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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.

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 crystal orientation effect on the fretting fatigue behavior in Ni-based single crystal superalloys (NBSX) was studied in this paper using crystal plasticity finite element (CPFE) simulation. The crystal plasticity constitutive model considering back stress was applied in the CPFE simulation, and four different crystal orientations were calculated to study the effect of crystal orientation. The CPFE simulation results showed significant differences between the four crystal orientations. The Mises stress and the cumulative shear strain under different crystal orientations was obtained. The change of stress and strain with time at different locations showed obvious difference under four crystal orientations. The simulation results showed obvious stress concentration near the contact edge, but the peak locations of cumulative shear strain would change under different crystal orientations.

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 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.

Slip deformation is the predominant fatigue damage form of nickel-based single crystals at intermediate temperature range (near 760℃). Under different crystal orientations, the dominant slip system may be an octahedral or cubic slip system. This paper explores the dominant slip systems under different crystal orientations through crystal plastic finite element analysis, and determines the fatigue damage parameter that can characterize both the octahedral deformation and the cubic deformation. The results show that within a certain orientation range close to the orientation, the dominant slip system of the nickel-based single crystal is the cubic slip system. For an orientation far deviating from the orientation, the dominant slip system is the octahedral slip system. Through finite element calculation, this paper gives the distribution diagram of the dominant slip system in the standard pole figure. The fatigue data of nickel-based single crystals with different orientations in the existing literature are used for verification, and the results show that the dominant slip system can be effectively determined by using the distribution diagram, and the proposed fatigue damage parameter can effectively characterize the fatigue damage of different slip systems. Compared with the condition where the distribution of the dominant slip system is not considered, the method proposed in this paper has effectively improved the fatigue life prediction ability.

Pure iron wires were obtained by a two-step process involving torsion and cold drawing. A steep change in grain size through the cross-section of wires was produced during the torsion stage. The Vickers hardness increased from 193 HV to 252 HV when measured from the inner to the outer layer. The microstructure of the outer region underwent more severe plastic deformation than the interior. The grain structure in the central region did not show significant change, while a large number of sub-grains were formed exhibiting an approximately equiaxed shape toward the wire surface. Under subsequent cold drawing, the grain gradient remained. The Vickers hardness increased from 220 HV to 274 HV when determined from the inner region to the surface. The grains in the interior formed a fibrous structure, while fine equiaxed grains were developed toward the surface with an average grain size of about 0.5 μm. The formation of small equiaxed grains at the near surface region is believed to originate from continuous dynamic recrystallization (CDRX). Moreover, a gradual change from low angle into high angle grain boundaries (HAGBs) was observed, facilitated by the gliding and clustering of geometrically necessary dislocations (GNDs). Consequently, the application of the two-step technique enhanced the tensile strength and elongation to failure of pure iron wires from 607 MPa to 808 MPa and 6.7% to 11.5%, respectively. In doing so, the pure iron wires achieved a desirable combination of high strength and ductility.

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.

The influence of orientation and temperature on fatigue short crack propagation behavior of Inconel 718 alloy was studied at both 25 °C and 650 °C by in-situ fatigue testing under scanning electron microscope. The fatigue crack growth rates (FCGR) showed evident dependence on orientation at both temperatures, following: XZ

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.

Scanning Electron Microscope (SEM) is an important observational device in scientific research. Over the past decades, scientists have been working on the high temperature deformation measurement and characterization of the critical alloy materials and components in hot sections by in situ observations with SEM. The in situ observations have both important theoretical and high engineering significance in understanding the mechanisms of the microstructure damage evolution of the alloys, understanding their high temperature failure mechanisms, extracting mechanical parameters and improving the accuracy of life prediction methods. However, in situ high temperature measurements in a SEM environment pose major challenges in high efficiency heat source design, establishment of stable high contrast microscopic images, and quantitative measurement of the deformation fields in high temperature environment. New methods and approaches must be therefore developed for measurement of mechanical properties in such harsh environment. In the study presented here, the major difficulties and challenges associated with high temperature experiments implemented in situ SEM are introduced firstly, especially the imaging failure at high temperature. The developments of high temperature in situ SEM technology in recent years are then reviewed, including high temperature in situ test instruments, high temperature imaging technology, and high temperature deformation measurement methods. On this basis, some new research results achieved by our group in the field of creep, fatigue and fracture of nickel based alloy materials at high temperature are introduced, such as the fatigue small and long crack growth behaviors at different temperature as well as in different crystallographic orientations. Nickel based alloy is the critical material for gas turbine and aero-engine due to their excellent creep, fatigue and corrosion resistance at high temperature. Finally, some conclusions of current development in this research field are presented, meanwhile works that need to be further extended in the future are also pointed out.