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In this paper fretting fatigue crack behavior in 1045 steel is studied by in-situ observation and finite element analysis. in-situ fretting fatigue experiments are conducted to capture real-time fretting fatigue crack formation and propagation process. The fretting fatigue tests under different load conditions are carried out, then the lifetime and fracture surface are obtained. The crack propagation rates under different loading conditions are measured by in-situ observations. With in-situ observation, crack initiation location and direction are analyzed. Finite element model is used to calculate J-integral which then is applied to fitting with experimental crack growth rate, and establishing crack growth rate model. From fitted S-N curve, it turns out that smaller load ratio leads to higher lifetime. Crack initiates slightly below the point equivalent to line contact of the contact surface in different test conditions, and crack direction shows no obvious relationship with load parameters. The established crack growth rate model well agrees with the test results.

The integrity of thermal barrier coatings (TBCs) plays an important role in the performance of turbine blades, nevertheless the effect of cooling hole on fracture of TBCs have been less studied. The present work aimed to study the stress evolution and fracture behavior of TBCs with cooling hole under cyclic thermal loadings. Cyclic thermal tests were conducted and the cracking behavior of TBCs around cooling hole was examined using scanning electron microscope (SEM). TBCs exhibited two types of cracks near the hole edge, i.e. surface crack in the top coat and interfacial crack, which could lead to local TBC spallation and shorten the TBC lifetime. To disclose the fracture mechanism, finite element (FE) analysis was also performed. The computed residual stress values were consistent with those measured by Raman spectroscopy tests. FE simulations indicated that the free-edge effect facilitated interfacial peeling and shear stresses near the cooling hole, and hence promoted the initiation of interfacial crack in TBCs. With the increase of thermal loading cycles, the interfacial crack propagated and then coalesced with the surface cracks in the top coat, leading to the final spallation of TBCs at the cooling hole.

Material failure of runway centerline lights was observed in Beijing Capital International Airport after a period of service. In the present paper, visual inspection indicated that cracks initiated from the threaded surface of the lamp, and propagated until fracture. The mechanical properties of three typical lamps were evaluated by tensile, fatigue and impact tests. Microstructural features were also observed. The effects of microstructure and chemical composition on the mechanical properties were identified. It is indicated that improving the material properties of runway centerline lights is beneficial for extending service life and avoiding similar failures.

The effect of crystal orientation on fretting fatigue induced crack initiation and dislocation distribution is studied by in-situ SEM observation and electron back-scattered diffraction (EBSD) in this paper. Cracks and slip lines are observed in the fretting contact area of Ni-based single-crystal (NBSX) superalloys. The in-situ SEM observation captures different crack and slip line behaviors under different crystal orientations. The EBSD analysis results show obvious misorientation and orientation deviation in the fretting contact area. For both crystal orientations, the geometrically necessary dislocation (GND) density distributions in the contact area are obtained by using Hough-based EBSD methods. The peak position of grain reference orientation deviation (GROD) and GND density matches with the fretting fatigue crack formation position. EBSD analysis shows that the dislocation density distribution on each slip system is closely related to the crack initiation direction. The direction of slip system with the maximum dislocation density agrees with the crack initiation direction obtained by in-situ observation.

To understand the influence of recrystallization on the low-cycle fatigue behavior of directionally solidified nickel-based DZ4 superalloy, crystal plasticity-based finite element (CPFE) analysis was carried out to study the cyclic plastic behaviors. Using the design of experiments (DOE) technique, a series of CPFE simulations were designed and performed for DZ4 superalloy with recrystallized grains to examine the cyclic plastic shear deformation at the carbide/matrix interface, at the recrystallized grain boundary/matrix, and in the matrix. Seven influence factors, i.e., applied strain, strain ratio, carbide modulus, dwell time, misorientation angle, carbide aspect ratio, and carbide size, were considered, and the effects of which were quantitatively evaluated by the maximum accumulated plastic shear strains via CPFE-based DOE analysis. The results showed that carbide and recrystallized grain boundary had competitively significant effects on fatigue crack initiation. Applied strain was the most influential parameter among all the studied parameters.

