Dynamic Embrittlement Research Articles

Stress corrosion cracking susceptibility of a new nickel‐based alloy in different environments

AbstractStress corrosion cracking (SCC) susceptibility of a new nickel‐based alloy C‐HRA‐2 under two types of superheated steam (SHS) (0.1 and 8 MPa) and supercritical water (SCW) (25 MPa) at 650°C was studied through slow strain rate tensile tests at a strain rate of 5 × 10−7 s−1. The results indicate that C‐HRA‐2 exhibits SCC susceptibility in both SHS and SCW, and the susceptibility increases with increasing pressure. Transgranular fracture (located inside) characterized by dimples/micropores and intergranular fracture (located at the edge) characterized by rock‐sugar‐like morphology was observed in all specimens. The brittleness of the alloy increases with the increase of medium pressure. A large number of cracks perpendicular to the loading axis were noticed on the gauge surface. The SCC mechanism of C‐HRA‐2 under steam and SCW is dynamic embrittlement.

Effect of different loading conditions on corrosion fatigue crack growth rate of a nickel-based alloy in supercritical water

Advanced ultra-supercritical (A-USC) technology improves the initial steam temperature to 650 °C and more. In such an extremely corrosive medium environment, metallic materials are susceptible to corrosion fatigue. In the present study, the aim is to evaluate the effect of maximum stress intensity factor (Kmax), stress ratio (R), waveform, frequency, and holding time (th) on the corrosion fatigue cracking behavior of C-HRA-2 (a candidate nickel-based alloy for A-USC units) in supercritical water (650 °C/25 MPa). The results show that with the increase of Kmax, the corrosion fatigue crack growth rate (CFCGR) increases and satisfies the exponential relationship. The decrease in R and frequency both cause the rise of CFCGR, which satisfies the logarithmic linear relationship. Sinusoidal and triangular wave loading have no significant effect on the CFCGR. Trapezoidal wave with th has a larger CFCGR while with the rise in th, the CFCGR increases. The crack propagation path is straight, perpendicular to the loading direction, and secondary cracks are also observed. Transgranular and intergranular mixed fractures occurred during continuous loading. The proportion of intergranular fracture increases significantly as holding time is applied. The corrosion fatigue crack growth of C-HRA-2 in supercritical water can be explained by dynamic embrittlement mechanism.

Temperature and strain-rate dependence of the elevated temperature ductility of Inconel 718 prepared by selective laser melting

The elevated temperature tensile properties of SLM IN718 were measured as a function of strain rate and test temperature to better understand the time-dependent and thermal aspects of environmental sensitivity. SLM and wrought materials were solution treated, aged, and tested over the range of 550–750 °C and 10−3 to 10−6 s−1. The SLM material tested across all conditions had inferior strength and ductility compared to wrought material of the same heat treatment because of environmental sensitivity in the form of dynamic embrittlement. SLM samples show evidence of brittle intergranular fracture, crack growth, and oxidized NbC particles on the fracture surface, which all contribute to the observed poor ductility. These features, and subsequent embrittlement, are intensified with decreasing strain rate and increasing temperature. Higher strain rates and lower temperatures are shown to improve the ductility in SLM IN718 but despite this recovery it remains susceptible to environmental attack even in the extreme cases of the current study. The prevalence of NbC at grain boundaries in the SLM material is the principal source of dynamic embrittlement and environmental sensitivity.

The influence of oxygen partial pressure on the crack propagation of superalloy under fatigue-creep-environment interaction

We elucidate here the effect of low oxygen partial pressure on the dwell fatigue crack growth rate (DFCGR) of 718Plus alloy at high temperature. In comparison under the air and vacuum conditions with lower oxygen partial pressure, the DFCGR of 718Plus alloy reduced by three orders of magnitude at 704 °C at lower oxygen partial pressure than that of in air condition. By analysis of microstructure, oxide, and crack path under different conditions, it was considered that the oxygen-induced dynamic embrittlement (DE) was the dominant impact mechanism for fatigue crack propagation at high-temperature in 718Plus alloy. Meanwhile, the stress-accelerated grain boundary oxidation (SAGBO) mechanism also played a significant role, especially in the long step-wise period. In addition, the types of oxide formed in creep process were directly related to the oxygen partial pressure and oxidation time.

