Fe–Ni alloy. Increasing the Ni atomic concentration above specific critical values, for example, 0.03 or 0.05, could enhance the fracture toughness.