A dovetail fixture for fretting fatigue test was designed. Fretting fatigue experiments were performed for Ni-based single crystal (NBSX) superalloys at 600 and 700 °C in two secondary orientations, [110] and [010]. The influence of secondary orientation and temperature on fretting fatigue behavior was investigated. The crystal plasticity finite element (CPFE) simulation was used to study the stress field at the contact region. The result indicates that the stress distribution is correlated to fretting damage, which is consistent with the crack initiation site. Fretting fatigue life is predicted based on critical plane method, which is in good agreement with the experimental result.

The fatigue fracture mechanism of a nickel-based single crystal (NBSC) superalloy with recrystallized grains was studied at 550 &deg, C by in situ observation with a scanning electron microscope (SEM) for the first time. Multiple crack initiations associated with recrystallized grain boundaries and carbides were observed. By analysis of the slip traces and crack propagation planes, the operated slip systems were identified to be octahedral for both single crystal substrate and recrystallized grains. Distinct crystallographic fractures dominated, accompanied by recrystallized grain boundary associated crack initiations. This is different from the widely reported solely intergranular cracking at high temperature. Fatigue crack growth rate curves showed evident fluctuation, due to the interaction of fatigue cracks with local microstructures and the crack coalescence mechanism. Both the recrystallized grains and the competition between different slip systems were responsible for the deceleration and acceleration of fatigue microstructurally small crack behavior.

This research focused on evaluating the mechanical properties change of Welding Metals (WMs) at the turbine-end (hot-end, working beyond 500 °C) of a retired CrMoV bainitic gas turbine rotor serving over 14 years, by comparison to those WMs at the compressor-end (cold-end, working below 300 °C). After long-term service, the room-temperature elastic modulus and the bainitic microstructure of hot-end WMs stayed almost the same as those of cold-end WMs, but the high-temperature elastic modulus and the dislocations density of the former became higher than those of the latter. The above change, especially the change of high-temperature elastic modulus produced significant influences on the tensile, fatigue and creep behaviors of hot-end WMs, which further led to a higher tensile and creep strength but a lower ductility, as well as a better fatigue crack propagation resistance under high-temperature condition. A correlation between the high-temperature elastic modulus change and the tensile/fatigue/creep behaviors change was established for hot-end WMs based on mechanics modelling, which provided a feasible way to monitor the rotor properties change.

Creep damage of austenitic stainless steels was evaluated by Electron Back-Scattered Diffraction (EBSD) analysis of creep deformation induced grain lattice distortion in this research. Based on the existing work of plasticity characterization by EBSD misorientation parameters, we used the grain lattice distortion parameter “Grain Reference Orientation Deviation (GROD)” obtained from EBSD analysis to estimate the creep strain e creep of austenitic stainless steels by their linear correlation e creep = ( 7.2 ∙ GROD − 2.4 ) % independent of creep conditions (stress σ 0 & temperature T) and average grain size D Grain . Then the creep damage ratio (creep time fraction t / t r ) was further evaluated from the estimated creep strain e creep by constitutive equation describing the e creep ~ t / t r creep curve, which takes the influence of creep conditions ( σ 0 , T ) into consideration. Compared with the previous work, this research developed a quantitative creep damage evaluation model with EBSD misorientation parameter GROD as input, creep damage ratio t / t r as output, and creep conditions ( σ 0 , T ) as environmental parameters. The final accuracy of the creep damage ratio evaluated by this new model can reach up to ± 30%.

The effect of applied cyclic load, including the maximum plastic strain e max p and the plastic strain amplitude e a p on final misorientation level of post-mortem low alloy steel after low cycle fatigue (LCF) test, was analyzed by Electron Back-Scattered Diffraction (EBSD) in this research at first. Two misorientation parameters widely used in EBSD analysis, Kernel Average Misorientation (KAM) and Grain Reference Orientation Deviation (GROD), are compared in terms of accumulated plasticity characterization after LCF failure, where a bilinear function is found between final KAM and applied cyclic load e max p & e a p . Then the cyclic load effect on final misorientation distribution of post-mortem low alloy steel after fatigue crack propagation (FCP) test was further analyzed by EBSD based on the above bilinear function between final KAM and e max p & e a p , as well as the equivalent e ¯ max p & Δ e ¯ p distribution functions given by HRR field. −1/2 power law is established to describe final KAM distribution near crack path. The area SKAM between final KAM distribution curve and undeformed KAM0 base line within a certain range, rather than SGROD, is proposed as a better measurement of fatigue crack tip driving force ΔK in constant load ratio R condition. Meanwhile, the influence of load ratio R on final misorientation distribution is also discussed.

