Creep-fatigue (CF) interaction in a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel was studied at 650 °C by introducing hold-time of up to 30 min at peak tensile strain ...of 0.7%. The symmetrical loops under pure fatigue/continuous cycling (PF/CC) became asymmetrical due to stress relaxation during hold-time. Moreover, this also resulted in a reduction of cyclic life. For the investigated hold-time durations, the increase in tensile hold (TH) period had a negligible effect on peak stresses, but led to a further reduction in lifetime. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to visualize microstructural evolution under both PF/CC and TH waveforms. In general, PF/CC resulted in: (1) rearrangement and/or annihilation of dislocations, (2) partial elimination of the original sub-grain structures, (3) grain growth, (4) M23C6 carbides coarsening and (5) Cr-W enriched Laves phase precipitation. Nevertheless, upon introducing tensile hold-time, no substantial additional microstructural changes were identified. To uncover reasons for specimens premature failure under TH waveforms, detailed investigations on their surfaces, cross-sections and fracture surfaces were carried out. These investigations led to two important conclusions. Firstly, due to comparatively longer high-temperature exposure, the extent of oxidation increased upon introducing TH which expedited damage progression. Secondly, TH induced intergranular damage in the form of creep cavities does not only provide additional crack initiation sites but also their growth/coalescence under tensile stresses act as a bridging link for accelerated crack propagation. These two findings are associated with a reduction of cyclic life due to introduction of hold-time. Hence, the effect of hold-times is primarily due to (1) oxidation-fatigue interaction and (2) creep-fatigue interaction.
We report on the low-cycle fatigue behavior of single-phase, face-centered cubic CoCrNi and CoCrFeMnNi at room temperature. Both alloys manifest cyclic hardening followed by softening and a near ...steady state until failure. CoCrNi exhibits higher strength, lower inelastic-strain, and longer lifetime than CoCrFeMnNi. For both alloys, microstructural investigations reveal no noticeable changes of texture, grain size and twin fraction. Nevertheless, CoCrNi exhibits planar dislocation structures, while CoCrFeMnNi shows well-defined wavy dislocation structures. This is due to CoCrNi lower stacking fault energy, which enhances planar slip and delays deformation localization leading to its superior fatigue resistance, compared to CoCrFeMnNi.
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This work provides a simple analytical formula with a set of best-fit coefficients for evaluating formation free energies of prismatic dislocation loops as a function of temperature in ...body-centered-cubic iron. The parameterization is made based on the anisotropic elastic self-energies and corresponding non-linear core-energies of edge dislocations. The variation of these two energy terms with temperature is estimated by including temperature-dependent elastic stiffness tensors in the anisotropic elasticity calculation, and based on the stability of C15 clusters and dislocation loops, respectively. The new analytical formula explicitly contains temperature and is valid for temperatures up to 1100 K.
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•W/Fe functionally graded material (FGM) are fabricated well by resistance sintering under ultra-high pressure (RSUHP).•The whole sintering time is less than 3min and cost-effective.•We research the ...W/Fe interface and the formation of intermetallic at the interface. In addition, we explain the possible method to avoid the formation of brittle intermetallic.•Vickers hardness of W/Fe FGM before and after heat treatment are investigated here.
W/Fe-based components are considered as primary structural materials for the future fusion reactor. A five-layer W/Fe functional graded material (FGM) with W volume fraction of 0%, 25%, 50%, 75% and 100%, respectively, have been fabricated by a novel sintering method combining resistance sintering with ultra-high pressure. The sintering was carried out under pressures of 9GPa and an input power of 11kW for 60s. The microstructure of cross-section observed by SEM shows a well-graded transition. The relative density of each layer is more than 97%. A brittle phase Fe7W6 is formed at the interface of W particles and Fe particles, and this intermetallic phase coats Fe particles homogeneously with a thickness of 1–5μm. It is possible to avoid the brittle phase by choosing appropriate particle size and sintering parameters. In addition, Vickers hardness of W/Fe FGM was investigated before and after heat treatment.
► Thermal stability of microstructures and its influences on mechanical properties. ► Investigations of inelastic behaviors of Hastelloy C-22 at elevated temperatures. ► Optimization of diffusion ...bonding parameters using a diffusion bonding model. ► Influences of surface impurities on the diffusion bonding quality. ► Verification of the optimized bonding parameters through bonding experiments.
The paper presents an optimization of diffusion bonding processes of Hastelloy C-22 based on the coupling of experiments and model simulations for manufacturing of micro heat exchangers for chemical applications. A systematic work is begun with experimental investigations of the influences of the bonding temperature on microstructures and mechanical properties of the material to be bonded. Based on these, the optimal bonding temperature is determined. To predict the remaining bonding parameters, a diffusion bonding model is used. The mechanical properties used as input parameters are provided by tensile and creep tests. The experimental investigations and model calculations yielded an optimized parameter set of 1050°C, 26MPa and 3.6h for a given surface roughness. This is verified by diffusion bonding using sheet material for various compression stresses, bonding durations and surface roughness. Bonding with this parameter set is successfully conducted, which in turn resulted in a sound bonding seam. The tensile strength was comparable to that of the base material and the toughness was higher than 50% of that of the base material.
The determination of fracture-mechanical properties is often very challenging, because the available standards like ASTM E1820 need specific size-requirements for the specimen dimensions to obtain ...valid fracture toughness. Especially in the ductile regime, where the presence of plasticity around the crack tip is affected by the multiaxial stress state and its triaxiality, the size-requirements are frequently not met. The fulfilment of the size-requirements needs the testing of big specimens, which is often not possible. If we now think of specimens, which are irradiated in test modules for future fusion reactors, their size cannot be as big as required, because the available volume for irradiation is restricted. This fact highlights the need of Small Specimens Test Techniques (SSTT) for the determination of fracture-mechanical properties in the ductile regime.
