Intergranular Cracking Research Articles

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.

Susceptibility of a Fe-Mn-Al-Ni-Cr shape memory alloy to environment-assisted corrosion cracking

In recent years, iron-based shape memory alloys (SMAs) have become increasingly interesting for application in civil engineering structures. Promising candidates in the field are iron-based Fe-Mn-Al-Ni-X (X = Ti, Cr) SMAs. These alloys have demonstrated the potential to be used in various types of applications. While the fully recoverable martensitic phase transformation can be used for structures where high damping capacities are required, i.e. for high rise buildings in earthquake prone areas, it has been successfully demonstrated that the material can also be used as prestressing element in concrete structures. However, for the application in civil engineering structures the problem of stress corrosion cracking and environment-assisted cracking has to be considered. The simultaneous occurrence of tensile stresses and unfavorable environmental conditions, such as carbonation of concrete or the penetration of salt, can lead to spontaneous failure of the building structure. In the present study, the susceptibility of Fe-Mn-Al-Ni-Cr to stress corrosion cracking was studied in neutral chloride containing solutions. For that purpose, tensile tests under constant load control at different stress levels have been conducted. The pitting corrosion that occurred during immersion in the electrolytic solution has a decisive influence on the martensitic phase transformation and the crack evolution. The high density of dislocations at the martensite-to-austenite interfaces promotes the formation of transgranular cracks as well as stress-induced corrosion cracks originating from corrosion pits. Assessment of the fracture surfaces revealed dominating intergranular crack growth, most likely induced by hydrogen embrittlement. The detrimental mechanisms lead to failure of the specimens well before the targeted test time of 1000 h.

Ameliorating dilution of Cr and Ni and improving PWSCC resistance of 52 M overlay by hot wire plasma arc cladding

The multi-scale chemical composition distribution and microstructure of Alloy 52 M overlay on low alloy steel by hot wire plasma arc cladding (HWPC) were characterized. Stress corrosion cracking (SCC) growth rates were quantified in the dilution zone (DZ) near the fusion boundary and in the bulk of Alloy 52 M using compact tension samples with a thickened cladding overlay. The DZ of the Alloy 52 M side exhibited a single macroscopic dilution zone containing approximately 27–28 wt% Cr and 50 wt% Ni, in contrast to the multiple dilution zones observed in the DZ of Alloy 52 M overlays or joints prepared using other welding technologies such as submerged arc welding (SMAW). The DZ of the HWPC overlay exhibited less dilution of Cr and Ni compared to SMAW weldment. This resulted in a shorter average SCC band and fewer local intergranular cracks in the DZ of HWPC, as observed from SCC crack growth rate specimens tested in simulated PWR primary water at 325 °C and 350 °C. These findings highlight the advantages of utilizing HWPC to enhance SCC resistance in the DZ of Alloy 52 M overlays over conventional cladding technologies.

Response of 11B enriched ZrB2 ultra-high temperature ceramic to neutron irradiation at elevated temperatures

ZrB2, an ultra-high temperature ceramic (UHTC) is being considered for use in fusion reactor first-wall structures, yet its response to irradiation remains poorly understood. This study employed scanning/transmission electron microscopy (S/TEM), synchrotron X-ray diffraction (XRD), finite element calculations, and thermal property measurements to thoroughly investigate the neutron-irradiation effects on 11B-enriched ZrB2. Neutron irradiations were conducted at 220 °C and 620 °C, with a neutron fluence of 2.2 × 1025 neutron/m2 (energy > 0.1 MeV), resulting in 3.9 dpa and 4200 appm He. The study revealed the unusual prevalence of prism loops and a > c anisotropic lattice swelling, likely linked to the low c/a ratio of ZrB2, leading to grain boundary microcracking. Reducing the grain sizes was effective in reducing intergranular cracking and macroscopic swelling. The observation of cavities in ZrB2 irradiated at 620 °C, as opposed to 220 °C, prompts questions about the temperature at which vacancies in ZrB2 become mobile, and the role of neutron absorption by 10B in elevating irradiation temperatures. Isotopic enrichment in 11B proves to be a viable strategy for mitigating helium production in transition-metal diborides, which is a critical consideration for nuclear applications. Irradiation-induced defects reduce the thermal diffusivity and conductivity of ZrB2 by a factor of 4–9, which has important implications for its role as a plasma-facing material in fusion reactors that drive high heat fluxes through first-wall materials. This comprehensive study lays the foundation for understanding ZrB2 behavior under neutron irradiation and highlights important phenomena to consider for various material applications.