Role of Impurity Sulphur in the Ductility Trough of Austenitic Iron-Nickel Alloys.

The role of impurity sulphur in the ductility trough of iron–nickel (Fe–Ni) alloys is investigated using hot tensile tests. A strong detrimental effect of some ppm levels of sulphur is demonstrated. In addition, it is shown that, in the ductility trough, material failure occurs through subcritical grain boundary crack propagation, involving dynamic embrittlement at the crack tip, due to the sulphur. Very high intergranular crack growth rates are observed. This is possible because plastic deformation accelerates the transport of sulphur to the crack tip, by several orders of magnitude, compared to normal bulk diffusion. The ductility is recovered at high strain rates, which correlates with a decrease in the sulphur concentration measured on the fracture surface. It is suggested that the main mechanism of sulphur transport is dragging by moving dislocations.

Subsurface Dynamic Deformation and Nano-structural Evolution in 40Cr Steel Under Dry Sliding Wear

Dry sliding tribological properties of normalized 40Cr steel pins against quenched and tempered GCr15 discs were investigated under various sliding distances through disc-on-pin contact configuration. Microstructural feature evolution and strain-hardening behavior of worn subsurface in 40Cr pins were systematically analyzed using scanning electron microscopy and transmission electron microscopy. It is found that based on the variation of wear rate, three stages can be divided throughout the friction process: initial wear, slow wear and stable wear. The corresponding microstructures beneath contact surface are subjected to significant strain-hardening effect and structural rearrangement. Nano-grains of 40–100 nm are produced in the topmost subsurface layer and the nanocrystallization mechanism during friction process is further elucidated. Pronounced vortex structures are generated in the localized subsurface zones due to high strain localization and shear instability. With the prolongation of sliding distances, vortex structures suffer dynamic embrittlement and exfoliation from the wear surface as a result of periodically excessive hardening effect, leading to the high wear rate in stable wear stage.

The sharp drop in fatigue crack growth life at a critical elevated temperature for a PM Ni-based superalloy FGH97

The fatigue crack growth (FCG) experiments for Ni-based powder metallurgy (PM) FGH97 superalloy were performed from RT to 800 °C in air. It has been found that the FCG life decreases in an accelerated manner with an increase in temperature in the service temperature range, and a sharp drop in FCG life was noticed at the inflection point temperature (Tc) of about 800 °C. The fractographic and microstructural analyses were carried out to investigate the primary reason for the phenomenon of observed sharp drop. The obtained results indicate that the fracture transforms from transgranular to intergranular mode during the crack propagation under elevated temperature, and the transition point of fracture mode (ΔKT) also declines rapidly with an increase in temperature. At 800 °C, ΔKT appears even from the crack starts to propagate, which is closely related to the significant decline of FCG life. Combined with the calculated apparent activation energies of FCG, it can be inferred that the sharp drop in FCG life is primarily attributed to the dynamic embrittlement induced weakening of grain boundary, which dominates in the intergranular FCG stage at 800 °C. Further, the evolution of the fracture mechanism governing the FCG process under the conditions of different temperatures and stress intensity factor range (ΔK) has been discussed. With an increase in temperature in the service temperature range, the dominant mechanism of the intergranular fractures transforms from stress assisted grain boundary oxidation to dynamic embrittlement, which results in the sharp decline of FCG life at Tc.