The misorientation of nickel-base single crystal (NBSX) superalloy is always ignored in the current research, which in fact has a certain influence on the fatigue life of NBSXs. To investigate this influence, the fatigue tests of NBSX superalloy DD5 with different misorientation angles have been carried out at 980 °C, and the failure mechanism has been investigated based on the fractographic observation and crystal plastic finite element (CPFE) analysis. The effect of real misorientation on the fatigue life of NBSX has been quantified, which is also reported for the first time.

The fatigue behavior of Fe44Mn36Co10Cr10 high entropy alloy (HEA) at room temperature and 300 °C was studied by in-situ SEM observation, electron back-scattered diffraction (EBSD) and crystal plasticity simulation. The microstructure evolution and deformation process was observed by in-situ SEM fatigue experiment. The misorientation and geometrically necessary dislocation (GND) density was analyzed by EBSD. A crystal plasticity finite element (CPFE) modeling method based on the real grain morphology was proposed. The cumulative plastic strain, dominant slip system and dominant slip plane distributions were obtained. CPFE simulation shows multiple dominant slip systems being activated, which agrees with the multiple slip phenomena observed by in-situ SEM.

Two metrics Kernel Average Misorientation (KAM) and Image Quality (IQ) widely used in Electron Back-Scatter Diffraction (EBSD) mapping are compared with each other on indicating the dislocations of type 40Cr low alloy steel induced by tensile and cyclic deformations approaching failure limits respectively. The tensile deformation up to 12 % just before necking increased the KAM and decreased the IQ obviously, thus a negative correlation was established between KAM and normalized IQ in this case. In contrast, the cyclic deformation with small strain amplitudes (±0.4 % and ±0.7 %) until fatigue failure only decreased the IQ obviously but almost produced no effect on KAM, thus the above negative correlation no longer existed in this case. The different correlations between KAM and normalized IQ in tensile and cyclic cases were attributed to the different ratios between Geometrically Necessary Dislocations (GNDs) and Statistically Storage Dislocations (SSDs). GNDs contribute to not only the lattice distortion that can be indicated by IQ mapping, but also the lattice curvature that can be indicated by KAM mapping, while SSDs only contribute to the lattice distortion. In tensile case, GNDs occupy the major part of total dislocations, thus KAM and IQ indicate the similar dislocations distribution. In cyclic case with the applied strain amplitude lower than ±0.7 %, SSDs play a leading role, thus KAM and IQ disagree with each other on indicating the dislocations distribution.

The effects of temperature and load on fretting fatigue induced geometrically necessary dislocation (GND) distribution in titanium alloy are studied by electron back-scattered diffraction (EBSD) in this paper. Fretting fatigue experiments under different temperature and load in TC11 titanium alloy are carried out and samples with small cracks are obtained. With Hough-based EBSD, the misorientation and GND densities induced by fretting fatigue are analyzed in both crack formation zone and crack tip. The results show that in the crack formation zone and crack tip, both temperature and load have obvious effects on the GND densities. With the increase of temperature and load, the GND densities rise significantly, which agrees with the decreasing tendency of fretting fatigue lives. The peak positions of GND density agree well with the crack formation locations, indicating the close relation between GND concentration and fretting fatigue crack formation. The relationship between GND density and distance from the crack path is studied. The results of individual GND densities on each slip system show that ‘Pyramidal ' and ‘Screw ' dislocations compose a majority of total GND density.

