Intergranular Fracture Research Articles

Dynamic response and damage behavior of impact wear for polycrystalline diamond compact under low kinetic energy impact

Polycrystalline diamond compact (PDC) cutters are the main load-bearing components in shale gas drilling, and their performance significantly affects the efficiency of shale gas extraction. One of the primary reasons for cutter failure is the dynamic impact between the rock and the cutter due to the complex geological conditions of shale gas extraction. Investigating the impact wear behavior and damage mechanisms is vital to designing superior-performing PDC structures and improving their effectiveness in engineering applications. This work investigates the kinetic response and damage behavior of PDCs under low kinetic energy impacts to explain the characteristics and mechanisms of PDC impact wear. The results show that the impact force fluctuates with impact owing to changes in the interface state. Owing to the elastic-plastic properties of PDC, 87 % of the kinetic energy is used for plastic deformation and fracture. Secondly, the contact time and kinetic energy absorption rate only relate to the impact mass. The contact time positively correlates with mass, while the kinetic energy absorption rate changes in the opposite direction. In addition, diamond fracture occurs with a low kinetic energy accumulation of only 0.059 J. Cobalt catalysis and stress effects were found to induce graphitization and carbon rehybridization jointly during impact wear. It is concluded that the PDC impact wear mechanism under low kinetic energy impact is a brittle fracture with a combination of transcrystalline fracture and intergranular fracture. This work provides referable basic research data for designing impact-wear resistant PDC structures and improving the efficiency of shale gas extraction.

Dynamic failure behavior of 7B52 laminated aluminum alloy subjected to deformable projectile impact

The ballistic response and failure mechanisms during the impact of a 304 steel-core projectile on a 7B52 laminated aluminum alloy plate were investigated. The impact resistance of the target plate was obtained through ballistic testing, and a detailed analysis of the fracture mechanism was conducted using microstructure characterization. Furthermore, an enhanced Gurson-Tvergaard-Needleman (GTN) model was employed to simulate the ballistic impact process, as well as the initiation and propagation of fractures. The results revealed significant deformation following the impact of the 304 steel-core projectile on the 7B52-T6 laminated plate. The target plate exhibited spalling failure, shear failure, and ductile hole expansion, with spalling failure being caused by intergranular fracture mechanisms. In the high stress triaxiality regions of the 7A62 alloy, intergranular cracks are prone to occur, and shear stress influences the expansion of the cracks. Fragments following layer cracking adhere to the 7A52 layer, thereby increasing the ballistic resistance surface area. The 7A52 layer on the rear side of the target plate effectively absorbed the projectile's energy through bending and stretching deformation, thereby offering enhanced protection. Furthermore, the spall resistance of the 7B52 laminated plate eliminate secondary damage caused by spall fragments.

Effect of quenching temperature on the microstructure evolutions and strength-ductility of W-Ni-Cu alloy at room-temperature and cryogenic

Tungsten (BCC phase) exhibits distinct ductile-to-brittle transition characteristics. As temperature decreases, brittleness increases, ductility decreases, significantly limits its engineering applications. Here, W-5Ni-2Cu alloy was prepared by powder metallurgy, and the effect of different quenching temperatures on its microstructure evolution and mechanical properties at room-temperature and cryogenic. According to the results, rapid quenching can significantly improve the strength-ductile of W-5Ni-2Cu, especially the cryogenic properties. After quenching at 1200 °C, the strength-ductility of W-5Ni-2Cu from room temperature to −43 °C did not deteriorate, and compared with the sintered alloy, the tensile strength and elongation were increased by 7.3% and 13.0% at room-temperature, and by 16.0% and 100.0% at cryogenic. The appropriate quenching temperature can effectively refine the W grain size, reducing the WW continuity and dihedral angle, while the quenching temperatures reaches 1300 °C, it will cause abnormal W grain growth and deteriorate the mechanical properties. Furthermore, rapid quenching changes the fracture mode from intergranular fractures to transgranular fractures. Especially, the W- 〈111〉∥γ-〈110〉 preferential orientation relationship is obviously improved at −43 °C, increasing the interfacial bonding strength and co-deformation ability of W-γ phase, thereby slowing down the hardening rate and increasing the strength and elongation.