Coupled stress-diffusion modelling of grain boundary segregation and dynamic embrittlement in a copper alloy

Intergranular fracture due to dynamic embrittlement by impurity segregation at grain boundaries is modelled and simulated in this work. The model is based upon: (i) a bulk diffusion equation including the effect of the hydrostatic pressure, (ii) a grain boundary diffusion equation including the effect of the normal cohesive traction, (iii) a constitutive law for microplasticity accounting for the viscoplastic slips inside the crystals, and (iv) a traction-separation law incorporating the degrading effect of impurity coverage on the cohesive strength. The numerical implementation of the coupled transient diffusion-mechanics model is done in the commercial finite element code Abaqus and tested with the sulfur/copper system. The model predictions for impurity diffusion from a surface constant source inside an isolated grain boundary exhibit reasonable agreement with known analytical solutions. An edge-notched specimen with contaminated crack flanks is then analysed under static loading, showing the stress-assisted penetration of the contaminant in the grain boundary and its effect on crack propagation. Finally, intergranular fracture occurrence in polycrystalline aggregates under tensile loading is simulated. The strong impurity segregation at the grain boundaries is demonstrated, and the phenomenon of ductile-to-brittle transition of the rupture mode, influenced by the temperature and strain rate parameters, is well reproduced by simulation.

Role of oxygen in enhanced fatigue cracking in a PM Ni-based superalloy: Stress assisted grain boundary oxidation or dynamic embrittlment?

The role of oxygen in enhanced fatigue cracking in an advanced Ni-based superalloy for turbine disc application has been evaluated in fatigue crack initiation and propagation stages along with static oxidation tests. It is found that the grain boundary oxide intrusion has a layered structure. The microstructure- and deformation-dependent grain boundary oxidation dominates the fatigue crack initiation and early propagation processes. As the crack propagates, this contribution arising from oxidation damage may gradually be overtaken by dynamic embrittlement processes until the mechanical damage outstrips the oxygen-related damage, resulting in a transition from intergranular to transgranular crack propagation.

Oxidation assisted intergranular cracking in 718 Nickel Superalloy: on the mechanism of dynamic embrittlement

Nickel Superalloy 718 was specially designed for high temperatures. However, due to an embrittlement phenomenon, its application is limited up to 650°C. This work presents a contribution to the understanding of the embrittlement mechanism focusing on the crack propagation kinetics responsible for the brittle fracture. This manifestation is known as OAIC (Oxidation Assisted Intergranular Cracking), although the phenomenology resulting in such degradation is not well understood. The approach to the problem was accomplished in the solubilized condition by mechanical testing, fractography and activation energy calculation.Generally, high temperature dissociation of NbC in the presence of oxygen is pointed as the source of niobium that ultimately lead to fracture by Nb2O5 formation at grain boundaries. However, metallographic analysis of fractured samples between 600 and 925°C at different strain rates indicated otherwise. Samples tested at 850°C showed minimum ductility and fast intergranular crack propagation, at a short time incompatible with carbide dissociation and Nb2O5 formation. Furthermore, there is no evidence of sufficient NbC along grain boundaries to supply elemental niobium to the formation of the Nb2O5 continuous film. Besides, onset to OAIC as a function of temperature and strain rate led to a 223kJ/mol activation energy, which can be associated to niobium self diffusion within the matrix. Considering that the dynamic aging phenomenon and the γ″ precipitation occur concomitantly with the deformation, the results suggest that niobium segregation together with preferential γ″ precipitation at grain boundary and the consequent stress state build up should play also a decisive role in the embrittlement.

Investigation and Modeling of Local Crack Arrest in Ferritic-Bainitic Steels Under Dynamic Loading

