Intergranular Cracking Research Articles

Quantitative analysis of local plasticity accompanying hydrogen-related fracture in low-carbon martensitic steel

In this study, the local plasticity accompanying hydrogen-related fracture in a low-carbon martensitic steel was quantitatively investigated by correlative microscopy analysis combining digital image correlation technique and electron backscatter diffraction. The results revealed that strain localization occurred mainly in the bulk of martensite blocks and at prior austenite grain boundaries, both with and without the presence of hydrogen. Moreover, hydrogen enhanced strain localization during deformation and facilitated crack nucleation and propagation. The quantitative analysis showed that the hydrogen-related quasi-cleavage cracking was closely related to the local plastic deformation, and increasing hydrogen content decreased the local strain level required for quasi-cleavage cracking. It was found that strain localization also occurred around the intergranular cracks, suggesting that hydrogen-induced intergranular cracking was not a classical simple decohesion process and strain localization was involved in the sequence of hydrogen-induced intergranular cracking. These findings shed light on the understanding of the intrinsic nature of hydrogen embrittlement.

Sensitization and stress corrosion cracking characteristics of 5059 alloy vis-à-vis 5083 alloy after reversion treatments

Moderately higher than room temperature exposures can sensitize marine grade 5xxx series aluminum alloys, making them susceptible to intergranular corrosion and stress corrosion cracking in marine environments. Sensitization occurs due to precipitation of active, magnesium-rich β phase along the grain boundaries. Higher strength, high-magnesium 5059 and 5083 grade alloys were artificially sensitized by thermal exposures at 60 °C, 80 °C and 150 °C for various durations of 1, 4, 15, 41, and 100 days. Sensitization and stress corrosion cracking responses are compared. 5059 alloy exhibited a higher degree of sensitization of 86 ± 1 mg/cm2 as compared to 64 ± 2 mg/cm2 of 5083 alloy, for the similar sensitization condition of 150 °C–15 days. This is attributed to the higher content and therefore super-saturation of magnesium in the former. In order to restore the corrosion resistance, reversion treatments involving re-exposing the sensitized alloys to elevated temperatures for short durations of 10 to 30 min were performed. Degree of sensitization after reversion treatment is evaluated leading to identification of conditions for obtaining safe values (<15 mg/cm2) without compromising the mechanical properties much. Optimum reversion treatment employed 30 min of exposures at 230 °C.

Enhanced corrosion resistance of 15–15Ti austenitic steel in liquid lead-bismuth eutectic at 550 ℃ by shot peening processing

The effect of shot peening on microstructure and corrosion of 15–15Ti austenitic steel in liquid LBE at 550 ℃ was investigated. The nanocrystallization and preferential recrystallization were observed, improving the resistance to corrosion and intergranular cracking in LBE. The intensive grain boundaries and dislocations on steel surface improved the compactness and adhesion of the formed oxide scale, efficiently preventing LBE penetration into steel. The Cr-rich oxide formed in boundaries of the recrystallized grains is Fe-Cr spinel with porous structure, it significantly prohibited the counter diffusion of elements, suppressing the growth of outer magnetite layer and inner transgranular oxide.

The effect of oxide scale and microstructural changes during cyclic hot corrosion on high-temperature tensile properties of Rene-80 superalloy

ABSTRACT The effect of oxide scale and microstructural changes during 10, 20, and 40 hot corrosion cycles on the high-temperature tensile properties of Rene-80 superalloy at 950 °C was investigated. Due to the formation of micro-cracks and micro-voids, compressive stresses produced from Cr2O3 and NiO growth, and tensile stresses stemming from NiMoO4 transformation and Al internal oxidation, the oxide scale spalled. By increasing the hot corrosion cycles, UTS (Ultimate Tensile Strength) and El.% (Elongation) first decreased and then increased due to the propagation of intergranular vertical cracks from the oxide scale to the Rene-80 after 20 cycles. During hot corrosion cycles, YS increased due to a rise in the density of near-surface intergranular cracks close to the Rene-80/oxide scale interface resulting from micro-void linkage and γ′-depleted zone. Due to the high area fraction and the small average size of secondary γ′, UTS and YS were the highest and lowest after ten cycles, respectively.