Tensile creep behavior of the Nb45Ta25Ti15Hf15 refractory high entropy alloy

The tensile creep behavior of a vacuum arc-melted Nb45Ta25Ti15Hf15 refractory high entropy alloy was investigated over a constant true stress range of 50–300 MPa at a temperature of 1173 K. Creep tests were carried out in both high vacuum (5 × 10−6 torr) and ultrahigh purity Ar gas to examine the environmental effect. The samples tested in vacuum exhibited power law behavior with a stress exponent of 4.1 and exceptional tensile creep ductility, whereas those tested in Ar suffered significant embrittlement due to HfO2 formation at grain boundaries, which was exacerbated at low applied stresses where extended exposure to residual O2 gas resulted in more extensive brittle intergranular fracture. Phase decomposition occurred after long-term thermal exposure, where a second Hf-rich body-centered cubic phase formed predominantly at grain boundaries but did not cause embrittlement. Compared to the equiatomic TaNbHfZrTi (Senkov alloy) and face-centered cubic multiple-principal element alloys, Nb45Ta25Ti15Hf15 has superior creep resistance, especially at high applied stresses, while maintaining excellent creep ductility. Transmission electron microscopy revealed that creep deformation in Nb45Ta25Ti15Hf15 at 1173 K is controlled by cross-kink collisions from screw dislocations that results in dipole drag at lower strain rates and jog drag at higher strain rates.

Effects of filler materials on the microstructural characteristics and mechanical properties of laser-welded lap joints of TZM alloys

The poor weldability of molybdenum (Mo) alloys has always been the main problem that impedes their application as large-sized structural components. Two types of laser-welded lap joints, namely LW-Ti and LW-Nb joints, were prepared using titanium (Ti) and niobium (Nb) as filler materials to investigate the alterations in microstructures and mechanical properties of TZM (Mo-0.5Ti-0.08Zr-0.02C) alloy joints. The results indicate that both Ti and Nb have excellent metallurgical compatibility with Mo, resulting in well-formed joints. Due to solid-solution strengthening, the microhardness of the fusion zone (FZ) significantly increases from 269.9 HV to 397.1 HV for LW-Ti joints and reaches 389.6 HV for LW-Nb joints. Furthermore, the shear strength of LW-Ti joints is approximately 2.05 times that of LW-Nb joints, measuring at 110.97 MPa. Fractures in LW-Ti joints primarily occur at the base metal (BM) and display lamellar ductile fracture characteristics, while fractures in LW-Nb joints mainly occur at the FZ and exhibit coarse brittle intergranular fractures. The mechanical properties of LW-Nb joints are compromised due to the larger maximum pore diameter (300 μm) in the FZ compared to LW-Ti joints (50 μm), which can be attributed to the higher viscosity of liquid niobium. Additionally, the average grain size in the FZ of LW-Nb joints (86.37 μm) is larger than that in LW-Ti joints (53.28 μm). The reduction in grain size observed in LW-Ti joints also increases the grain boundary area, decreases oxygen concentration along the grain boundaries, and enhances binding strength at Mo grain boundaries. Given Ti's stronger affinity for oxygen compared to Mo, the abundant formation of second-phase TiO2 particles within LW-Ti joints hinders the specific formation of MoO2 along grain boundaries. Consequently, the proposed welding technology was employed to weld a large-scale cylinder-flange component made from Mo alloy, which is expected to greatly facilitate the widespread utilization of welded structures made from Mo alloys.

Experimental and theoretical investigation of failure mechanism in Al2O3/SiC composite under plane shock waves

The investigation of fracture and fragmentation is essential for evaluating the protection performance and structural design of ceramic armour. This paper focused on the dynamic failure and fracture mechanisms of the fine-grained Al2O3/SiC composite subjected to plane shock waves. The macroscopic shock response characteristic was determined through a series of plane impact tests, which included the Hugoniot curves of the composite. The results indicated that the fracture mechanism is significantly influenced by specific stress characteristics. The transgranular fracture in large grains and intergranular fracture in small grains exhibit obvious local plastic deformation characteristics. Moreover, the presence of second-phase SiC particles markedly influences both crack propagation and the microfracture mode of the composite. Lastly, a statistical analysis of the ceramic fragments was conducted after the soft recovery of specimens in impact experiments, and analytical models were developed and modified to predict the distribution of fragments under plane shock loading.