Local crack arrest is usually irrelevant under quasi-static loading conditions in the ductile to brittle transition region. Elevated loading rates, however, allow cleavage fracture due to the dynamic embrittlement also at higher testing temperatures compared to static loading. This behavior is generally accompanied by local crack arrest events. In addition, adiabatic heating processes in the crack tip region increase local temperature as well, which further promotes crack arrest. This complex interaction between crack initiation and crack arrest at elevated loading rates substantially changes macroscopic fracture behavior, whereas its investigation is the core of this work. An experimental database of dynamic fracture mechanics experiments for the reactor pressure vessel steel 22NiMoCr3-7 is examined in this work that was previously tested at crack tip loading rates of about 103 to 105 MPa√m /s. Recent fractographic examinations and statistics regarding the occurrence and characteristics of local crack arrest incidences are shown for different loading rates and testing temperatures. Furthermore, an existing local probabilistic cleavage fracture model is used to describe macroscopic fracture behavior for the provided experimental database, and also compared to other assessment methods (i.e. Master Curve). The shortcomings in the numerical assessment methods can be linked to the amount of observed local crack arrest incidences, and a micromechanically motivated model modification is proposed to consider the mechanism of local crack arrest. The agreement between experimental results and numerical cleavage fracture assessment can be significantly increased by using the modified model.

Effects of oxygen-related damage on dwell-fatigue crack propagation in a P/M Ni-based superalloy: From 2D to 3D assessment

Effects of oxygen-related damage (i.e. oxidation and dynamic embrittlement) on fatigue crack propagation behavior in an advanced disc alloy have been assessed in air and vacuum under dwell-fatigue conditions at 725°C. The enhanced fatigue crack propagation is closely related to oxygen-related damage at/ahead of the crack tip, which is determined by the testing environment, the dwell period and the crack propagation rate itself based on two dimensional (2D) observation of the crack tip in an optical microscope and scanning electron microscope. X-ray computed tomography has also been employed to examine the differences between three dimension (3D) crack morphology in air and vacuum conditions, and the crack features have been quantified in terms of crack opening displacements, secondary cracks and uncracked bridging ligaments. The results show that the fatigue crack propagation rate is related to the amount of secondary cracks, and the crack length increment in a loading cycle is related to the breaking/cracking of the uncracked bridging ligaments within the discontinuous cracking zone ahead of the crack tip as oxygen-related damage preferentially occurs in these highly deformed regions. By combination of 3D X-ray computed tomography and traditional 2D observation, a deeper understanding is provided of the mechanisms of oxygen-enhanced fatigue crack propagation behavior.

Intergranular Oxidation Effects During Dwell-Time Fatigue of High-Strength Superalloys

The present paper summarizes experimental work to identify the mechanisms of dwell-time cracking during service operation of polycrystalline nickel-base superalloys, such as Alloy 718 and AD730. By means of crack growth monitoring during various kinds of cyclic loading in vacuum and in air using the potential drop technique, it was shown that the combination of sustained tensile stress, load reversal, and oxidizing atmosphere leads to an increase in the crack propagation rate by orders of magnitude, as compared to cyclic reference tests without dwell time and/or under vacuum conditions. By careful metallographic and theoretical analysis, the embrittling effect was attributed to stress-induced oxygen diffusion ahead of the intergranular crack tip followed by decohesion in a nanometer scale and had been termed “dynamic embrittlement.” More recently, atom probe tomography of the near-crack tip region revealed that the damage zone consists of Cr-rich transition oxides rather than elemental oxygen. This is in qualitative agreement with TGA measurements on Alloy 718 specimens without mechanical loading, which shows that crack propagation velocities of 50 µm/s do not allow massive Cr2O3 or NiO scale formation. By means of a quantitative analysis of the fracture surface, it became evident that grain-boundary attack depends on the grain-boundary character. This observation was supported by four-point bending experiments on grain-boundary-engineered samples with a high fraction of coincident site lattice boundaries and bicrystalline samples with well-defined grain-boundary misorientation relationships with respect to the loading axis. Taking the experimental results into account, semiquantitative modeling concepts have been developed to correlate crack propagation rates with the oxygen grain-boundary diffusivity, the local microstructure, and the mechanical stress states. These concepts are discussed in terms to adapt grain size and precipitate microstructure of polycrystalline superalloys.