Exploring the effect of bio-silica on the mechanical, microstructural, and corrosion properties of aluminium metal matrix composites

The role of silica in the aluminium alloy is to enhance its mechanical properties. Silica is an eco-friendly material that lowers the melting temperature which in turn enhances the fluidity of alloys. Low-cost synthesis, abundant natural resources, and mass production are other merits of silicon. In this investigation, plant-based bio-silica particles were incorporated in aluminium 7075 hybrid composites. The rice husk is rich in silica, and when it is burned or processed, it turns into ash, known as rice husk ash (RHA). After purification, the silica in RHA can be extracted using alkali fusion. Stir casting processes were used to fabricate hybrid composite material. Aluminium 7075 hybrid composites reinforced with different wt.% (0, 3, 6, and 9) of bio-silica extracted from rice husk were fabricated. Mechanical properties such as tensile, hardness, and impact were evaluated. Also, corrosion resistance was studied for the fabricated composites. The samples with different proportional values such as AlB (Al7075), AlBS1 (97 wt. % Al7075 + 3 wt. % bio-silica), AlBS2 (94% Al7075 + 6 wt. % bio-silica), and AlBS3 (91 wt. % Al7075 + 9 wt. % bio-silica) were fabricated by the stir casting process. Detailed microstructure characterization has been investigated using scanning electron microscopy (SEM). AlBS3 hybrid composites demonstrate a notable enhancement of 303.66 Mpa tensile strength and we observed a remarkable 10% increase in ductility compared to other composites. It was noticed that the sample AlBS3 shows an increased hardness of 162.4 HV and an impact energy of 26.67 kJ/mm2 due to the increased number of bio-silica particles. SEM-based fractography analysis of tensile and impact test specimens revealed the presence of dimples, cleavage facets, and intergranular cracks offering valuable insights into the failure mode.

Thermo-mechanical creep-fatigue damage evolution and life assessment of TiAl alloy

Thermo-mechanical creep-fatigue (TMCF) behaviors of a TiAl alloy were investigated by conducting various creep, creep-fatigue, and thermo-mechanical tests and performing detailed microstructural analysis. A new creep model was proposed to unify primary strain hardening and tertiary accelerating damage effects. The TMCF failure of TiAl alloys is accompanied by the crack initiation from surface oxides/carbides and mixed transgranular and intergranular cracking. It revealed that introducing thermo-mechanical cycles significantly reduces the creep damage rate during the creep dwell stage and the fatigue process, i.e., the creep-delaying effect, which is attributed to the non-proportional strengthening effect induced by thermal cycles. A life model based on the average minimum creep strain rate was proposed to predict the creep-dominated TMCF life. This model incorporates the complex creep-fatigue interactions on creep damage and demonstrated good agreement with experimental results under varying loading conditions. The present work provides new insights into understanding the creep-dominant thermo-mechanical damage evolution of TiAl alloys.

Insights into the effects of coating and single crystallization on the rate performance and cycle life of LiNi0.9Mn0.1O2 cathode

Coating and single crystal are two common strategies for cobalt-free nickel-rich layered oxides to solve its poor rate performance and cycle stability. However, the action mechanism of different modification protocols to suppress the attenuation are unclear yet. Herein, the Li2MoO4 layer-coated polycrystalline LiNi0.9Mn0.1O2 (1.0 %-Mo + NM91) and single crystal LiNi0.9Mn0.1O2 (SC-NM91) are prepared to investigate this difference, respectively. By focusing on the interior of particles, the relationship between structure evolution and electrochemical behavior is systematically studied, and the intrinsic mechanism of coating/single-crystallization modifications on suppressing the attenuation is clarified. The results show that microcracks in LiNi0.9Mn0.1O2 (NM91) are the main culprit leading to the rate capability decay, and the coating can effectively prevent the radial diffusion of microcracks from the center to surface, inhibiting the generation of surface side reactions. Therefore, the coating has a more advantage in improving the rate performance at 5.0C, the discharge capacity of 1.0 %-Mo + NM91 (130.6 mAh/g) is 7.9 % higher than that of SC-NM91 (121.0 mAh/g). In contrast, the single-crystallization can effectively prevent the formation of intergranular cracks arising from the anisotropic stress in NM91, which causes the severe cycle degradation. Correspondingly, the grain boundary-free SC-NM91 shows superior cyclability. The capacity retention rate of SC-NM91 (80.8 %) at 0.2C after 100cycles is 6.3 % higher than that of 1.0 %-Mo + NM91 (74.5 %). This work concludes the effect difference of different modification methods on enhancing the electrochemical performance, which provides theoretical and technical guidance for the optimized and targeted modification design in the cobalt-free high nickel cathode materials.