High temperature embrittlement of Inconel 625 alloy manufactured by laser powder bed fusion

The comprehensive mechanical behavior of Inconel 625 alloy fabricated by laser powder bed fusion (LPBF) and hot isostatic pressing (HIP) was studied. Tensile experiments were performed at six temperatures ranging from room temperature to 1000 °C to explore the effect of temperature on the mechanical properties, and the microstructure and fracture mechanism was investigated by combining electron backscatter diffraction, transmission electron microscopy, and atom probe tomography. In comparison with the properties of conventionally fabricated Inconel 625 alloys, the mechanical properties at room temperature are essentially the same. However, the plasticity of the Inconel 625 alloys fabricated by LPBF is drastically decreased with increasing temperature, especially when the temperature exceeds 700 °C due to the transformation from transgranular ductile fracture to intergranular fracture. Detailed characterization of cracks and grain boundaries (GBs) reveals significant segregation of non-metallic elements at GBs, which impedes the movement of GBs and decreases their strength, thus enhancing the intergranular embrittlement and resulting in the drastic reduction of ductility. Moreover, the irregular grain morphologies result in numerous GBs directional sharp changes and steps, as well as GBs triple junctions, in which the stress concentration is further enlarged, thus promoting the nucleation of voids and cracks and its associated high temperature embrittlement. The underlying mechanism of ductility deterioration of LPBF-fabricated Inconel 625 alloy at elevated temperatures would provide important insights into its performance at high temperatures.

Microscopic-plastic deformation behavior of grain boundary precipitates in an Al–Zn–Mg alloy

The nano-precipitates with minimal misfit strengthen Al–Zn–Mg alloys by impeding dislocation motion. However, grain boundary precipitates (GBPs) are known to have a completely incoherent relationship with the matrix, making them less effective in strengthening the alloys. Instead, GBPs are recognized for preoccupying elements such as Zn or Mg, thereby diminishing the quantity of nano-strengthening phases. GBPs significantly influence the deformation and fracture behaviors of Al–Zn–Mg alloys by accelerating stress localization and crack formation. In this study, we investigated how the accumulation of dislocations in the GBP areas leads to stress localization and subsequent cracking, using micropillar tests and crystal plasticity finite element method (CP-FEM) analysis. The dislocation pile-up in the GBPs causes stress localization around the grain boundaries, inducing crack initiation and leading to intergranular fracture. These findings enhance our understanding of the role of GBPs in the deformation and cracking of Al–Zn–Mg alloys and may pave the way for new concepts in managing GBPs.

Effects of build direction and microstructure on fatigue crack growth behavior of Ti6Al4V fabricated by laser powder bed fusion

AbstractThis study compares the fatigue crack growth (FCG) behavior of Ti6Al4V parts fabricated by laser powder bed melting (LPBF) with four build directions (0°, 30°, 60°, and 90°) and two heat treatments and analyzed the effect of build direction and microstructure on FCG. The results show that the build direction has a significant effect on the FCG rate. Particularly, specimens with 90° build direction have the best FCG resistance while the 0° specimens have the worst. Furthermore, the fragile deposition layer boundary is the main cause of the crack deflection. The microstructure shows that the columnar β grains have less effect on FCG. The increase in resistance to FCG after heat treatment is attributed to the lath‐like α grains consuming the energy for FCG. As compared to needle‐like α′ grains, the lath‐like α grains are more likely to initiate intergranular fracture and secondary cracking, leading to rugged crack tip paths.

Theoretical and experimental study on mixed-mode I + II fracture characteristics of thermally treated granite under liquid nitrogen cooling

Liquid nitrogen fracturing technology is widely used in the exploitation of hot dry rock (HDR) geothermal resources. It is necessary to understand the fracture characteristics of hot dry rocks under liquid nitrogen cooling. In this study, the mixed-mode I + II fracture characteristics of the thermally treated granite under liquid nitrogen cooling were investigated. The results indicate that as the exposed temperature increases, the number of internal microcracks increases, while the fracture energy and fracture toughness of the granites decrease. From 25 ℃ to 600 ℃, mode I fracture toughness decreases from 1.286MP·m1/2 to 0.612MP·m1/2; mode Ⅱ fracture toughness decreases from 0.830MP·m1/2 to 0.413MP·m1/2; mixed-mode Ⅰ+Ⅱ fracture toughness decreases from 1.082 MP·m1/2 to 0.530MP·m1/2 at Me = 0.705 and from 0.914 MP·m1/2 to 0.445 MP·m1/2 at Me = 0.427, respectively. Additionally, the intergranular fracture behavior increases as temperature increases, resulting in an increase in irregularity of fracture surfaces. The generalized maximum tangential stress (GMTS) criterion is used for predicting the fracture characteristics (fracture initiation angle and fracture toughness) of thermal damaged granites. The accuracy of the GMTS criterion in predicting crack resistance when mode II loading dominates has been improved by considering the variation of critical damage radius.