The presented work focuses on an approach for the determination of fracture-mechanical properties in the ductile regime including stable crack growth and crack-resistance behavior. The authors have developed the initial approach some years ago and within this work the approach was simplified as much as possible. The basic idea of the approach is, that the crack growth can be simulated using Finite Element Method combined with a cohesive zone model. The cohesive zone model is a two parametric model, namely the cohesive stress σc and the cohesive energy Γc, which are identified on small specimens only. The new simplified approach was now validated on ferritic-martensitic steel Eurofer97 at room temperature.
In the past, the approach used complicated features like a CCD camera system and has now been simplified in a way that no CCD camera system is required. The main part of the approach is the identification of cohesive zone parameters (cohesive stress σc and energy Γc) on small specimens. The cohesive stress σc can be determined on notched round tensile specimens with different notch root radius to account for different stress states or stress triaxialities in the specimen. With dedicated Finite Element modelling a local fracture stress dependent on stress triaxiality can be identified. The cohesive energy Γc can be carried out by simulating the small fracture-mechanical specimen using the Finite Element Method combined with the cohesive zone model and parameter fitting to experimental results. The cohesive energy Γc is treated to be identified, if the simulated crack-resistance curve describes the experimental behavior.
After identification of these parameters, a big fracture-mechanical specimen can be simulated using the cohesive zone parameters already determined on small specimens. Finally, the crack-resistance curve of a big specimen can be predicted and a valid fracture toughness can be identified if the size-requirements of the big specimens are met. In case the requirements are not fulfilled, a bigger specimen geometry can be simulated until all size criteria are met. With this method, the testing of big specimens can be avoided. For the future there is a Round Robin exercise planned including defined test matrices to demonstrate the general applicability of the approach.
•Dislocation loops of type are observed more frequently than type 1/2 after HFR irradiation. Loop density steadily increases with dose and dose rate, mean loop size increases mainly with dose.•Voids ...show a homogeneous spatial distribution after HFR and BOR 60 irradiation in the temperature range between 300 and 330 °C. A high volume fraction of voids is observed after mixed spectrum HFR irradiation, which can be related to helium production.•Precipitates of types MX and M23C6 are observed in EUROFER 97, which show a clear growth with dose due to neutron irradiation.
Characterization of irradiation induced microstructural evolution is essential for assessing the applicability of structural steels like the Reduced Activation Ferritic/Martensitic steel EUROFER 97 in upcoming fusion reactors. In this work Transmission Electron Microscopy (TEM) is used to determine the defect microstructure after different neutron irradiation conditions. In particular dislocation loops, voids and precipitates are analyzed concerning defect nature, density and size distribution after irradiation to 15 dpa at 300 °C in the mixed spectrum High Flux Reactor (HFR). New results are combined with previously obtained data from irradiation in the fast spectrum BOR-60 reactor (15 and 32 dpa, 330 °C), which allows for assessment of dose and dose rate effects on the aforementioned irradiation induced defects and microstructural characteristics.
•Mechanical behavior under isothermal uniaxial stress-controlled cyclic loading are collected for Eurofer97 at 550 °C.•Ratcheting behavior of Eurofer97 is compared with that of mod.9Cr-1Mo ferritic ...martensitic steel at 550 °C.•Parameters values in a previously developed constitutive model are fitted to simulate the ratcheting behavior of Eurofer97 at 550 °C.
The components in reactor are usually exposed to cyclic loading caused by frequent startups, shutdown and load fluctuations which lead to ratcheting effect. Eurofer97 is a structural material candidate for in-vessel components of future fusion reactor. To investigate the ratcheting behavior of Eurofer97, uniaxial stress-controlled cyclic loading tests are performed at 550 °C. Ratcheting rates under various loading conditions, including various peak stresses, stress ratios and stress rates are determined to build the database of Eurofer97 for future application in the blankets of fusion reactor. An already developed unified visco-plastic constitutive model for mod.9Cr-1Mo FM steels is further applied to adapt the ratcheting behavior of Eurofer97. The developed model describes both cyclic softening in strain-controlled LCF tests and its impact on the ratcheting behavior of mod.9Cr-1Mo FM steel fairly good, at both room temperature and 550 °C. Verifying whether the current model can also describe the ratcheting behavior of Eurofer97 is a further aim of the work reported here.
In this work, we used Dislocation Dynamics (DD) simulations to investigate the role of the hierarchical defects microstructure of ferritic-martensitic steel Eurofer97 in determining its hardening ...behavior. A Representative Volume Element (RVE) for DD simulation is identified based on the typical martensitic lath size. Material properties for DD simulations in b.c.c Eurofer97 are determined, including the dislocation mobility parameters. The dependence of material parameters on temperature is fitted to experimental yield strength measurements carried out at room temperature and 330 °C, respectively. Voids and precipitates observed in the microstructure, such as M23C6 and Tantalum-rich MX, are considered in our DD simulations as inclusions with realistic size distribution and volume density, while 〈111〉 -and 〈100〉 -type irradiation loops are included directly in the DD simulations. The lath structure, together with its typical precipitates arrangement and the different crystallographic orientation of the martensitic blocks can also be captured in the simulations. DD simulations are used to extract microstructure-specific hardening parameters, which can be used to simulate the properties of Eurofer97 at the engineering scale.
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