This study investigates the healing behavior of microcracksin a Ni-based superalloy using high-density pulsed electric current (HDPEC). Experimental results demonstrate that a single high-energy HDPEC treatment achieved up to 70 % healing efficiency. A finite element model based on the actual microcrack geometry was developed to simulate the coupled electrical, thermal, and mechanical responses. Unlike macrocracks, microcracks do not exhibit significant temperature and thermal stress concentration at the crack tip due to rapid thermal conduction. Instead, their healing is driven by the synergistic effects of elevated temperature, thermal compressive stress, and enhanced atomic diffusion. A critical crack length of 100 mu m was identified, below or above which different repair mechanisms dominate. Based on these insights, tailored strategies for repairing micro-and macro-scale cracks are proposed. This work offers a fundamental understanding of scale-dependent crack healing mechanisms and provides a practical route for extending the fatigue life of structural components in aerospace, energy, and electronics applications.

Gas turbines require proper operation and maintenance, and predicting the creep crack initiation life of turbine blades made of a solidification-controlled Ni-based superalloy is crucial. A unified life assessment method was developed to assess creep crack initiation life at various stress-concentrated regions, using a misorientation parameter which is equivalent to the relative notch opening displacement. This parameter can predict creep crack initiation life using crystal orientation distribution measured by the EBSD method. In this study, the influence of the specimen plate thickness was investigated, and it was confirmed that there is no issues with practical applicability.

This study explores the efficacy of high-density pulsed electric current (HDPEC) in healing plastic deformation damage in single-crystal Ni-based superalloys. Single-crystal specimens are prepared from directionally solidified nickel-based superalloy to assess the potential of electric current treatment in restoring plastic deformation capability. Experiments involving various current densities and pulse durations reveal that a pulsed electric current of 17 ms at 400 A mm-2 yields optimal results, enhancing fracture elongation from 7.8% to 12.3% without compromising strength. The study conducts a comprehensive analysis of microstructural changes induced by pulsed electric current, employing quasi-in-situ electron backscatter diffraction and transmission electron microscopy techniques. The results demonstrate that HDPEC disrupts the planar dislocation network, homogenizes dislocation distribution, and promotes dislocation entanglement. Consequently, microdefects in the alloy are eliminated, restoring the material's ductility. The findings suggest that this technology holds significant promise for repairing fatigued components and underscore the potential for further research in this domain.

This study examined the effect of high-density pulsed electric current (HDPEC) treatment on the residual stress relief of martensitic stainless steel SCS6. The findings indicate that applying HDPEC at 100 A/mm2 for 370 ms, or equivalent current conditions at a maximum temperature of approximately 735 °C, can nearly eliminate surface residual stresses. Moreover, this treatment resulted in a 7 pct increase in strength without deteriorating the ductility. X-ray diffraction and electron backscatter diffraction analysis found that defects (i.e., dislocations) within the grains were remarkably removed, which was the main reason for the residual stress removal. This study demonstrates the feasibility of HDPEC in eliminating residual stresses of martensitic stainless steels, presenting a promising alternative to traditional heat treatment. In addition, this method introduces possibilities for localized non-destructive treatments on substantial components, enabling rapid and straightforward repairs.