Comparison between slow strain rate test and constant load test on notched specimens for HSLA steels HE susceptibility evaluation

The assessment of hydrogen embrittlement (HE) susceptibility in high-strength steels, intended for use in subsea components that are exposed to cathodic protection for corrosion control, is of great importance. Therefore, the present study investigates the behavior of AISI 4340 steel with hardness of 32, 36 and 40 HRC. The investigation involves employing slow strain rate test (nSSRT) and constant load test (CLT) on notched specimens with three different notch root radii. These tests were carried out under two different levels of cathodic protection (−1100 and −950 mVAg|AgCl) in a 3.5 % wt NaCl aqueous solution. The outcomes from the tests on notched specimens were compared with the hydrostatic stress profile obtained through finite element analysis (FEA). Additionally, the post-test fracture surfaces were analyzed using scanning electron microscopy (SEM). The test results revealed a higher hydrogen embrittlement index (HEI) with increasing material hardness and the applied cathodic potential. Moreover, a smaller notch radius was associated with a higher HEI. A clear correlation was observed between HEI and the extent of intergranular fracture in the test samples. Furthermore, the results from the nSSRT tests were correlated with the threshold stress (σth) obtained from CLT. It was evident that a relationship of σth = 90 %σnSSRT_min could be employed for a faster determination of this parameter.

Study on the microstructure and properties of TiB2-CoCrFeNiW0.2 metal ceramic composites prepared by pressureless sintering

Three types of TiB2-HEA ceramic composite were prepared using a non-equimolar ratio CoCrFeNiW0.2 high entropy alloy (HEA) as a binder, mechanical alloying (MA), and 1600 °C pressureless sintering (PS) processes. The phase analysis results indicate the presence of TiB2, FCC, and TCP phases in the TiB2-HEA composite. A detailed study was conducted on the microstructure of TiB2-HEA composite. The results indicate that there is a certain solid solubility between TiB2 and HEA. The Ti and B elements in TiB2 will enter HEA, and the elements in HEA will also enter TiB2. Along with the discovery of many lattice distortion regions in TiB2, a diffraction pattern of P-43m cubic structure was also observed in HEA. HEA has good wettability with TiB2, so it can not only inhibit the grain growth of TiB2, but also improve the performance of TiB2-HEA composite by utilizing the inherent excellent properties of HEA. There are both intergranular and transgranular fractures in the TiB2-HEA composite. The relative density, flexural strength, Vickers hardness, fracture toughness, and electrical resistivity of TiB2-30 wt%HEA are 99.7%, 680 MPa, 18.6 GPa, 6.4 MPa m1/2, and 5.7 × 10−7 Ω m, respectively.

Advancement of microstructural evolution and deformation mechanisms in AA6082 aluminum alloy under elevated-temperature tensile loading

This study explores the intricate interplay between strain-induced precipitation (SIP) and the underlying mechanisms that govern hot deformation in AA6082 aluminum alloy. Uniaxial tensile tests were conducted at elevated temperatures, ranging from 200 °C to 400 °C, and varied strain rates from 0.01 s−1 to 10 s−1. Employing advanced methodologies such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), energy dispersive X-ray spectrometry (EDS), and transmission electron microscopy (TEM), the research investigates the relationship between deformation, precipitation, and structural evolution. At lower strain rates, longer deformation times promote dynamic recrystallization, thereby promoting the growth of substructures with enhanced ductility. On the contrary, at higher strain rates (10 s−1), shorter deformation times inhibit such processes, resulting in a clear predominance of dislocation motion along specific slip systems and consequently influencing the mechanical response of the alloy. The results also reveal temperature-dependent deformation mechanisms, such as intragrain slip, continuous dynamic recrystallization (CDRX), and SIP phenomena, which influence the alloy's mechanical response. At 200 °C, prominent work hardening dominates, while 400 °C exhibits dynamic softening mechanisms. EBSD analysis elucidate temperature-specific deformation mechanisms, including intragrain slip at 200 °C and transitional phenomena at 300 °C. Microstructural analysis at 200 °C reveals the absence of needle- or rod-shaped precipitates. On the contrary, at 300 °C, the presence of β/β' precipitates significantly influences the nucleation of β'' precipitates at the grain boundaries. At 400 °C, substantial rod-shaped β' phase precipitates appear, demonstrating a complex interplay between diffusion mechanisms and strain-induced effects. In addition, the study also investigated the fracture behavior of aluminum alloy at different tensile temperatures, revealing the transition from transgranular fracture at 200 °C to intergranular fracture at 300 °C, as well as the shear effect of β/β' phase precipitates at this temperature. The findings provide a comprehensive understanding of the alloy behavior under diverse conditions, essential to optimize its processing parameters and mechanical properties.