Fretting fatigue behavior of Ni-base single crystal (NBSX) superalloys was investigated based on in-situ scanning electron microscope (SEM) observations. Evolutions of slip lines and initiations of cracks were observed. Slip lines were found on specimen free surface along three typical directions. Short cracks were observed to initiate along the slip line. Crystal plasticity constitutive model considering cyclic hardening effect was employed. Crystal plasticity finite element method (CPFEM) simulation showed the activations of crystallographic slip systems at contact region. The slip plane with the maximum plastic slip was determined as dominate slip plane. Equivalent plastic strain was proved effective to predict crack initiation. The simulation results including predicted slip lines, crack initiation locations and orientations were in good agreements with observations.

In the present work, based on the Dugdale criterion, a closed-form theoretical crack tip driving force model of simple expressions was proposed for static and fatigue crack propagation along a linear yield strength gradient (YSG). Qualitative and quantitative analyses of the YSG effect on the crack tip driving force are given. The crack tip driving force depends on the yield strength at the crack tip, the YSG within the plastic zone and the applied load. A positive (or negative) YSG has a shielding (or amplifying) effect on the crack tip driving force. A difference of material-toughening mechanisms between smooth and sharp strength gradient was revealed both theoretically and numerically. In the linear YSG case, when a fatigue crack propagates along the YSG at a constant applied cyclic crack tip stress intensity factor, the cyclic crack tip opening displacement decreases (or increases) when the crack propagates towards a higher (or lower)-strength region; however, the cyclic crack tip stress intensity factor increases, no matter when the crack propagates towards a higher- or lower-strength region. This abnormal phenomenon causes theoretical and experimental challenges for characterizing the smooth strength gradient effect on the crack tip driving force.

In situ observations by scanning electron microscopy show that small fatigue cracks in a nickel-based single crystal superalloy are inclined to the loading direction and propagate in dominant crystallographic manners. In order to evaluate the driving forces for inclined crack propagation, three-dimensional anisotropic linear elastic finite-element analysis is conducted. The plastic zone size on the dominant slip plane has been calculated and proposed to correlate the fatigue crack growth. It is shown that this parameter takes into account both material anisotropy and octahedral fracture modes, and it can effectively characterize small crack propagation behaviour.

The crack closure behavior of a nickel-based single crystal (SC) superalloy was studied based on in-situ SEM fatigue tests. Specimens in two different secondary orientations were tested at 600 °C. A model considering the anisotropy of SC superalloys was developed to analyze crack closure behavior. The results show that specimens in two orientations exhibit different cracking modes. For specimen [0 0 1]/[0 1 0], roughness-induced crack closure effect is dominant in crack closure behavior and crack closure effect is constant during crack propagation process. However, for specimen [0 0 1]/[1 1 0], plasticity-induced crack closure effect is dominant in crack closure which becomes enhanced as crack grows.

Fifteen multiaxial critical-plane fatigue damage parameters of isotropic metal materials are applied to the life evaluation of nickel-base single crystal superalloys. These parameters are classified into stress-, strain- and strain-energy parameters, and their predictive abilities are evaluated by 12 groups of single crystal fatigue data obtained from mainly an intermediate temperature range (500–760 °C) where the slip deformation is the main damage mode. The results show that the strain energy group exhibits the best fitting characteristics with correlation coefficients. A topologized resolved shear stress method (TRSS) considering the interaction of main slip systems is further proposed. The dispersion degree of resolved shear stress among the main slip systems is quantified by a dispersion factor, and the real driving capacity of resolved shear stress is amended according to the topology of driving force. The obtained results show that the proposed method has achieved an improvement of correlation ability.

The fretting fatigue behavior of Ni-based single crystal (NBSX) superalloy specimens with different crystal orientations ([110] and [010]) was investigated by in-situ scanning electron microscopy at 400 °C. The slip lines on the surface of the specimens were captured during the test, and crack paths were recorded. These results show that the dominant deformation mode of fretting fatigue at 400 °C is the crystallographic slip. Fretting contact conditions were simulated by the crystal plasticity finite element (CPFE) method. It was found that the activated slip systems and planes in the numerical results are correlated to crystal orientations. The predicted slip lines corresponding to the dominant slip planes obtained by the simulations are in good agreement with the experimental observations. And the crack initiation sites in the experiment coincide with the equivalent plastic strain in the simulation.