Study on the effect of grain size on the microstructure evolution of Q345 steel plate under high-speed impact

This study used a two-stage light gas gun to launch reactive fragments at high speeds into steel plates with different grain sizes (large and small). In addition, utilizing material characterization technologies like SEM, EBSD, and TEM, the microstructure evolution of steel plates with varying grain sizes was measured and analyzed both before and after impact. The results indicate that small-grain steel plates are more prone to intergranular or transgranular crack propagation, and they produce more macroscopic and microscopic cracks on the perforated surface under high-speed impact than large-grain steel plates. At the grain boundaries of both large and small grains of steel plate, as well as high-density dislocations, the impact process produces a significant number of minor angles with local misorientation ranging from 2 to 10°. A small-grain steel plate has an obvious grain boundary-strengthening effect. Small grains are more likely than large grains to cause changes in the crystal structure of BCC-HCP close to the perforation, and some grains form a lot of nanoscale elongated structures with the same crystal structure as the matrix. The interaction between dislocations or the emission of a large number of dislocation cells from grain boundaries may occur within some of the larger grains. The study's findings, which are illustrated in this article, can be used as a reference to understand how grain size influences the steel plates' microstructure changes after impacts.

Deformation modes and fracture behaviors of peak-aged Mg-Gd-Y alloys with different grain structures

The ductility and toughness of peak-aged (PA) Mg-RE alloys are significantly influenced by their grain structure characteristics. To investigate this issue, we examined PA Mg-8.24Gd-2.68Y (wt.%) alloys with two distinct grain structures: an extruded-PA sample with dynamic recrystallized (DRXed) fine grains and coarse hot-worked grains, and an extrusion-solution treated and PA sample with grown large equiaxed grains. The results showed that the extruded-PA sample demonstrated a favorable combination of tensile strength (426 MPa) and ductility (7.0 %). Although intergranular microcracks nucleated in the DRXed region due to strain incompatibility, crack propagation was impeded by the DRXed fine grains, inducing intrinsic and extrinsic toughening mechanisms. On the other hand, the hot-worked grains in the extruded-PA sample initiated transgranular cracks after a relatively high strain, attributed to the strain partitioning effect, ultimately leading to failure. In comparison, the solution-treated-PA sample exhibited lower tensile strength and ductility (338 MPa and 3.7 %, respectively). Intergranular cracks nucleated in the CG sample before necking, and the readily formed critical crack, facilitated by the large grain size, exhibited unstable crack growth, resulting in premature failure. This work offers valuable insights for designing high-performance PA Mg-RE alloys and preventing premature failure in practical applications.

Effect of plastic strain under roller bending on fatigue properties ofextruded 6063 T66 aluminium alloy profile

The tensile plastic strain of material sampled from profiled areas under roller bending and pre-strained material was analyzed. The experimental data of high-cycle fatigue were approximated by a probabilistic Weibull cumulative distribution (P−S−N curve). The fatigue life prediction model was implemented for numerical simulations of roller bending in the LS-Dyna solver and experimental data. The cyclic behaviour of the material shows a decrease in the fatigue life distribution with increasing static failure levels. The fractographic analysis of the fracture surface shows an increased level of intergranular cracking for the tensile pre-strain as a result of the dislocation accumulation in the neighbourhood of grain boundaries. The estimation results of the proposed model show good correlation of probabilistic scatter bands (error below 10%).

A multi-scale formulation for polycrystalline materials accounting for cohesive micro-cracks: Homogenisation of the traction-separation law

This contribution investigates the modelling of failure in polycrystalline materials due to the nucleation and propagation of cohesive cracks at the micro-scale. First, a two-scale formulation that enables the analysis of failure, even when the evolution and propagation of cohesive micro-cracks induce material instability, is proposed. The development of this macro-continuous/micro-discontinuous formulation is based on the Method of Multi-Scale Virtual Power, from which dynamic equilibrium and homogenisation equations are derived, ensuring a variationally consistent scale transition. The respective minimal kinematical constraint is retrieved and enforced with the Lagrange multiplier method. The formulation is also extended for periodic boundary conditions. The discretisation of the resulting equilibrium equations, with the finite element method and the generalised-α method, is described in detail.Second, a fracture-based computational homogenisation procedure is developed to obtain homogenised traction-separation laws and fracture properties from microstructural volume elements. It is derived from a crack-based Hill-Mandel principle and employs a novel energetic-based damage variable, within the Park-Paulino-Roesler cohesive model, proposed to define the crack domain and compute the crack homogenised quantities. A strategy for accurately computing the homogenised unit normal vector of the equivalent macroscopic crack is also suggested.Numerical examples demonstrate the formulation’s ability to analyse fracture problems, including a study on the impact of different microscopic boundary conditions. This work also reveals the convergence of the traction-separation law with increasing microstructure model size when using the fracture-based computational homogenisation. Furthermore, it investigates how advanced deformation mechanisms, like slip plasticity and martensitic transformation, affect intergranular crack propagation mechanisms.