Investigation of Hydrogen Embrittlement of Haynes 617 and Hastelloy X Alloys Using Electrochemical Hydrogen Charging

This study explores the hydrogen embrittlement behaviour of two Ni-based superalloys using electrochemical hydrogen charging. Two types of tensile specimens with different geometry for the Haynes 617 and Hastelloy X alloys were electrochemically hydrogen-charged, and then a slow strain rate test was conducted to investigate the hydrogen embrittlement behaviour. Unlike the ASTM standard specimens, two-step dog-bone specimens with a higher surface-area-to-volume ratio showed higher sensitivity to hydrogen embrittlement because hydrogen atoms are distributed mostly on the surface area. On the other hand, the Haynes 617 alloy had a lower hydrogen embrittlement resistance than that of the Hastelloy X alloy due to its relatively large grain size and the presence of precipitates at grain boundaries. The Haynes 617 alloy primarily showed an intergranular fracture mode with cracks from the slip band, whereas the Hastelloy X alloy exhibited a combination of transgranular and intergranular fracture behavior under hydrogen-charged conditions.

Effects of Oxygen Content on Microstructure and Creep Property of Powder Metallurgy Superalloy

The effects of oxygen content on the microstructure and creep properties of the FGH96 superalloy were investigated. When oxygen content increased from 135 ppm to 341 ppm, the prior particle boundary (PPB) rose from degree 2 to degree 3, the size of the γ′ phase on PPB enlarged from 1.07 μm to 1.27 μm, and the MC carbide size grew from 77.4 nm to 104.0 nm. Meanwhile, the steady creep rate accelerated from 4.34 × 10−3 h−1 to 1.87 × 10−2 h−1, and the creep rupture life shortened from 176 h to 94 h, the creep rupture mode transferred from intergranular and transgranular mixed fracture to along PPB fracture. During creep, the micro-twin formation and gliding will be restrained by ∑3 boundaries. FGH96 superalloy with higher oxygen content contains less ∑3 boundaries, and its micro-twins cross-slipped instead of single-direction slip in lower oxygen content superalloy. Consequently, samples with a higher oxygen content crept faster and ruptured earlier.

Pre-hydrogenation metallurgy for Ti-3Al-5Mo-4.5V alloy with high density and mechanical properties

The present work proposes a novel master-alloy pre-hydrogenation powder metallurgy (MAPH) method to fabricate the TC16 (Ti-3Al-5Mo-4.5V) alloy, which overcomes the challenge of low sintering density due to the high percentage of alloying powders (APs) and the presence of Mo element. The relative density of sintered TC16 alloys increases with the Ti-addition in APs, i.e., ∼99.2% and ∼99.4% for sintered TC16 alloys based on pre-hydrogenated 7.5Ti-3Al-5Mo-4.5V (7.5TiMA-Hx) and 12.5Ti-3Al-5Mo-4.5V (12.5TiMA-Hx) powders, respectively. The APs were successfully modified to be brittle and hydrogen-containing, which exhibits similar pressing and sintering characteristics as TiH2 powders. The increased relative density could be ascribed to the brittleness of 7.5/12.5TiMA-Hx, in which particles show sharp corners that can form interlocked structures with TiH2 particles during pressing. Furthermore, the 7.5/12.5TiMA-Hx and TiH2 powder particles release H2 and shrink simultaneously during sintering, which minimizes the difference in volume mismatch between particles and promotes elemental diffusion. The sintered TC16-7.5TiMA-Hx alloy achieves excellent mechanical properties, i.e., an ultimate tensile strength of ∼1004.6 MPa, a yield strength of ∼934.2 MPa and an elongation of ∼15.8%, due to the low porosity and acceptable impurity contents. However, the excessive O and N elements in the sintered TC16-12.5TiMA-Hx alloy can induce premature intergranular fracture, resulting in decreased ductility. Therefore, the innovative MAPH technique is promising in obtaining high density sintered Ti alloys with excellent mechanical properties.