The indentation behaviours of Ni-based single crystal (NBSX) superalloy in different crystal orientations are studied by indentation tests and Electron Backscattered Diffraction (EBSD) analysis. The effects of crystal orientations on pile-up modes, dominant slip system distribution and geometrically necessary dislocation (GND) density distribution are revealed by experiments and crystal plasticity finite element (CPFE) simulation. The results and analysis of pyramid and cone indentations show that the pile-up modes produced by different azimuths are significantly different due to the off-plane displacement distribution. The differences of crystallographic slip behaviors under different crystal orientations induce different slip line and indentation symmetry patterns. EBSD observation and CPFE analysis indicate the dependence of GND density distribution on crystal orientations. The results of indentation tests, EBSD observation and CPFE simulation achieve good agreements.

In-situ Scanning Electron Microscopy (SEM) fatigue experiments were carried out to study short fatigue crack propagation (FCP) behavior of various regions (weld zone, interface region and heat affected zone (HAZ)) in a domestic dissimilar metal welded joint of nuclear power plant. The local microstructural effect on short fatigue crack initiation and propagation behavior was investigated with its influence on both material fatigue and structure fatigue analyzed. Considering material fatigue, in the weld region, crack grows along δ ferrites when propagating parallel to the dendrite, and deflects or branches along δ ferrite, γ austenite dendrite, δ/γ interface and grain boundaries when propagating perpendicular to the dendrite; in safe ends, the crack grows along slip lines and coalesces with secondary cracks; in A508 HAZ, the crack propagates or branches along martensite transgranularly. In terms of structural fatigue, the crack tends to deflect when propagating across the weld/A508 interface or weld/316 L interface with the influence of local microstructure, and the weld/A508 interface region has a resistance to FCP due to its high strength. The fatigue crack propagation rate of each region was compared and analyzed. The fatigue fractography was also characterized under SEM to analyze the crack propagation process. Keywords: In-situ SEM, Short fatigue crack propagation, Dissimilar metal welded joint

Rafting and creep modeling of single crystal superalloys at high temperatures are important for the safety assessment and life prediction in practice. In this research, a new model has been developed to describe the rafting evolution and incorporated into the Cailletaud single crystal plasticity model to simulate the creep behavior. The driving force of rafting is assumed to be the relaxation of the strain energy, and it is calculated with the local stress state, a superposition of the external and misfit stress tensors. In addition, the isotropic coarsening is introduced by the cube root dependence of the microstructure periodicity on creep time based on Ostwal ripening. Then the influence of rafting on creep deformation is taken into account as the Orowan stress in the single crystal plasticity model. The capability of the proposed model is validated with creep experiments of CMSX-4 at 950 °C and 1050 °C. It is able to predict the rafting direction at complex loading conditions and evaluate the channel width during rafting. For [001] tensile creep tests, good agreement has been shown between the model predictions and experimental results at different temperatures and stress levels. The creep acceleration can be captured with this model and is attributed to the microstructure degradation caused by the precipitate coarsening.

Fatigue crack growth (FCG) behaviors of a nickel-based single crystal superalloy were tested in 6 crystal orientations at 600 °C, and the effect of secondary orientations on FCG behaviors were investigated through theoretical analysis. FCG test results showed significant differences in FCG rates between different secondary orientations, albeit SEM observations on their fracture surfaces did not reveal evident distinctions. To evaluate the anisotropic FCG behaviors, a methodology was implemented based on the elastic energy release rate, which is successful in reducing the scatter caused by various orientations. In addition, the proposed energy release rate coefficient is proven to be effective in evaluating the resistance to long fatigue crack growth in different secondary crystal orientations.