Technologies for evaluating the creep damage of Ni-base superalloys of Directionally Solidified (DS) materials or single crystal materials, used as gas turbine blades, have been recently developed. One notable inspection technique is the electron back-scatter diffraction (EBSD) method which uses the misorientation analysis within grains of the Ni-base superalloy. However, this is a destructive technique. Another notable inspection technique is the rocking curve method, which uses the relationship between the full width at half maximum (FWHM) of a rocking curve by X-ray diffractometry and the creep damage of the Ni-base superalloy. It requires rotating the sample. Hence, these techniques have problems in practical applications. We have proposed an exclusive technique by applying the transmission Laue method to Ni-base superalloys, to address the problems with the above techniques. All of these techniques utilize a master curve, made by evaluating test pieces damaged at several stages and used as a standard for sample evaluation, however how to evaluate the creep damage of sample by each method is different as described above. There are four advantages to the new technique (patent pending), as follows; Creep damage of a sample is evaluated 1. without moving the sample. 2. non-destructively. 3. regardless of the thickness. 4. in a conventional laboratory. The first advantage uses the Laue method with polychromatic X-rays, which allows the sample to be measured without moving it. Second, the high energy X-ray microbeam probes the sample below any surface coatings by transmission. Thrird, our measurement technique probes the grains within the sample crystal regardless of their depth. Fourth, our new apparatus for generating high energy X-rays allows the samples measured in a conventional laboratory. We are calling this technique "the crystallite orientation deviation method". In this study, the new technique is primally verified through the theoretical examination for applying the Laue method to the DS or the single crystal material by considering elastic and inelastic crystallite orientation deviations. Then, results of creep tests for test pieces of a Ni-base superalloy of DS material were compared with experimental results obtained from making two master curves by COD using the same test pieces cut out into two sets, respectively one set without thermal barrier coating (TBC) and the other set with TBC. As results, it was found that tendances between the results of COD and the results of creep tests had a good agreement with each other. That is, it validates that COD can successfully evaluate the creep damage of the Ni-base superalloy. Furthermore, it was also found that the results of test pieces without TBC had a good agreement with those with TBC. It shows that the TBC coating on the surface of Ni-base superalloy was correctly penetrated by the polychromatic X-rays based on COD. We expect that this technique COD also can be applied to polycrystalline materials in addition to the single crystals, especially the materials having coarse grains. It suggests that many aircraft parts may be evaluated by COD.

The subcritical crack growth behavior under sustained loading and its microscopic mechanism in <100> oriented single crystal (SC) Ni-based superalloys subjected to a relatively low temperature condition (500 °C) are critically investigated. An evident time-dependent crack growth (TDCG) phenomenon is observed even at such a temperature condition, but its rate (da/dt) is significantly small and weakly dependent on the crack length, i.e. the stress intensity (K) value. The cross-sectional observation of a crack tip region by electron microscopy (SEM/TEM), together with the conventional fractography, reveals the following points: the macroscopic crack path shows pronounced branching influenced by the {100} planes; most dislocations in the γ’ phase near the crack tip are rectilinearly aligned and locked on the {100} planes; the crack tip dislocations provide the microscopically preferable oxidation paths. These points strongly suggest that the dislocation-mediated micro-oxidation process plays a major role in the observed anisotropic TDCG mechanism that eventually leads to the anomalous da/dt - K property.

In this study, using a C(T) specimen for 12Cr steel used for a steam turbine rotor, crack growth tests under creep-fatigue conditions with and without overloading were conducted and the fundamental behaviours of deformation and crack growth were investigated experimentally. As a result, load line displacement was found to be suppressed under overload condition regardless of temperature and stress holding time. Crack initiation life under high temperature and low stress condition with overloading was delayed as compared with no overload condition. However, under low temperature and high stress condition with short stress holding time, crack initiation was accelerated remarkably. Furthermore, although crack growth life under creep-fatigue conditions with overloading was found to be shortened as compared with no overloading, the effect of overloading on crack growth life was decreased with an increase in stress holding time.

This study investigates the inhibitory effects of high-density pulsed electric current (HDPEC) treatment on fatigue crack initiation in directionally solidified nickel-based superalloy. Experiments were conducted under fully reversed conditions (R = -1) using low-cycle fatigue tests to assess the impact of HDPEC on crack initiation. The distribution of misorientation at different stages before and after crack initiation of untreated and HDPEC-treated samples was analyzed through kernel average misorientation maps by electron backscatter diffraction method. The results demonstrate that the HDPEC treatment significantly delays the initiation life of cracks, extending from approximately 2000 cycles to 5000 cycles. Additionally, the average fatigue life of the samples increased from 3851 cycles to 10881 cycles, more than doubling the fatigue life. Furthermore, the formation of persistent slip markings on the surface of the HDPEC-treated samples was completely suppressed, indicating changes in the persistent slip band's structure. This phenomenon led to a distinct pattern of crack formation and propagation compared to untreated samples, suggesting that HDPEC treatment effectively alters the pattern of crack initiation and propagation by homogenizing dislocation distribution and alleviating residual stresses caused by cyclic loading. The result confirms the potential of HDPEC technology to enhance the fatigue life of nickel-based superalloys, and underscores the potential for further research in this domain.