Electrolyte Optimization to Improve the High-Voltage Operation of Single-Crystal LiNi0.83Co0.11Mn0.06O2 in Lithium-Ion Batteries

Single-crystal Ni-rich layered oxide materials LiNi1−x−yCoxMnyO2 (NCM, 1 – x − y ≥ 0.6) are emerging as promising cathode materials that do not show intergranular cracks as a result of the lack of grain boundaries and anisotropy of the bulk structure, enabling extended cyclability in lithium-ion batteries (LIBs) operating at high voltage. However, SC-NCM materials still suffer from capacity fading upon extended cycling. This degradation of capacity can be attributed to a reconstruction of the surface. A phase transformation from layered structures to disordered spinel/rock-salt structures was found to be responsible for impedance growth and capacity loss. Film-forming additives are a straightforward approach for the mitigation of surface reconstruction via the formation of a robust protection layer at the cathode’s surface. In this work, we investigate various additives on the electrochemical performance of single-crystal LiNi0.83Co0.11Mn0.06O2 (SC-NCM83). The results demonstrate that the use of 1% lithium difluoroxalate borate (LiDFOB) and 1% lithium difluorophosphate (LiPO2F2) additives substantially enhanced the cycling performance (with a capacity retention of 93.6% after 150 cycles) and rate capability in comparison to the baseline electrolyte (72.7%) as well as electrolytes using 1% LiDFOB (90.5%) or 1% LiPO2F2 (88.3%) individually. The superior cycling stability of the cell using the combination of both additives was attributed to the formation of a conformal cathode/electrolyte interface (CEI) layer, resulting in a stabilized bulk structure and decreased impedance upon long-term cycling, as evidenced via a combination of state-of-the-art analytical techniques.

The effect of solutionizing heat treatment on ductility loss and damage evolution in AA7075 aluminum sheet deformed at 450°C

The high-temperature deformation behavior of AA7075 sheet was investigated using hot tensile testing. The specimens were solutionized at 475 °C for 15 min prior to cooling to a deformation temperature between 300 and 450 °C. AA7075 exhibited a progressive rise in ductility with increasing deformation temperature until a sudden drop in ductility was observed at 450 °C. The loss of ductility is attributed to localized melting at the grain boundaries during the solutionizing step. Increasing the holding time at 450 °C prior to the onset of deformation resulted in the gradual restoration of the ductility of the material as a result of isothermal solidification. Samples which are directly heated to 450 °C, without solutionizing at 475 °C, did not experience any loss of ductility. The elongation of non-solutionized sample was always greater than that of the solutionized one when deformation is carried out at 450 °C. This is attributed to the different distribution of particles on the grain-boundaries within the two materials. Trace analysis revealed that intergranular crack propagation proceeds preferentially on higher-indexed random grain boundary planes such as (533), (420), and (531).

Investigations on the microstructural evolution and mechanical properties of novel Al 6061/7075–granite–SiC metal matrix composites fabricated through two-stage stir casting

This study aims to explore the untapped potential of scrap granite particles as reinforcement to enhance the performance of monolithic Al alloy. Through a two-stage stir casting process, the aluminium alloy matrix (AA6061/AA7075) is reinforced with waste granite particles (GP) (5 wt.% and 2 wt.%) and silicon carbide (SiC) (2 wt.%). The microstructural, mechanical and tribological properties of aluminium-based single and hybrid reinforced composites are investigated using ASTM standards. Microstructural characterisation, including optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), confirms the presence of grain refinement and microstructural strain in the composites. The tensile, hardness, impact energy and wear resistance properties of M2 (AA6061, 2 wt.% GP) and Z1 (AA7075, 2 wt.% SiC) composites surpassed those of M1 (AA6061, 5 wt.% GP) and Z2 (AA7075, 2 wt.% SiC, 2 wt.% GP) composites, underscoring their enhanced performance. M2 composite demonstrates a significant 14.97% increase in hardness and a precise 0.82% increase in tensile strength compared to M1. Similarly, the Z1 composite showcases a remarkable 50.1% improvement in hardness and 22.2% enhancement in tensile strength compared to their respective counterpart (Z2). SEM-based fractography analysis of tensile and impact test specimens revealed the presence of dimples, cleavage facets, river lines, intergranular cracks and disbonding, offering valuable insights into the failure mode. Furthermore, the worn surfaces exhibited wear mechanisms involving abrasion and adhesion-delamination. The utilisation of granite particles as reinforcement in these composites resulted in significantly improved properties, positioning them as a promising alternative for future sustainable materials.