Creep-fatigue interaction is extremely important for the life prediction of structural components subjected to high temperature conditions. The widely used life prediction methods such as time fraction rule and ductility exhaustion method are found to be insufficient in long time regimes and cannot give a clear physical explanation for the material failure. In this paper, a generalized life prediction model is proposed on the basis of the hysteresis energy and law of energy conservation. The influence of hold time on the cyclic stress–strain behavior is described based on isochronous stress–strain curves and stress relaxation during hold time is evaluated with the time hardening rule. An analytical approach is set up to calculate the hysteresis energy for the creep-fatigue tests. The generalized hysteresis energy is used as a model parameter to predict the fatigue and creep-fatigue life of 316L(N) and it is found to have a power law relationship with the number of cycles to failure. The fatigue and creep-fatigue life predicted with the new energy-based method shows a good correlation with the experimental results from different researchers.

The effect of Weibull distribution-based stochastic fiber/matrix interface strength on the transverse tensile strength and failure process of long fiber reinforced composites was numerically investigated in represent volume elements. The transverse tensile strength and failure strain of the stochastic interface strength models are much lower than the identical interface strength cases, indicating that transverse tensile strength calculated from the identical interface strength model will lead to an overestimation. In the stochastic interface strength model, failure path goes along interfaces of lower strength but not always the lowest strength, and variation of fiber location distribution will lead to a different failure process. Influence of the two Weibull parameters on the transverse tensile strength was also investigated. The transverse tensile strength improves with the increase of Weibull scale parameter and shape parameter. A small value of shape parameter will lead to a scattered interface strength distribution and much lower transverse tensile strength.

Fatigue crack growth (FCG) behaviors of a widely used nickel-based directionally solidified (DS) superalloy have been investigated. Standard compact tension (CT) specimens in longitudinal, transverse and diagonal directions are cast and tested at 25 °C, 600 °C and 850 °C to reveal the orientation and temperature dependence. The post-test fractography is observed through scanning electron microscope (SEM) and optical microscope (OM) to understand the underlying mechanism responsible for the fracture modes. Results indicate that cracks in all three orientations exhibit a similar propagating behavior, while the temperature shows a significant effect on the crack propagation regardless of the influence of orientation. It has been found that a higher temperature leads to a faster propagation rate in the initial stage due to the cyclic softening response of materials. However, the FCG rates of specimens at lower temperature speed up more rapidly and exceed those at higher temperature in the following stage. This is attributed to the crack closure effect induced by the oxidation at a much higher temperature. Therefore, a new model based on thermal activation is proposed to get a better ability for the FCG rate prediction of the DS superalloy under different temperatures.

Electrical Discharge Machining (EDM) is the machining technique of discharging corrosion of the material surface. The effect of the EDM pulse current on the micro structure and properties of the P20 specimen surface were investigated. Three layers were observed on the specimen surface after discharging: ablation layer, molten layer and recrystallized layer. The experimental results show that with the increment of the pulse current, the thickness of the molten layer and recrystallized layer increased. The contents of the oxygen and carbon on the outside of the molten layer were higher than the base metal for the diffusion during the machining. The micro hardness reduced from the out layer to the base metal after the heat treatment, indicating the mechanical properties changing of the machining surface. In the molten layer, micro fracture is caused by the residual thermal stress during the machining.

The influence of partial recrystallization (RX) on the low cycle fatigue (LCF) behavior of directionally solidified DZ4 superalloy was studied using in situ scanning electron microscopy. Three typical recrystallization microstructures were prepared on the fatigue samples, via shot peening and subsequent heat treatment. The effect of RX on LCF life is evidently related to the specific RX microstructure. The RX layer does not necessarily reduce the LCF life. The fatigue crack initiation and short crack propagation behaviors for both raw DZ4 and recrystallized samples were in situ examined, which rationalized the observed effect of recrystallization on the LCF life variation.

Small fatigue crack growth behaviors of a nickel-base single crystal (SX) are studied by in situ observations with scanning electron microscope (SEM). Two different crack propagation modes are identified, i.e., cracks at room temperature and 300 °C prefer non-crystallographic propagation mode, while those at a higher temperature of 600 °C propagate preferentially along crystallographic slip systems, besides, evident “small crack anomaly” has been partly found. Further computation of crack growth rate reveals that conventional Paris law is found no longer capable of characterizing the crystallographic crack propagation behaviors. Subsequently, with the help of finite element analysis (FEA), the crack tip stress fields are reconstructed and resolved shear stresses in each octahedral slip systems are calculated. It is found that the maximum resolved shear stress intensity parameter, first proposed by Chen and Liu, is able to characterize the growth behaviors of those small cracks.