Numerical modeling and simulation of intergranular fracture due to dynamic embrittlement for a Cu[sbnd]Ni[sbnd]Si alloy

The involvement of the dynamic embrittlement phenomenon during tensile tests of a CuNiSi alloy leads to decohesion of grain boundaries, which is dependent on the energy decrease at grain boundaries caused by sulfur segregation. The severity of dynamic embrittlement increases with the increasing of temperature and/or the decreasing of straining rate. A crystal plasticity approach associated with the cohesive zone method is used in order to analyze the fracture caused by dynamic embrittlement. A cohesive zone model is used to describe the grain boundary cracking and one of its main parameters is identified using a homogenization scheme for the nonlinear response of a polycrystal in presence of degradable grain boundaries. Numerical simulations are carried out in order to simulate the mechanical behavior related to dynamic embrittlement under different conditions of strain rate and temperature. The obtained results such as tensile stress-strain curves and intergranular crack distribution are compared with experimental observations.

Effect of dynamic embrittlement on high temperature fatigue crack propagation in IN718 – experimental characterisation and mechanism-based modelling

At elevated temperatures, the nickel-base superalloy IN718 is prone to the failure type ‘dynamic embrittlement’. Dynamic embrittlement is assumed to be driven by tensile stress-controlled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries in front of a crack and results in a fast brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, high temperature fatigue crack propagation tests were carried out at 650 °C in vacuum and air applying various dwell times and testing frequencies. The crack growth was monitored by the ACPD technique and a far-field microscope. The observations show that at low stress intensity factor ranges, crack propagation mainly occurs in the unloading and loading parts of the cycle and only minor crack propagation takes place during the dwell time. With increasing dwell time, the contribution of the crack propagation at constant stress increases. A mechanism-based model was developed on the basis of these findings which allows for a quantitative description of the effect of dynamic embrittlement on fatigue crack propagation rate. The results of respective simulations correspond very satisfactorily to the experimental data. Hence, the model is suitable for lifetime assessment.

On the mechanism of dynamic embrittlement and its effect on fatigue crack propagation in IN718 at 650°C

IN718 is a commonly used nickel-base alloy for high temperature applications, e.g., in gas and steam turbines. At elevated temperatures, this and other superalloys are prone to the failure mechanism "dynamic embrittlement". Dynamic embrittlement can be considered as a kind of stress corrosion cracking, driven by tensile-stress-controlled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries by weakening the grain boundary cohesion resulting in fast and brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, high-temperature fatigue crack propagation tests were carried out at 650°C applying various dwell times and testing frequencies. Most of the tests were performed in laboratory air, but some experiments were run in vacuum as well, in order to eliminate environmental effects and, hence, to define the reference fatigue crack propagation behaviour. The observations show that at low stress intensity factor ranges ΔKI, continuous crack growth occurs. At intermediate values of ΔKI, no crack propagation takes place during the dwell part of the cycle. Rather, the crack extends during unloading and reloading between subsequent hold times. The time necessary to grow the crack under sustained load during the dwell time was found to decrease with increasing stress intensity factor. Therefore, at high values of ΔKI, there is a contribution of the crack propagation at constant stress, since the incubation time is shorter than the dwell time. A mechanism-based model was developed for the range of test parameters, where intergranular and transgranular areas exist side by side in the fracture surface. The total crack growth per cycle is calculated by a linear combination of the intergranular and the transgranular contribution using the corresponding area fractions as weighting factors. It is shown that simulation calculations based on this model approach correspond very reasonably to the experimental observations. Hence, the model provides a quantitative mechanismen-related description of the effect of dynamic embrittlement on fatigue crack propagation rate.

