5 Werkstofftechnik
Filtern
Dokumenttyp
- Zeitschriftenartikel (42) (entfernen)
Referierte Publikation
- ja (42) (entfernen)
Schlagworte
- CALPHAD (4)
- Creep (3)
- Finite element analysis (3)
- HIP (3)
- Phase-field simulation (3)
- Characterization (2)
- Density-based Phase-Field Modelling (2)
- Densty-based Thermodynamics (2)
- Dislocation (2)
- Environmental stress cracking (ESC) (2)
- Fracture mechanics (2)
- Full notch creep test (FNCT) (2)
- Laser scanning microscopy (LSM) (2)
- Low cycle fatigue (2)
- Microstrucrue Design (2)
- Microstructure (2)
- Nickel-base superalloys (2)
- Phase-field (2)
- Residual stress (2)
- Single-crystals (2)
- Superalloys (2)
- Thermodynamics (2)
- 9-12%Cr steel (1)
- Accelerated temporal integration (1)
- Additive manufacturing (1)
- Ageing (1)
- Aging (1)
- Al alloys (1)
- Al-Cu alloy (1)
- Alloy Safety (1)
- Alloy design (1)
- Alloy microstructure (1)
- Alloys' Safety (1)
- Atomistic Simulation (1)
- Burst (1)
- CALPHAD database analysis (1)
- Ceramic multilayers (1)
- Chemically complex alloy (1)
- Chemo-mechanical coupling (1)
- Co-firings (1)
- Contact mechanics (1)
- Crack growth (1)
- Crack propagation (1)
- Crack tip opening displacement (1)
- Craze-crack mechanism (1)
- Crystal plasticity (1)
- Cyclic softening (1)
- Defects engineering (1)
- Deformation mechanisms (1)
- Degradation (1)
- Density-based Thermodynamics (1)
- Density-based model (1)
- Differential scanning calorimetry (DSC) (1)
- Diffusion (1)
- Digitalization (1)
- Directional grain growth (1)
- EXAFS (1)
- Elastic constants (1)
- Elastic energy (1)
- Elasticity (1)
- Electron microscopy (1)
- End of life (1)
- Environmentally assisted cracking (1)
- Failure rate (1)
- Fatigue (1)
- Fe-Mn steels (1)
- Ferritic–martensitic steel (1)
- Fourier series (1)
- Fracture surfaces (1)
- Fullerite (1)
- Global stability criterion (1)
- Grade S960QL steel (1)
- Gradient-enhanced fatigue model (1)
- Grain Boundary (1)
- Grain Boundary Phase Diagram (1)
- Grain Boundary Segregation (1)
- Grain Boundary Spinodal (1)
- Grain boundaries (1)
- Grain boundary (1)
- Grain boundary engineering (1)
- Grain boundary phase diagram (1)
- Grain boundary structure (1)
- Grain boundary thermodynamics (1)
- Grain growth (1)
- Hertzian cracks (1)
- High Entropy Alloys (1)
- High-Entropy Alloys (1)
- High-Entropy Materials (1)
- High-entropy alloys (1)
- Incremental lifetime models (1)
- Interfacial anisotropy (1)
- Internal stresses (1)
- Isostatic hot pressing (HIP) (1)
- Laser beam melting (LBM) (1)
- Lattice distortions (1)
- Liquid Metal Embrittlement (1)
- Liquid-metal embrittlement (1)
- Long-term aging (1)
- Low strain (1)
- Machine learning (1)
- Mass transport (1)
- Material modeling (1)
- Materials Modelling (1)
- Mean-field modelling (1)
- Mechanical anisotropy (1)
- Microstructural characterization (1)
- Microstructure Design (1)
- Misfitting precipitate (1)
- Model (1)
- Nanocrystalline alloys (1)
- Nanoindentation (1)
- Nickel alloys (1)
- Nickel-base alloy (1)
- Optical criterion of brittleness (1)
- Ordering (1)
- Ostwald ripening (1)
- P92 (1)
- Parametric modeling (1)
- Phase Diagram (1)
- Phase field model (1)
- Phase stability (1)
- Phase-Field Simulations (1)
- Phase-field modelling (1)
- Pore (1)
- Precipitate shape (1)
- Precipitation (1)
- Production scatter (1)
- Refractory high entropy alloys (1)
- Refractory superalloys (1)
- Relaxation fatigue (1)
- Resistance spot welding (1)
- S-phase (1)
- Safety assessment (1)
- Scanning electron microscopy (1)
- Segregation engineering (1)
- Selective laser melting (SLM) (1)
- Short-range order (1)
- Single-Crystal (1)
- Single-crystal (1)
- Slow crack growth (SCG) (1)
- Spinodal decomposition (1)
- Steels (1)
- Strain energy (1)
- Superalloy (1)