As the typical intragranular misorientation parameters, Kernel Averaged Misorientation (KAM) and Grain Reference Orientation Deviation (GROD) are widely used in diffraction-based misorientation mapping, while their evolution laws under various conditions and physical meanings in continuum mechanics description are rarely investigated systematically. Therefore, we designed several comparative experiments considering the influences of grain boundaries (single-crystalline vs. poly-crystalline), sample geometries (smooth vs. notched) and plastic strain modes (tension vs. buckling) on KAM & GROD evolution, and captured the intragranular misorientation, dislocation density, and material distortion synchronously based on coupled EBSD-ECCI-DIC mapping in this research. Meanwhile, we also discussed the physical meanings of KAM & GROD based on continuum mechanics description and provided the theoretical explanations to phenomena observed in the above experiments. KAM results from three in-surface invariants ( ρ GND I , ρ GND II & ρ GND III ) of GND density tensor ρGND induced by plastic strain distribution incompatible with the activated slip systems in unloaded elastic-plastic condition, which cause the same lattice curvature effects as three elastic strain modes with non-zero curl (buckling, in-surface bending & torsion) in purely elastic condition. GROD reflects neither local plastic strain nor local material rotation alone, but its “V-type” distribution near the neutral surface reflects the buckling curvature of single-crystal. Besides, KAM ¯ & GROD ¯ averaged over multiple grains can be used to estimate the nominal plastic strain applied in poly-crystal.

In situ observation and measurement of the high-temperature deformation of advanced hot-section materials using scanning electron microscopy (SEM) is beneficial for understanding their microscopic failure mechanisms. Through in-situ measurement and subsequent analysis, crack nucleation site, growth rate and propagation behavior can be determined as well as the interaction between crack and microstructure, such as inclusion, pore or grain boundary. However, these measurements are challenging above 700 °C as the SEM images are severely degraded by the thermal electron emission. In this work, an in situ mechanical testing system for extremely high temperatures (up to 1000 °C) inside an SEM was developed and used to investigate the small fatigue crack growth behavior of a nickel-based single-crystal superalloy from room temperature to 980 °C. The fatigue crack propagation modes and growth rates were investigated at different temperatures. The mechanism of the propagation mode transition was discussed in detail. Conventional Paris law was found no longer capable of characterizing the growth behavior of small crack. Subsequently, with the help of the digital image correlation technique, plastic zone size of the crack tip was obtained, and it was found that it could characterize the growth behavior of the small crack.

This paper aims to understand the effect of local recrystallization (RX) on the low cycle fatigue fracture of a turbine-blade single crystal nickel-based superalloy. The fatigue life of the single crystal superalloy was evidently decreased by local recrystallization. In single crystal specimens, casting porosity is the preferential fatigue crack initiation site, which is followed by crystallographic crack propagation along one or several octahedral slip planes. For all RX specimens, fatigue cracks preferred to initiate from local recrystallized grains and propagated through the recrystallized grains in a transgranular manner, followed by crystallographic crack propagation in the substrate single crystal superalloy. Moreover, fatigue tests indicated that locally recrystallized specimens exhibited temperature dependent fracture modes, i.e., transgranular cracking dominated at 550 &deg, C, whereas intergranular cracking was preferred at 850 &deg, C. Evident oxidation of fracture surfaces and strength degradation of grain boundaries at 850 &deg, C was evidenced by scanning electronic microscopic observations. The present study emphasized the need to evaluate the effect of recrystallization according to the working conditions of turbine components, i.e., the local temperature.

This work used in-situ scanning electron microscopy method to investigate the small fatigue crack behavior of the aluminum alloy 2024 laminated structure produced by ultrasonic consolidation process. The influence of local microstructure on the crack propagation of the laminates after different heat treatments (T4 and T62) has been compared and discussed. It was found that both the precipitations and the interfaces of the layers could retard crack growth and alter the growth direction, and the effect of interfaces was much stronger. Due to the higher ratio of the toughness to bond strength, the barrier effect of the interfaces on impeding the fatigue crack growth in specimens after the T4 heat treatment was more evident.