True triaxial test and DEM simulation of rock mechanical behaviors, meso-cracking mechanism and precursor subject to underground excavation disturbance

Mechanical excavation or blasting generates stress waves that rapidly dissipate in the surrounding high-geostress rock, causing microdynamic disturbances. These disturbances trigger microcracks within the damaged rock, even inducing engineering disasters. However, the fracture behaviors and mechanisms of rockmass subjected to excavation disturbances under three-dimensional geostress remains unclear. Therefore, this study proposed a true triaxial static–dynamic combined loading method to capture the entire process of tunnel excavation damage and the continuous fracturing induced by microdynamic disturbances. True triaxial static–dynamic combined tests and numerical simulations using PFC3D-GBM were employed systematically to analyze the influence of principal stresses σ1, σ2, and σ3 on the disturbances mechanical behaviors of the gabbro. The disturbance failure under true triaxial stress indicated a three-stage pattern of deformation: deceleration, constant velocity, and acceleration. With increased σ1 or decreasing σ2 and σ3, the disturbance bearing capacity of the gabbro significantly decreased. Moreover, an increase in σ1 and σ3 corresponded to an increased proportion of intergranular shear cracks, whereas an increase in σ2 resulted in a notable increase in intragranular tensile cracks. Small-magnitude events tended to disperse during the deceleration and constant-velocity stages, whereas larger-magnitude events were concentrated during the acceleration stage. The AE parameter b-value initially increased and then decreased during disturbance fracture process. The excavation disturbance of the tunnel intensified the depth of the damage zone and the energy released, thereby increasing the risk of a catastrophic deep fracture.

The finding of anisotropic compatibility of grain boundaries in a directionally solidified superalloy

The anisotropic compatibility of grain boundaries (GBs) is found in a directionally solidified superalloy, when loading is aligned with (L sample) or transverse (T sample) to the grain growth direction. The greater compatibility of GBs is shown in the L sample by calculating the parameter m′ of the GBs using two developed models. The first model considers the theoretical scenario that grains grow along the perfect [001] orientation, and the compatibility between the neighbouring grains with all kinds of orientation difference is calculated. The second model, termed as Schmid-factor-tolerance model, considers the effects of asymmetrical slip systems caused by the deviation of grain growth direction from the [001] orientation in a real microstructure. Validation of the model is achieved through fatigue tests, demonstrating its capability to interpret and predict the crack initiation behaviours at grain boundaries. Combined with the fatigue tests to illustrate the anisotropic fracture behaviours of the L and T specimens, it shows that the intergranular cracks are more likely to occur in the T specimen with poor compatibility, which proves the capability of predicting the intergranular cracks for the Schmid-factor-tolerance model.

Determining the Hot Workability and Microstructural Evolution of an Fe-Cr-Mo-Mn Steel Using 3D Processing Maps.

The Laasraoui segmented and Arrhenius flow stress model, dynamic recrystallization (DRX) model, grain size prediction model, and hot processing map (HPM) of Fe-Cr-Mo-Mn steels were established through isothermal compression tests. The models and HPM were proven by experiment to be highly accurate. As the deformation temperature decreased or the strain rate increased, the flow stress increased and the grain size of the Fe-Cr-Mo-Mn steel decreased, while the volume fraction of DRX (Xdrx) decreased. The optimal range of the hot processing was determined to be 1050-1200 °C/0.369-1 s-1. Zigzag-like grain boundaries (GBs) and intergranular cracks were found in the unstable region, in which the disordered martensitic structure was observed. The orderly packet martensite was formed in the general processing region, and the mixed structure with incomplete DRX grains was composed of coarse and fine grains. The microstructure in the optimum processing region was composed of DRX grains and the multistage martensite. The validity of the Laasraoui segmented flow stress model, DRX model, grain size prediction model, and HPM was verified by upsetting tests.