- Symmetric dwell periods (1)
- Thermomechanical fatigue (1)
- Transmission electron microscopy (1)
- Transmission electron microscopy (TEM) (1)
- Turbine disk (1)
- Viscoplasticity (1)
- X-ray computed tomography (CT) (1)
- γ″ phase (1)
- θ′ phase (1)
- θ′-(Al2Cu) precipitate phase (1)
Organisationseinheit der BAM
- 5.5 Materialmodellierung (42) (entfernen)
Paper des Monats
- ja (3)
A key limitation of the most constitutive models that reproduce a Degradation of quasi-brittle materials is that they generally do not address issues related to fatigue. One reason is the huge computational costs to resolve each load cycle on the structural level. The goal of this paper is the development of a temporal Integration scheme, which significantly increases the computational efficiency of the finite element method in comparison to conventional temporal integrations.
The essential constituent of the fatigue model is an implicit gradient-enhanced formulation of the damage rate. The evolution of the field variables is computed as amultiscale Fourier series in time.On a microchronological scale attributed to single cycles, the initial boundary value problem is approximated by linear BVPs with respect to the Fourier coefficients. Using the adaptive cycle jump concept, the obtained damage rates are transferred to a coarsermacrochronological scale associated with the duration of material deterioration. The performance of the developedmethod is hence improved due to an efficient numerical treatment of the microchronological problem in combination with the cycle jump technique on the macrochronological scale. Validation examples demonstrate the convergence of the obtained solutions to the reference simulations while significantly reducing the computational costs.
Systematic microstructure design requires reliable thermodynamic descriptions of each and all microstructure elements. While such descriptions are well established for most bulk phases, thermodynamic assessment of microstructure defects is challenging because of their individualistic nature. In this paper, a model is devised for assessing grain boundary thermodynamics based on available bulk thermodynamic data. We propose a continuous relative atomic density field and its spatial gradients to describe the grain boundary region with reference to the homogeneous bulk and derive the grain boundary Gibbs free energy functional. The grain boundary segregation isotherm and phase diagram are computed for a regular binary solid solution, and qualitatively benchmarked for the Pt–Au system. The relationships between the grain boundary's atomic density, excess free volume, and misorientation angle are discussed. Combining the current density-based model with available bulk thermodynamic databases enables constructing databases, phase diagrams, and segregation isotherms for grain boundaries, opening possibilities for studying and designing heterogeneous microstructures.
A phenomenological criterion for an optical assessment of PE-HD fracture surfaces obtained from FNCT
(2021)
The full-notch creep test (FNCT) is a common test method to evaluate the environmental stress cracking (ESC) behavior of high-density polyethylene (PE-HD), e.g. for container materials. The test procedure as specified in ISO 16770 provides a comparative measure of the resistance against ESC using the time to failure of PE-HD specimens under constant mechanical load in a well-defined liquid test environment. Since the craze-crack damage mechanism underlying the ESC phenomenon is associated with brittle failure, the occurrence of a predominantly brittle fracture surface is a prerequisite to consider an FNCT measurement as representative for ESC, i.e. a time to failure dominated by craze-crack propagation.