12Cr steel is used as one of the materials for steam turbine rotor. Steam turbine rotor is an important component and high reliability is required. In actual turbine rotor which has central hole, thermal stress induced by inflow of steam is generated at start-up. At this time, an overload greater than the load level during steady-state operation is exerted on the stress concentration area near the central hole. Therefore, it is important to evaluate the effect of overload on the creep crack initiation and growth behavior. In this study, using a C(T) specimen for 12Cr steel used for steam turbine rotor, crack growth tests under creep and initial overload creep conditions were conducted and behaviors of creep deformation, crack initiation and growth were investigated. In addition, these results were evaluated by various fracture mechanics parameters. As a result, it was found that the initial overload suppressed creep deformation and delayed creep crack initiation. Especially, this tendency was more pronounced at the low temperature and high stress condition. However, the initial overload does not significantly affect the creep crack growth rate.

The microscopic mechanism of time-dependent crack growth (TDCG) in single crystal (SC) Ni-based superalloys under a relatively high-temperature condition (900 °C) is critically investigated. Two alloy types are compared: alloy A containing 3% rhenium (Re) and alloy B containing no Re. With an initial stress intensity (K) value of 40 MPam1/2, both alloys show similar two step TDCG, i.e. a steady stage followed by an acceleration stage. The total life to failure (tf) of alloy B is, however, an order of magnitude shorter than alloy A. In addition to the conventional fractography, cross-sectional analyses (EBSD and EDS) are also conducted for an interrupted crack tip. Two distinct fracture modes are then identified in both alloys: {111}<112> twin-induced shear fracture and non-crystallographic fracture along dendrite boundaries. The twin systems observed are the secondary ones having a relatively smaller Schmid factor compared to the primary ones that remain inactive under the <100> tension. In the case of alloy A, the twin induces local phase transformation involving TCP precipitation, decomposition of γ (Ni)/γ’ (Ni3Al) structure and recrystallization, which eventually enhances local ductility and dominates the crack growth rate. In the case of alloy B, the macroscopic appearances of these fracture modes are much the same, but they are significantly facilitated by micro-voids formed exclusively in γ channel whose stability is much lower than that in alloy A. The apparently brittle TDCG nature of alloy B compared to alloy A can be interpreted as an enhanced localization of plasticity at the crack tip.

Crystal orientation measurements using electron backscatter diffraction (EBSD) have been utilized to quantify plastic strain. However, detectability and accuracy of quantification of the plastic strain were not clear. In this study, a round robin measurement using deformed Type 316 stainless steel specimens was launched between 11 organizations using the same specimens. The magnitude of induced plastic strain was 0.25%–10.28%. Two kinds of misorientation parameters, the local misorientation and intra-grain misorientation, were quoted as the measure of the plastic strain. The intra-grain misorientation exhibited a clearer correlation with the degree of plastic strain than the local misorientation did. It was found that the EBSD measurement could detect the plastic strain of more than 1%. It was shown that the variation of the misorientation parameters between the organizations largely depended on the accuracy of crystal orientation identification, which was quantified by the background noise Bn. The error could be reduced by applying the smoothing filter. The EBSD measurements were concluded to be capable of quantifying the degree of plastic strain with accuracy of 0.5% for the strain less than 2%.