Impact of the surface quality on the thermal shock performance of beryllium armor tiles for first wall applications

Beryllium will be applied as first wall armor material in ITER. The armor has to sustain high steady state and transient power fluxes. For transient events like edge localized modes, these transient power fluxes rise up to 1.0GWm−2 with a duration of 0.5–0.75ms in the divertor region and a significant fraction of this power flux is deposited on the first wall as well. In the present work, the reference beryllium grade for the ITER first wall application S-65 was prepared with various surface conditions and subjected to transient power fluxes (thermal shocks) with ITER relevant loading parameters. After 1000 thermal shocks, a crucial destruction of the entire loaded area was observed and linked to the stress accelerated grain boundary oxidation (SAGBO)/dynamic embrittlement (DE) effect. Furthermore, the study revealed that the majority of the thermally induced cracks formed between 1 and 10 pulses and then grew wider and deeper with increasing pulse number. The surface quality did not influence the cracking behavior of beryllium in any detectable way. However, the polished surface demonstrated the highest resistance against the observed crucial destruction mechanism.

Analytical homogenization modeling and computational simulation of intergranular fracture in polycrystals

Failure at grain boundaries has a critical effect on the overall fracture behaviour of polycrystalline aggregates, as is the case in many metals. In the case of dynamic embrittlement, segregation of impurities occurs at grain boundaries, lowering their cohesive strength. The material response is then dominantly determined by grain boundary properties. The self-consistent scheme is extended to account for grain boundary decohesion by using a cohesive law to represent crack initiation and propagation. After introducing the imperfect interface conditions into the Eshelby’s equivalent inclusion solution, the effective tensile response and brittle intergranular fracture of a Cu–Ni–Si alloy is predicted. The proposed analytical model allows for the identification of parameters for both crystal plasticity and cohesive constitutive laws, from a single macroscopic tensile curve. Afterwards, multiscale computations of artificial microstructures are done using the analytically calibrated values of material parameters. Comparison of the results with experimental data shows a satisfactory agreement.

A Grain Boundary Fracture Model for Predicting Dynamic Embrittlement and Oxidation-Induced Cracking in Superalloys

Nickel-based superalloys are sometimes susceptible to oxygen embrittlement in the form of dynamic embrittlement or oxidation-induced grain boundary cracking during services at elevated temperatures. Dynamic embrittlement is a fracture process that involves the ingress and diffusion of atomic oxygen to induce time-dependent decohesion of grain boundaries. A related fracture process, also a time-dependent process, is stress-accelerated grain boundary oxidation and oxide-induced cracking along grain boundaries. In this paper, a micromechanical model is developed to treat both dynamic embrittlement and oxidation-induced crack growth in Ni-based superalloys. The model is utilized to assess: (1) the conditions where dynamic embrittlement are dominant, (2) the conditions where oxidation-induced crack growth are dominant, and (3) the role of oxidation in suppressing dynamic embrittlement in Ni-based superalloys. For illustration, the grain boundary fracture model is applied to predict the onset of dynamic embrittlement and oxidation-induced crack growth in superalloys such as IN 718.

Creep–Environment Interactions in Dwell-Fatigue Crack Growth of Nickel Based Superalloys

A multi-scale, mechanistic model is developed to describe and predict the dwell-fatigue crack growth rate in the P/M disk superalloy, ME3, as a function of creep–environment interactions. In this model, the time-dependent cracking mechanisms involve grain boundary sliding and dynamic embrittlement, which are identified by the grain boundary activation energy, as well as, the slip/grain boundary interactions in both air and vacuum. Modeling of the damage events is achieved by adapting a cohesive zone (CZ) approach which considers the deformation behavior of the grain boundary element at the crack tip. The deformation response of this element is controlled by the surrounding continuum in both far field (internal state variable model) and near field (crystal plasticity model) regions and the intrinsic grain boundary viscosity which defines the mobility of the element by scaling up the motion of dislocations into a mesoscopic scale. This intergranular cracking process is characterized by the rate at which the grain boundary sliding reaches a critical displacement. A damage criterion is introduced by considering the grain boundary mobility limit in the tangential direction leading to strain incompatibility and failure. Results of simulated intergranular crack growth rate using the CZ model are generated for temperatures ranging from 923 K to 1073 K (650 °C to 800 °C), in both air and vacuum. These results are compared with those experimentally obtained and analysis of the model sensitivity to loading conditions, particularly temperature and oxygen partial pressure, are presented.