Effect of material chemical composition on oxidation and stress corrosion cracking of the submerged arc welding Alloy 52M overlay in a simulated PWR primary water

Compact tension (CT) specimens from submerged arc welding Alloy 52 M overlay are used to determine the stress corrosion cracking (SCC) resistance of the dilution zones and bulk overlay metal on the 52 M side in a simulated pressurized water reactor (PWR) primary water. SEM-EDS and X-ray fluorescence spectroscopy (XRF) results show three dilution zones (DZs) on the 52 M side. Oxidation test results show that the thickness of the compact oxide layer and maximum local oxidation penetration depth decrease for 1st DZ, 2nd DZ, and bulk 52 M, which are consistent with the average SCC growth rate and the number of local intergranular SCC cracks in these regions. The dilution zones with low Cr but high Fe content possess lower oxidation and SCC resistance than the bulk 52 M.

Simulation experiment for tellurium-induced intergranular cracking in oxidizing molten salt environment

One user-friendly laboratory investigation method for tellurium-induced intergranular cracking has been preliminarily developed in this study. The Te exposure experiments were carried out using FLiNaK salts with NiF2 and Cr3Te4 at 700 °C in GH3535 alloy tubes. Te exists in the form of intergranular segregation without the formation of surface tellurides, and Te-induced intergranular cracking can occur under unstrained conditions, which agrees with the observations in molten salt reactors. The acceleration effect of NiF2 can be ascribed to its reaction with Cr3Te4 to form Te metal.

Unravelling the effect of F phase on hydrogen-assisted intergranular cracking in nickel-based Alloy 725: Experimental and DFT study

The effect of the scarcely reported F phase on hydrogen-assisted cracking in nickel-based Alloy 725 was thoroughly studied by combining tensile tests, advanced characterization, and density functional theory (DFT) calculations. The results show grain boundary precipitate F phase promotes intergranular fracture in a hydrogen environment. DFT calculations further indicates hydrogen atoms lower the binding strength of the F phase and Ni matrix interfaces. More importantly, our study showed for the first time that the addition of approximately 0.01 wt.% boron can effectively suppress F phase precipitation, thereby elevating the hydrogen resistance of Alloy 725.

Evolution of tungsten degradation under different cyclic ELM-like high heat flux plasma

A millisecond pulsed repetitive plasma generator based on capillary discharge is utilized to produce ELM-like heat load. Based on this device, the evolution of tungsten degradation under heat loads with different heat fluxes, pulses, and frequencies is investigated. With an increase of heat flux and pulse, the tungsten surface experiences an increasing melting and cracking damage. The cross-sections of samples are analyzed to investigate the mechanisms of crack formation and propagation. There are numerous small cracks of ∼20 μm length in the melting layer, which may extend to form the primary cracks under further stress. Among the primary cracks, the central crack in the heat-affected layer initially propagates perpendicularly to the surface and grows parallel to the surface, converging with the edge cracks under cyclic high heat loads. And the microstructures near cracks also reveal the mechanism of crack formation, including the intergranular primary cracks and partial transgranular secondary cracks. This work provides a novel approach to simulating ELM-like thermal load and corresponding tungsten cracking behavior under repetitive high heat flux plasma.

Cause analysis of surface cracks in flash butt welding joint of high manganese steel frog

Surface cracking is one of the typical failures in butt welding joints of high manganese steel railway frogs. Understanding the formation mechanism of such cracks can effectively prevent similar failures from occurring. Herein, assisted with multiple macro and micro characterization tools, we discussed the causes of surface crack formation for a flash butt welding joint of a high manganese steel frog. The results show that the surface cracks are typical intergranular stress corrosion cracks (IGSCC) distributed near the fusion line between the carbon steel rail and the stainless steel medium in the stainless steel side. Acicular martensite exists near the fusion line between carbon steel rail and stainless steel medium, and the residual stress is large. At the same time, there are continuous network chromium carbide precipitates on the austenitic grain boundary of stainless steel. The above two points should be the main reasons for the initiation of cracks on the weld surface. Reduce the proportion of martensite structure by reasonably optimizing the welding cooling rate. Properly temper martensite to reduce the hardness and internal stress in the transition zone. These measures can prevent the occurrence of intergranular cracks on the weld surface.