The craze-crack propagation continuously reduces the effective residual cross-sectional area of the specimen during the test, which results in a corresponding increase of the effective mechanical stress. Thus, a transition to ductile shear deformation is inevitable at later stages of the test, leading usually to a pronounced central ligament.
Therefore, an optical evaluation of FNCT fracture surfaces concerning their brittleness is essential. An enhanced imaging analysis of FNCT fracture surfaces enables a detailed assessment of craze-crack Propagation during ESC. In this study, laser scanning microscopy (LSM) was employed to evaluate whether FNCT fracture surfaces are representative with respect to craze-crack propagation and ESC. Based on LSM height data, a phenomenological criterion is proposed to assess the validity of distinct FNCT measurements. This criterion is
supposed to facilitate a quick evaluation of FNCT results in practical routine testing. Its applicability is verified on a sample basis for seven different commercial PE-HD container materials.
The sound numerical prediction of welding-induced thermal stresses, residual stresses, and distortions strongly depends on the accurate description of a welded material’s thermomechanical deformation behaviour. In this work, we provide experimental data on the viscoplastic deformation behaviour of a grade-s960ql steel up to a temperature of 1000 ◦C. In addition, a multi-phase viscoplastic material model is proposed, which accounts for the experimentally observed isothermal deformation behaviour of grade-s960ql steel base and austenitised material, as well as for athermal contributions that originate from solid-state phase transformations. The multi-phase viscoplastic and a classic rateindependent isotropic hardening material model were applied in the numerical simulations of both-ends-fixed bar Satoh tests and a single-pass gas metal arc weld. The influence of material modelling choices on the agreement between numerical simulation and experimental results is discussed, and recommendations for further work are given.
Aero-engine turbine disks are safety-relevant components which are operated under high thermal and mechanical stress conditions. The actual part qualification and certification procedures make use of spin-tests conducted on production-similar disks. While these tests provide, on the one hand, a reliable definition of the critical conditions for real components, on the other hand they represent a relevant cost item for engine manufacturers. The aim of this work is to present two alternative burst speed assessment methods under development based on the Failure Assessment Diagram (FAD) and a global stability criterion, respectively. In the scope of the fracture mechanics assessment, the failure modes hoop-burst and rim-peeling are investigated with semicircular surface cracks modelled at the critical regions on the turbine disk. The comparison of the predicted critical rotational speed shows good agreement between the assessment methods.
Along with the desire for developing novel multi-principal element alloys, also known as high-entropy alloys, the concern about their safe application is also increasingly growing. This relates to the alloys’ phase stability, in particular, the control required over unexpected phase decompositions resulting from solute segregation at grain boundaries. Yet, the mechanisms of co-segregation and grain boundary phase decomposition in multi-component alloys are rather challenging to explore. In fact, quantitative investigation of grain boundary behaviors is mostly conducted for binary and a few ternary alloys. In this work, we apply the recently introduced CALPHAD-integrated density-based formalism [RSC Advances 10 (2020) 26728-26741] for considering co-segregation phenomena in alloys with an arbitrary number of components —the term ‘co-segregation’ here refers to co-evolution and any mutual interplay among the solute atoms during their interaction with a grain boundary. Quaternary Fe-Co-Mn-Cr alloy system is studied. We present two major advances beyond previous results: First, a co-segregation-induced multi-component grain boundary spinodal decomposition is quantitatively simulated for the first time. We found that in addition to its low cohesive energy and asymmetrical mixing enthalpy due to magnetic ordering, Mn plays a leading role in triggering interfacial phase decomposition by having a relatively large, concentration-dependent atomic mobility. Second, as an alternative to grain boundary phase diagrams proposed for binary and ternary alloys, we introduce the concept of co-segregation maps for grain boundary segregation screening and design in multi-component alloys. Applying the co-segregation maps, the nonlinear Mn and Cr co-segregation are discussed. Depicted on the alloying composition and phase space, the co-segregation maps enable the required insights to guide a safer, more controlled design of high-entropy alloys.