The microscopic mechanism of high temperature (900 degrees C) fatigue crack growth in a single crystal Ni-based superalloy is investigated. Particular attention is focused to its relation to the cyclic misorientation markings (CMMs), formerly termed EBSD (electron backscattered diffraction) striation, left along the crack wake. Observation of a cross-sectioned crack tip is conducted by various electron microscopy i.e. SEM, EBSD, highvoltage TEM and STEM-EDS. A {100} fatigue crack subjected to a relatively high Delta K (40 MPam(1/2)) shows CMMs with an interval corresponding to crack growth rate (2-4 mu m/cycle), consisting of heavily dislocated gamma (Ni) phase and less dislocated gamma' (Ni3Al) phase. It is also revealed that local oxidation along the gamma/gamma' interface at the crack tip existed at the current temperature/Delta K level. On the bases of these findings, a rational yet qualitative crack growth model that incorporated both brittle/ductile processes is proposed.

Needless to say, it is important to estimate the stress applied to a material when conducting failure analysis. In recent years, a material assessment method using electron back-scatter diffraction (EBSD) has been developed. It has been reported that a characteristic misorientation distribution corresponding to the fracture mode is seen in cross-sectional EBSD observation near the fracture surface of a Ni-based superalloy. Furthermore, the authors discovered EBSD striations on the crack cross-section, which is formed with each fatigue crack growth during a turbine shut-down process. This was discovered in misorientation analysis on a single-crystal superalloy blade used in a commercial land-based gas turbine. Since Ni-based superalloys have high deformation resistance, they do not undergo enough ductile deformation to form striations at the crack tip on the fracture surface during fatigue crack growth, and, as a result, striations corresponding to cyclic loadings are rarely observed in fractography. Even in such a Ni-based superalloy with brittle crack growth, the fatigue crack growth rate and the applied stress can be estimated by measuring EBSD striation spacing in misorientation analysis. However, a practical problem in assessment is that the resolution limit fixed with field emission scanning electron microscopes (FE-SEM) determine the range in which crack growth rate can be assessed. Hence, it is difficult to clearly discriminate the EBSD striations when the fatigue crack growth rate is too low, such as in the low stress intensity factor range (ΔK) region. The applied stress can be calculated from ΔK. Therefore, in this paper, in order to estimate the applied stress during fatigue crack growth, we focused on estimating ΔK by measuring the plastic zone size along the crack growth.

単結晶Ni基超合金に高温大気中で導入したクリープき裂先端をTEM，EDS，EBSD等により詳細に観察した．き裂先端の前方には双晶帯およびこれに沿った微細な析出物が認められ，これが高温酸化を誘発していると見られる．

Changes in misorientation with deformation were measured by various misorientation analysis methods using the electron backscattered diffraction (EBSD) method, and quantitative assessments were attempted to estimate the amount of strain or damage. Misorientations were correlated with macroscopic plastic or creep strains for comparative well-strained materials such as austenitic stainless steels. Ni-base superalloys used for components requiring high temperature strength such as gas turbine blades, have low ductility with precipitation of the γ’ phase in grains, therefore the change of crystal orientation was considered to be extremely suppressed in comparison with austenitic stainless steels. In addition, it was anticipated that the extremely large grains of Ni-base superalloys made it difficult to properly assess the damage as misorientation. However, with the current advances in the EBSD acquisition systems in conjunction with scanning electron microscopy, it has become possible to make unprecedented resolved measurements of the local crystal structure distribution at a millimeter scale. In particular, in order to assess the damage of gas turbine blades, the complex blade inner cooling system complicates the distribution of temperatures and stresses in the blades, which implies that it is required to assess the influence of geometry at stress concentrated regions in addition to the condition of temperatures, stresses and creep fatigue wave forms. To date, in the case of the conventional casting material or the same geometry notched specimen of the directionally solidified (DS) superalloy, the average misorientation which means the grain reference orientation deviation (GROD) within grains in a certain predetermined evaluation area including the notch increases linearly up to the initiation of creep cracks regardless of the testing temperatures, strain rates and the effect of fatigue under the creep dominant condition. However, the different notch geometry of the DS superalloy shows the different characteristics of the misorientation development. This paper focuses on a misorientation parameter which can assess the creep crack initiation life independent of the geometry at stress concentrated regions. In order to assess the creep crack initiation life at various stress concentrated areas of the DS superalloy, the development of a unified life assessment method independent of the individual notch geometries was discussed. As a result of this study, a parameter dividing the GROD by the initial notch opening value, φ0, was proposed and it was confirmed that the proposed parameter, GROD/φ0 shows similar characteristics with the relative notch opening displacement (RNOD) curves which correspond to the local strain energy and the initiation of creep crack at the notch tip independent of the geometry at a stress concentrated region.