Synergetic softening-strengthening behaviors of gradient nano-grained CoCrFeMnNi high-entropy alloys located in the inverse Hall-Petch range revealed by atomistic simulations

Gradient-structured CoCrFeMnNi high-entropy alloy (HEA) with all grain sizes lying in the inverse Hall-Petch range exhibits a significant synergistic softening behavior, with the yield strength even lower than the rule of mixtures (ROM) under uniaxial tensile loading. The softening of grain boundaries (GB) is very pronounced in hard domains due to the extremely small grain size. Moreover, the coordinated deformation of heterostructure promotes grain rotation in soft domains with large grain sizes. However, the extra strength generated by hetero-deformation induced (HDI) strengthening increases with increasing volume fraction of hard domain, contributing to a gradual decrease in the gap between yield strength and ROM. Here, we identify multiple deformation mechanisms promoted by mechanical incompatibility through molecular dynamics (MD) simulations. On the one hand, the higher volume fraction of the hard domain encourages the adaptation of dislocation-mediated activities to plastic strains, thus reducing the possibility of GB sliding or intergranular brittle cracking. More importantly, soft domains are more susceptible to detwinning under additional compressive stress, which induces grain refinement in localized regions and contributes extra strength. Observation of the strain distribution reveals that the banded shear bands (SBs) are spread throughout the soft domains, which effectively improves strain gradient strengthening. It has been verified that the gradient structure has the highest extra strength at the hard domain volume fraction of 40–50%. The findings provide insights into the engineering applications of gradient-structured HEA.

Forming limit and failure characterization of as-received and bi-axial prestrained Cu-Cr-Zr-Ti alloy sheets in the context of multistage forming

The characterization of forming limit utilizing Cu-Cr-Zr-Ti alloy sheets during the multistage forming process is imperative for successfully fabricating thrust chamber liners used in satellite launch vehicles. Therefore, the present work studies the effect of prestrain on the forming limit and failure behaviour of this alloy under different deformation paths. A stretch-forming setup was utilized to induce 8% equi-biaxial prestrain (EBP), and subsequently, these prestrained specimens were deformed till the onset of failure using the same setup. After prestraining, a shift in the failure strains was observed in the principal strain space, whereas it was found to be absent in the polar effective plastic strain, principal stress and effective plastic strain-stress triaxiality loci. The cup height along different deformation paths was marginally decreased in the prestrained specimens because of the reduction in failure strains, and the fracture occurred in the prestrained region. Further, failure analysis revealed intergranular cracks in the specimens deformed along plane strain and uniaxial deformation paths due to the presence of micron-size Cr-rich precipitates along grain boundaries. However, intergranular and transgranular cracks were present in the prestrained specimens, which were deformed equi-biaxially similar to the as-received specimens. The presence of nano-size Cr-rich precipitates inside the grains acted as nucleation sites for voids during equi-biaxial stretching, leading to transgranular cracks. It was also observed that failure in the prestrained and as-received specimens occurred at a similar effective plastic strain value due to the combined effect of void density and void size. Based on experimental observations, it was inferred that the forming limit of Cu-Cr-Zr-Ti sheets under a two-stage forming process was primarily affected by the presence of micro and nano-size Cr-rich precipitates.

Study on the Meso-Failure Mechanism of Granite under Real-Time High Temperature by Numerical Simulation

In the development of geothermal resources in hot dry rocks, deep underground rock masses are typically subjected to real-time high-temperature environments. High temperatures alter the physical and mechanical properties of the rocks, directly affecting the safe and efficient utilization of hot dry rock resources. Therefore, a grain-based model (GBM) of particle flow code (PFC) was constructed based on uniaxial compression tests, and the model was verified according to macroscopic mechanical parameters and damage modes, in order to carry out the simulation study of the uniaxial compression of granite and explore the meso-failure mechanism of granite under real-time high temperature. The relationships between stress–strain curves and crack derivation, the evolution of microcracks, and the characteristics of acoustic emission activity and energy changes at different temperatures were investigated in conjunction with the results of laboratory tests. The results show that crack development, acoustic emission activity, and energy evolution during uniaxial compression include four main stages: initial compression, elasticity, plastic strengthening, and post-peak damage. The failure of granite is primarily controlled by mica and feldspar. During loading, intergranular tensile cracks first emerge within the granite, followed by intragranular tensile cracks, with shear cracks appearing last. As the temperature increases, the total number of microcracks continuously rises, the frequency of acoustic emission events increases, and both dissipated energy and boundary energy gradually decrease, showing an upward trend in the energy dissipation ratio, indicating an increase in thermal damage due to high temperatures. At 400 °C, the rate of microcrack formation increases significantly, with intergranular and intragranular cracks starting to coalesce into macroscopic cracks that extend outward. In the post-peak stage, the phenomenon of multiple peaks in acoustic emission events begins to appear. At 600 °C, the rate of microcrack formation reaches its maximum, with cracks extending throughout the sample to form a network of fractures, resulting in the granite exhibiting ductile failure characteristics.