This paper aimed to investigate the influence of welding technique and temperature on fatigue properties of heat-resistant steel with hardfacing coatings. The plasma transferred arc welding (PTAW) and the oxy-acetylene welding (OAW) were employed. The rotating bending fatigue tests were performed at room temperature (RT) and 500 °C. It was found the fatigue strength with 10 7 cycles of OAW specimens at RT was lower than that of PTAW ones, possibly resulting from the higher amount of carbides in OAW coatings. The fatigue strength with 10 7 cycles at 500 °C was higher than that at RT, which was mainly due to the interface delamination and the increase in ductility with increasing temperature. Two failure modes, i.e. the coating failure mode at RT and the coating-interface failure mode at 500 °C, were proposed. The fatigue life was predicted with the model considering the characteristic geometry of inclusions, the average hardness of coating, and the effect of external stress.

The effect of high temperature hold time at 1050 °C on thermal fatigue properties and failure characteristics of electron beam-physical vapor deposited (EB-PVD) thermal barrier coated samples (TBCs) was investigated in this work. To clarify this effect, the microstructure, especially that near the interface after certain thermal cycles was characterized by scanning electron microscopy (SEM). Results revealed that with increasing hold time at high temperature, fatigue life first increased then decreased and the failure mode diverted from the interfacial failure mode to one that consisted of both the interfacial failure mode and failure within the TBC. Failure was determined by strain energy density in the ceramic coating and the TGO, the fracture toughness of the ceramic coating, the TGO and the interface, and their correlations with microstructure defects near the interface. Based on analysis of the failure mechanism and the microstructure evolution near the interface, and by combining the simulation modeling of the thermal cycling response, a damage accumulation life prediction model was developed in terms of the TGO thickness. This model, which considered the evolution of the fatigue stress due to the increase in TGO thickness, was able to predict thermal fatigue life of the TBCs/nickel based superalloy system under different thermal cycling histories.

Small fatigue crack behaviors in a nickel-based directionally solidified superalloy DZ4 were studied by in situ Scanning Electron Microscopy. The crack initiation and propagation manners were identified under different temperatures, i.e. 25 °C, 350 °C, 700 °C. Fatigue crack growth occurred preferentially along slip bands at 25 °C and 350 °C but by Mode-I type at 700 °C. The crack growth rate generally increased with temperature, especially between room temperature and 350 °C. The anomalous small crack growth was analyzed by in situ examining the effect of microstructure. The small cracks were found to be primarily microstructurally small and secondly mechanically small.

The effect of recrystallization on the low cycle fatigue life of DZ4 directionally solidified superalloy was investigated for specimens with three different recrystallized layers, which were generated by shot peening (0.1MPa, 0.3MPa and 0.5MPa respectively) and a subsequent annealing heat treatment. The fatigue life showed a decrease for recrystallized specimens with shot-peening of 0.1 MPa and 0.3 MPa, and an unusual increase for that of 0.5MPa, in comparison with the original DZ4 specimen. In-situ SEM observations were performed on the short crack growth behaviors for both original and recrystallized specimens, which revealed the fracture mechanism and the interaction with microstructure. Quantitative analysis of fatigue crack growth rates rationalized the influence of recrystallization on the low-cycle fatigue life of DZ4.

3D non-equilibrium molecular dynamics (NEMD) simulations using embedded atom potentials method (EAM) are performed to identify the dynamics processes of atomic-scale interfacial friction taking places in metal tribopairs. A block–block sliding simulation model for soft-to-hard (Cu/Fe) and soft-to-soft (Cu/Ag) tribopairs with is built. The microstructural evolution and temperature variation of the two tribopairs are analyzed at different sliding speeds. The results show that the average temperature of the two different tribopairs both increases rapidly during the transient sliding period. The different microstructural changes for the two tribopairs, including extensive plastic deformation, mechanical mixing and material transfer are observed when the temperature rapidly increases. The characteristics of the friction effects for the two tribopairs are also revealed by analyzing the friction force evolution as a function of time and velocity.