Computational methods for lifetime prediction of metallic components under high-temperature fatigue
(2019)
The issue of service life prediction of hot metallic components subjected to cyclic loadings is addressed. Two classes of lifetime models are considered, namely, the incremental lifetime rules and the parametric models governed by the fracture mechanics concept. Examples of application to an austenitic cast iron are presented. In addition, computational techniques to accelerate the time integration of the incremental models throughout the fatigue loading history are discussed. They efficiently solve problems where a stabilized response of a component is not observed, for example due to the plastic strain which is no longer completely reversed and accumulates throughout the fatigue history. The performance of such an accelerated Integration technique is demonstrated for a finite element simulation of a viscoplastic solid under repeating loading–unloading cycles.
A density-based phase field model is developed where the free energy functional is explicitly linked with molecular dynamics and is referred to as the Molecular Phase Field Method (MoPF). MoPF simulations involve expressing interatomic potentials in terms of density to form a density based free energy functional. Inputs to this functional are taken from atomistics such that the phase field density profile matches the corresponding density profile from atomistic simulations. We analyze our results by comparing the MoPF calculated excess interfacial energies with excess interfacial energies calculated using molecular dynamics associated with several nickel grain boundaries. Additionally, a comparison is made between our results and the interfacial energies of a \Sigma7 boundary across a variety of FCC systems simulated using density functional theory. The MoPF method is able to successfully predict grain boundary free energy trends between grain boundary and material types offering an atomistically informed mesoscale formulation for studying grain boundary physics.
The creep behavior of single crystals of the nickel-based superalloy CMSX-4 was investigated at 1288 °C, which is the temperature of the hot isostatic pressing treatment applied to this superalloy in the industry. It was found that at this super-solvus temperature, where no gammaPrime-strengthening occurs, the superalloy is very soft and rapidly deforms under stresses between 4 and 16 MPa. The creep resistance was found to be very anisotropic, e.g., the creep rate of [001] crystals was about 11 times higher than that of a [111] crystal. The specimens of different orientations also showed a very different necking behavior. The reduction of the cross-sectional area psi of [001] crystals reached nearly 100 pct, while for a [111] crystal psi = 62 pct. The EBSD analysis of deformed specimens showed that despite such a large local strain the [001] crystals did not recrystallize, while a less deformed [111] crystal totally recrystallized within the necking zone. The recrystallization degree was found to be correlated with deformation behavior as well as with dwell time at high temperature. From the analysis of the obtained results (creep anisotropy, stress dependence of the creep rate, traces of shear deformation, and TEM observations), it was concluded that the main strain contribution resulted from <01-1>{111} octahedral slip.
A γ-analogue of the superalloy CMSX-4 that does not contain the strengthening γ′ -phase and only consists of the γ-solid solution of nickel has been designed, solidified as single-crystals of different orientations, and tested under creep conditions in the temperature range between 1150 and 1288 °C. The tests have revealed a very high creep anisotropy of this alloy, as was previously found for CMSX-4 at supersolvus temperature of 1288 °C. This creep anisotropy could be explained by the dominance of 〈011〉{111} octahedral slip. Furthermore, the analysis of the creep data has yielded a high value of the creep activation energy, Qc≈442 kJ/mol, which correlates with the high activation energy of Re diffusion in Ni. This supports the hypothesis that dislocation motion in the γ-matrix of Re-containing superalloys is controlled by the diffusion of the Re atoms segregating at the dislocation core. The Norton stress exponent n is close to 5, which is a typical value for pure metals and their alloys. The absence of γ′ -reprecipitation after high-temperature creep tests facilitates microstructural investigations. It has been shown by EBSD that creep deformation results in an increasing misorientation of the existing low angle boundaries. In addition, according to TEM, new low angle boundaries appear due to reactions of the a/2 〈011〉 mobile dislocations and knitting of new networks.