Effect of heat treatment on microcracking behaviors and Mode-I fracture characteristics of granite: An experimental and numerical investigation

Combining thermal stimulation and tensile fracture induced by cold water injection can synergistically enhance hydraulic fracturing, improving geothermal reservoir connectivity. To investigate the heat effect on the Mode-I fracture characteristics of granite, notched semi-circular bend (SCB) granite specimens were prepared and heat-treated at five temperatures ranging from 25 to 600 ℃. Mineralogical analysis and three-point bending tests were performed on the SCB specimens, and a grain-based model (GBM) incorporating four types of mineral particles was developed. The model was simulated for heat treatment and Mode-I loading and then validated against experimental results. The behavior and evolution of intra- and intergranular microcracks in the SCB specimens during heating, cooling, and mechanical loading were analyzed. Moreover, the fracture surface morphology and microcracking mechanisms observed in the SCB specimens at different heat-treatment temperatures were discussed. The results reveal a nonlinear trend in the Mode-I fracture toughness of the SCB specimens, initially increasing and then decreasing with heat-treatment temperature. The number of thermally induced cracks escalates with increasing heat-treatment temperature, predominantly as intergranular tensile cracks, followed by intragranular tensile cracks. Under Mode-I loading, the SCB specimens heat-treated below 450 ℃ predominantly exhibit intergranular cracks, while intragranular tensile cracks become more prevalent at 600 ℃. Thermally induced cracks can alter the propagation direction and path of mechanically induced cracks, causing macrocracks to deviate from the initial straight propagation path. Scanning electron microscope (SEM) analysis illustrates that at 300 ℃ and above, the fracture surface morphology becomes more fragmented and longer microcracks appear.

Review on Environmentally Assisted Static and Fatigue Cracking of Al-Mg-Si-(Cu) Alloys

This paper reviews the relevant literature and covers the main aspects of the environmentally assisted cracking of Al-Mg-Si-(Cu) alloys. Apart from a brief overview of the major microstructural and mechanical properties, it presents research results on the corrosion sensitivity and stress corrosion susceptibility of Al-Mg-Si alloys. Possible mechanisms of stress corrosion cracking and corrosion fatigue in aluminum alloys, such as anodic dissolution and/or interaction with hydrogen, are considered. A number of factors, including atmospheric or solution conditions, applied stress, and material properties, can affect these mechanisms, leading to environmentally assisted cracking. Specific attention is given to Al-Mg-Si alloys with copper, which may increase the sensitivity to intergranular corrosion. The susceptibility to both intergranular corrosion and stress corrosion cracking of Cu-containing Al-Mg-Si alloys is mostly associated with a very thin layer (segregation) of Cu on the grain boundaries. However, the effect of Cu on the corrosion fatigue and fatigue crack growth rate of Al-Mg-Si alloys has received limited attention in the literature. At the current state of the research, it has not yet been holistically assessed, although a few studies have shown that a certain content of copper can improve the resistance of aluminum alloys to the environment with regard to corrosion fatigue. Furthermore, considerations of the synergistic actions of various factors remain essential for further studying environmentally assisted cracking phenomena in aluminum alloys.