The operating lifetime of the components made of a large-scale forged material, such as the power generating components in a thermal power plant, is influenced by the initiation of defects such as cracks. Peening is known to be effective for the improvement of fatigue strength and the prevention of the stress corrosion cracking initiation. Shot peening that generates compressive residual stress on the material surface is applied to the stress concentration region of the high-strength large-scale forged material in order to relax the tensile stress, prevents crack initiation and extends operation lifetime. To confirm the durability of the compressive residual stress generated by peening, the influence of thermal aging treatment on the compressive residual stress is evaluated. As a result, it is reported that compression is maintained though compressive residual stress relaxes. The purpose of this study is to develop evaluation method of the relaxation behavior of residual stress generated by shot peening and laser peening during actual plant operations. Shot peening and laser peening were performed on the specimens of Ni-base superalloy Alloy 706, which is used as the material of the discs of gas turbines, and the thermal aging treatment test was performed. It shows that the compressive residual stress decreases to an elastic limit after heating and then relaxes due to creep strain generation. It is found that residual stress was almost determined by the residual stress just after heating.

Crystal orientations of creep damaged Type 316 stainless steel were measured by 10 organizations using the same specimens, passed in a round robin, in order to investigate the scatter in material damage assessment using the electron backscatter diffraction (EBSD) technique. The measurements were performed according to the EBSD measurement guideline issued by the Society of Material Science, Japan. Two misorientation parameters, the local and intra-grain misorientations, were calculated using mapping data of measured crystal orientations. It was shown that the area averaged local and intra-grain misorientations correlated well with the degree of the inelastic strain caused by the creep damage. Although the area averaged local misorientation showed eminent scatter, the scatter in the area averaged intra-grain misorientation was relatively small The scatter in the area averaged local misorientation was deduced to be brought about by the error in the crystal orientation measurements. Since the accuracy of the crystal orientation measurement depends on various factors and is difficult to control, the correlation between the degree of the creep damage and the local misorientations obtained by one SEM/EBSD system is difficult to apply to other SEM/EBSD systems. On the other hand, the area averaged intra-grain misorientation is not affected much by the error in the crystal orientation measurements and the values obtained by various organizations using different SEM/EBSD systems were almost the same. It was concluded that the area averaged intra-grain misorientation can be used for measurement of the creep damage (inelastic strain). The empirical relationship between the area averaged intra-grain misorientation and the degree of the creep damage can be shared regardless of the SEM/EBSD system used.

Some cracks were found at the first stage discs of gas turbine rotors made of Ni-base superalloy. There were inter-granular cracking and observed to be highly stressed and damage sensitive locations with less potential for oxidation, which is thought to occur due to hold-time cracking. In this study, laser peening technology for gas turbine components was developed to improve material properties of Ni-base superalloy Alloy 706, and the effects on the material properties were examined. Laser peening is a novel process to induce compressive residual stress on material surface by irradiating focused high-power laser pulses. Several durability tests, such as thermal aging treatment test and stress aging test, were performed under thermal power plant operation conditions. The value of residual stress was over -500 MPa after exposed 3000 hours at 500℃. In addition, surface residual stress was almost the same value in case of applied stress was less than 800 MPa at 500 ℃. Therefore, the effectiveness of laser peening treatment under the running temperature was confirmed.