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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 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.

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.

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.

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.

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.

A model was proposed to investigate the critical condition of Portevin–Le Chatelier (PLC) effect in the current paper. Hahner's limitation of the additional activation energy under completely aged condition for the existence of PLC regime was extended to a more general case, which allowed to simultaneously account for the effects of strain hardening rate and the dynamic strain aging (DSA) exponent. The dependences of the critical strain on the strain rate and temperature were also derived based on the current model. The strain rate dependent stress predicted by the current model was similar to that by Jiang's model, but with less model parameters. The predictions of critical strain evolution with strain rate and temperature were compared with experimental data published in literature. The good agreement between these results indicated that the current model can well capture the critical strain in a wide range of strain rates and temperatures.

The dynamic compressive properties of SiC particle reinforced pure Al matrix composites, fabricated by spark plasma sintering technique with mixture powders prepared by mechanical alloying process, were tested in this paper. Two different average SiC particle sizes of 12 μm and 45 μm were adopted, and the compressive tests of these composites at strain rates ranging from 800/s to 5200/s were conducted by split Hopkinson pressure bar. The damage mechanism of the SiCp/Al composites was analyzed through the microstructural observations and high-precision density measurements. Results show that the dynamic properties and damage accumulation of these composites are significantly affected by the particle distribution, size, particle cracking, particle/matrix interface debonding and adiabatic heat softening. The composites containing smaller SiC particles exhibit higher flow stress, lower strain rate sensitivity, and less damage at high strain rate deformation.

The aim of this paper is to investigate the initiation stage of stress corrosion of an aluminum alloy. Three-point bending experiments are carried out under displacement-controlled mode. The tested specimens are Al2024-T3 alloy with thickness of 2 mm which are immersed in the 3.5% sodium chloride solution in the experiments. The solution temperature was controlled from 20 to 80 °C and the exerted stresses were 0, 174 and 348 MPa, respectively, to study the influences of the temperature and the stress in the stress corrosion process. The subjective speckle method has been employed to observe the processes of the stress corrosion in microregions by means of measuring the local surface roughness of the specimens. Treatments of employing both average intensity and an autocorrelation coefficient of the subject speckle fields confirmed that the subjective speckle method is effective to characterize the stress corrosion initiation process of the tested aluminum alloy. The experimental results show that the temperature influences the rate of corrosion distinctly and meanwhile the rate of the corrosion also depends on the stress state considerably.

The purpose of this paper is to study the effects of inserting an expanded hole or a non-expanded hole at the tip of a fatigue crack on the subsequent delay of fatigue crack propagation. Single edge pre-cracked specimens of two aluminum alloys were used to perform the crack arresting tests under cyclic stress loading. An obvious increase in the delay of crack propagation is obtained for the specimens with expanded hole. The compressive residual stresses produced by the hole expansion process play an important role in increasing the fatigue resistance of the material. The observation of micro-structure at the fatigue failure section reveals that the hole expansion process modifies the condition and properties of the surface layer of materials and affects, significantly the length of the nucleation stage of the fatigue process.

High temperature isothermal fatigue and in-phase thermomechanical fatigue (TMF) tests in stress control were performed on a molybdenum-based alloy, TZM. Two temperature levels of 350 and 500°C were chosen for isothermal fatigue tests and a temperature interval of 350–500°C was applied to thermomechanical fatigue tests. The stress–strain response and the life cycle of the material were measured during the tests. It appears that temperature variations influence the fatigue life of the alloy and the material is further damaged with combined stress and temperature cycles. The analysis of the hysteresis loops showed that a ratcheting phenomenon and a cyclic softening behavior of the material occurred during each kind of the fatigue tests. The observations of the microstructure of the damaged specimens demonstrated that the fracture of the alloy was, on the whole, a brittle process under higher cyclic stress conditions.