Accelerated corrosion tests of a contact pair austenitic stainless steel – titanium alloy

The results of accelerated corrosion tests of a pair of austenitic stainless steel — titanium alloy under conditions of elevated temperatures and high concentrations of corrosive agents are presented, conditions for the developing the mechanism of local corrosion being the same. The proposed approach included provoking the appearance of microstress concentrators by three-point loading of test samples with subsequent forced development of local types of corrosion through simultaneous exposure to an increased concentration of corrosive agents and temperatures at a fixed exposure in an autoclave. The results of modeling the corrosion process in a titanium alloy being in contact with stainless steel are assessed proceeding from the data obtained from metallographic studies of the state of surface layers of the metal and the morphology of corrosion products on prepared sections of the surface of templates cut from the samples under study. We considred a pair of steel 08Kh18N10T — titanium alloy VT-5. It is shown that the contact of samples under conditions of accelerated corrosion tests does not lead to a significant development of local types of corrosion of the titanium alloy. On the contrary, analysis of the state of the surface layers of the samples of stainless steel revealed the presence of deep corrosion pits, triggering the intergranular corrosion. In this case, corrosion of austenitic steel, both in the presence and in the absence of contact with titanium, proceeded in three stages: the initiation and fusion of pitting; formation of knife corrosion ulcers; propagation of intergranular cracks into the depth of steel. The proposed methodology of corrosion testing can be used to predict the possible nature of the development of metal corrosion under operating conditions that form local zones of static stresses in structural materials, and for prompt response to extend the design life of the equipment at thermal and nuclear power facilities.

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.

Laboratory investigation on physical and mechanical behaviors of granite after heating and different cooling rates

As an important formation of geothermal reservoirs, granite has very complex microstructure and strong heterogeneity which play an important role in the damage and failure mechanical behavior of granite under different temperatures and cooling rates. In order to better understand the influence of microstructure on the multiscale physical and mechanical properties of granite at evaluated temperatures and cooling rates, this paper conducted a series of macroscopic and microscopic experimental investigations on the granite samples after a heating with two cooling rates treatment. The results show that the failure mode changes from brittle to ductile at 400 °C which is the critical temperature for both macroscopic and microscopic properties. The physical and mechanical properties of specimen after air cooling treatment increase at 200 °C due to the transition of macropores into micropores. Transgranular cracks take place in feldspar at 400 °C and numbers of intergranular cracks develop inside all minerals at 500 °C. The number and width of microcracks in water-cooled samples are much greater than those of air-cooled simples. Based on these experimental investigations, a cross-scale characterization method of Young’s modulus is proposed by introducing a thermal damage factor into the Mori–Tanaka method to consider the thermal damage induced by the thermal treatment.

High-temperature low-cycle fatigue and fatigue–creep behaviour of Inconel 718 superalloy: Damage and deformation mechanisms

In this article, strain-controlled Low-Cycle Fatigue (LCF) and fatigue–creep tests were performed on Inconel 718 nickel-based superalloy at temperatures of 650 °C and 730 °C. LCF tests at elevated temperatures were performed with a mechanical strain rate of 1×10−3/s, while fatigue–creep tests involved either tensile or compressive strain dwell. Both the LCF and fatigue–creep tests revealed cyclic softening, with the mean stress evolving oppositely to the applied strain dwell in the fatigue–creep tests. Investigations into the damage mechanisms identified intergranular cracking as the predominant failure mode. Fatigue–creep loading with a compressive dwell resulted in multiple crack initiations from transgranular oxide intrusions, along with multiple creep cavities during loading at 730 °C. Deformation features such as persistent slip bands and deformation nanotwins were observed during cycling at 650 °C. In addition, fatigue–creep tests at 730 °C exhibited δ phase precipitation and a coarsening of strengthening precipitates, contributing to additional softening that increased over prolonged test durations. Finally, the observed lifetime during LCF tests decreased with increasing temperatures, and fatigue–creep loading was observed to be more damaging than LCF. On the other hand, fatigue–creep loading with a tensile strain dwell demonstrated a higher lifetime compared to LCF at 730 °C.

The effect of surface grinding on the stress corrosion cracking initiation of 316LN stainless steel in 600 °C supercritical CO2

The effect of surface grinding on the stress corrosion cracking initiation of 316LN stainless steel in 600 °C supercritical CO2 was studied through constant extension rate tensile test. The ground surface exhibited much higher oxidation resistance than the polished surface as the deformation layer can greatly enhance the diffusion of active elements and promote the formation of more protective oxide scale. Moreover, the intergranular cracking was effectively suppressed on the ground surface as the ingress of oxidant along the original grain boundary was inhibited.