Failure analysis of oil refinery heater treater’s fractured fire tube

Overheating, corrosive degradation, hydrogen embrittlement, creep, erosion, and corrosion are the cause of most failure problems in fire tubes. Herein, we have evaluated the macrostructural, microstructural, and mechanical properties at welded U-bend of the fractured fire tube to analyze the failure of the SA 387 grade 22 steel heater treater fire tube that has been under continuous use for 18 years at an oil refinery. We have found that the embrittlement factor at U-bend proved to be the most significant factor in its failure. The embrittlement X factor for weld metal is 45, which is three times more than the recommended value (≤15). It is apprehended that the fractured metal part could be susceptible to temper embrittlement. There is a significant higher hardness value of weld metal as compared to base metal, which is due to metallic phase transition at high temperature. The Charpy impact energy absorbed by the fractured metal is 13 J, which is lower than impact energy absorbed by the base metal 300 J. The macroscopic examination reveals that the crack initiated on the emulsion side and propagated radially in welded U-bend before extending further into the base metal in the axial direction. The scanning electron microscopy demonstrated intergranular crack propagation, discontinuous cracking along grain boundaries, and blunted cracks at grain interiors. The fractography is performed on the fracture surface of the impact test specimens, and it indicated heavily cavitated intergranular facets along the grain boundaries and deep secondary cracks on the fracture surface. Energy disperse spectroscopy identified the carbides formation at the U-bend cracking parts, which counter confirm the microstructural phase transition. These findings suggested that the severe cracking observed in the fire tube at the U-bend is likely due to temper embrittlement (TE) induced by exposure of the fire tube material to elevated temperatures.

Effect of texture evolution and defect on the crack initiation of a Ni‐based superalloy electron beam welded joint

AbstractElectron beam welding (EBW), a high‐quality and precise joining process, is widely used in aerospace manufacturing. In this study, through in situ scanning electron microscope (SEM) fatigue experiments and X‐ray computed tomography (CT) scanning technology, we revealed the fatigue crack initiation and propagation mechanism in GH4169 superalloy EBW joints under different post‐welding heat treatment conditions. The welded pores have a decisive effect on the fatigue properties of direct aging treatment welded joints. Fatigue cracks initiate from pore defects on the surface or in the subsurface. Due to the heterogeneous microstructure in weld seam, fatigue cracks tend to grow along the primary dendrite axis and secondary dendrite arms. Besides, in homogenization and aging treatment specimen, intergranular fatigue cracks tend to nucleate in the slip zone of large grains and twin boundaries. Although homogenization treatment can eliminate segregation, it also results in significant grain growth in both weld seam and base material.

Metallographic Evaluation of Increased Susceptibility to Intermediate Embrittlement of Engine Valve Forgings Made of NCF 3015 High Nickel and Chromium Steel.

This paper focused on determining the increased tendency of cracking after the die forging process of high nickel and chromium steel. The increase in carbon content in austenitic nickel-chromium steel promoted the tendency of valve forgings to forging intergranular crack on the valve head. Attention was paid to issues related to the chemical composition of the material to be considered when hot forming nickel-chromium steel components. Optical and scanning electron microscopies were used to examine the microstructure and fracture features of the samples removed from a fractured valve head. The embrittlement was due to microcavity formation at grain boundaries. Creep theory at grain boundaries was used to explain crack formation. The tensile behavior was interpreted from the evolution of the microstructure during deformation and referred to intermediate brittleness to explain the effect of carbon. It was found that the increased carbon content of the nickel-chromium steel and the strong undercooling observed at the edges of the valve head are factors that promote a reduction in grain boundary cohesion and enhance intermediate temperature embrittlement. Finally, it was found that the formation of a heterogeneous structure manifested by the presence of grain boundary M23C6-type carbides in the austenitic matrix was most likely related to the occurring brittleness.

Precipitation Behavior and Corrosion Properties of Stirred Zone in FSWed AA5083 Al-Mg Alloy after Sensitization

This paper investigated the Mg-rich phase precipitation behavior and the corrosion performance throughout the thickness direction within the stirred zone (SZ) of friction stir welded (FSW) AA5083 alloy after 175 °C/100 h sensitization. For the as-welded SZ, the recrystallized grain size gradually decreased from the top surface (5.5 μm) to the bottom (3.7 μm). The top and bottom of the SZ maintained relatively high levels of deformed grains and accumulated strain induced by either shoulder pressing or pin stirring. After 175 °C/100 h sensitization, 100 nm thick β′-Al3Mg2 precipitates were present along the grain boundaries (GBs) in the SZ. The bottom of the SZ exhibited more continuous precipitates along GBs due to the fine grain size and the large fraction of high-angle grain boundaries (0.724%). Although the as-welded SZ exhibited excellent corrosion resistance, it became extremely vulnerable to intergranular cracking (IGC) and stress corrosion cracking (SCC) after sensitization. The large SCC susceptibility indices of the SZ samples ranged from 66.9% to 73.1%. These findings suggest that sensitization can strongly deteriorate the corrosion resistance of the Al-Mg FSW joint, which is of critical importance for the safety and reliability of marine applications.