Enhancing the recrystallization resistance and strength-ductility trade-off of Al–Mg–Si–Cu–Mn alloys by three-step homogenization

This study explores a three-step homogenization regime tailored to optimize the dispersoids precipitation, recrystallization resistance, and consequently, the mechanical performance of an Al–Mg–Si–Cu–Mn alloy. Comparative analyses against conventional one-step homogenization treatment reveal a substantial improvement in the precipitation of α-Al(Mn,Fe)Si dispersoids, with a 77.2% increase in number density and a 9.1% reduction in average size. This enhancement is attributed to the optimized nucleation kinetics achieved at an intermediate temperature during the three-step homogenization process. Samples subjected to one-step homogenization treatment experience complete recrystallization after a 1 h solution treatment at 560 °C, whereas three-step homogenized samples exhibit superior resistance to recrystallization, maintaining a well-preserved recovery structure. Pinning force analysis highlights the enhanced effectiveness of dispersoids obtained with the three-step homogenization treatment in inhibiting recrystallization. In terms of mechanical performance, samples underwent the three-step homogenization treatment demonstrate a synergistic enhancement in strength and ductility compared to the one-step homogenized sample. This enhancement is attributed to the intergranular fracture and low working hardening rate induced by the large recrystallized grains present in the samples treated with one-step homogenization.

Dynamic responses and failure behaviors of submerged reefs containing pre-drilled hole subjected to repetitive impacting

The drilling-hammer method is adopted for the breaking over submerged reefs in waterway regulation engineering. However, rocks with pre-drilled holes are highly susceptible to the repetitive impacts from drop hammers or drilling equipment. Exploring the dynamic responses and failure behaviors of drilled-hole rocks under repetitive impacting is important for promoting implementation of this technique. In this study, the repetitive impacting tests were conducted on the drilled rocks by the split Hopkinson pressure bar to investigate the rock bearing capacity, deformation property, failure behavior, and energy evolution. The results indicate that the presence of pre-drilled holes and the decreasing loading rates attenuate the bearing capacity of rocks subjected to repetitive impacting. In contrast, the rock deformation capacity is impacted subtly by the impact loading rate and increases with the number of impacts and borehole size. The drilled rock failure occurred with the prevalence of intergranular fractures; subsequently, the proportion of transgranular fractures increased, contributing to the loading rate-dependent bearing capacity. The inverted S-shaped damage evolution curve was assessed using the inverted logistic growth model. The specimen absorbed few incident energies for fracture generation and propagation, and the rock with a larger drill size displays higher energy utilization and larger fragments.

Strengthening behavior and model of ultra-high strength Ti–15Mo–2.7Nb–3Al–0.2Si titanium alloy

The microstructure evolution and strengthening behavior of the ultra-high strength Ti–15Mo–2.7Nb– 3Al–0.2Si titanium alloy were studied utilizing XRD, OM, SEM, and TEM analyses. The results show that the dislocation-strengthening and precipitation-strengthening effects could mostly affect the yield strength of this alloy. The highest yield strength of 1518 MPa was obtained under a combined process of cold rolling + recrystallization + cold rolling + duplex aging. This trend is mainly due to the high density of remaining dislocations, as well as dense and thin secondary α phases in microstructures. A theoretical composite-strengthening model was built, and the prediction error is within 16.6%. Furthermore, it is found that increasing the content of the secondary α phase can continuously strengthen the intragrain zone. This feature causes the intergranular fracture to appear and gradually dominate the fracture surface.

2D and 3D characterization of oxidation‐fatigue mechanisms in an advanced Ni‐based superalloy: Effects of microstructure and elevated temperature

AbstractThe effects of varying γ′ size and grain boundary carbide distribution at elevated temperatures on the fatigue behavior of turbine disk alloy RR1000 were examined through combining 2D and 3D imaging characterization. The fatigue crack growth rate at 725°C is higher than at 650°C due to enhanced oxidation damage at higher temperature, dominated by an intergranular fracture mode. This is more marked with longer dwell time (linked to diffusion time) and when carbides have formed continuously at grain boundaries. The fatigue crack path was characterized by X‐ray CT scan to present the complex 3D crack propagation morphology. It is noted that a discontinuous crack path and uncracked ligaments can be discerned ahead of the crack tip particularly in the microstructure where carbides are continuously formed at grain boundaries. This is linked to embrittled zones ahead of the crack tip formed during longer dwell times at the maximum stress during cycling at elevated temperature.